diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index d2828669d43..f918facc86d 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -150,7 +150,10 @@ impl LlvmType for CastTarget {
         // Simplify to a single unit or an array if there's no prefix.
         // This produces the same layout, but using a simpler type.
         if self.prefix.iter().all(|x| x.is_none()) {
-            if rest_count == 1 {
+            // We can't do this if is_consecutive is set and the unit would get
+            // split on the target. Currently, this is only relevant for i128
+            // registers.
+            if rest_count == 1 && (!self.rest.is_consecutive || self.rest.unit != Reg::i128()) {
                 return rest_ll_unit;
             }
 
diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index 8ec1f5a99e7..a6894a7e089 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
@@ -1524,8 +1524,8 @@ extern "C" LLVMValueRef LLVMRustBuildCall(LLVMBuilderRef B, LLVMTypeRef Ty, LLVM
 }
 
 extern "C" LLVMValueRef LLVMRustGetInstrProfIncrementIntrinsic(LLVMModuleRef M) {
-  return wrap(llvm::Intrinsic::getDeclaration(unwrap(M),
-              (llvm::Intrinsic::ID)llvm::Intrinsic::instrprof_increment));
+  return wrap(llvm::Intrinsic::getDeclaration(
+      unwrap(M), llvm::Intrinsic::instrprof_increment));
 }
 
 extern "C" LLVMValueRef LLVMRustBuildMemCpy(LLVMBuilderRef B,
diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
index 4a7720b38f8..f8f59fbeab4 100644
--- a/compiler/rustc_middle/src/ty/instance.rs
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -520,7 +520,10 @@ impl<'tcx> Instance<'tcx> {
                 // Reify `Trait::method` implementations if KCFI is enabled
                 // FIXME(maurer) only reify it if it is a vtable-safe function
                 _ if tcx.sess.is_sanitizer_kcfi_enabled()
-                    && tcx.associated_item(def_id).trait_item_def_id.is_some() =>
+                    && tcx
+                        .opt_associated_item(def_id)
+                        .and_then(|assoc| assoc.trait_item_def_id)
+                        .is_some() =>
                 {
                     // If this function could also go in a vtable, we need to `ReifyShim` it with
                     // KCFI because it can only attach one type per function.
diff --git a/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs b/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
index de43f9faff9..320d8fd3977 100644
--- a/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
+++ b/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
@@ -58,16 +58,24 @@
 //! borrowing from the outer closure, and we simply peel off a `deref` projection
 //! from them. This second body is stored alongside the first body, and optimized
 //! with it in lockstep. When we need to resolve a body for `FnOnce` or `AsyncFnOnce`,
-//! we use this "by move" body instead.
+//! we use this "by-move" body instead.
+//!
+//! ## How does this work?
+//!
+//! This pass essentially remaps the body of the (child) closure of the coroutine-closure
+//! to take the set of upvars of the parent closure by value. This at least requires
+//! changing a by-ref upvar to be by-value in the case that the outer coroutine-closure
+//! captures something by value; however, it may also require renumbering field indices
+//! in case precise captures (edition 2021 closure capture rules) caused the inner coroutine
+//! to split one field capture into two.
 
-use itertools::Itertools;
-
-use rustc_data_structures::unord::UnordSet;
+use rustc_data_structures::unord::UnordMap;
 use rustc_hir as hir;
+use rustc_middle::hir::place::{PlaceBase, Projection, ProjectionKind};
 use rustc_middle::mir::visit::MutVisitor;
 use rustc_middle::mir::{self, dump_mir, MirPass};
 use rustc_middle::ty::{self, InstanceDef, Ty, TyCtxt, TypeVisitableExt};
-use rustc_target::abi::FieldIdx;
+use rustc_target::abi::{FieldIdx, VariantIdx};
 
 pub struct ByMoveBody;
 
@@ -116,32 +124,116 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
             .tuple_fields()
             .len();
 
-        let mut by_ref_fields = UnordSet::default();
-        for (idx, (coroutine_capture, parent_capture)) in tcx
+        let mut field_remapping = UnordMap::default();
+
+        // One parent capture may correspond to several child captures if we end up
+        // refining the set of captures via edition-2021 precise captures. We want to
+        // match up any number of child captures with one parent capture, so we keep
+        // peeking off this `Peekable` until the child doesn't match anymore.
+        let mut parent_captures =
+            tcx.closure_captures(parent_def_id).iter().copied().enumerate().peekable();
+        // Make sure we use every field at least once, b/c why are we capturing something
+        // if it's not used in the inner coroutine.
+        let mut field_used_at_least_once = false;
+
+        for (child_field_idx, child_capture) in tcx
             .closure_captures(coroutine_def_id)
             .iter()
+            .copied()
             // By construction we capture all the args first.
             .skip(num_args)
-            .zip_eq(tcx.closure_captures(parent_def_id))
             .enumerate()
         {
-            // This upvar is captured by-move from the parent closure, but by-ref
-            // from the inner async block. That means that it's being borrowed from
-            // the outer closure body -- we need to change the coroutine to take the
-            // upvar by value.
-            if coroutine_capture.is_by_ref() && !parent_capture.is_by_ref() {
-                assert_ne!(
-                    coroutine_kind,
-                    ty::ClosureKind::FnOnce,
-                    "`FnOnce` coroutine-closures return coroutines that capture from \
-                    their body; it will always result in a borrowck error!"
-                );
-                by_ref_fields.insert(FieldIdx::from_usize(num_args + idx));
-            }
+            loop {
+                let Some(&(parent_field_idx, parent_capture)) = parent_captures.peek() else {
+                    bug!("we ran out of parent captures!")
+                };
 
-            // Make sure we're actually talking about the same capture.
-            // FIXME(async_closures): We could look at the `hir::Upvar` instead?
-            assert_eq!(coroutine_capture.place.ty(), parent_capture.place.ty());
+                let PlaceBase::Upvar(parent_base) = parent_capture.place.base else {
+                    bug!("expected capture to be an upvar");
+                };
+                let PlaceBase::Upvar(child_base) = child_capture.place.base else {
+                    bug!("expected capture to be an upvar");
+                };
+
+                assert!(
+                    child_capture.place.projections.len() >= parent_capture.place.projections.len()
+                );
+                // A parent matches a child they share the same prefix of projections.
+                // The child may have more, if it is capturing sub-fields out of
+                // something that is captured by-move in the parent closure.
+                if parent_base.var_path.hir_id != child_base.var_path.hir_id
+                    || !std::iter::zip(
+                        &child_capture.place.projections,
+                        &parent_capture.place.projections,
+                    )
+                    .all(|(child, parent)| child.kind == parent.kind)
+                {
+                    // Make sure the field was used at least once.
+                    assert!(
+                        field_used_at_least_once,
+                        "we captured {parent_capture:#?} but it was not used in the child coroutine?"
+                    );
+                    field_used_at_least_once = false;
+                    // Skip this field.
+                    let _ = parent_captures.next().unwrap();
+                    continue;
+                }
+
+                // Store this set of additional projections (fields and derefs).
+                // We need to re-apply them later.
+                let child_precise_captures =
+                    &child_capture.place.projections[parent_capture.place.projections.len()..];
+
+                // If the parent captures by-move, and the child captures by-ref, then we
+                // need to peel an additional `deref` off of the body of the child.
+                let needs_deref = child_capture.is_by_ref() && !parent_capture.is_by_ref();
+                if needs_deref {
+                    assert_ne!(
+                        coroutine_kind,
+                        ty::ClosureKind::FnOnce,
+                        "`FnOnce` coroutine-closures return coroutines that capture from \
+                        their body; it will always result in a borrowck error!"
+                    );
+                }
+
+                // Finally, store the type of the parent's captured place. We need
+                // this when building the field projection in the MIR body later on.
+                let mut parent_capture_ty = parent_capture.place.ty();
+                parent_capture_ty = match parent_capture.info.capture_kind {
+                    ty::UpvarCapture::ByValue => parent_capture_ty,
+                    ty::UpvarCapture::ByRef(kind) => Ty::new_ref(
+                        tcx,
+                        tcx.lifetimes.re_erased,
+                        parent_capture_ty,
+                        kind.to_mutbl_lossy(),
+                    ),
+                };
+
+                field_remapping.insert(
+                    FieldIdx::from_usize(child_field_idx + num_args),
+                    (
+                        FieldIdx::from_usize(parent_field_idx + num_args),
+                        parent_capture_ty,
+                        needs_deref,
+                        child_precise_captures,
+                    ),
+                );
+
+                field_used_at_least_once = true;
+                break;
+            }
+        }
+
+        // Pop the last parent capture
+        if field_used_at_least_once {
+            let _ = parent_captures.next().unwrap();
+        }
+        assert_eq!(parent_captures.next(), None, "leftover parent captures?");
+
+        if coroutine_kind == ty::ClosureKind::FnOnce {
+            assert_eq!(field_remapping.len(), tcx.closure_captures(parent_def_id).len());
+            return;
         }
 
         let by_move_coroutine_ty = tcx
@@ -157,7 +249,7 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
             );
 
         let mut by_move_body = body.clone();
-        MakeByMoveBody { tcx, by_ref_fields, by_move_coroutine_ty }.visit_body(&mut by_move_body);
+        MakeByMoveBody { tcx, field_remapping, by_move_coroutine_ty }.visit_body(&mut by_move_body);
         dump_mir(tcx, false, "coroutine_by_move", &0, &by_move_body, |_, _| Ok(()));
         by_move_body.source = mir::MirSource::from_instance(InstanceDef::CoroutineKindShim {
             coroutine_def_id: coroutine_def_id.to_def_id(),
@@ -168,7 +260,7 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
 
 struct MakeByMoveBody<'tcx> {
     tcx: TyCtxt<'tcx>,
-    by_ref_fields: UnordSet<FieldIdx>,
+    field_remapping: UnordMap<FieldIdx, (FieldIdx, Ty<'tcx>, bool, &'tcx [Projection<'tcx>])>,
     by_move_coroutine_ty: Ty<'tcx>,
 }
 
@@ -183,24 +275,59 @@ impl<'tcx> MutVisitor<'tcx> for MakeByMoveBody<'tcx> {
         context: mir::visit::PlaceContext,
         location: mir::Location,
     ) {
+        // Initializing an upvar local always starts with `CAPTURE_STRUCT_LOCAL` and a
+        // field projection. If this is in `field_remapping`, then it must not be an
+        // arg from calling the closure, but instead an upvar.
         if place.local == ty::CAPTURE_STRUCT_LOCAL
-            && let Some((&mir::ProjectionElem::Field(idx, ty), projection)) =
+            && let Some((&mir::ProjectionElem::Field(idx, _), projection)) =
                 place.projection.split_first()
-            && self.by_ref_fields.contains(&idx)
+            && let Some(&(remapped_idx, remapped_ty, needs_deref, additional_projections)) =
+                self.field_remapping.get(&idx)
         {
-            let (begin, end) = projection.split_first().unwrap();
-            // FIXME(async_closures): I'm actually a bit surprised to see that we always
-            // initially deref the by-ref upvars. If this is not actually true, then we
-            // will at least get an ICE that explains why this isn't true :^)
-            assert_eq!(*begin, mir::ProjectionElem::Deref);
-            // Peel one ref off of the ty.
-            let peeled_ty = ty.builtin_deref(true).unwrap().ty;
+            // As noted before, if the parent closure captures a field by value, and
+            // the child captures a field by ref, then for the by-move body we're
+            // generating, we also are taking that field by value. Peel off a deref,
+            // since a layer of reffing has now become redundant.
+            let final_deref = if needs_deref {
+                let Some((mir::ProjectionElem::Deref, projection)) = projection.split_first()
+                else {
+                    bug!(
+                        "There should be at least a single deref for an upvar local initialization, found {projection:#?}"
+                    );
+                };
+                // There may be more derefs, since we may also implicitly reborrow
+                // a captured mut pointer.
+                projection
+            } else {
+                projection
+            };
+
+            // The only thing that should be left is a deref, if the parent captured
+            // an upvar by-ref.
+            std::assert_matches::assert_matches!(final_deref, [] | [mir::ProjectionElem::Deref]);
+
+            // For all of the additional projections that come out of precise capturing,
+            // re-apply these projections.
+            let additional_projections =
+                additional_projections.iter().map(|elem| match elem.kind {
+                    ProjectionKind::Deref => mir::ProjectionElem::Deref,
+                    ProjectionKind::Field(idx, VariantIdx::ZERO) => {
+                        mir::ProjectionElem::Field(idx, elem.ty)
+                    }
+                    _ => unreachable!("precise captures only through fields and derefs"),
+                });
+
+            // We start out with an adjusted field index (and ty), representing the
+            // upvar that we get from our parent closure. We apply any of the additional
+            // projections to make sure that to the rest of the body of the closure, the
+            // place looks the same, and then apply that final deref if necessary.
             *place = mir::Place {
                 local: place.local,
                 projection: self.tcx.mk_place_elems_from_iter(
-                    [mir::ProjectionElem::Field(idx, peeled_ty)]
+                    [mir::ProjectionElem::Field(remapped_idx, remapped_ty)]
                         .into_iter()
-                        .chain(end.iter().copied()),
+                        .chain(additional_projections)
+                        .chain(final_deref.iter().copied()),
                 ),
             };
         }
diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index 69b48bf0aff..353c3f41ed8 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -34,8 +34,7 @@ use unescape_error_reporting::{emit_unescape_error, escaped_char};
 rustc_data_structures::static_assert_size!(rustc_lexer::Token, 12);
 
 #[derive(Clone, Debug)]
-pub struct UnmatchedDelim {
-    pub expected_delim: Delimiter,
+pub(crate) struct UnmatchedDelim {
     pub found_delim: Option<Delimiter>,
     pub found_span: Span,
     pub unclosed_span: Option<Span>,
diff --git a/compiler/rustc_parse/src/lexer/tokentrees.rs b/compiler/rustc_parse/src/lexer/tokentrees.rs
index a506f98bf3a..b5a5a2a90ee 100644
--- a/compiler/rustc_parse/src/lexer/tokentrees.rs
+++ b/compiler/rustc_parse/src/lexer/tokentrees.rs
@@ -77,7 +77,6 @@ impl<'psess, 'src> TokenTreesReader<'psess, 'src> {
         for &(_, sp) in &self.diag_info.open_braces {
             err.span_label(sp, "unclosed delimiter");
             self.diag_info.unmatched_delims.push(UnmatchedDelim {
-                expected_delim: Delimiter::Brace,
                 found_delim: None,
                 found_span: self.token.span,
                 unclosed_span: Some(sp),
@@ -163,9 +162,8 @@ impl<'psess, 'src> TokenTreesReader<'psess, 'src> {
                             candidate = Some(*brace_span);
                         }
                     }
-                    let (tok, _) = self.diag_info.open_braces.pop().unwrap();
+                    let (_, _) = self.diag_info.open_braces.pop().unwrap();
                     self.diag_info.unmatched_delims.push(UnmatchedDelim {
-                        expected_delim: tok,
                         found_delim: Some(close_delim),
                         found_span: self.token.span,
                         unclosed_span: unclosed_delimiter,
diff --git a/compiler/rustc_target/src/abi/call/aarch64.rs b/compiler/rustc_target/src/abi/call/aarch64.rs
index f99f6a3b721..04020d13f22 100644
--- a/compiler/rustc_target/src/abi/call/aarch64.rs
+++ b/compiler/rustc_target/src/abi/call/aarch64.rs
@@ -31,7 +31,7 @@ where
             RegKind::Vector => size.bits() == 64 || size.bits() == 128,
         };
 
-        valid_unit.then_some(Uniform { unit, total: size })
+        valid_unit.then_some(Uniform::consecutive(unit, size))
     })
 }
 
@@ -60,7 +60,7 @@ where
     let size = ret.layout.size;
     let bits = size.bits();
     if bits <= 128 {
-        ret.cast_to(Uniform { unit: Reg::i64(), total: size });
+        ret.cast_to(Uniform::new(Reg::i64(), size));
         return;
     }
     ret.make_indirect();
@@ -100,9 +100,9 @@ where
     };
     if size.bits() <= 128 {
         if align.bits() == 128 {
-            arg.cast_to(Uniform { unit: Reg::i128(), total: size });
+            arg.cast_to(Uniform::new(Reg::i128(), size));
         } else {
-            arg.cast_to(Uniform { unit: Reg::i64(), total: size });
+            arg.cast_to(Uniform::new(Reg::i64(), size));
         }
         return;
     }
diff --git a/compiler/rustc_target/src/abi/call/arm.rs b/compiler/rustc_target/src/abi/call/arm.rs
index 95f6691d42a..9371e1b3958 100644
--- a/compiler/rustc_target/src/abi/call/arm.rs
+++ b/compiler/rustc_target/src/abi/call/arm.rs
@@ -21,7 +21,7 @@ where
             RegKind::Vector => size.bits() == 64 || size.bits() == 128,
         };
 
-        valid_unit.then_some(Uniform { unit, total: size })
+        valid_unit.then_some(Uniform::consecutive(unit, size))
     })
 }
 
@@ -49,7 +49,7 @@ where
     let size = ret.layout.size;
     let bits = size.bits();
     if bits <= 32 {
-        ret.cast_to(Uniform { unit: Reg::i32(), total: size });
+        ret.cast_to(Uniform::new(Reg::i32(), size));
         return;
     }
     ret.make_indirect();
@@ -78,7 +78,7 @@ where
 
     let align = arg.layout.align.abi.bytes();
     let total = arg.layout.size;
-    arg.cast_to(Uniform { unit: if align <= 4 { Reg::i32() } else { Reg::i64() }, total });
+    arg.cast_to(Uniform::consecutive(if align <= 4 { Reg::i32() } else { Reg::i64() }, total));
 }
 
 pub fn compute_abi_info<'a, Ty, C>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>)
diff --git a/compiler/rustc_target/src/abi/call/csky.rs b/compiler/rustc_target/src/abi/call/csky.rs
index 8b4328db52e..7951f28beea 100644
--- a/compiler/rustc_target/src/abi/call/csky.rs
+++ b/compiler/rustc_target/src/abi/call/csky.rs
@@ -18,7 +18,7 @@ fn classify_ret<Ty>(arg: &mut ArgAbi<'_, Ty>) {
         if total.bits() > 64 {
             arg.make_indirect();
         } else if total.bits() > 32 {
-            arg.cast_to(Uniform { unit: Reg::i32(), total });
+            arg.cast_to(Uniform::new(Reg::i32(), total));
         } else {
             arg.cast_to(Reg::i32());
         }
@@ -38,7 +38,7 @@ fn classify_arg<Ty>(arg: &mut ArgAbi<'_, Ty>) {
     if arg.layout.is_aggregate() {
         let total = arg.layout.size;
         if total.bits() > 32 {
-            arg.cast_to(Uniform { unit: Reg::i32(), total });
+            arg.cast_to(Uniform::new(Reg::i32(), total));
         } else {
             arg.cast_to(Reg::i32());
         }
diff --git a/compiler/rustc_target/src/abi/call/loongarch.rs b/compiler/rustc_target/src/abi/call/loongarch.rs
index 35d4b331cb4..943b12a9fbf 100644
--- a/compiler/rustc_target/src/abi/call/loongarch.rs
+++ b/compiler/rustc_target/src/abi/call/loongarch.rs
@@ -195,7 +195,7 @@ where
         if total.bits() <= xlen {
             arg.cast_to(xlen_reg);
         } else {
-            arg.cast_to(Uniform { unit: xlen_reg, total: Size::from_bits(xlen * 2) });
+            arg.cast_to(Uniform::new(xlen_reg, Size::from_bits(xlen * 2)));
         }
         return false;
     }
@@ -278,10 +278,10 @@ fn classify_arg<'a, Ty, C>(
     if total.bits() > xlen {
         let align_regs = align > xlen;
         if is_loongarch_aggregate(arg) {
-            arg.cast_to(Uniform {
-                unit: if align_regs { double_xlen_reg } else { xlen_reg },
-                total: Size::from_bits(xlen * 2),
-            });
+            arg.cast_to(Uniform::new(
+                if align_regs { double_xlen_reg } else { xlen_reg },
+                Size::from_bits(xlen * 2),
+            ));
         }
         if align_regs && is_vararg {
             *avail_gprs -= *avail_gprs % 2;
diff --git a/compiler/rustc_target/src/abi/call/mips.rs b/compiler/rustc_target/src/abi/call/mips.rs
index 57ccfe2152b..0e5a7f37a09 100644
--- a/compiler/rustc_target/src/abi/call/mips.rs
+++ b/compiler/rustc_target/src/abi/call/mips.rs
@@ -27,7 +27,7 @@ where
 
     if arg.layout.is_aggregate() {
         let pad_i32 = !offset.is_aligned(align);
-        arg.cast_to_and_pad_i32(Uniform { unit: Reg::i32(), total: size }, pad_i32);
+        arg.cast_to_and_pad_i32(Uniform::new(Reg::i32(), size), pad_i32);
     } else {
         arg.extend_integer_width_to(32);
     }
diff --git a/compiler/rustc_target/src/abi/call/mips64.rs b/compiler/rustc_target/src/abi/call/mips64.rs
index 2700f67b209..b2a2c34b980 100644
--- a/compiler/rustc_target/src/abi/call/mips64.rs
+++ b/compiler/rustc_target/src/abi/call/mips64.rs
@@ -68,7 +68,7 @@ where
         }
 
         // Cast to a uniform int structure
-        ret.cast_to(Uniform { unit: Reg::i64(), total: size });
+        ret.cast_to(Uniform::new(Reg::i64(), size));
     } else {
         ret.make_indirect();
     }
@@ -139,7 +139,7 @@ where
     let rest_size = size - Size::from_bytes(8) * prefix_index as u64;
     arg.cast_to(CastTarget {
         prefix,
-        rest: Uniform { unit: Reg::i64(), total: rest_size },
+        rest: Uniform::new(Reg::i64(), rest_size),
         attrs: ArgAttributes {
             regular: ArgAttribute::default(),
             arg_ext: ArgExtension::None,
diff --git a/compiler/rustc_target/src/abi/call/mod.rs b/compiler/rustc_target/src/abi/call/mod.rs
index cdd3f0afd79..4502df339d1 100644
--- a/compiler/rustc_target/src/abi/call/mod.rs
+++ b/compiler/rustc_target/src/abi/call/mod.rs
@@ -255,11 +255,16 @@ pub struct Uniform {
     ///   for 64-bit integers with a total size of 20 bytes. When the argument is actually passed,
     ///   this size will be rounded up to the nearest multiple of `unit.size`.
     pub total: Size,
+
+    /// Indicate that the argument is consecutive, in the sense that either all values need to be
+    /// passed in register, or all on the stack. If they are passed on the stack, there should be
+    /// no additional padding between elements.
+    pub is_consecutive: bool,
 }
 
 impl From<Reg> for Uniform {
     fn from(unit: Reg) -> Uniform {
-        Uniform { unit, total: unit.size }
+        Uniform { unit, total: unit.size, is_consecutive: false }
     }
 }
 
@@ -267,6 +272,18 @@ impl Uniform {
     pub fn align<C: HasDataLayout>(&self, cx: &C) -> Align {
         self.unit.align(cx)
     }
+
+    /// Pass using one or more values of the given type, without requiring them to be consecutive.
+    /// That is, some values may be passed in register and some on the stack.
+    pub fn new(unit: Reg, total: Size) -> Self {
+        Uniform { unit, total, is_consecutive: false }
+    }
+
+    /// Pass using one or more consecutive values of the given type. Either all values will be
+    /// passed in registers, or all on the stack.
+    pub fn consecutive(unit: Reg, total: Size) -> Self {
+        Uniform { unit, total, is_consecutive: true }
+    }
 }
 
 /// Describes the type used for `PassMode::Cast`.
diff --git a/compiler/rustc_target/src/abi/call/nvptx64.rs b/compiler/rustc_target/src/abi/call/nvptx64.rs
index 5c040ce9c3b..f85fa2419f0 100644
--- a/compiler/rustc_target/src/abi/call/nvptx64.rs
+++ b/compiler/rustc_target/src/abi/call/nvptx64.rs
@@ -35,7 +35,7 @@ where
             16 => Reg::i128(),
             _ => unreachable!("Align is given as power of 2 no larger than 16 bytes"),
         };
-        arg.cast_to(Uniform { unit, total: Size::from_bytes(2 * align_bytes) });
+        arg.cast_to(Uniform::new(unit, Size::from_bytes(2 * align_bytes)));
     } else {
         // FIXME: find a better way to do this. See https://github.com/rust-lang/rust/issues/117271.
         arg.make_direct_deprecated();
diff --git a/compiler/rustc_target/src/abi/call/powerpc64.rs b/compiler/rustc_target/src/abi/call/powerpc64.rs
index 2d41f77e50e..11a6cb52bab 100644
--- a/compiler/rustc_target/src/abi/call/powerpc64.rs
+++ b/compiler/rustc_target/src/abi/call/powerpc64.rs
@@ -2,7 +2,7 @@
 // Alignment of 128 bit types is not currently handled, this will
 // need to be fixed when PowerPC vector support is added.
 
-use crate::abi::call::{ArgAbi, FnAbi, Reg, RegKind, Uniform};
+use crate::abi::call::{Align, ArgAbi, FnAbi, Reg, RegKind, Uniform};
 use crate::abi::{Endian, HasDataLayout, TyAbiInterface};
 use crate::spec::HasTargetSpec;
 
@@ -37,7 +37,7 @@ where
             RegKind::Vector => arg.layout.size.bits() == 128,
         };
 
-        valid_unit.then_some(Uniform { unit, total: arg.layout.size })
+        valid_unit.then_some(Uniform::consecutive(unit, arg.layout.size))
     })
 }
 
@@ -81,7 +81,7 @@ where
             Reg::i64()
         };
 
-        ret.cast_to(Uniform { unit, total: size });
+        ret.cast_to(Uniform::new(unit, size));
         return;
     }
 
@@ -108,18 +108,20 @@ where
     }
 
     let size = arg.layout.size;
-    let (unit, total) = if size.bits() <= 64 {
+    if size.bits() <= 64 {
         // Aggregates smaller than a doubleword should appear in
         // the least-significant bits of the parameter doubleword.
-        (Reg { kind: RegKind::Integer, size }, size)
+        arg.cast_to(Reg { kind: RegKind::Integer, size })
     } else {
-        // Aggregates larger than a doubleword should be padded
-        // at the tail to fill out a whole number of doublewords.
-        let reg_i64 = Reg::i64();
-        (reg_i64, size.align_to(reg_i64.align(cx)))
+        // Aggregates larger than i64 should be padded at the tail to fill out a whole number
+        // of i64s or i128s, depending on the aggregate alignment. Always use an array for
+        // this, even if there is only a single element.
+        let reg = if arg.layout.align.abi.bytes() > 8 { Reg::i128() } else { Reg::i64() };
+        arg.cast_to(Uniform::consecutive(
+            reg,
+            size.align_to(Align::from_bytes(reg.size.bytes()).unwrap()),
+        ))
     };
-
-    arg.cast_to(Uniform { unit, total });
 }
 
 pub fn compute_abi_info<'a, Ty, C>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>)
diff --git a/compiler/rustc_target/src/abi/call/riscv.rs b/compiler/rustc_target/src/abi/call/riscv.rs
index 6a38496dc57..5d4b3a9d245 100644
--- a/compiler/rustc_target/src/abi/call/riscv.rs
+++ b/compiler/rustc_target/src/abi/call/riscv.rs
@@ -201,7 +201,7 @@ where
         if total.bits() <= xlen {
             arg.cast_to(xlen_reg);
         } else {
-            arg.cast_to(Uniform { unit: xlen_reg, total: Size::from_bits(xlen * 2) });
+            arg.cast_to(Uniform::new(xlen_reg, Size::from_bits(xlen * 2)));
         }
         return false;
     }
@@ -284,10 +284,10 @@ fn classify_arg<'a, Ty, C>(
     if total.bits() > xlen {
         let align_regs = align > xlen;
         if is_riscv_aggregate(arg) {
-            arg.cast_to(Uniform {
-                unit: if align_regs { double_xlen_reg } else { xlen_reg },
-                total: Size::from_bits(xlen * 2),
-            });
+            arg.cast_to(Uniform::new(
+                if align_regs { double_xlen_reg } else { xlen_reg },
+                Size::from_bits(xlen * 2),
+            ));
         }
         if align_regs && is_vararg {
             *avail_gprs -= *avail_gprs % 2;
diff --git a/compiler/rustc_target/src/abi/call/sparc.rs b/compiler/rustc_target/src/abi/call/sparc.rs
index 57ccfe2152b..0e5a7f37a09 100644
--- a/compiler/rustc_target/src/abi/call/sparc.rs
+++ b/compiler/rustc_target/src/abi/call/sparc.rs
@@ -27,7 +27,7 @@ where
 
     if arg.layout.is_aggregate() {
         let pad_i32 = !offset.is_aligned(align);
-        arg.cast_to_and_pad_i32(Uniform { unit: Reg::i32(), total: size }, pad_i32);
+        arg.cast_to_and_pad_i32(Uniform::new(Reg::i32(), size), pad_i32);
     } else {
         arg.extend_integer_width_to(32);
     }
diff --git a/compiler/rustc_target/src/abi/call/sparc64.rs b/compiler/rustc_target/src/abi/call/sparc64.rs
index cbed5b4afc1..acdcd5cc0d4 100644
--- a/compiler/rustc_target/src/abi/call/sparc64.rs
+++ b/compiler/rustc_target/src/abi/call/sparc64.rs
@@ -192,7 +192,7 @@ where
 
                 arg.cast_to(CastTarget {
                     prefix: data.prefix,
-                    rest: Uniform { unit: Reg::i64(), total: rest_size },
+                    rest: Uniform::new(Reg::i64(), rest_size),
                     attrs: ArgAttributes {
                         regular: data.arg_attribute,
                         arg_ext: ArgExtension::None,
@@ -205,7 +205,7 @@ where
         }
     }
 
-    arg.cast_to(Uniform { unit: Reg::i64(), total });
+    arg.cast_to(Uniform::new(Reg::i64(), total));
 }
 
 pub fn compute_abi_info<'a, Ty, C>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>)
diff --git a/compiler/rustc_target/src/abi/call/wasm.rs b/compiler/rustc_target/src/abi/call/wasm.rs
index a7a2b314a94..a773fb1e814 100644
--- a/compiler/rustc_target/src/abi/call/wasm.rs
+++ b/compiler/rustc_target/src/abi/call/wasm.rs
@@ -1,4 +1,4 @@
-use crate::abi::call::{ArgAbi, FnAbi, Uniform};
+use crate::abi::call::{ArgAbi, FnAbi};
 use crate::abi::{HasDataLayout, TyAbiInterface};
 
 fn unwrap_trivial_aggregate<'a, Ty, C>(cx: &C, val: &mut ArgAbi<'a, Ty>) -> bool
@@ -10,7 +10,7 @@ where
         if let Some(unit) = val.layout.homogeneous_aggregate(cx).ok().and_then(|ha| ha.unit()) {
             let size = val.layout.size;
             if unit.size == size {
-                val.cast_to(Uniform { unit, total: size });
+                val.cast_to(unit);
                 return true;
             }
         }
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 10c03387a5b..837b784f272 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -40,7 +40,7 @@ pub struct ImplCandidate<'tcx> {
 
 enum GetSafeTransmuteErrorAndReason {
     Silent,
-    Error { err_msg: String, safe_transmute_explanation: String },
+    Error { err_msg: String, safe_transmute_explanation: Option<String> },
 }
 
 struct UnsatisfiedConst(pub bool);
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index afdc2c5a7f7..1b8b09ddda1 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -558,7 +558,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                                 GetSafeTransmuteErrorAndReason::Error {
                                     err_msg,
                                     safe_transmute_explanation,
-                                } => (err_msg, Some(safe_transmute_explanation)),
+                                } => (err_msg, safe_transmute_explanation),
                             }
                         } else {
                             (err_msg, None)
@@ -3068,28 +3068,33 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             return GetSafeTransmuteErrorAndReason::Silent;
         };
 
+        let dst = trait_ref.args.type_at(0);
+        let src = trait_ref.args.type_at(1);
+        let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
+
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             obligation.cause,
             src_and_dst,
             assume,
         ) {
             Answer::No(reason) => {
-                let dst = trait_ref.args.type_at(0);
-                let src = trait_ref.args.type_at(1);
-                let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
                 let safe_transmute_explanation = match reason {
                     rustc_transmute::Reason::SrcIsNotYetSupported => {
-                        format!("analyzing the transmutability of `{src}` is not yet supported.")
+                        format!("analyzing the transmutability of `{src}` is not yet supported")
                     }
 
                     rustc_transmute::Reason::DstIsNotYetSupported => {
-                        format!("analyzing the transmutability of `{dst}` is not yet supported.")
+                        format!("analyzing the transmutability of `{dst}` is not yet supported")
                     }
 
                     rustc_transmute::Reason::DstIsBitIncompatible => {
                         format!("at least one value of `{src}` isn't a bit-valid value of `{dst}`")
                     }
 
+                    rustc_transmute::Reason::DstUninhabited => {
+                        format!("`{dst}` is uninhabited")
+                    }
+
                     rustc_transmute::Reason::DstMayHaveSafetyInvariants => {
                         format!("`{dst}` may carry safety invariants")
                     }
@@ -3135,14 +3140,23 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         format!("`{dst}` has an unknown layout")
                     }
                 };
-                GetSafeTransmuteErrorAndReason::Error { err_msg, safe_transmute_explanation }
+                GetSafeTransmuteErrorAndReason::Error {
+                    err_msg,
+                    safe_transmute_explanation: Some(safe_transmute_explanation),
+                }
             }
             // Should never get a Yes at this point! We already ran it before, and did not get a Yes.
             Answer::Yes => span_bug!(
                 span,
                 "Inconsistent rustc_transmute::is_transmutable(...) result, got Yes",
             ),
-            other => span_bug!(span, "Unsupported rustc_transmute::Answer variant: `{other:?}`"),
+            // Reached when a different obligation (namely `Freeze`) causes the
+            // transmutability analysis to fail. In this case, silence the
+            // transmutability error message in favor of that more specific
+            // error.
+            Answer::If(_) => {
+                GetSafeTransmuteErrorAndReason::Error { err_msg, safe_transmute_explanation: None }
+            }
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 2c8116b779b..98d5b466cd0 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -119,7 +119,9 @@ pub fn predicates_for_generics<'tcx>(
 
 /// Determines whether the type `ty` is known to meet `bound` and
 /// returns true if so. Returns false if `ty` either does not meet
-/// `bound` or is not known to meet bound.
+/// `bound` or is not known to meet bound (note that this is
+/// conservative towards *no impl*, which is the opposite of the
+/// `evaluate` methods).
 pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
     infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
@@ -127,8 +129,50 @@ pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
     def_id: DefId,
 ) -> bool {
     let trait_ref = ty::TraitRef::new(infcx.tcx, def_id, [ty]);
-    let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, trait_ref);
-    infcx.predicate_must_hold_modulo_regions(&obligation)
+    pred_known_to_hold_modulo_regions(infcx, param_env, trait_ref)
+}
+
+/// FIXME(@lcnr): this function doesn't seem right and shouldn't exist?
+///
+/// Ping me on zulip if you want to use this method and need help with finding
+/// an appropriate replacement.
+#[instrument(level = "debug", skip(infcx, param_env, pred), ret)]
+fn pred_known_to_hold_modulo_regions<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    pred: impl ToPredicate<'tcx>,
+) -> bool {
+    let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, pred);
+
+    let result = infcx.evaluate_obligation_no_overflow(&obligation);
+    debug!(?result);
+
+    if result.must_apply_modulo_regions() {
+        true
+    } else if result.may_apply() {
+        // Sometimes obligations are ambiguous because the recursive evaluator
+        // is not smart enough, so we fall back to fulfillment when we're not certain
+        // that an obligation holds or not. Even still, we must make sure that
+        // the we do no inference in the process of checking this obligation.
+        let goal = infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
+        infcx.probe(|_| {
+            let ocx = ObligationCtxt::new(infcx);
+            ocx.register_obligation(obligation);
+
+            let errors = ocx.select_all_or_error();
+            match errors.as_slice() {
+                // Only known to hold if we did no inference.
+                [] => infcx.shallow_resolve(goal) == goal,
+
+                errors => {
+                    debug!(?errors);
+                    false
+                }
+            }
+        })
+    } else {
+        false
+    }
 }
 
 #[instrument(level = "debug", skip(tcx, elaborated_env))]
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index f98a1714a3f..25ba985397e 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -314,12 +314,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     .flat_map(|cond| flatten_answer_tree(tcx, obligation, predicate, cond))
                     .collect(),
                 Condition::IfTransmutable { src, dst } => {
-                    let trait_def_id = obligation.predicate.def_id();
+                    let transmute_trait = obligation.predicate.def_id();
                     let assume_const = predicate.trait_ref.args.const_at(2);
-                    let make_obl = |from_ty, to_ty| {
-                        let trait_ref1 = ty::TraitRef::new(
+                    let make_transmute_obl = |from_ty, to_ty| {
+                        let trait_ref = ty::TraitRef::new(
                             tcx,
-                            trait_def_id,
+                            transmute_trait,
                             [
                                 ty::GenericArg::from(to_ty),
                                 ty::GenericArg::from(from_ty),
@@ -331,17 +331,45 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             obligation.cause.clone(),
                             obligation.recursion_depth + 1,
                             obligation.param_env,
-                            trait_ref1,
+                            trait_ref,
                         )
                     };
 
+                    let make_freeze_obl = |ty| {
+                        let trait_ref = ty::TraitRef::new(
+                            tcx,
+                            tcx.lang_items().freeze_trait().unwrap(),
+                            [ty::GenericArg::from(ty)],
+                        );
+                        Obligation::with_depth(
+                            tcx,
+                            obligation.cause.clone(),
+                            obligation.recursion_depth + 1,
+                            obligation.param_env,
+                            trait_ref,
+                        )
+                    };
+
+                    let mut obls = vec![];
+
+                    // If the source is a shared reference, it must be `Freeze`;
+                    // otherwise, transmuting could lead to data races.
+                    if src.mutability == Mutability::Not {
+                        obls.extend([make_freeze_obl(src.ty), make_freeze_obl(dst.ty)])
+                    }
+
                     // If Dst is mutable, check bidirectionally.
                     // For example, transmuting bool -> u8 is OK as long as you can't update that u8
                     // to be > 1, because you could later transmute the u8 back to a bool and get UB.
                     match dst.mutability {
-                        Mutability::Not => vec![make_obl(src.ty, dst.ty)],
-                        Mutability::Mut => vec![make_obl(src.ty, dst.ty), make_obl(dst.ty, src.ty)],
+                        Mutability::Not => obls.push(make_transmute_obl(src.ty, dst.ty)),
+                        Mutability::Mut => obls.extend([
+                            make_transmute_obl(src.ty, dst.ty),
+                            make_transmute_obl(dst.ty, src.ty),
+                        ]),
                     }
+
+                    obls
                 }
             }
         }
diff --git a/compiler/rustc_transmute/src/layout/dfa.rs b/compiler/rustc_transmute/src/layout/dfa.rs
index b8922696e30..77d5a48f158 100644
--- a/compiler/rustc_transmute/src/layout/dfa.rs
+++ b/compiler/rustc_transmute/src/layout/dfa.rs
@@ -35,6 +35,7 @@ impl<R> Transitions<R>
 where
     R: Ref,
 {
+    #[allow(dead_code)]
     fn insert(&mut self, transition: Transition<R>, state: State) {
         match transition {
             Transition::Byte(b) => {
@@ -82,6 +83,7 @@ impl<R> Dfa<R>
 where
     R: Ref,
 {
+    #[allow(dead_code)]
     pub(crate) fn unit() -> Self {
         let transitions: Map<State, Transitions<R>> = Map::default();
         let start = State::new();
diff --git a/compiler/rustc_transmute/src/layout/nfa.rs b/compiler/rustc_transmute/src/layout/nfa.rs
index 78fcceb5f2c..3c5963202c6 100644
--- a/compiler/rustc_transmute/src/layout/nfa.rs
+++ b/compiler/rustc_transmute/src/layout/nfa.rs
@@ -160,6 +160,7 @@ where
         Self { transitions, start, accepting }
     }
 
+    #[allow(dead_code)]
     pub(crate) fn edges_from(&self, start: State) -> Option<&Map<Transition<R>, Set<State>>> {
         self.transitions.get(&start)
     }
diff --git a/compiler/rustc_transmute/src/layout/tree.rs b/compiler/rustc_transmute/src/layout/tree.rs
index f6bc224c7e7..12c984f1603 100644
--- a/compiler/rustc_transmute/src/layout/tree.rs
+++ b/compiler/rustc_transmute/src/layout/tree.rs
@@ -173,16 +173,20 @@ pub(crate) mod rustc {
     use super::Tree;
     use crate::layout::rustc::{Def, Ref};
 
+    use rustc_middle::ty::layout::HasTyCtxt;
+    use rustc_middle::ty::layout::LayoutCx;
     use rustc_middle::ty::layout::LayoutError;
+    use rustc_middle::ty::layout::LayoutOf;
     use rustc_middle::ty::AdtDef;
-    use rustc_middle::ty::GenericArgsRef;
-    use rustc_middle::ty::ParamEnv;
+    use rustc_middle::ty::AdtKind;
+    use rustc_middle::ty::List;
     use rustc_middle::ty::ScalarInt;
-    use rustc_middle::ty::VariantDef;
     use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
     use rustc_span::ErrorGuaranteed;
-    use rustc_target::abi::Align;
-    use std::alloc;
+    use rustc_target::abi::FieldsShape;
+    use rustc_target::abi::Size;
+    use rustc_target::abi::TyAndLayout;
+    use rustc_target::abi::Variants;
 
     #[derive(Debug, Copy, Clone)]
     pub(crate) enum Err {
@@ -206,176 +210,64 @@ pub(crate) mod rustc {
         }
     }
 
-    trait LayoutExt {
-        fn clamp_align(&self, min_align: Align, max_align: Align) -> Self;
-    }
-
-    impl LayoutExt for alloc::Layout {
-        fn clamp_align(&self, min_align: Align, max_align: Align) -> Self {
-            let min_align = min_align.bytes().try_into().unwrap();
-            let max_align = max_align.bytes().try_into().unwrap();
-            Self::from_size_align(self.size(), self.align().clamp(min_align, max_align)).unwrap()
-        }
-    }
-
-    struct LayoutSummary {
-        total_align: Align,
-        total_size: usize,
-        discriminant_size: usize,
-        discriminant_align: Align,
-    }
-
-    impl LayoutSummary {
-        fn from_ty<'tcx>(ty: Ty<'tcx>, ctx: TyCtxt<'tcx>) -> Result<Self, &'tcx LayoutError<'tcx>> {
-            use rustc_middle::ty::ParamEnvAnd;
-            use rustc_target::abi::{TyAndLayout, Variants};
-
-            let param_env = ParamEnv::reveal_all();
-            let param_env_and_type = ParamEnvAnd { param_env, value: ty };
-            let TyAndLayout { layout, .. } = ctx.layout_of(param_env_and_type)?;
-
-            let total_size: usize = layout.size().bytes_usize();
-            let total_align: Align = layout.align().abi;
-            let discriminant_align: Align;
-            let discriminant_size: usize;
-
-            if let Variants::Multiple { tag, .. } = layout.variants() {
-                discriminant_align = tag.align(&ctx).abi;
-                discriminant_size = tag.size(&ctx).bytes_usize();
-            } else {
-                discriminant_align = Align::ONE;
-                discriminant_size = 0;
-            };
-
-            Ok(Self { total_align, total_size, discriminant_align, discriminant_size })
-        }
-
-        fn into(&self) -> alloc::Layout {
-            alloc::Layout::from_size_align(
-                self.total_size,
-                self.total_align.bytes().try_into().unwrap(),
-            )
-            .unwrap()
-        }
-    }
-
     impl<'tcx> Tree<Def<'tcx>, Ref<'tcx>> {
-        pub fn from_ty(ty: Ty<'tcx>, tcx: TyCtxt<'tcx>) -> Result<Self, Err> {
-            use rustc_middle::ty::FloatTy::*;
-            use rustc_middle::ty::IntTy::*;
-            use rustc_middle::ty::UintTy::*;
+        pub fn from_ty(
+            ty_and_layout: TyAndLayout<'tcx, Ty<'tcx>>,
+            cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
+        ) -> Result<Self, Err> {
             use rustc_target::abi::HasDataLayout;
 
-            if let Err(e) = ty.error_reported() {
+            if let Err(e) = ty_and_layout.ty.error_reported() {
                 return Err(Err::TypeError(e));
             }
 
-            let target = tcx.data_layout();
+            let target = cx.tcx.data_layout();
+            let pointer_size = target.pointer_size;
 
-            match ty.kind() {
+            match ty_and_layout.ty.kind() {
                 ty::Bool => Ok(Self::bool()),
 
-                ty::Int(I8) | ty::Uint(U8) => Ok(Self::u8()),
-                ty::Int(I16) | ty::Uint(U16) => Ok(Self::number(2)),
-                ty::Int(I32) | ty::Uint(U32) | ty::Float(F32) => Ok(Self::number(4)),
-                ty::Int(I64) | ty::Uint(U64) | ty::Float(F64) => Ok(Self::number(8)),
-                ty::Int(I128) | ty::Uint(U128) => Ok(Self::number(16)),
-                ty::Int(Isize) | ty::Uint(Usize) => {
-                    Ok(Self::number(target.pointer_size.bytes_usize()))
+                ty::Float(nty) => {
+                    let width = nty.bit_width() / 8;
+                    Ok(Self::number(width as _))
                 }
 
-                ty::Tuple(members) => {
-                    if members.len() == 0 {
-                        Ok(Tree::unit())
-                    } else {
-                        Err(Err::NotYetSupported)
-                    }
+                ty::Int(nty) => {
+                    let width = nty.normalize(pointer_size.bits() as _).bit_width().unwrap() / 8;
+                    Ok(Self::number(width as _))
                 }
 
-                ty::Array(ty, len) => {
-                    let len = len
-                        .try_eval_target_usize(tcx, ParamEnv::reveal_all())
-                        .ok_or(Err::NotYetSupported)?;
-                    let elt = Tree::from_ty(*ty, tcx)?;
+                ty::Uint(nty) => {
+                    let width = nty.normalize(pointer_size.bits() as _).bit_width().unwrap() / 8;
+                    Ok(Self::number(width as _))
+                }
+
+                ty::Tuple(members) => Self::from_tuple(ty_and_layout, members, cx),
+
+                ty::Array(inner_ty, len) => {
+                    let FieldsShape::Array { stride, count } = &ty_and_layout.fields else {
+                        return Err(Err::NotYetSupported);
+                    };
+                    let inner_ty_and_layout = cx.layout_of(*inner_ty)?;
+                    assert_eq!(*stride, inner_ty_and_layout.size);
+                    let elt = Tree::from_ty(inner_ty_and_layout, cx)?;
                     Ok(std::iter::repeat(elt)
-                        .take(len as usize)
+                        .take(*count as usize)
                         .fold(Tree::unit(), |tree, elt| tree.then(elt)))
                 }
 
-                ty::Adt(adt_def, args_ref) => {
-                    use rustc_middle::ty::AdtKind;
-
-                    // If the layout is ill-specified, halt.
-                    if !(adt_def.repr().c() || adt_def.repr().int.is_some()) {
-                        return Err(Err::NotYetSupported);
+                ty::Adt(adt_def, _args_ref) if !ty_and_layout.ty.is_box() => {
+                    match adt_def.adt_kind() {
+                        AdtKind::Struct => Self::from_struct(ty_and_layout, *adt_def, cx),
+                        AdtKind::Enum => Self::from_enum(ty_and_layout, *adt_def, cx),
+                        AdtKind::Union => Self::from_union(ty_and_layout, *adt_def, cx),
                     }
-
-                    // Compute a summary of the type's layout.
-                    let layout_summary = LayoutSummary::from_ty(ty, tcx)?;
-
-                    // The layout begins with this adt's visibility.
-                    let vis = Self::def(Def::Adt(*adt_def));
-
-                    // And is followed the layout(s) of its variants
-                    Ok(vis.then(match adt_def.adt_kind() {
-                        AdtKind::Struct => Self::from_repr_c_variant(
-                            ty,
-                            *adt_def,
-                            args_ref,
-                            &layout_summary,
-                            None,
-                            adt_def.non_enum_variant(),
-                            tcx,
-                        )?,
-                        AdtKind::Enum => {
-                            trace!(?adt_def, "treeifying enum");
-                            let mut tree = Tree::uninhabited();
-
-                            for (idx, variant) in adt_def.variants().iter_enumerated() {
-                                let tag = tcx.tag_for_variant((ty, idx));
-                                tree = tree.or(Self::from_repr_c_variant(
-                                    ty,
-                                    *adt_def,
-                                    args_ref,
-                                    &layout_summary,
-                                    tag,
-                                    variant,
-                                    tcx,
-                                )?);
-                            }
-
-                            tree
-                        }
-                        AdtKind::Union => {
-                            // is the layout well-defined?
-                            if !adt_def.repr().c() {
-                                return Err(Err::NotYetSupported);
-                            }
-
-                            let ty_layout = layout_of(tcx, ty)?;
-
-                            let mut tree = Tree::uninhabited();
-
-                            for field in adt_def.all_fields() {
-                                let variant_ty = field.ty(tcx, args_ref);
-                                let variant_layout = layout_of(tcx, variant_ty)?;
-                                let padding_needed = ty_layout.size() - variant_layout.size();
-                                let variant = Self::def(Def::Field(field))
-                                    .then(Self::from_ty(variant_ty, tcx)?)
-                                    .then(Self::padding(padding_needed));
-
-                                tree = tree.or(variant);
-                            }
-
-                            tree
-                        }
-                    }))
                 }
 
                 ty::Ref(lifetime, ty, mutability) => {
-                    let layout = layout_of(tcx, *ty)?;
-                    let align = layout.align();
-                    let size = layout.size();
+                    let ty_and_layout = cx.layout_of(*ty)?;
+                    let align = ty_and_layout.align.abi.bytes() as usize;
+                    let size = ty_and_layout.size.bytes_usize();
                     Ok(Tree::Ref(Ref {
                         lifetime: *lifetime,
                         ty: *ty,
@@ -389,80 +281,143 @@ pub(crate) mod rustc {
             }
         }
 
-        fn from_repr_c_variant(
-            ty: Ty<'tcx>,
-            adt_def: AdtDef<'tcx>,
-            args_ref: GenericArgsRef<'tcx>,
-            layout_summary: &LayoutSummary,
-            tag: Option<ScalarInt>,
-            variant_def: &'tcx VariantDef,
-            tcx: TyCtxt<'tcx>,
+        /// Constructs a `Tree` from a tuple.
+        fn from_tuple(
+            ty_and_layout: TyAndLayout<'tcx, Ty<'tcx>>,
+            members: &'tcx List<Ty<'tcx>>,
+            cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
         ) -> Result<Self, Err> {
-            let mut tree = Tree::unit();
-
-            let repr = adt_def.repr();
-            let min_align = repr.align.unwrap_or(Align::ONE);
-            let max_align = repr.pack.unwrap_or(Align::MAX);
-
-            let variant_span = trace_span!(
-                "treeifying variant",
-                min_align = ?min_align,
-                max_align = ?max_align,
-            )
-            .entered();
-
-            let mut variant_layout = alloc::Layout::from_size_align(
-                0,
-                layout_summary.total_align.bytes().try_into().unwrap(),
-            )
-            .unwrap();
-
-            // The layout of the variant is prefixed by the tag, if any.
-            if let Some(tag) = tag {
-                let tag_layout =
-                    alloc::Layout::from_size_align(tag.size().bytes_usize(), 1).unwrap();
-                tree = tree.then(Self::from_tag(tag, tcx));
-                variant_layout = variant_layout.extend(tag_layout).unwrap().0;
-            }
-
-            // Next come fields.
-            let fields_span = trace_span!("treeifying fields").entered();
-            for field_def in variant_def.fields.iter() {
-                let field_ty = field_def.ty(tcx, args_ref);
-                let _span = trace_span!("treeifying field", field = ?field_ty).entered();
-
-                // begin with the field's visibility
-                tree = tree.then(Self::def(Def::Field(field_def)));
-
-                // compute the field's layout characteristics
-                let field_layout = layout_of(tcx, field_ty)?.clamp_align(min_align, max_align);
-
-                // next comes the field's padding
-                let padding_needed = variant_layout.padding_needed_for(field_layout.align());
-                if padding_needed > 0 {
-                    tree = tree.then(Self::padding(padding_needed));
+            match &ty_and_layout.fields {
+                FieldsShape::Primitive => {
+                    assert_eq!(members.len(), 1);
+                    let inner_ty = members[0];
+                    let inner_ty_and_layout = cx.layout_of(inner_ty)?;
+                    assert_eq!(ty_and_layout.layout, inner_ty_and_layout.layout);
+                    Self::from_ty(inner_ty_and_layout, cx)
                 }
-
-                // finally, the field's layout
-                tree = tree.then(Self::from_ty(field_ty, tcx)?);
-
-                // extend the variant layout with the field layout
-                variant_layout = variant_layout.extend(field_layout).unwrap().0;
+                FieldsShape::Arbitrary { offsets, .. } => {
+                    assert_eq!(offsets.len(), members.len());
+                    Self::from_variant(Def::Primitive, None, ty_and_layout, ty_and_layout.size, cx)
+                }
+                FieldsShape::Array { .. } | FieldsShape::Union(_) => Err(Err::NotYetSupported),
             }
-            drop(fields_span);
-
-            // finally: padding
-            let padding_span = trace_span!("adding trailing padding").entered();
-            if layout_summary.total_size > variant_layout.size() {
-                let padding_needed = layout_summary.total_size - variant_layout.size();
-                tree = tree.then(Self::padding(padding_needed));
-            };
-            drop(padding_span);
-            drop(variant_span);
-            Ok(tree)
         }
 
-        pub fn from_tag(tag: ScalarInt, tcx: TyCtxt<'tcx>) -> Self {
+        /// Constructs a `Tree` from a struct.
+        ///
+        /// # Panics
+        ///
+        /// Panics if `def` is not a struct definition.
+        fn from_struct(
+            ty_and_layout: TyAndLayout<'tcx, Ty<'tcx>>,
+            def: AdtDef<'tcx>,
+            cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
+        ) -> Result<Self, Err> {
+            assert!(def.is_struct());
+            let def = Def::Adt(def);
+            Self::from_variant(def, None, ty_and_layout, ty_and_layout.size, cx)
+        }
+
+        /// Constructs a `Tree` from an enum.
+        ///
+        /// # Panics
+        ///
+        /// Panics if `def` is not an enum definition.
+        fn from_enum(
+            ty_and_layout: TyAndLayout<'tcx, Ty<'tcx>>,
+            def: AdtDef<'tcx>,
+            cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
+        ) -> Result<Self, Err> {
+            assert!(def.is_enum());
+            let layout = ty_and_layout.layout;
+
+            if let Variants::Multiple { tag_field, .. } = layout.variants() {
+                // For enums (but not coroutines), the tag field is
+                // currently always the first field of the layout.
+                assert_eq!(*tag_field, 0);
+            }
+
+            let variants = def.discriminants(cx.tcx()).try_fold(
+                Self::uninhabited(),
+                |variants, (idx, ref discriminant)| {
+                    let tag = cx.tcx.tag_for_variant((ty_and_layout.ty, idx));
+                    let variant_def = Def::Variant(def.variant(idx));
+                    let variant_ty_and_layout = ty_and_layout.for_variant(&cx, idx);
+                    let variant = Self::from_variant(
+                        variant_def,
+                        tag,
+                        variant_ty_and_layout,
+                        layout.size,
+                        cx,
+                    )?;
+                    Result::<Self, Err>::Ok(variants.or(variant))
+                },
+            )?;
+
+            return Ok(Self::def(Def::Adt(def)).then(variants));
+        }
+
+        /// Constructs a `Tree` from a 'variant-like' layout.
+        ///
+        /// A 'variant-like' layout includes those of structs and, of course,
+        /// enum variants. Pragmatically speaking, this method supports anything
+        /// with `FieldsShape::Arbitrary`.
+        ///
+        /// Note: This routine assumes that the optional `tag` is the first
+        /// field, and enum callers should check that `tag_field` is, in fact,
+        /// `0`.
+        fn from_variant(
+            def: Def<'tcx>,
+            tag: Option<ScalarInt>,
+            ty_and_layout: TyAndLayout<'tcx, Ty<'tcx>>,
+            total_size: Size,
+            cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
+        ) -> Result<Self, Err> {
+            // This constructor does not support non-`FieldsShape::Arbitrary`
+            // layouts.
+            let FieldsShape::Arbitrary { offsets, memory_index } = ty_and_layout.layout.fields()
+            else {
+                return Err(Err::NotYetSupported);
+            };
+
+            // When this function is invoked with enum variants,
+            // `ty_and_layout.size` does not encompass the entire size of the
+            // enum. We rely on `total_size` for this.
+            assert!(ty_and_layout.size <= total_size);
+
+            let mut size = Size::ZERO;
+            let mut struct_tree = Self::def(def);
+
+            // If a `tag` is provided, place it at the start of the layout.
+            if let Some(tag) = tag {
+                size += tag.size();
+                struct_tree = struct_tree.then(Self::from_tag(tag, cx.tcx));
+            }
+
+            // Append the fields, in memory order, to the layout.
+            let inverse_memory_index = memory_index.invert_bijective_mapping();
+            for (memory_idx, field_idx) in inverse_memory_index.iter_enumerated() {
+                // Add interfield padding.
+                let padding_needed = offsets[*field_idx] - size;
+                let padding = Self::padding(padding_needed.bytes_usize());
+
+                let field_ty_and_layout = ty_and_layout.field(&cx, field_idx.as_usize());
+                let field_tree = Self::from_ty(field_ty_and_layout, cx)?;
+
+                struct_tree = struct_tree.then(padding).then(field_tree);
+
+                size += padding_needed + field_ty_and_layout.size;
+            }
+
+            // Add trailing padding.
+            let padding_needed = total_size - size;
+            let trailing_padding = Self::padding(padding_needed.bytes_usize());
+
+            Ok(struct_tree.then(trailing_padding))
+        }
+
+        /// Constructs a `Tree` representing the value of a enum tag.
+        fn from_tag(tag: ScalarInt, tcx: TyCtxt<'tcx>) -> Self {
             use rustc_target::abi::Endian;
             let size = tag.size();
             let bits = tag.to_bits(size).unwrap();
@@ -479,24 +434,42 @@ pub(crate) mod rustc {
             };
             Self::Seq(bytes.iter().map(|&b| Self::from_bits(b)).collect())
         }
-    }
 
-    fn layout_of<'tcx>(
-        ctx: TyCtxt<'tcx>,
-        ty: Ty<'tcx>,
-    ) -> Result<alloc::Layout, &'tcx LayoutError<'tcx>> {
-        use rustc_middle::ty::ParamEnvAnd;
-        use rustc_target::abi::TyAndLayout;
+        /// Constructs a `Tree` from a union.
+        ///
+        /// # Panics
+        ///
+        /// Panics if `def` is not a union definition.
+        fn from_union(
+            ty_and_layout: TyAndLayout<'tcx, Ty<'tcx>>,
+            def: AdtDef<'tcx>,
+            cx: LayoutCx<'tcx, TyCtxt<'tcx>>,
+        ) -> Result<Self, Err> {
+            assert!(def.is_union());
 
-        let param_env = ParamEnv::reveal_all();
-        let param_env_and_type = ParamEnvAnd { param_env, value: ty };
-        let TyAndLayout { layout, .. } = ctx.layout_of(param_env_and_type)?;
-        let layout = alloc::Layout::from_size_align(
-            layout.size().bytes_usize(),
-            layout.align().abi.bytes().try_into().unwrap(),
-        )
-        .unwrap();
-        trace!(?ty, ?layout, "computed layout for type");
-        Ok(layout)
+            let union_layout = ty_and_layout.layout;
+
+            // This constructor does not support non-`FieldsShape::Union`
+            // layouts. Fields of this shape are all placed at offset 0.
+            let FieldsShape::Union(fields) = union_layout.fields() else {
+                return Err(Err::NotYetSupported);
+            };
+
+            let fields = &def.non_enum_variant().fields;
+            let fields = fields.iter_enumerated().try_fold(
+                Self::uninhabited(),
+                |fields, (idx, ref field_def)| {
+                    let field_def = Def::Field(field_def);
+                    let field_ty_and_layout = ty_and_layout.field(&cx, idx.as_usize());
+                    let field = Self::from_ty(field_ty_and_layout, cx)?;
+                    let trailing_padding_needed = union_layout.size - field_ty_and_layout.size;
+                    let trailing_padding = Self::padding(trailing_padding_needed.bytes_usize());
+                    let field_and_padding = field.then(trailing_padding);
+                    Result::<Self, Err>::Ok(fields.or(field_and_padding))
+                },
+            )?;
+
+            Ok(Self::def(Def::Adt(def)).then(fields))
+        }
     }
 }
diff --git a/compiler/rustc_transmute/src/lib.rs b/compiler/rustc_transmute/src/lib.rs
index b6f937ebe47..12312271646 100644
--- a/compiler/rustc_transmute/src/lib.rs
+++ b/compiler/rustc_transmute/src/lib.rs
@@ -1,6 +1,6 @@
 #![feature(alloc_layout_extra)]
 #![feature(never_type)]
-#![allow(dead_code, unused_variables)]
+#![allow(unused_variables)]
 
 #[macro_use]
 extern crate tracing;
@@ -49,6 +49,8 @@ pub enum Reason<T> {
     DstIsNotYetSupported,
     /// The layout of the destination type is bit-incompatible with the source type.
     DstIsBitIncompatible,
+    /// The destination type is uninhabited.
+    DstUninhabited,
     /// The destination type may carry safety invariants.
     DstMayHaveSafetyInvariants,
     /// `Dst` is larger than `Src`, and the excess bytes were not exclusively uninitialized.
diff --git a/compiler/rustc_transmute/src/maybe_transmutable/mod.rs b/compiler/rustc_transmute/src/maybe_transmutable/mod.rs
index 16d15580a05..2789fe8f6b1 100644
--- a/compiler/rustc_transmute/src/maybe_transmutable/mod.rs
+++ b/compiler/rustc_transmute/src/maybe_transmutable/mod.rs
@@ -33,6 +33,9 @@ mod rustc {
     use super::*;
     use crate::layout::tree::rustc::Err;
 
+    use rustc_middle::ty::layout::LayoutCx;
+    use rustc_middle::ty::layout::LayoutOf;
+    use rustc_middle::ty::ParamEnv;
     use rustc_middle::ty::Ty;
     use rustc_middle::ty::TyCtxt;
 
@@ -43,12 +46,20 @@ mod rustc {
         pub fn answer(self) -> Answer<<TyCtxt<'tcx> as QueryContext>::Ref> {
             let Self { src, dst, assume, context } = self;
 
+            let layout_cx = LayoutCx { tcx: context, param_env: ParamEnv::reveal_all() };
+            let layout_of = |ty| {
+                layout_cx
+                    .layout_of(ty)
+                    .map_err(|_| Err::NotYetSupported)
+                    .and_then(|tl| Tree::from_ty(tl, layout_cx))
+            };
+
             // Convert `src` and `dst` from their rustc representations, to `Tree`-based
             // representations. If these conversions fail, conclude that the transmutation is
             // unacceptable; the layouts of both the source and destination types must be
             // well-defined.
-            let src = Tree::from_ty(src, context);
-            let dst = Tree::from_ty(dst, context);
+            let src = layout_of(src);
+            let dst = layout_of(dst);
 
             match (src, dst) {
                 (Err(Err::TypeError(_)), _) | (_, Err(Err::TypeError(_))) => {
@@ -86,6 +97,10 @@ where
         // references.
         let src = src.prune(&|def| false);
 
+        if src.is_inhabited() && !dst.is_inhabited() {
+            return Answer::No(Reason::DstUninhabited);
+        }
+
         trace!(?src, "pruned src");
 
         // Remove all `Def` nodes from `dst`, additionally...
diff --git a/compiler/rustc_transmute/src/maybe_transmutable/query_context.rs b/compiler/rustc_transmute/src/maybe_transmutable/query_context.rs
index 54ed03d44e6..1ccb6f36c8e 100644
--- a/compiler/rustc_transmute/src/maybe_transmutable/query_context.rs
+++ b/compiler/rustc_transmute/src/maybe_transmutable/query_context.rs
@@ -5,8 +5,6 @@ pub(crate) trait QueryContext {
     type Def: layout::Def;
     type Ref: layout::Ref;
     type Scope: Copy;
-
-    fn min_align(&self, reference: Self::Ref) -> usize;
 }
 
 #[cfg(test)]
@@ -31,10 +29,6 @@ pub(crate) mod test {
         type Def = Def;
         type Ref = !;
         type Scope = ();
-
-        fn min_align(&self, reference: !) -> usize {
-            unimplemented!()
-        }
     }
 }
 
@@ -48,9 +42,5 @@ mod rustc {
         type Ref = layout::rustc::Ref<'tcx>;
 
         type Scope = Ty<'tcx>;
-
-        fn min_align(&self, reference: Self::Ref) -> usize {
-            unimplemented!()
-        }
     }
 }
diff --git a/library/core/src/iter/adapters/step_by.rs b/library/core/src/iter/adapters/step_by.rs
index b8b96417d13..616dd0afc51 100644
--- a/library/core/src/iter/adapters/step_by.rs
+++ b/library/core/src/iter/adapters/step_by.rs
@@ -1,6 +1,7 @@
 use crate::{
     intrinsics,
     iter::{from_fn, TrustedLen, TrustedRandomAccess},
+    num::NonZeroUsize,
     ops::{Range, Try},
 };
 
@@ -22,7 +23,11 @@ pub struct StepBy<I> {
     /// Additionally this type-dependent preprocessing means specialized implementations
     /// cannot be used interchangeably.
     iter: I,
-    step: usize,
+    /// This field is `step - 1`, aka the correct amount to pass to `nth` when iterating.
+    /// It MUST NOT be `usize::MAX`, as `unsafe` code depends on being able to add one
+    /// without the risk of overflow.  (This is important so that length calculations
+    /// don't need to check for division-by-zero, for example.)
+    step_minus_one: usize,
     first_take: bool,
 }
 
@@ -31,7 +36,16 @@ impl<I> StepBy<I> {
     pub(in crate::iter) fn new(iter: I, step: usize) -> StepBy<I> {
         assert!(step != 0);
         let iter = <I as SpecRangeSetup<I>>::setup(iter, step);
-        StepBy { iter, step: step - 1, first_take: true }
+        StepBy { iter, step_minus_one: step - 1, first_take: true }
+    }
+
+    /// The `step` that was originally passed to `Iterator::step_by(step)`,
+    /// aka `self.step_minus_one + 1`.
+    #[inline]
+    fn original_step(&self) -> NonZeroUsize {
+        // SAFETY: By type invariant, `step_minus_one` cannot be `MAX`, which
+        // means the addition cannot overflow and the result cannot be zero.
+        unsafe { NonZeroUsize::new_unchecked(intrinsics::unchecked_add(self.step_minus_one, 1)) }
     }
 }
 
@@ -81,8 +95,8 @@ where
     // The zero-based index starting from the end of the iterator of the
     // last element. Used in the `DoubleEndedIterator` implementation.
     fn next_back_index(&self) -> usize {
-        let rem = self.iter.len() % (self.step + 1);
-        if self.first_take { if rem == 0 { self.step } else { rem - 1 } } else { rem }
+        let rem = self.iter.len() % self.original_step();
+        if self.first_take { if rem == 0 { self.step_minus_one } else { rem - 1 } } else { rem }
     }
 }
 
@@ -209,7 +223,7 @@ unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
 
     #[inline]
     default fn spec_next(&mut self) -> Option<I::Item> {
-        let step_size = if self.first_take { 0 } else { self.step };
+        let step_size = if self.first_take { 0 } else { self.step_minus_one };
         self.first_take = false;
         self.iter.nth(step_size)
     }
@@ -217,22 +231,22 @@ unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
     #[inline]
     default fn spec_size_hint(&self) -> (usize, Option<usize>) {
         #[inline]
-        fn first_size(step: usize) -> impl Fn(usize) -> usize {
-            move |n| if n == 0 { 0 } else { 1 + (n - 1) / (step + 1) }
+        fn first_size(step: NonZeroUsize) -> impl Fn(usize) -> usize {
+            move |n| if n == 0 { 0 } else { 1 + (n - 1) / step }
         }
 
         #[inline]
-        fn other_size(step: usize) -> impl Fn(usize) -> usize {
-            move |n| n / (step + 1)
+        fn other_size(step: NonZeroUsize) -> impl Fn(usize) -> usize {
+            move |n| n / step
         }
 
         let (low, high) = self.iter.size_hint();
 
         if self.first_take {
-            let f = first_size(self.step);
+            let f = first_size(self.original_step());
             (f(low), high.map(f))
         } else {
-            let f = other_size(self.step);
+            let f = other_size(self.original_step());
             (f(low), high.map(f))
         }
     }
@@ -247,10 +261,9 @@ unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
             }
             n -= 1;
         }
-        // n and self.step are indices, we need to add 1 to get the amount of elements
+        // n and self.step_minus_one are indices, we need to add 1 to get the amount of elements
         // When calling `.nth`, we need to subtract 1 again to convert back to an index
-        // step + 1 can't overflow because `.step_by` sets `self.step` to `step - 1`
-        let mut step = self.step + 1;
+        let mut step = self.original_step().get();
         // n + 1 could overflow
         // thus, if n is usize::MAX, instead of adding one, we call .nth(step)
         if n == usize::MAX {
@@ -288,8 +301,11 @@ unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
         R: Try<Output = Acc>,
     {
         #[inline]
-        fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
-            move || iter.nth(step)
+        fn nth<I: Iterator>(
+            iter: &mut I,
+            step_minus_one: usize,
+        ) -> impl FnMut() -> Option<I::Item> + '_ {
+            move || iter.nth(step_minus_one)
         }
 
         if self.first_take {
@@ -299,7 +315,7 @@ unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
                 Some(x) => acc = f(acc, x)?,
             }
         }
-        from_fn(nth(&mut self.iter, self.step)).try_fold(acc, f)
+        from_fn(nth(&mut self.iter, self.step_minus_one)).try_fold(acc, f)
     }
 
     default fn spec_fold<Acc, F>(mut self, mut acc: Acc, mut f: F) -> Acc
@@ -307,8 +323,11 @@ unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
         F: FnMut(Acc, Self::Item) -> Acc,
     {
         #[inline]
-        fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
-            move || iter.nth(step)
+        fn nth<I: Iterator>(
+            iter: &mut I,
+            step_minus_one: usize,
+        ) -> impl FnMut() -> Option<I::Item> + '_ {
+            move || iter.nth(step_minus_one)
         }
 
         if self.first_take {
@@ -318,7 +337,7 @@ unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
                 Some(x) => acc = f(acc, x),
             }
         }
-        from_fn(nth(&mut self.iter, self.step)).fold(acc, f)
+        from_fn(nth(&mut self.iter, self.step_minus_one)).fold(acc, f)
     }
 }
 
@@ -336,7 +355,7 @@ unsafe impl<I: DoubleEndedIterator + ExactSizeIterator> StepByBackImpl<I> for St
         // is out of bounds because the length of `self.iter` does not exceed
         // `usize::MAX` (because `I: ExactSizeIterator`) and `nth_back` is
         // zero-indexed
-        let n = n.saturating_mul(self.step + 1).saturating_add(self.next_back_index());
+        let n = n.saturating_mul(self.original_step().get()).saturating_add(self.next_back_index());
         self.iter.nth_back(n)
     }
 
@@ -348,16 +367,16 @@ unsafe impl<I: DoubleEndedIterator + ExactSizeIterator> StepByBackImpl<I> for St
         #[inline]
         fn nth_back<I: DoubleEndedIterator>(
             iter: &mut I,
-            step: usize,
+            step_minus_one: usize,
         ) -> impl FnMut() -> Option<I::Item> + '_ {
-            move || iter.nth_back(step)
+            move || iter.nth_back(step_minus_one)
         }
 
         match self.next_back() {
             None => try { init },
             Some(x) => {
                 let acc = f(init, x)?;
-                from_fn(nth_back(&mut self.iter, self.step)).try_fold(acc, f)
+                from_fn(nth_back(&mut self.iter, self.step_minus_one)).try_fold(acc, f)
             }
         }
     }
@@ -371,16 +390,16 @@ unsafe impl<I: DoubleEndedIterator + ExactSizeIterator> StepByBackImpl<I> for St
         #[inline]
         fn nth_back<I: DoubleEndedIterator>(
             iter: &mut I,
-            step: usize,
+            step_minus_one: usize,
         ) -> impl FnMut() -> Option<I::Item> + '_ {
-            move || iter.nth_back(step)
+            move || iter.nth_back(step_minus_one)
         }
 
         match self.next_back() {
             None => init,
             Some(x) => {
                 let acc = f(init, x);
-                from_fn(nth_back(&mut self.iter, self.step)).fold(acc, f)
+                from_fn(nth_back(&mut self.iter, self.step_minus_one)).fold(acc, f)
             }
         }
     }
@@ -424,8 +443,7 @@ macro_rules! spec_int_ranges {
             fn spec_next(&mut self) -> Option<$t> {
                 // if a step size larger than the type has been specified fall back to
                 // t::MAX, in which case remaining will be at most 1.
-                // The `+ 1` can't overflow since the constructor substracted 1 from the original value.
-                let step = <$t>::try_from(self.step + 1).unwrap_or(<$t>::MAX);
+                let step = <$t>::try_from(self.original_step().get()).unwrap_or(<$t>::MAX);
                 let remaining = self.iter.end;
                 if remaining > 0 {
                     let val = self.iter.start;
@@ -474,7 +492,7 @@ macro_rules! spec_int_ranges {
             {
                 // if a step size larger than the type has been specified fall back to
                 // t::MAX, in which case remaining will be at most 1.
-                let step = <$t>::try_from(self.step + 1).unwrap_or(<$t>::MAX);
+                let step = <$t>::try_from(self.original_step().get()).unwrap_or(<$t>::MAX);
                 let remaining = self.iter.end;
                 let mut acc = init;
                 let mut val = self.iter.start;
@@ -500,7 +518,7 @@ macro_rules! spec_int_ranges_r {
             fn spec_next_back(&mut self) -> Option<Self::Item>
                 where Range<$t>: DoubleEndedIterator + ExactSizeIterator,
             {
-                let step = (self.step + 1) as $t;
+                let step = self.original_step().get() as $t;
                 let remaining = self.iter.end;
                 if remaining > 0 {
                     let start = self.iter.start;
diff --git a/src/bootstrap/src/core/builder.rs b/src/bootstrap/src/core/builder.rs
index e8977a4b173..e3b27600c5e 100644
--- a/src/bootstrap/src/core/builder.rs
+++ b/src/bootstrap/src/core/builder.rs
@@ -645,27 +645,6 @@ pub enum Kind {
 }
 
 impl Kind {
-    pub fn parse(string: &str) -> Option<Kind> {
-        // these strings, including the one-letter aliases, must match the x.py help text
-        Some(match string {
-            "build" | "b" => Kind::Build,
-            "check" | "c" => Kind::Check,
-            "clippy" => Kind::Clippy,
-            "fix" => Kind::Fix,
-            "fmt" => Kind::Format,
-            "test" | "t" => Kind::Test,
-            "bench" => Kind::Bench,
-            "doc" | "d" => Kind::Doc,
-            "clean" => Kind::Clean,
-            "dist" => Kind::Dist,
-            "install" => Kind::Install,
-            "run" | "r" => Kind::Run,
-            "setup" => Kind::Setup,
-            "suggest" => Kind::Suggest,
-            _ => return None,
-        })
-    }
-
     pub fn as_str(&self) -> &'static str {
         match self {
             Kind::Build => "build",
diff --git a/src/bootstrap/src/lib.rs b/src/bootstrap/src/lib.rs
index bcb8260b15a..1a8322c0dfd 100644
--- a/src/bootstrap/src/lib.rs
+++ b/src/bootstrap/src/lib.rs
@@ -1911,15 +1911,6 @@ impl Compiler {
     pub fn is_snapshot(&self, build: &Build) -> bool {
         self.stage == 0 && self.host == build.build
     }
-
-    /// Returns if this compiler should be treated as a final stage one in the
-    /// current build session.
-    /// This takes into account whether we're performing a full bootstrap or
-    /// not; don't directly compare the stage with `2`!
-    pub fn is_final_stage(&self, build: &Build) -> bool {
-        let final_stage = if build.config.full_bootstrap { 2 } else { 1 };
-        self.stage >= final_stage
-    }
 }
 
 fn envify(s: &str) -> String {
diff --git a/tests/codegen/powerpc64le-struct-align-128.rs b/tests/codegen/powerpc64le-struct-align-128.rs
new file mode 100644
index 00000000000..0096c6d3138
--- /dev/null
+++ b/tests/codegen/powerpc64le-struct-align-128.rs
@@ -0,0 +1,93 @@
+// Test that structs aligned to 128 bits are passed with the correct ABI on powerpc64le.
+// This is similar to aarch64-struct-align-128.rs, but for ppc.
+
+//@ compile-flags: --target powerpc64le-unknown-linux-gnu
+//@ needs-llvm-components: powerpc
+
+#![feature(no_core, lang_items)]
+#![crate_type = "lib"]
+#![no_core]
+
+#[lang="sized"]
+trait Sized { }
+#[lang="freeze"]
+trait Freeze { }
+#[lang="copy"]
+trait Copy { }
+
+#[repr(C)]
+pub struct Align8 {
+    pub a: u64,
+    pub b: u64,
+}
+
+#[repr(transparent)]
+pub struct Transparent8 {
+    a: Align8
+}
+
+#[repr(C)]
+pub struct Wrapped8 {
+    a: Align8,
+}
+
+extern "C" {
+    // CHECK: declare void @test_8([2 x i64], [2 x i64], [2 x i64])
+    fn test_8(a: Align8, b: Transparent8, c: Wrapped8);
+}
+
+#[repr(C)]
+#[repr(align(16))]
+pub struct Align16 {
+    pub a: u64,
+    pub b: u64,
+}
+
+#[repr(transparent)]
+pub struct Transparent16 {
+    a: Align16
+}
+
+#[repr(C)]
+pub struct Wrapped16 {
+    pub a: Align16,
+}
+
+extern "C" {
+    // It's important that this produces [1 x i128]  rather than just i128!
+    // CHECK: declare void @test_16([1 x i128], [1 x i128], [1 x i128])
+    fn test_16(a: Align16, b: Transparent16, c: Wrapped16);
+}
+
+#[repr(C)]
+#[repr(align(32))]
+pub struct Align32 {
+    pub a: u64,
+    pub b: u64,
+    pub c: u64,
+}
+
+#[repr(transparent)]
+pub struct Transparent32 {
+    a: Align32
+}
+
+#[repr(C)]
+pub struct Wrapped32 {
+    pub a: Align32,
+}
+
+extern "C" {
+    // CHECK: declare void @test_32([2 x i128], [2 x i128], [2 x i128])
+    fn test_32(a: Align32, b: Transparent32, c: Wrapped32);
+}
+
+pub unsafe fn main(
+    a1: Align8, a2: Transparent8, a3: Wrapped8,
+    b1: Align16, b2: Transparent16, b3: Wrapped16,
+    c1: Align32, c2: Transparent32, c3: Wrapped32,
+) {
+    test_8(a1, a2, a3);
+    test_16(b1, b2, b3);
+    test_32(c1, c2, c3);
+}
diff --git a/tests/codegen/step_by-overflow-checks.rs b/tests/codegen/step_by-overflow-checks.rs
new file mode 100644
index 00000000000..43e8514a8b7
--- /dev/null
+++ b/tests/codegen/step_by-overflow-checks.rs
@@ -0,0 +1,26 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+use std::iter::StepBy;
+use std::slice::Iter;
+
+// The constructor for `StepBy` ensures we can never end up needing to do zero
+// checks on denominators, so check that the code isn't emitting panic paths.
+
+// CHECK-LABEL: @step_by_len_std
+#[no_mangle]
+pub fn step_by_len_std(x: &StepBy<Iter<i32>>) -> usize {
+    // CHECK-NOT: div_by_zero
+    // CHECK: udiv
+    // CHECK-NOT: div_by_zero
+    x.len()
+}
+
+// CHECK-LABEL: @step_by_len_naive
+#[no_mangle]
+pub fn step_by_len_naive(x: Iter<i32>, step_minus_one: usize) -> usize {
+    // CHECK: udiv
+    // CHECK: call{{.+}}div_by_zero
+    x.len() / (step_minus_one + 1)
+}
diff --git a/tests/ui/async-await/async-closures/mut-ref-reborrow.rs b/tests/ui/async-await/async-closures/mut-ref-reborrow.rs
new file mode 100644
index 00000000000..9f2cbd7ce1c
--- /dev/null
+++ b/tests/ui/async-await/async-closures/mut-ref-reborrow.rs
@@ -0,0 +1,27 @@
+//@ aux-build:block-on.rs
+//@ run-pass
+//@ check-run-results
+//@ revisions: e2021 e2018
+//@[e2018] edition:2018
+//@[e2021] edition:2021
+
+#![feature(async_closure)]
+
+extern crate block_on;
+
+async fn call_once(f: impl async FnOnce()) { f().await; }
+
+pub async fn async_closure(x: &mut i32) {
+    let c = async move || {
+        *x += 1;
+    };
+    call_once(c).await;
+}
+
+fn main() {
+    block_on::block_on(async {
+        let mut x = 0;
+        async_closure(&mut x).await;
+        assert_eq!(x, 1);
+    });
+}
diff --git a/tests/ui/async-await/async-closures/overlapping-projs.rs b/tests/ui/async-await/async-closures/overlapping-projs.rs
new file mode 100644
index 00000000000..6dd00b16103
--- /dev/null
+++ b/tests/ui/async-await/async-closures/overlapping-projs.rs
@@ -0,0 +1,27 @@
+//@ aux-build:block-on.rs
+//@ edition:2021
+//@ run-pass
+//@ check-run-results
+
+#![feature(async_closure)]
+
+extern crate block_on;
+
+async fn call_once(f: impl async FnOnce()) {
+    f().await;
+}
+
+async fn async_main() {
+    let x = &mut 0;
+    let y = &mut 0;
+    let c = async || {
+        *x = 1;
+        *y = 2;
+    };
+    call_once(c).await;
+    println!("{x} {y}");
+}
+
+fn main() {
+    block_on::block_on(async_main());
+}
diff --git a/tests/ui/async-await/async-closures/overlapping-projs.run.stdout b/tests/ui/async-await/async-closures/overlapping-projs.run.stdout
new file mode 100644
index 00000000000..8d04f961a03
--- /dev/null
+++ b/tests/ui/async-await/async-closures/overlapping-projs.run.stdout
@@ -0,0 +1 @@
+1 2
diff --git a/tests/ui/async-await/async-closures/precise-captures.call.run.stdout b/tests/ui/async-await/async-closures/precise-captures.call.run.stdout
new file mode 100644
index 00000000000..6062556837c
--- /dev/null
+++ b/tests/ui/async-await/async-closures/precise-captures.call.run.stdout
@@ -0,0 +1,29 @@
+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+untouched
+
+after call
+drop first
+after await
+uncaptured
+
+after call
+drop first
+after await
+uncaptured
diff --git a/tests/ui/async-await/async-closures/precise-captures.call_once.run.stdout b/tests/ui/async-await/async-closures/precise-captures.call_once.run.stdout
new file mode 100644
index 00000000000..ddb02d47600
--- /dev/null
+++ b/tests/ui/async-await/async-closures/precise-captures.call_once.run.stdout
@@ -0,0 +1,29 @@
+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+fixed
+after await
+uncaptured
+
+after call
+after await
+fixed
+untouched
+
+after call
+drop first
+after await
+uncaptured
+
+after call
+drop first
+after await
+uncaptured
diff --git a/tests/ui/async-await/async-closures/precise-captures.force_once.run.stdout b/tests/ui/async-await/async-closures/precise-captures.force_once.run.stdout
new file mode 100644
index 00000000000..ddb02d47600
--- /dev/null
+++ b/tests/ui/async-await/async-closures/precise-captures.force_once.run.stdout
@@ -0,0 +1,29 @@
+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+fixed
+after await
+uncaptured
+
+after call
+after await
+fixed
+untouched
+
+after call
+drop first
+after await
+uncaptured
+
+after call
+drop first
+after await
+uncaptured
diff --git a/tests/ui/async-await/async-closures/precise-captures.rs b/tests/ui/async-await/async-closures/precise-captures.rs
new file mode 100644
index 00000000000..e82dd1dbaf0
--- /dev/null
+++ b/tests/ui/async-await/async-closures/precise-captures.rs
@@ -0,0 +1,157 @@
+//@ aux-build:block-on.rs
+//@ edition:2021
+//@ run-pass
+//@ check-run-results
+//@ revisions: call call_once force_once
+
+// call - Call the closure regularly.
+// call_once - Call the closure w/ `async FnOnce`, so exercising the by_move shim.
+// force_once - Force the closure mode to `FnOnce`, so exercising what was fixed
+//   in <https://github.com/rust-lang/rust/pull/123350>.
+
+#![feature(async_closure)]
+#![allow(unused_mut)]
+
+extern crate block_on;
+
+#[cfg(any(call, force_once))]
+macro_rules! call {
+    ($c:expr) => { ($c)() }
+}
+
+#[cfg(call_once)]
+async fn call_once(f: impl async FnOnce()) {
+    f().await
+}
+
+#[cfg(call_once)]
+macro_rules! call {
+    ($c:expr) => { call_once($c) }
+}
+
+#[cfg(not(force_once))]
+macro_rules! guidance {
+    ($c:expr) => { $c }
+}
+
+#[cfg(force_once)]
+fn infer_fnonce(c: impl async FnOnce()) -> impl async FnOnce() { c }
+
+#[cfg(force_once)]
+macro_rules! guidance {
+    ($c:expr) => { infer_fnonce($c) }
+}
+
+#[derive(Debug)]
+struct Drop(&'static str);
+
+impl std::ops::Drop for Drop {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+
+struct S {
+    a: i32,
+    b: Drop,
+    c: Drop,
+}
+
+async fn async_main() {
+    // Precise capture struct
+    {
+        let mut s = S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture &mut struct
+    {
+        let s = &mut S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture struct by move
+    {
+        let mut s = S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async move || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture &mut struct by move
+    {
+        let s = &mut S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async move || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        // `s` is still captured fully as `&mut S`.
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture struct, consume field
+    {
+        let mut s = S { a: 1, b: Drop("drop first"), c: Drop("untouched") };
+        let c = guidance!(async move || {
+            // s.a = 2; // FIXME(async_closures): Figure out why this fails
+            drop(s.b);
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture struct by move, consume field
+    {
+        let mut s = S { a: 1, b: Drop("drop first"), c: Drop("untouched") };
+        let c = guidance!(async move || {
+            // s.a = 2; // FIXME(async_closures): Figure out why this fails
+            drop(s.b);
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+}
+
+fn main() {
+    block_on::block_on(async_main());
+}
diff --git a/tests/ui/sanitizer/cfi-method-fn-ptr-cast.rs b/tests/ui/sanitizer/cfi-fn-ptr.rs
similarity index 85%
rename from tests/ui/sanitizer/cfi-method-fn-ptr-cast.rs
rename to tests/ui/sanitizer/cfi-fn-ptr.rs
index 8f79de11748..505b4b8e7f0 100644
--- a/tests/ui/sanitizer/cfi-method-fn-ptr-cast.rs
+++ b/tests/ui/sanitizer/cfi-fn-ptr.rs
@@ -1,4 +1,4 @@
-// Verifies that casting a method to a function pointer works.
+// Verifies that casting to a function pointer works.
 
 //@ revisions: cfi kcfi
 // FIXME(#122848) Remove only-linux once OSX CFI binaries work
@@ -46,6 +46,8 @@ impl Trait1 for Type1 {
     fn foo(&self) {}
 }
 
+fn foo<T>(_: &T) {}
+
 fn main() {
     let type1 = Type1 {};
     let f = <Type1 as Trait1>::foo;
@@ -53,5 +55,7 @@ fn main() {
     // Check again with different optimization barriers
     S2 { f: <S as Foo>::foo }.foo(&S);
     // Check mismatched #[track_caller]
-    S2 { f: <S as Foo>::bar }.foo(&S)
+    S2 { f: <S as Foo>::bar }.foo(&S);
+    // Check non-method functions
+    S2 { f: foo }.foo(&S)
 }
diff --git a/tests/ui/traits/pred-known-to-hold-modulo-regions-unsized-tail.rs b/tests/ui/traits/pred-known-to-hold-modulo-regions-unsized-tail.rs
new file mode 100644
index 00000000000..4e8c19d600d
--- /dev/null
+++ b/tests/ui/traits/pred-known-to-hold-modulo-regions-unsized-tail.rs
@@ -0,0 +1,244 @@
+// This is a non-regression test for issues #108721 and its duplicate #123275 (hopefully, because
+// the test is still convoluted and the ICE is fiddly).
+//
+// `pred_known_to_hold_modulo_regions` prevented "unexpected unsized tail" ICEs with warp/hyper but
+// was unknowingly removed in #120463.
+
+//@ build-pass: the ICE happened in codegen
+
+use std::future::Future;
+trait TryFuture: Future {
+    type Ok;
+}
+impl<F, T> TryFuture for F
+where
+    F: ?Sized + Future<Output = Option<T>>,
+{
+    type Ok = T;
+}
+trait Executor {}
+struct Exec {}
+trait HttpBody {
+    type Data;
+}
+trait ConnStreamExec<F> {}
+impl<F> ConnStreamExec<F> for Exec where H2Stream<F>: Send {}
+impl<E, F> ConnStreamExec<F> for E where E: Executor {}
+struct H2Stream<F> {
+    _fut: F,
+}
+trait NewSvcExec<S, E, W: Watcher<S, E>> {
+    fn execute_new_svc(&mut self, _fut: NewSvcTask<S, E, W>) {
+        unimplemented!()
+    }
+}
+impl<S, E, W> NewSvcExec<S, E, W> for Exec where W: Watcher<S, E> {}
+trait Watcher<S, E> {
+    type Future;
+}
+struct NoopWatcher;
+impl<S, E> Watcher<S, E> for NoopWatcher
+where
+    S: HttpService,
+    E: ConnStreamExec<S::Future>,
+{
+    type Future = Option<<<S as HttpService>::ResBody as HttpBody>::Data>;
+}
+trait Service<Request> {
+    type Response;
+    type Future;
+}
+trait HttpService {
+    type ResBody: HttpBody;
+    type Future;
+}
+struct Body {}
+impl HttpBody for Body {
+    type Data = String;
+}
+impl<S> HttpService for S
+where
+    S: Service<(), Response = ()>,
+{
+    type ResBody = Body;
+    type Future = S::Future;
+}
+trait MakeServiceRef<Target> {
+    type ResBody;
+    type Service: HttpService<ResBody = Self::ResBody>;
+}
+impl<T, Target, S, F> MakeServiceRef<Target> for T
+where
+    T: for<'a> Service<&'a Target, Response = S, Future = F>,
+    S: HttpService,
+{
+    type Service = S;
+    type ResBody = S::ResBody;
+}
+fn make_service_fn<F, Target, Ret>(_f: F) -> MakeServiceFn<F>
+where
+    F: FnMut(&Target) -> Ret,
+    Ret: Future,
+{
+    unimplemented!()
+}
+struct MakeServiceFn<F> {
+    _func: F,
+}
+impl<'t, F, Ret, Target, Svc> Service<&'t Target> for MakeServiceFn<F>
+where
+    F: FnMut(&Target) -> Ret,
+    Ret: Future<Output = Option<Svc>>,
+{
+    type Response = Svc;
+    type Future = Option<()>;
+}
+struct AddrIncoming {}
+struct Server<I, S, E> {
+    _incoming: I,
+    _make_service: S,
+    _protocol: E,
+}
+impl<I, S, E, B> Server<I, S, E>
+where
+    S: MakeServiceRef<(), ResBody = B>,
+    B: HttpBody,
+    E: ConnStreamExec<<S::Service as HttpService>::Future>,
+    E: NewSvcExec<S::Service, E, NoopWatcher>,
+{
+    fn serve(&mut self) {
+        let fut = NewSvcTask::new();
+        self._protocol.execute_new_svc(fut);
+    }
+}
+fn serve<S>(_make_service: S) -> Server<AddrIncoming, S, Exec> {
+    unimplemented!()
+}
+struct NewSvcTask<S, E, W: Watcher<S, E>> {
+    _state: State<S, E, W>,
+}
+struct State<S, E, W: Watcher<S, E>> {
+    _fut: W::Future,
+}
+impl<S, E, W: Watcher<S, E>> NewSvcTask<S, E, W> {
+    fn new() -> Self {
+        unimplemented!()
+    }
+}
+trait Filter {
+    type Extract;
+    type Future;
+    fn map<F>(self, _fun: F) -> MapFilter<Self, F>
+    where
+        Self: Sized,
+    {
+        unimplemented!()
+    }
+    fn wrap_with<W>(self, _wrapper: W) -> W::Wrapped
+    where
+        Self: Sized,
+        W: Wrap<Self>,
+    {
+        unimplemented!()
+    }
+}
+fn service<F>(_filter: F) -> FilteredService<F>
+where
+    F: Filter,
+{
+    unimplemented!()
+}
+struct FilteredService<F> {
+    _filter: F,
+}
+impl<F> Service<()> for FilteredService<F>
+where
+    F: Filter,
+{
+    type Response = ();
+    type Future = FilteredFuture<F::Future>;
+}
+struct FilteredFuture<F> {
+    _fut: F,
+}
+struct MapFilter<T, F> {
+    _filter: T,
+    _func: F,
+}
+impl<T, F> Filter for MapFilter<T, F>
+where
+    T: Filter,
+    F: Func<T::Extract>,
+{
+    type Extract = F::Output;
+    type Future = MapFilterFuture<T, F>;
+}
+struct MapFilterFuture<T: Filter, F> {
+    _extract: T::Future,
+    _func: F,
+}
+trait Wrap<F> {
+    type Wrapped;
+}
+fn make_filter_fn<F, U>(_func: F) -> FilterFn<F>
+where
+    F: Fn() -> U,
+{
+    unimplemented!()
+}
+struct FilterFn<F> {
+    _func: F,
+}
+impl<F, U> Filter for FilterFn<F>
+where
+    F: Fn() -> U,
+    U: TryFuture,
+    U::Ok: Send,
+{
+    type Extract = U::Ok;
+    type Future = Option<U>;
+}
+fn trace<F>(_func: F) -> Trace<F>
+where
+    F: Fn(),
+{
+    unimplemented!()
+}
+struct Trace<F> {
+    _func: F,
+}
+impl<FN, F> Wrap<F> for Trace<FN> {
+    type Wrapped = WithTrace<FN, F>;
+}
+struct WithTrace<FN, F> {
+    _filter: F,
+    _trace: FN,
+}
+impl<FN, F> Filter for WithTrace<FN, F>
+where
+    F: Filter,
+{
+    type Extract = ();
+    type Future = (F::Future, fn(F::Extract));
+}
+trait Func<Args> {
+    type Output;
+}
+impl<F, R> Func<()> for F
+where
+    F: Fn() -> R,
+{
+    type Output = R;
+}
+fn main() {
+    let make_service = make_service_fn(|_| {
+        let tracer = trace(|| unimplemented!());
+        let filter = make_filter_fn(|| std::future::ready(Some(())))
+            .map(|| "Hello, world")
+            .wrap_with(tracer);
+        let svc = service(filter);
+        std::future::ready(Some(svc))
+    });
+    let mut server = serve(make_service);
+    server.serve();
+}
diff --git a/tests/ui/transmutability/arrays/huge-len.stderr b/tests/ui/transmutability/arrays/huge-len.stderr
index 37160c5c959..3fc652f47c1 100644
--- a/tests/ui/transmutability/arrays/huge-len.stderr
+++ b/tests/ui/transmutability/arrays/huge-len.stderr
@@ -2,7 +2,7 @@ error[E0277]: `()` cannot be safely transmuted into `ExplicitlyPadded`
   --> $DIR/huge-len.rs:21:41
    |
 LL |     assert::is_maybe_transmutable::<(), ExplicitlyPadded>();
-   |                                         ^^^^^^^^^^^^^^^^ values of the type `ExplicitlyPadded` are too big for the current architecture
+   |                                         ^^^^^^^^^^^^^^^^ analyzing the transmutability of `ExplicitlyPadded` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/huge-len.rs:8:14
@@ -17,7 +17,7 @@ error[E0277]: `ExplicitlyPadded` cannot be safely transmuted into `()`
   --> $DIR/huge-len.rs:24:55
    |
 LL |     assert::is_maybe_transmutable::<ExplicitlyPadded, ()>();
-   |                                                       ^^ values of the type `ExplicitlyPadded` are too big for the current architecture
+   |                                                       ^^ analyzing the transmutability of `ExplicitlyPadded` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/huge-len.rs:8:14
diff --git a/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr b/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr
index e486928a445..b4cd70142c4 100644
--- a/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr
+++ b/tests/ui/transmutability/arrays/should_require_well_defined_layout.stderr
@@ -2,7 +2,7 @@ error[E0277]: `[String; 0]` cannot be safely transmuted into `()`
   --> $DIR/should_require_well_defined_layout.rs:25:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `[String; 0]` is not yet supported.
+   |                                                    ^^ analyzing the transmutability of `[String; 0]` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:12:14
@@ -23,7 +23,7 @@ error[E0277]: `u128` cannot be safely transmuted into `[String; 0]`
   --> $DIR/should_require_well_defined_layout.rs:26:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `[String; 0]` is not yet supported.
+   |                                               ^^^^^^^^^ analyzing the transmutability of `[String; 0]` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:12:14
@@ -44,7 +44,7 @@ error[E0277]: `[String; 1]` cannot be safely transmuted into `()`
   --> $DIR/should_require_well_defined_layout.rs:31:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `[String; 1]` is not yet supported.
+   |                                                    ^^ analyzing the transmutability of `[String; 1]` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:12:14
@@ -65,7 +65,7 @@ error[E0277]: `u128` cannot be safely transmuted into `[String; 1]`
   --> $DIR/should_require_well_defined_layout.rs:32:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `[String; 1]` is not yet supported.
+   |                                               ^^^^^^^^^ analyzing the transmutability of `[String; 1]` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:12:14
@@ -86,7 +86,7 @@ error[E0277]: `[String; 2]` cannot be safely transmuted into `()`
   --> $DIR/should_require_well_defined_layout.rs:37:52
    |
 LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `[String; 2]` is not yet supported.
+   |                                                    ^^ analyzing the transmutability of `[String; 2]` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:12:14
@@ -107,7 +107,7 @@ error[E0277]: `u128` cannot be safely transmuted into `[String; 2]`
   --> $DIR/should_require_well_defined_layout.rs:38:47
    |
 LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `[String; 2]` is not yet supported.
+   |                                               ^^^^^^^^^ analyzing the transmutability of `[String; 2]` is not yet supported
    |
 note: required by a bound in `is_maybe_transmutable`
   --> $DIR/should_require_well_defined_layout.rs:12:14
diff --git a/tests/ui/transmutability/enums/niche_optimization.rs b/tests/ui/transmutability/enums/niche_optimization.rs
new file mode 100644
index 00000000000..23f57ecad75
--- /dev/null
+++ b/tests/ui/transmutability/enums/niche_optimization.rs
@@ -0,0 +1,156 @@
+//@ check-pass
+//! Checks that niche optimizations are encoded correctly.
+#![crate_type = "lib"]
+#![feature(transmutability)]
+#![allow(dead_code, incomplete_features, non_camel_case_types)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+
+    pub fn is_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, {
+            Assume {
+                alignment: false,
+                lifetimes: false,
+                safety: true,
+                validity: false,
+            }
+        }>
+    {}
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, {
+            Assume {
+                alignment: false,
+                lifetimes: false,
+                safety: true,
+                validity: true,
+            }
+        }>
+    {}
+}
+
+#[repr(u8)] enum V0 { V = 0 }
+#[repr(u8)] enum V1 { V = 1 }
+#[repr(u8)] enum V2 { V = 2 }
+#[repr(u8)] enum V253 { V = 253 }
+#[repr(u8)] enum V254 { V = 254 }
+#[repr(u8)] enum V255 { V = 255 }
+
+fn bool() {
+    enum OptionLike {
+        A(bool),
+        B,
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<OptionLike>() == 1);
+    };
+
+    assert::is_transmutable::<OptionLike, u8>();
+
+    assert::is_transmutable::<bool, OptionLike>();
+    assert::is_transmutable::<V0, OptionLike>();
+    assert::is_transmutable::<V1, OptionLike>();
+    assert::is_transmutable::<V2, OptionLike>();
+}
+
+fn one_niche() {
+    #[repr(u8)]
+    enum N1 {
+        S = 0,
+        E = 255 - 1,
+    }
+
+    enum OptionLike {
+        A(N1),
+        B,
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<OptionLike>() == 1);
+    };
+
+    assert::is_transmutable::<OptionLike, u8>();
+    assert::is_transmutable::<V0, OptionLike>();
+    assert::is_transmutable::<V254, OptionLike>();
+    assert::is_transmutable::<V255, OptionLike>();
+}
+
+fn one_niche_alt() {
+    #[repr(u8)]
+    enum N1 {
+        S = 1,
+        E = 255 - 1,
+    }
+
+    enum OptionLike {
+        A(N1),
+        B,
+        C,
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<OptionLike>() == 1);
+    };
+
+    assert::is_transmutable::<OptionLike, u8>();
+    assert::is_transmutable::<V0, OptionLike>();
+    assert::is_transmutable::<V254, OptionLike>();
+    assert::is_transmutable::<V255, OptionLike>();
+}
+
+fn two_niche() {
+    #[repr(u8)]
+    enum Niche {
+        S = 0,
+        E = 255 - 2,
+    }
+
+    enum OptionLike {
+        A(Niche),
+        B,
+        C,
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<OptionLike>() == 1);
+    };
+
+    assert::is_transmutable::<OptionLike, u8>();
+    assert::is_transmutable::<V0, OptionLike>();
+    assert::is_transmutable::<V253, OptionLike>();
+    assert::is_transmutable::<V254, OptionLike>();
+    assert::is_transmutable::<V255, OptionLike>();
+}
+
+fn no_niche() {
+    use std::mem::MaybeUninit;
+
+    #[repr(u8)]
+    enum Niche {
+        S = 0,
+        E = 255 - 1,
+    }
+
+    enum OptionLike {
+        A(Niche),
+        B,
+        C,
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<OptionLike>() == 2);
+    };
+
+    #[repr(C)]
+    struct Pair<T, U>(T, U);
+
+    assert::is_transmutable::<V0, Niche>();
+    assert::is_transmutable::<V254, Niche>();
+    assert::is_transmutable::<Pair<V0, Niche>, OptionLike>();
+    assert::is_transmutable::<Pair<V1, MaybeUninit<u8>>, OptionLike>();
+    assert::is_transmutable::<Pair<V2, MaybeUninit<u8>>, OptionLike>();
+}
diff --git a/tests/ui/transmutability/enums/repr/padding_differences.rs b/tests/ui/transmutability/enums/repr/padding_differences.rs
new file mode 100644
index 00000000000..d0e1502b5e2
--- /dev/null
+++ b/tests/ui/transmutability/enums/repr/padding_differences.rs
@@ -0,0 +1,80 @@
+//@ check-pass
+//! Adapted from https://rust-lang.github.io/unsafe-code-guidelines/layout/enums.html#explicit-repr-annotation-without-c-compatibility
+#![crate_type = "lib"]
+#![feature(transmutability)]
+#![allow(dead_code, incomplete_features, non_camel_case_types)]
+
+use std::mem::MaybeUninit;
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+
+    pub fn is_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, {
+            Assume {
+                alignment: false,
+                lifetimes: false,
+                safety: true,
+                validity: false,
+            }
+        }>
+    {}
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, {
+            Assume {
+                alignment: false,
+                lifetimes: false,
+                safety: true,
+                validity: true,
+            }
+        }>
+    {}
+}
+
+#[repr(u8)] enum V0 { V = 0 }
+#[repr(u8)] enum V1 { V = 1 }
+
+fn repr_u8() {
+    #[repr(u8)]
+    enum TwoCases {
+        A(u8, u16),     // 0x00 INIT INIT INIT
+        B(u16),         // 0x01 PADD INIT INIT
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<TwoCases>() == 4);
+    };
+
+    #[repr(C)] struct TwoCasesA(V0, u8, u8, u8);
+    #[repr(C)] struct TwoCasesB(V1, MaybeUninit<u8>, u8, u8);
+
+    assert::is_transmutable::<TwoCasesA, TwoCases>();
+    assert::is_transmutable::<TwoCasesB, TwoCases>();
+
+    assert::is_maybe_transmutable::<TwoCases, TwoCasesA>();
+    assert::is_maybe_transmutable::<TwoCases, TwoCasesB>();
+}
+
+fn repr_c_u8() {
+    #[repr(C, u8)]
+    enum TwoCases {
+        A(u8, u16),     // 0x00 PADD INIT PADD INIT INIT
+        B(u16),         // 0x01 PADD INIT INIT PADD PADD
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<TwoCases>() == 6);
+    };
+
+    #[repr(C)] struct TwoCasesA(V0, MaybeUninit<u8>, u8, MaybeUninit<u8>, u8, u8);
+    #[repr(C)] struct TwoCasesB(V1, MaybeUninit<u8>, u8, u8, MaybeUninit<u8>, MaybeUninit<u8>);
+
+    assert::is_transmutable::<TwoCasesA, TwoCases>();
+    assert::is_transmutable::<TwoCasesB, TwoCases>();
+
+    assert::is_maybe_transmutable::<TwoCases, TwoCasesA>();
+    assert::is_maybe_transmutable::<TwoCases, TwoCasesB>();
+}
diff --git a/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.rs b/tests/ui/transmutability/enums/repr/should_handle_all.rs
similarity index 80%
rename from tests/ui/transmutability/enums/repr/should_require_well_defined_layout.rs
rename to tests/ui/transmutability/enums/repr/should_handle_all.rs
index 630e662b926..a8ec86fa40d 100644
--- a/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.rs
+++ b/tests/ui/transmutability/enums/repr/should_handle_all.rs
@@ -1,5 +1,4 @@
-//! An enum must have a well-defined layout to participate in a transmutation.
-
+//@ check-pass
 #![crate_type = "lib"]
 #![feature(repr128)]
 #![feature(transmutability)]
@@ -21,23 +20,17 @@ mod assert {
     {}
 }
 
-fn should_reject_repr_rust() {
-    fn void() {
-        enum repr_rust {}
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
-    }
-
+fn should_accept_repr_rust() {
     fn singleton() {
         enum repr_rust { V }
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_rust, ()>();
+        assert::is_maybe_transmutable::<u128, repr_rust>();
     }
 
     fn duplex() {
         enum repr_rust { A, B }
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_rust, ()>();
+        assert::is_maybe_transmutable::<u128, repr_rust>();
     }
 }
 
@@ -116,7 +109,7 @@ fn should_accept_primitive_reprs()
     }
 }
 
-fn should_accept_repr_C() {
+fn should_accept_repr_c() {
     #[repr(C)] enum repr_c { V }
     assert::is_maybe_transmutable::<repr_c, ()>();
     assert::is_maybe_transmutable::<i128, repr_c>();
diff --git a/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.stderr b/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.stderr
deleted file mode 100644
index 2a683de6a65..00000000000
--- a/tests/ui/transmutability/enums/repr/should_require_well_defined_layout.stderr
+++ /dev/null
@@ -1,135 +0,0 @@
-error[E0277]: `void::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:27:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `void::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:13:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `void::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:28:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `void::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:13:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `singleton::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:33:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `singleton::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:13:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `singleton::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:34:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `singleton::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:13:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `duplex::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:39:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `duplex::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:13:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `duplex::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:40:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `duplex::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:13:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error: aborting due to 6 previous errors
-
-For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/transmutability/malformed-program-gracefulness/unknown_src_field.stderr b/tests/ui/transmutability/malformed-program-gracefulness/unknown_src_field.stderr
index eeed8a62a2a..cabc7bcfef7 100644
--- a/tests/ui/transmutability/malformed-program-gracefulness/unknown_src_field.stderr
+++ b/tests/ui/transmutability/malformed-program-gracefulness/unknown_src_field.stderr
@@ -8,7 +8,7 @@ error[E0277]: `Src` cannot be safely transmuted into `Dst`
   --> $DIR/unknown_src_field.rs:19:36
    |
 LL |     assert::is_transmutable::<Src, Dst>();
-   |                                    ^^^ `Dst` has an unknown layout
+   |                                    ^^^ analyzing the transmutability of `Dst` is not yet supported
    |
 note: required by a bound in `is_transmutable`
   --> $DIR/unknown_src_field.rs:12:14
diff --git a/tests/ui/transmutability/maybeuninit.rs b/tests/ui/transmutability/maybeuninit.rs
new file mode 100644
index 00000000000..77c3381c774
--- /dev/null
+++ b/tests/ui/transmutability/maybeuninit.rs
@@ -0,0 +1,43 @@
+#![crate_type = "lib"]
+#![feature(transmutability)]
+#![allow(dead_code, incomplete_features, non_camel_case_types)]
+
+use std::mem::MaybeUninit;
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }>
+    {}
+}
+
+fn validity() {
+    // An initialized byte is a valid uninitialized byte.
+    assert::is_maybe_transmutable::<u8, MaybeUninit<u8>>();
+
+    // An uninitialized byte is never a valid initialized byte.
+    assert::is_maybe_transmutable::<MaybeUninit<u8>, u8>(); //~ ERROR: cannot be safely transmuted
+}
+
+fn padding() {
+    #[repr(align(8))]
+    struct Align8;
+
+    #[repr(u8)]
+    enum ImplicitlyPadded {
+        A(Align8),
+    }
+
+    #[repr(u8)]
+    enum V0 {
+        V0 = 0,
+    }
+
+    #[repr(C)]
+    struct ExplicitlyPadded(V0, MaybeUninit<[u8; 7]>);
+
+    assert::is_maybe_transmutable::<ExplicitlyPadded, ImplicitlyPadded>();
+    assert::is_maybe_transmutable::<ImplicitlyPadded, ExplicitlyPadded>();
+}
diff --git a/tests/ui/transmutability/maybeuninit.stderr b/tests/ui/transmutability/maybeuninit.stderr
new file mode 100644
index 00000000000..be7dcaf35ea
--- /dev/null
+++ b/tests/ui/transmutability/maybeuninit.stderr
@@ -0,0 +1,18 @@
+error[E0277]: `MaybeUninit<u8>` cannot be safely transmuted into `u8`
+  --> $DIR/maybeuninit.rs:21:54
+   |
+LL |     assert::is_maybe_transmutable::<MaybeUninit<u8>, u8>();
+   |                                                      ^^ at least one value of `MaybeUninit<u8>` isn't a bit-valid value of `u8`
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/maybeuninit.rs:12:14
+   |
+LL |     pub fn is_maybe_transmutable<Src, Dst>()
+   |            --------------------- required by a bound in this function
+LL |     where
+LL |         Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }>
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/transmutability/references/unsafecell.rs b/tests/ui/transmutability/references/unsafecell.rs
new file mode 100644
index 00000000000..a8a1f969fb4
--- /dev/null
+++ b/tests/ui/transmutability/references/unsafecell.rs
@@ -0,0 +1,47 @@
+#![crate_type = "lib"]
+#![feature(transmutability)]
+#![allow(dead_code, incomplete_features, non_camel_case_types)]
+
+use std::cell::UnsafeCell;
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }>
+    {}
+}
+
+fn value_to_value() {
+    // We accept value-to-value transmutations of `UnsafeCell`-containing types,
+    // because owning a value implies exclusive access.
+    assert::is_maybe_transmutable::<UnsafeCell<u8>, u8>();
+    assert::is_maybe_transmutable::<u8, UnsafeCell<u8>>();
+    assert::is_maybe_transmutable::<UnsafeCell<u8>, UnsafeCell<u8>>();
+}
+
+fn ref_to_ref() {
+    // We forbid `UnsafeCell`-containing ref-to-ref transmutations, because the
+    // two types may use different, incompatible synchronization strategies.
+    assert::is_maybe_transmutable::<&'static u8, &'static UnsafeCell<u8>>(); //~ ERROR: cannot be safely transmuted
+
+    assert::is_maybe_transmutable::<&'static UnsafeCell<u8>, &'static UnsafeCell<u8>>(); //~ ERROR: cannot be safely transmuted
+}
+
+fn mut_to_mut() {
+    // `UnsafeCell` does't matter for `&mut T` to `&mut U`, since exclusive
+    // borrows can't be used for shared access.
+    assert::is_maybe_transmutable::<&'static mut u8, &'static mut UnsafeCell<u8>>();
+    assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static mut u8>();
+    assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static mut UnsafeCell<u8>>();
+}
+
+fn mut_to_ref() {
+    // We don't care about `UnsafeCell` for transmutations in the form `&mut T
+    // -> &U`, because downgrading a `&mut T` to a `&U` deactivates `&mut T` for
+    // the lifetime of `&U`.
+    assert::is_maybe_transmutable::<&'static mut u8, &'static UnsafeCell<u8>>();
+    assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static u8>();
+    assert::is_maybe_transmutable::<&'static mut UnsafeCell<u8>, &'static UnsafeCell<u8>>();
+}
diff --git a/tests/ui/transmutability/references/unsafecell.stderr b/tests/ui/transmutability/references/unsafecell.stderr
new file mode 100644
index 00000000000..651eb8ceb26
--- /dev/null
+++ b/tests/ui/transmutability/references/unsafecell.stderr
@@ -0,0 +1,33 @@
+error[E0277]: `&u8` cannot be safely transmuted into `&UnsafeCell<u8>`
+  --> $DIR/unsafecell.rs:27:50
+   |
+LL |     assert::is_maybe_transmutable::<&'static u8, &'static UnsafeCell<u8>>();
+   |                                                  ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Freeze` is not implemented for `&'static UnsafeCell<u8>`
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/unsafecell.rs:12:14
+   |
+LL |     pub fn is_maybe_transmutable<Src, Dst>()
+   |            --------------------- required by a bound in this function
+LL |     where
+LL |         Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }>
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
+
+error[E0277]: `&UnsafeCell<u8>` cannot be safely transmuted into `&UnsafeCell<u8>`
+  --> $DIR/unsafecell.rs:29:62
+   |
+LL |     assert::is_maybe_transmutable::<&'static UnsafeCell<u8>, &'static UnsafeCell<u8>>();
+   |                                                              ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Freeze` is not implemented for `&'static UnsafeCell<u8>`
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/unsafecell.rs:12:14
+   |
+LL |     pub fn is_maybe_transmutable<Src, Dst>()
+   |            --------------------- required by a bound in this function
+LL |     where
+LL |         Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }>
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.rs b/tests/ui/transmutability/structs/repr/should_handle_all.rs
similarity index 54%
rename from tests/ui/transmutability/structs/repr/should_require_well_defined_layout.rs
rename to tests/ui/transmutability/structs/repr/should_handle_all.rs
index 2e673601baf..52c24eecf12 100644
--- a/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.rs
+++ b/tests/ui/transmutability/structs/repr/should_handle_all.rs
@@ -1,3 +1,4 @@
+//@ check-pass
 //! A struct must have a well-defined layout to participate in a transmutation.
 
 #![crate_type = "lib"]
@@ -20,47 +21,47 @@ mod assert {
     {}
 }
 
-fn should_reject_repr_rust()
+fn should_accept_repr_rust()
 {
     fn unit() {
         struct repr_rust;
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_rust, ()>();
+        assert::is_maybe_transmutable::<u128, repr_rust>();
     }
 
     fn tuple() {
         struct repr_rust();
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_rust, ()>();
+        assert::is_maybe_transmutable::<u128, repr_rust>();
     }
 
     fn braces() {
         struct repr_rust{}
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_rust, ()>();
+        assert::is_maybe_transmutable::<u128, repr_rust>();
     }
 
     fn aligned() {
         #[repr(align(1))] struct repr_rust{}
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_rust, ()>();
+        assert::is_maybe_transmutable::<u128, repr_rust>();
     }
 
     fn packed() {
         #[repr(packed)] struct repr_rust{}
-        assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_rust, ()>();
+        assert::is_maybe_transmutable::<u128, repr_rust>();
     }
 
     fn nested() {
         struct repr_rust;
         #[repr(C)] struct repr_c(repr_rust);
-        assert::is_maybe_transmutable::<repr_c, ()>(); //~ ERROR cannot be safely transmuted
-        assert::is_maybe_transmutable::<u128, repr_c>(); //~ ERROR cannot be safely transmuted
+        assert::is_maybe_transmutable::<repr_c, ()>();
+        assert::is_maybe_transmutable::<u128, repr_c>();
     }
 }
 
-fn should_accept_repr_C()
+fn should_accept_repr_c()
 {
     fn unit() {
         #[repr(C)] struct repr_c;
diff --git a/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr b/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
deleted file mode 100644
index 77788f72c21..00000000000
--- a/tests/ui/transmutability/structs/repr/should_require_well_defined_layout.stderr
+++ /dev/null
@@ -1,267 +0,0 @@
-error[E0277]: `should_reject_repr_rust::unit::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:27:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `should_reject_repr_rust::unit::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::unit::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:28:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `should_reject_repr_rust::unit::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `should_reject_repr_rust::tuple::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:33:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `should_reject_repr_rust::tuple::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::tuple::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:34:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `should_reject_repr_rust::tuple::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `should_reject_repr_rust::braces::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:39:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `should_reject_repr_rust::braces::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::braces::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:40:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `should_reject_repr_rust::braces::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `aligned::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:45:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `aligned::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `aligned::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:46:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `aligned::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `packed::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:51:52
-   |
-LL |         assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                    ^^ analyzing the transmutability of `packed::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `packed::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:52:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                               ^^^^^^^^^ analyzing the transmutability of `packed::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `nested::repr_c` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:58:49
-   |
-LL |         assert::is_maybe_transmutable::<repr_c, ()>();
-   |                                                 ^^ analyzing the transmutability of `nested::repr_c` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `nested::repr_c`
-  --> $DIR/should_require_well_defined_layout.rs:59:47
-   |
-LL |         assert::is_maybe_transmutable::<u128, repr_c>();
-   |                                               ^^^^^^ analyzing the transmutability of `nested::repr_c` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error: aborting due to 12 previous errors
-
-For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.rs b/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.rs
index 4c285a616b3..64110753832 100644
--- a/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.rs
+++ b/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.rs
@@ -22,4 +22,5 @@ fn should_pad_explicitly_packed_field() {
     //~^ ERROR: recursive type
 
     assert::is_maybe_transmutable::<ExplicitlyPadded, ()>();
+    //~^ ERROR: cannot be safely transmuted
 }
diff --git a/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.stderr b/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.stderr
index 7fb051f6625..ebfb5361143 100644
--- a/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.stderr
+++ b/tests/ui/transmutability/structs/repr/transmute_infinitely_recursive_type.stderr
@@ -15,7 +15,22 @@ error[E0391]: cycle detected when computing layout of `should_pad_explicitly_pac
    = note: cycle used when evaluating trait selection obligation `(): core::mem::transmutability::BikeshedIntrinsicFrom<should_pad_explicitly_packed_field::ExplicitlyPadded, core::mem::transmutability::Assume { alignment: false, lifetimes: false, safety: false, validity: false }>`
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
-error: aborting due to 2 previous errors
+error[E0277]: `ExplicitlyPadded` cannot be safely transmuted into `()`
+  --> $DIR/transmute_infinitely_recursive_type.rs:24:55
+   |
+LL |     assert::is_maybe_transmutable::<ExplicitlyPadded, ()>();
+   |                                                       ^^ analyzing the transmutability of `ExplicitlyPadded` is not yet supported
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/transmute_infinitely_recursive_type.rs:14:14
+   |
+LL |     pub fn is_maybe_transmutable<Src, Dst>()
+   |            --------------------- required by a bound in this function
+LL |     where
+LL |         Dst: BikeshedIntrinsicFrom<Src>,
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_maybe_transmutable`
 
-Some errors have detailed explanations: E0072, E0391.
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0072, E0277, E0391.
 For more information about an error, try `rustc --explain E0072`.
diff --git a/tests/ui/transmutability/transmute-padding-ice.rs b/tests/ui/transmutability/transmute-padding-ice.rs
index 3f3f75bc086..f5935a0009e 100644
--- a/tests/ui/transmutability/transmute-padding-ice.rs
+++ b/tests/ui/transmutability/transmute-padding-ice.rs
@@ -1,7 +1,13 @@
+//@ check-pass
+//! This UI test was introduced as check-fail by a buggy bug-fix for an ICE. In
+//! fact, this transmutation should be valid.
+
 #![crate_type = "lib"]
 #![feature(transmutability)]
 #![allow(dead_code)]
 
+use std::mem::size_of;
+
 mod assert {
     use std::mem::{Assume, BikeshedIntrinsicFrom};
 
@@ -22,6 +28,7 @@ fn test() {
     #[repr(C)]
     struct B(u8, u8);
 
+    assert_eq!(size_of::<A>(), size_of::<B>());
+
     assert::is_maybe_transmutable::<B, A>();
-    //~^ ERROR cannot be safely transmuted
 }
diff --git a/tests/ui/transmutability/transmute-padding-ice.stderr b/tests/ui/transmutability/transmute-padding-ice.stderr
deleted file mode 100644
index 4c121d463c6..00000000000
--- a/tests/ui/transmutability/transmute-padding-ice.stderr
+++ /dev/null
@@ -1,22 +0,0 @@
-error[E0277]: `B` cannot be safely transmuted into `A`
-  --> $DIR/transmute-padding-ice.rs:25:40
-   |
-LL |     assert::is_maybe_transmutable::<B, A>();
-   |                                        ^ the size of `B` is smaller than the size of `A`
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/transmute-padding-ice.rs:10:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<
-   |  ______________^
-LL | |             Src,
-LL | |             { Assume { alignment: true, lifetimes: true, safety: true, validity: true } },
-LL | |         >,
-   | |_________^ required by this bound in `is_maybe_transmutable`
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/transmutability/uninhabited.rs b/tests/ui/transmutability/uninhabited.rs
new file mode 100644
index 00000000000..b61b110f6a1
--- /dev/null
+++ b/tests/ui/transmutability/uninhabited.rs
@@ -0,0 +1,71 @@
+#![crate_type = "lib"]
+#![feature(transmutability)]
+#![allow(dead_code, incomplete_features, non_camel_case_types)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+
+    pub fn is_maybe_transmutable<Src, Dst>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, {
+            Assume {
+                alignment: true,
+                lifetimes: true,
+                safety: true,
+                validity: true,
+            }
+        }>
+    {}
+}
+
+fn void() {
+    enum Void {}
+
+    // This transmutation is vacuously acceptable; since one cannot construct a
+    // `Void`, unsoundness cannot directly arise from transmuting a void into
+    // anything else.
+    assert::is_maybe_transmutable::<Void, u128>();
+
+    assert::is_maybe_transmutable::<(), Void>(); //~ ERROR: cannot be safely transmuted
+}
+
+// Non-ZST uninhabited types are, nonetheless, uninhabited.
+fn yawning_void() {
+    enum Void {}
+
+    struct YawningVoid(Void, u128);
+
+    const _: () = {
+        assert!(std::mem::size_of::<YawningVoid>() == std::mem::size_of::<u128>());
+        // Just to be sure the above constant actually evaluated:
+        assert!(false); //~ ERROR: evaluation of constant value failed
+    };
+
+    // This transmutation is vacuously acceptable; since one cannot construct a
+    // `Void`, unsoundness cannot directly arise from transmuting a void into
+    // anything else.
+    assert::is_maybe_transmutable::<YawningVoid, u128>();
+
+    assert::is_maybe_transmutable::<(), Void>(); //~ ERROR: cannot be safely transmuted
+}
+
+// References to uninhabited types are, logically, uninhabited, but for layout
+// purposes are not ZSTs, and aren't treated as uninhabited when they appear in
+// enum variants.
+fn distant_void() {
+    enum Void {}
+
+    enum DistantVoid {
+        A(&'static Void)
+    }
+
+    const _: () = {
+        assert!(std::mem::size_of::<DistantVoid>() == std::mem::size_of::<usize>());
+        // Just to be sure the above constant actually evaluated:
+        assert!(false); //~ ERROR: evaluation of constant value failed
+    };
+
+    assert::is_maybe_transmutable::<DistantVoid, ()>();
+    assert::is_maybe_transmutable::<DistantVoid, &'static Void>();
+    assert::is_maybe_transmutable::<u128, DistantVoid>(); //~ ERROR: cannot be safely transmuted
+}
diff --git a/tests/ui/transmutability/uninhabited.stderr b/tests/ui/transmutability/uninhabited.stderr
new file mode 100644
index 00000000000..60219b0f263
--- /dev/null
+++ b/tests/ui/transmutability/uninhabited.stderr
@@ -0,0 +1,86 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/uninhabited.rs:41:9
+   |
+LL |         assert!(false);
+   |         ^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: false', $DIR/uninhabited.rs:41:9
+   |
+   = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/uninhabited.rs:65:9
+   |
+LL |         assert!(false);
+   |         ^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: false', $DIR/uninhabited.rs:65:9
+   |
+   = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: `()` cannot be safely transmuted into `void::Void`
+  --> $DIR/uninhabited.rs:29:41
+   |
+LL |     assert::is_maybe_transmutable::<(), Void>();
+   |                                         ^^^^ `void::Void` is uninhabited
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/uninhabited.rs:10:14
+   |
+LL |       pub fn is_maybe_transmutable<Src, Dst>()
+   |              --------------------- required by a bound in this function
+LL |       where
+LL |           Dst: BikeshedIntrinsicFrom<Src, {
+   |  ______________^
+LL | |             Assume {
+LL | |                 alignment: true,
+LL | |                 lifetimes: true,
+...  |
+LL | |             }
+LL | |         }>
+   | |__________^ required by this bound in `is_maybe_transmutable`
+
+error[E0277]: `()` cannot be safely transmuted into `yawning_void::Void`
+  --> $DIR/uninhabited.rs:49:41
+   |
+LL |     assert::is_maybe_transmutable::<(), Void>();
+   |                                         ^^^^ `yawning_void::Void` is uninhabited
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/uninhabited.rs:10:14
+   |
+LL |       pub fn is_maybe_transmutable<Src, Dst>()
+   |              --------------------- required by a bound in this function
+LL |       where
+LL |           Dst: BikeshedIntrinsicFrom<Src, {
+   |  ______________^
+LL | |             Assume {
+LL | |                 alignment: true,
+LL | |                 lifetimes: true,
+...  |
+LL | |             }
+LL | |         }>
+   | |__________^ required by this bound in `is_maybe_transmutable`
+
+error[E0277]: `u128` cannot be safely transmuted into `DistantVoid`
+  --> $DIR/uninhabited.rs:70:43
+   |
+LL |     assert::is_maybe_transmutable::<u128, DistantVoid>();
+   |                                           ^^^^^^^^^^^ at least one value of `u128` isn't a bit-valid value of `DistantVoid`
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/uninhabited.rs:10:14
+   |
+LL |       pub fn is_maybe_transmutable<Src, Dst>()
+   |              --------------------- required by a bound in this function
+LL |       where
+LL |           Dst: BikeshedIntrinsicFrom<Src, {
+   |  ______________^
+LL | |             Assume {
+LL | |                 alignment: true,
+LL | |                 lifetimes: true,
+...  |
+LL | |             }
+LL | |         }>
+   | |__________^ required by this bound in `is_maybe_transmutable`
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0080, E0277.
+For more information about an error, try `rustc --explain E0080`.
diff --git a/tests/ui/transmutability/unions/repr/should_handle_align.rs b/tests/ui/transmutability/unions/repr/should_handle_align.rs
index 8668cca3cb5..ba4e904e161 100644
--- a/tests/ui/transmutability/unions/repr/should_handle_align.rs
+++ b/tests/ui/transmutability/unions/repr/should_handle_align.rs
@@ -25,13 +25,13 @@ fn should_pad_explicitly_aligned_field() {
     #[derive(Clone, Copy)] #[repr(u8)] enum V0u8 { V = 0 }
     #[derive(Clone, Copy)] #[repr(u8)] enum V1u8 { V = 1 }
 
-    #[repr(C)]
+    #[repr(align(1))]
     pub union Uninit {
         a: (),
         b: V1u8,
     }
 
-    #[repr(C, align(2))]
+    #[repr(align(2))]
     pub union align_2 {
         a: V0u8,
     }
diff --git a/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.rs b/tests/ui/transmutability/unions/repr/should_handle_all.rs
similarity index 58%
rename from tests/ui/transmutability/unions/repr/should_require_well_defined_layout.rs
rename to tests/ui/transmutability/unions/repr/should_handle_all.rs
index 8495b0ea88f..85d48dd9b7f 100644
--- a/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.rs
+++ b/tests/ui/transmutability/unions/repr/should_handle_all.rs
@@ -1,7 +1,7 @@
-//! A struct must have a well-defined layout to participate in a transmutation.
+//@ check-pass
 
 #![crate_type = "lib"]
-#![feature(transmutability)]
+#![feature(transmutability, transparent_unions)]
 #![allow(dead_code, incomplete_features, non_camel_case_types)]
 
 mod assert {
@@ -20,17 +20,17 @@ mod assert {
     {}
 }
 
-fn should_reject_repr_rust()
+fn should_accept_repr_rust()
 {
     union repr_rust {
         a: u8
     }
 
-    assert::is_maybe_transmutable::<repr_rust, ()>(); //~ ERROR cannot be safely transmuted
-    assert::is_maybe_transmutable::<u128, repr_rust>(); //~ ERROR cannot be safely transmuted
+    assert::is_maybe_transmutable::<repr_rust, ()>();
+    assert::is_maybe_transmutable::<u128, repr_rust>();
 }
 
-fn should_accept_repr_C()
+fn should_accept_repr_c()
 {
     #[repr(C)]
     union repr_c {
@@ -41,3 +41,15 @@ fn should_accept_repr_C()
     assert::is_maybe_transmutable::<repr_c, ()>();
     assert::is_maybe_transmutable::<u128, repr_c>();
 }
+
+
+fn should_accept_transparent()
+{
+    #[repr(transparent)]
+    union repr_transparent {
+        a: u8
+    }
+
+    assert::is_maybe_transmutable::<repr_transparent, ()>();
+    assert::is_maybe_transmutable::<u128, repr_transparent>();
+}
diff --git a/tests/ui/transmutability/unions/repr/should_handle_packed.rs b/tests/ui/transmutability/unions/repr/should_handle_packed.rs
index 4af6c1d3a61..fc06eba4353 100644
--- a/tests/ui/transmutability/unions/repr/should_handle_packed.rs
+++ b/tests/ui/transmutability/unions/repr/should_handle_packed.rs
@@ -27,7 +27,6 @@ fn should_pad_explicitly_packed_field() {
     #[derive(Clone, Copy)] #[repr(u8)] enum V2u8 { V = 2 }
     #[derive(Clone, Copy)] #[repr(u32)] enum V3u32 { V = 3 }
 
-    #[repr(C)]
     pub union Uninit {
         a: (),
         b: V1u8,
diff --git a/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.stderr b/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.stderr
deleted file mode 100644
index bec07f13103..00000000000
--- a/tests/ui/transmutability/unions/repr/should_require_well_defined_layout.stderr
+++ /dev/null
@@ -1,47 +0,0 @@
-error[E0277]: `should_reject_repr_rust::repr_rust` cannot be safely transmuted into `()`
-  --> $DIR/should_require_well_defined_layout.rs:29:48
-   |
-LL |     assert::is_maybe_transmutable::<repr_rust, ()>();
-   |                                                ^^ analyzing the transmutability of `should_reject_repr_rust::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error[E0277]: `u128` cannot be safely transmuted into `should_reject_repr_rust::repr_rust`
-  --> $DIR/should_require_well_defined_layout.rs:30:43
-   |
-LL |     assert::is_maybe_transmutable::<u128, repr_rust>();
-   |                                           ^^^^^^^^^ analyzing the transmutability of `should_reject_repr_rust::repr_rust` is not yet supported.
-   |
-note: required by a bound in `is_maybe_transmutable`
-  --> $DIR/should_require_well_defined_layout.rs:12:14
-   |
-LL |       pub fn is_maybe_transmutable<Src, Dst>()
-   |              --------------------- required by a bound in this function
-LL |       where
-LL |           Dst: BikeshedIntrinsicFrom<Src, {
-   |  ______________^
-LL | |             Assume {
-LL | |                 alignment: true,
-LL | |                 lifetimes: true,
-...  |
-LL | |             }
-LL | |         }>
-   | |__________^ required by this bound in `is_maybe_transmutable`
-
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0277`.