Auto merge of #136389 - matthiaskrgr:rollup-x453dy9, r=matthiaskrgr

Rollup of 6 pull requests

Successful merges:

 - #130514 (Implement MIR lowering for unsafe binders)
 - #135684 (docs: Documented Send and Sync requirements for Mutex + MutexGuard)
 - #136307 (Implement all mix/max functions in a (hopefully) more optimization amendable way)
 - #136360 (Stabilize `once_wait`)
 - #136364 (document that ptr cmp is unsigned)
 - #136374 (Add link attribute for Enzyme's LLVMRust FFI)

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_ast/src/ast.rs

@@ -1657,7 +1657,7 @@ impl GenBlockKind {
 }
 
 /// Whether we're unwrapping or wrapping an unsafe binder
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 #[derive(Encodable, Decodable, HashStable_Generic)]
 pub enum UnsafeBinderCastKind {
     // e.g. `&i32` -> `unsafe<'a> &'a i32`

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3915,7 +3915,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                         ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::Subslice { .. }
                         | ProjectionElem::Subtype(_)
-                        | ProjectionElem::Index(_) => kind,
+                        | ProjectionElem::Index(_)
+                        | ProjectionElem::UnwrapUnsafeBinder(_) => kind,
                     },
                     place_ty.projection_ty(tcx, elem),
                 )

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -370,6 +370,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 ProjectionElem::Downcast(..) => (),
                 ProjectionElem::OpaqueCast(..) => (),
                 ProjectionElem::Subtype(..) => (),
+                ProjectionElem::UnwrapUnsafeBinder(_) => (),
                 ProjectionElem::Field(field, _ty) => {
                     // FIXME(project-rfc_2229#36): print capture precisely here.
                     if let Some(field) = self.is_upvar_field_projection(PlaceRef {
@@ -450,9 +451,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     PlaceRef { local, projection: proj_base }.ty(self.body, self.infcx.tcx)
                 }
                 ProjectionElem::Downcast(..) => place.ty(self.body, self.infcx.tcx),
-                ProjectionElem::Subtype(ty) | ProjectionElem::OpaqueCast(ty) => {
+                ProjectionElem::Subtype(ty)
-                    PlaceTy::from_ty(*ty)
+                | ProjectionElem::OpaqueCast(ty)
-                }
+                | ProjectionElem::UnwrapUnsafeBinder(ty) => PlaceTy::from_ty(*ty),
                 ProjectionElem::Field(_, field_type) => PlaceTy::from_ty(*field_type),
             },
         };

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -167,7 +167,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                         | ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::OpaqueCast { .. }
                         | ProjectionElem::Subslice { .. }
-                        | ProjectionElem::Downcast(..),
+                        | ProjectionElem::Downcast(..)
+                        | ProjectionElem::UnwrapUnsafeBinder(_),
                     ],
             } => bug!("Unexpected immutable place."),
         }

compiler/rustc_borrowck/src/lib.rs

@@ -1398,6 +1398,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                     self.consume_operand(location, (operand, span), state);
                 }
             }
+
+            Rvalue::WrapUnsafeBinder(op, _) => {
+                self.consume_operand(location, (op, span), state);
+            }
         }
     }
 
@@ -1770,7 +1774,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 // So it's safe to skip these.
                 ProjectionElem::OpaqueCast(_)
                 | ProjectionElem::Subtype(_)
-                | ProjectionElem::Downcast(_, _) => (),
+                | ProjectionElem::Downcast(_, _)
+                | ProjectionElem::UnwrapUnsafeBinder(_) => (),
             }
 
             place_ty = place_ty.projection_ty(tcx, elem);
@@ -2004,6 +2009,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 // FIXME: is this true even if P is an adt with a dtor?
                 { }
 
+                ProjectionElem::UnwrapUnsafeBinder(_) => {
+                    check_parent_of_field(self, location, place_base, span, state);
+                }
+
                 // assigning to (*P) requires P to be initialized
                 ProjectionElem::Deref => {
                     self.check_if_full_path_is_moved(
@@ -2384,7 +2393,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                     | ProjectionElem::Subslice { .. }
                     | ProjectionElem::Subtype(..)
                     | ProjectionElem::OpaqueCast { .. }
-                    | ProjectionElem::Downcast(..) => {
+                    | ProjectionElem::Downcast(..)
+                    | ProjectionElem::UnwrapUnsafeBinder(_) => {
                         let upvar_field_projection = self.is_upvar_field_projection(place);
                         if let Some(field) = upvar_field_projection {
                             let upvar = &self.upvars[field.index()];

compiler/rustc_borrowck/src/places_conflict.rs

@@ -250,7 +250,8 @@ fn place_components_conflict<'tcx>(
                 | (ProjectionElem::Subslice { .. }, _, _)
                 | (ProjectionElem::OpaqueCast { .. }, _, _)
                 | (ProjectionElem::Subtype(_), _, _)
-                | (ProjectionElem::Downcast { .. }, _, _) => {
+                | (ProjectionElem::Downcast { .. }, _, _)
+                | (ProjectionElem::UnwrapUnsafeBinder(_), _, _) => {
                     // Recursive case. This can still be disjoint on a
                     // further iteration if this a shallow access and
                     // there's a deref later on, e.g., a borrow
@@ -519,5 +520,9 @@ fn place_projection_conflict<'tcx>(
             pi1_elem,
             pi2_elem
         ),
+
+        (ProjectionElem::UnwrapUnsafeBinder(_), _) => {
+            todo!()
+        }
     }
 }

compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs

@@ -325,6 +325,10 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
                     self.consume_operand(location, operand);
                 }
             }
+
+            Rvalue::WrapUnsafeBinder(op, _) => {
+                self.consume_operand(location, op);
+            }
         }
     }
 

compiler/rustc_borrowck/src/prefixes.rs

@@ -66,6 +66,10 @@ impl<'tcx> Iterator for Prefixes<'tcx> {
                             self.next = Some(cursor_base);
                             return Some(cursor);
                         }
+                        ProjectionElem::UnwrapUnsafeBinder(_) => {
+                            self.next = Some(cursor_base);
+                            return Some(cursor);
+                        }
                         ProjectionElem::Downcast(..)
                         | ProjectionElem::Subslice { .. }
                         | ProjectionElem::OpaqueCast { .. }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -302,6 +302,25 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
                     )
                     .unwrap();
             }
+            ProjectionElem::UnwrapUnsafeBinder(ty) => {
+                let ty::UnsafeBinder(binder_ty) = *base_ty.ty.kind() else {
+                    unreachable!();
+                };
+                let found_ty = self.typeck.infcx.instantiate_binder_with_fresh_vars(
+                    self.body().source_info(location).span,
+                    BoundRegionConversionTime::HigherRankedType,
+                    binder_ty.into(),
+                );
+                self.typeck
+                    .relate_types(
+                        ty,
+                        context.ambient_variance(),
+                        found_ty,
+                        location.to_locations(),
+                        ConstraintCategory::Boring,
+                    )
+                    .unwrap();
+            }
             ProjectionElem::Subtype(_) => {
                 bug!("ProjectionElem::Subtype shouldn't exist in borrowck")
             }
@@ -2233,6 +2252,27 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 self.check_operand(right, location);
             }
 
+            Rvalue::WrapUnsafeBinder(op, ty) => {
+                self.check_operand(op, location);
+                let operand_ty = op.ty(self.body, self.tcx());
+
+                let ty::UnsafeBinder(binder_ty) = *ty.kind() else {
+                    unreachable!();
+                };
+                let expected_ty = self.infcx.instantiate_binder_with_fresh_vars(
+                    self.body().source_info(location).span,
+                    BoundRegionConversionTime::HigherRankedType,
+                    binder_ty.into(),
+                );
+                self.sub_types(
+                    operand_ty,
+                    expected_ty,
+                    location.to_locations(),
+                    ConstraintCategory::Boring,
+                )
+                .unwrap();
+            }
+
             Rvalue::RawPtr(..)
             | Rvalue::ThreadLocalRef(..)
             | Rvalue::Len(..)
@@ -2258,7 +2298,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             | Rvalue::NullaryOp(..)
             | Rvalue::CopyForDeref(..)
             | Rvalue::UnaryOp(..)
-            | Rvalue::Discriminant(..) => None,
+            | Rvalue::Discriminant(..)
+            | Rvalue::WrapUnsafeBinder(..) => None,
 
             Rvalue::Aggregate(aggregate, _) => match **aggregate {
                 AggregateKind::Adt(_, _, _, user_ty, _) => user_ty,
@@ -2450,7 +2491,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 | ProjectionElem::OpaqueCast(..)
                 | ProjectionElem::Index(..)
                 | ProjectionElem::ConstantIndex { .. }
-                | ProjectionElem::Subslice { .. } => {
+                | ProjectionElem::Subslice { .. }
+                | ProjectionElem::UnwrapUnsafeBinder(_) => {
                     // other field access
                 }
                 ProjectionElem::Subtype(_) => {

compiler/rustc_codegen_cranelift/src/base.rs

@@ -925,6 +925,10 @@ fn codegen_stmt<'tcx>(
                     }
                     crate::discriminant::codegen_set_discriminant(fx, lval, variant_index);
                 }
+                Rvalue::WrapUnsafeBinder(ref operand, _to_ty) => {
+                    let operand = codegen_operand(fx, operand);
+                    lval.write_cvalue_transmute(fx, operand);
+                }
             }
         }
         StatementKind::StorageLive(_)
@@ -993,7 +997,9 @@ pub(crate) fn codegen_place<'tcx>(
                 cplace = cplace.place_deref(fx);
             }
             PlaceElem::OpaqueCast(ty) => bug!("encountered OpaqueCast({ty}) in codegen"),
-            PlaceElem::Subtype(ty) => cplace = cplace.place_transmute_type(fx, fx.monomorphize(ty)),
+            PlaceElem::Subtype(ty) | PlaceElem::UnwrapUnsafeBinder(ty) => {
+                cplace = cplace.place_transmute_type(fx, fx.monomorphize(ty));
+            }
             PlaceElem::Field(field, _ty) => {
                 cplace = cplace.place_field(fx, field);
             }

compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs

@@ -4,6 +4,8 @@ use libc::{c_char, c_uint};
 
 use super::ffi::{BasicBlock, Metadata, Module, Type, Value};
 use crate::llvm::Bool;
+
+#[link(name = "llvm-wrapper", kind = "static")]
 extern "C" {
     // Enzyme
     pub fn LLVMRustHasMetadata(I: &Value, KindID: c_uint) -> bool;
@@ -12,10 +14,13 @@ extern "C" {
     pub fn LLVMRustDIGetInstMetadata(I: &Value) -> Option<&Metadata>;
     pub fn LLVMRustEraseInstFromParent(V: &Value);
     pub fn LLVMRustGetTerminator<'a>(B: &BasicBlock) -> &'a Value;
+    pub fn LLVMRustVerifyFunction(V: &Value, action: LLVMRustVerifierFailureAction) -> Bool;
+}
+
+extern "C" {
+    // Enzyme
     pub fn LLVMDumpModule(M: &Module);
     pub fn LLVMDumpValue(V: &Value);
-    pub fn LLVMRustVerifyFunction(V: &Value, action: LLVMRustVerifierFailureAction) -> Bool;
-
     pub fn LLVMGetFunctionCallConv(F: &Value) -> c_uint;
     pub fn LLVMGetReturnType(T: &Type) -> &Type;
     pub fn LLVMGetParams(Fnc: &Value, parms: *mut &Value);

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -502,6 +502,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     bug!("encountered OpaqueCast({ty}) in codegen")
                 }
                 mir::ProjectionElem::Subtype(ty) => cg_base.project_type(bx, self.monomorphize(ty)),
+                mir::ProjectionElem::UnwrapUnsafeBinder(ty) => {
+                    cg_base.project_type(bx, self.monomorphize(ty))
+                }
                 mir::ProjectionElem::Index(index) => {
                     let index = &mir::Operand::Copy(mir::Place::from(index));
                     let index = self.codegen_operand(bx, index);

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -823,6 +823,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
                 OperandRef { val: OperandValue::Immediate(val), layout: box_layout }
             }
+            mir::Rvalue::WrapUnsafeBinder(ref operand, binder_ty) => {
+                let operand = self.codegen_operand(bx, operand);
+                let binder_ty = self.monomorphize(binder_ty);
+                let layout = bx.cx().layout_of(binder_ty);
+                OperandRef { val: operand.val, layout }
+            }
         }
     }
 
@@ -1123,7 +1129,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::Rvalue::Discriminant(..) |
             mir::Rvalue::NullaryOp(..) |
             mir::Rvalue::ThreadLocalRef(_) |
-            mir::Rvalue::Use(..) => // (*)
+            mir::Rvalue::Use(..) |
+            mir::Rvalue::WrapUnsafeBinder(..) => // (*)
                 true,
             // Arrays are always aggregates, so it's not worth checking anything here.
             // (If it's really `[(); N]` or `[T; 0]` and we use the place path, fine.)

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -728,6 +728,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     );
                 }
             }
+
+            Rvalue::WrapUnsafeBinder(..) => {
+                // Unsafe binders are always trivial to create.
+            }
         }
     }
 

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -258,6 +258,8 @@ where
             in_place::<Q, _>(cx, in_local, place.as_ref())
         }
 
+        Rvalue::WrapUnsafeBinder(op, _) => in_operand::<Q, _>(cx, in_local, op),
+
         Rvalue::Aggregate(kind, operands) => {
             // Return early if we know that the struct or enum being constructed is always
             // qualified.
@@ -297,7 +299,8 @@ where
             | ProjectionElem::ConstantIndex { .. }
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Downcast(_, _)
-            | ProjectionElem::Index(_) => {}
+            | ProjectionElem::Index(_)
+            | ProjectionElem::UnwrapUnsafeBinder(_) => {}
         }
 
         let base_ty = place_base.ty(cx.body, cx.tcx);

compiler/rustc_const_eval/src/check_consts/resolver.rs

@@ -202,7 +202,8 @@ where
             | mir::Rvalue::NullaryOp(..)
             | mir::Rvalue::UnaryOp(..)
             | mir::Rvalue::Discriminant(..)
-            | mir::Rvalue::Aggregate(..) => {}
+            | mir::Rvalue::Aggregate(..)
+            | mir::Rvalue::WrapUnsafeBinder(..) => {}
         }
     }
 

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -381,6 +381,7 @@ where
             OpaqueCast(ty) => {
                 span_bug!(self.cur_span(), "OpaqueCast({ty}) encountered after borrowck")
             }
+            UnwrapUnsafeBinder(target) => base.transmute(self.layout_of(target)?, self)?,
             // We don't want anything happening here, this is here as a dummy.
             Subtype(_) => base.transmute(base.layout(), self)?,
             Field(field, _) => self.project_field(base, field.index())?,

compiler/rustc_const_eval/src/interpret/step.rs

@@ -277,6 +277,13 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let discr = self.discriminant_for_variant(op.layout.ty, variant)?;
                 self.write_immediate(*discr, &dest)?;
             }
+
+            WrapUnsafeBinder(ref op, _ty) => {
+                // Constructing an unsafe binder acts like a transmute
+                // since the operand's layout does not change.
+                let op = self.eval_operand(op, None)?;
+                self.copy_op_allow_transmute(&op, &dest)?;
+            }
         }
 
         trace!("{:?}", self.dump_place(&dest));

compiler/rustc_hir_typeck/src/expr.rs

@@ -1658,8 +1658,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         hir_ty: Option<&'tcx hir::Ty<'tcx>>,
         expected: Expectation<'tcx>,
     ) -> Ty<'tcx> {
-        self.dcx().span_err(inner_expr.span, "unsafe binder casts are not fully implemented");
-
         match kind {
             hir::UnsafeBinderCastKind::Wrap => {
                 let ascribed_ty =

compiler/rustc_middle/src/mir/pretty.rs

@@ -1248,6 +1248,10 @@ impl<'tcx> Debug for Rvalue<'tcx> {
             ShallowInitBox(ref place, ref ty) => {
                 with_no_trimmed_paths!(write!(fmt, "ShallowInitBox({place:?}, {ty})"))
             }
+
+            WrapUnsafeBinder(ref op, ty) => {
+                with_no_trimmed_paths!(write!(fmt, "wrap_binder!({op:?}; {ty})"))
+            }
         }
     }
 }
@@ -1308,6 +1312,9 @@ fn pre_fmt_projection(projection: &[PlaceElem<'_>], fmt: &mut Formatter<'_>) ->
             ProjectionElem::Index(_)
             | ProjectionElem::ConstantIndex { .. }
             | ProjectionElem::Subslice { .. } => {}
+            ProjectionElem::UnwrapUnsafeBinder(_) => {
+                write!(fmt, "unwrap_binder!(")?;
+            }
         }
     }
 
@@ -1356,6 +1363,9 @@ fn post_fmt_projection(projection: &[PlaceElem<'_>], fmt: &mut Formatter<'_>) ->
             ProjectionElem::Subslice { from, to, from_end: false } => {
                 write!(fmt, "[{from:?}..{to:?}]")?;
             }
+            ProjectionElem::UnwrapUnsafeBinder(ty) => {
+                write!(fmt, "; {ty})")?;
+            }
         }
     }
 

compiler/rustc_middle/src/mir/statement.rs

@@ -62,7 +62,8 @@ impl<V, T> ProjectionElem<V, T> {
             | Self::Subtype(_)
             | Self::ConstantIndex { .. }
             | Self::Subslice { .. }
-            | Self::Downcast(_, _) => false,
+            | Self::Downcast(_, _)
+            | Self::UnwrapUnsafeBinder(..) => false,
         }
     }
 
@@ -76,7 +77,8 @@ impl<V, T> ProjectionElem<V, T> {
             | Self::Subtype(_)
             | Self::ConstantIndex { .. }
             | Self::Subslice { .. }
-            | Self::Downcast(_, _) => true,
+            | Self::Downcast(_, _)
+            | Self::UnwrapUnsafeBinder(..) => true,
         }
     }
 
@@ -102,6 +104,9 @@ impl<V, T> ProjectionElem<V, T> {
             | Self::Subtype(_)
             | Self::OpaqueCast(_)
             | Self::Subslice { .. } => false,
+
+            // FIXME(unsafe_binders): Figure this out.
+            Self::UnwrapUnsafeBinder(..) => false,
         }
     }
 }
@@ -443,7 +448,8 @@ impl<'tcx> Rvalue<'tcx> {
             | Rvalue::UnaryOp(_, _)
             | Rvalue::Discriminant(_)
             | Rvalue::Aggregate(_, _)
-            | Rvalue::ShallowInitBox(_, _) => true,
+            | Rvalue::ShallowInitBox(_, _)
+            | Rvalue::WrapUnsafeBinder(_, _) => true,
         }
     }
 }

compiler/rustc_middle/src/mir/syntax.rs

@@ -1276,6 +1276,10 @@ pub enum ProjectionElem<V, T> {
     /// requiring an intermediate variable.
     OpaqueCast(T),
 
+    /// A transmute from an unsafe binder to the type that it wraps. This is a projection
+    /// of a place, so it doesn't necessarily constitute a move out of the binder.
+    UnwrapUnsafeBinder(T),
+
     /// A `Subtype(T)` projection is applied to any `StatementKind::Assign` where
     /// type of lvalue doesn't match the type of rvalue, the primary goal is making subtyping
     /// explicit during optimizations and codegen.
@@ -1493,6 +1497,9 @@ pub enum Rvalue<'tcx> {
     /// optimizations and codegen backends that previously had to handle deref operations anywhere
     /// in a place.
     CopyForDeref(Place<'tcx>),
+
+    /// Wraps a value in an unsafe binder.
+    WrapUnsafeBinder(Operand<'tcx>, Ty<'tcx>),
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]

compiler/rustc_middle/src/mir/tcx.rs

@@ -146,6 +146,11 @@ impl<'tcx> PlaceTy<'tcx> {
             ProjectionElem::Subtype(ty) => {
                 PlaceTy::from_ty(handle_opaque_cast_and_subtype(&self, ty))
             }
+
+            // FIXME(unsafe_binders): Rename `handle_opaque_cast_and_subtype` to be more general.
+            ProjectionElem::UnwrapUnsafeBinder(ty) => {
+                PlaceTy::from_ty(handle_opaque_cast_and_subtype(&self, ty))
+            }
         };
         debug!("projection_ty self: {:?} elem: {:?} yields: {:?}", self, elem, answer);
         answer
@@ -241,6 +246,7 @@ impl<'tcx> Rvalue<'tcx> {
             },
             Rvalue::ShallowInitBox(_, ty) => Ty::new_box(tcx, ty),
             Rvalue::CopyForDeref(ref place) => place.ty(local_decls, tcx).ty,
+            Rvalue::WrapUnsafeBinder(_, ty) => ty,
         }
     }
 

compiler/rustc_middle/src/mir/type_foldable.rs

@@ -1,6 +1,7 @@
 //! `TypeFoldable` implementations for MIR types
 
 use rustc_ast::InlineAsmTemplatePiece;
+use rustc_hir::UnsafeBinderCastKind;
 use rustc_hir::def_id::LocalDefId;
 
 use super::*;
@@ -21,6 +22,7 @@ TrivialTypeTraversalImpls! {
     SwitchTargets,
     CoroutineKind,
     CoroutineSavedLocal,
+    UnsafeBinderCastKind,
 }
 
 TrivialTypeTraversalImpls! {

compiler/rustc_middle/src/mir/visit.rs

@@ -781,6 +781,11 @@ macro_rules! make_mir_visitor {
                         self.visit_operand(operand, location);
                         self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));
                     }
+
+                    Rvalue::WrapUnsafeBinder(op, ty) => {
+                        self.visit_operand(op, location);
+                        self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));
+                    }
                 }
             }
 
@@ -1151,6 +1156,11 @@ macro_rules! visit_place_fns {
                     self.visit_ty(&mut new_ty, TyContext::Location(location));
                     if ty != new_ty { Some(PlaceElem::Subtype(new_ty)) } else { None }
                 }
+                PlaceElem::UnwrapUnsafeBinder(ty) => {
+                    let mut new_ty = ty;
+                    self.visit_ty(&mut new_ty, TyContext::Location(location));
+                    if ty != new_ty { Some(PlaceElem::UnwrapUnsafeBinder(new_ty)) } else { None }
+                }
                 PlaceElem::Deref
                 | PlaceElem::ConstantIndex { .. }
                 | PlaceElem::Subslice { .. }
@@ -1219,7 +1229,8 @@ macro_rules! visit_place_fns {
             match elem {
                 ProjectionElem::OpaqueCast(ty)
                 | ProjectionElem::Subtype(ty)
-                | ProjectionElem::Field(_, ty) => {
+                | ProjectionElem::Field(_, ty)
+                | ProjectionElem::UnwrapUnsafeBinder(ty) => {
                     self.visit_ty(ty, TyContext::Location(location));
                 }
                 ProjectionElem::Index(local) => {

compiler/rustc_middle/src/thir.rs

@@ -489,6 +489,19 @@ pub enum ExprKind<'tcx> {
         user_ty: UserTy<'tcx>,
         user_ty_span: Span,
     },
+    /// An unsafe binder cast on a place, e.g. `unwrap_binder!(*ptr)`.
+    PlaceUnwrapUnsafeBinder {
+        source: ExprId,
+    },
+    /// An unsafe binder cast on a value, e.g. `unwrap_binder!(rvalue())`,
+    /// which makes a temporary.
+    ValueUnwrapUnsafeBinder {
+        source: ExprId,
+    },
+    /// Construct an unsafe binder, e.g. `wrap_binder(&ref)`.
+    WrapUnsafeBinder {
+        source: ExprId,
+    },
     /// A closure definition.
     Closure(Box<ClosureExpr<'tcx>>),
     /// A literal.

compiler/rustc_middle/src/thir/visit.rs

@@ -136,6 +136,9 @@ pub fn walk_expr<'thir, 'tcx: 'thir, V: Visitor<'thir, 'tcx>>(
         | ValueTypeAscription { source, user_ty: _, user_ty_span: _ } => {
             visitor.visit_expr(&visitor.thir()[source])
         }
+        PlaceUnwrapUnsafeBinder { source }
+        | ValueUnwrapUnsafeBinder { source }
+        | WrapUnsafeBinder { source } => visitor.visit_expr(&visitor.thir()[source]),
         Closure(box ClosureExpr {
             closure_id: _,
             args: _,

compiler/rustc_mir_build/messages.ftl

@@ -361,6 +361,18 @@ mir_build_unreachable_pattern = unreachable pattern
     .unreachable_pattern_let_binding = there is a binding of the same name; if you meant to pattern match against the value of that binding, that is a feature of constants that is not available for `let` bindings
     .suggestion = remove the match arm
 
+mir_build_unsafe_binder_cast_requires_unsafe =
+    unsafe binder cast is unsafe and requires unsafe block
+    .label = unsafe binder cast
+    .note = casting to or from an `unsafe<...>` binder type is unsafe since it erases lifetime
+        information that may be required to uphold safety guarantees of a type
+
+mir_build_unsafe_binder_cast_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
+    unsafe binder cast is unsafe and requires unsafe block or unsafe fn
+    .label = unsafe binder cast
+    .note = casting to or from an `unsafe<...>` binder type is unsafe since it erases lifetime
+        information that may be required to uphold safety guarantees of a type
+
 mir_build_unsafe_field_requires_unsafe =
     use of unsafe field is unsafe and requires unsafe block
     .note = unsafe fields may carry library invariants

compiler/rustc_mir_build/src/builder/expr/as_place.rs

@@ -105,7 +105,8 @@ fn convert_to_hir_projections_and_truncate_for_capture(
             ProjectionElem::OpaqueCast(_) | ProjectionElem::Subtype(..) => continue,
             ProjectionElem::Index(..)
             | ProjectionElem::ConstantIndex { .. }
-            | ProjectionElem::Subslice { .. } => {
+            | ProjectionElem::Subslice { .. }
+            | ProjectionElem::UnwrapUnsafeBinder(_) => {
                 // We don't capture array-access projections.
                 // We can stop here as arrays are captured completely.
                 break;
@@ -523,6 +524,20 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.and(PlaceBuilder::from(temp))
             }
 
+            ExprKind::PlaceUnwrapUnsafeBinder { source } => {
+                let place_builder = unpack!(
+                    block = this.expr_as_place(block, source, mutability, fake_borrow_temps,)
+                );
+                block.and(place_builder.project(PlaceElem::UnwrapUnsafeBinder(expr.ty)))
+            }
+            ExprKind::ValueUnwrapUnsafeBinder { source } => {
+                let source_expr = &this.thir[source];
+                let temp = unpack!(
+                    block = this.as_temp(block, source_expr.temp_lifetime, source, mutability)
+                );
+                block.and(PlaceBuilder::from(temp).project(PlaceElem::UnwrapUnsafeBinder(expr.ty)))
+            }
+
             ExprKind::Array { .. }
             | ExprKind::Tuple { .. }
             | ExprKind::Adt { .. }
@@ -560,7 +575,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::OffsetOf { .. }
             | ExprKind::Yield { .. }
             | ExprKind::ThreadLocalRef(_)
-            | ExprKind::Call { .. } => {
+            | ExprKind::Call { .. }
+            | ExprKind::WrapUnsafeBinder { .. } => {
                 // these are not places, so we need to make a temporary.
                 debug_assert!(!matches!(Category::of(&expr.kind), Some(Category::Place)));
                 let temp =
@@ -776,7 +792,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     | ProjectionElem::OpaqueCast(..)
                     | ProjectionElem::Subtype(..)
                     | ProjectionElem::ConstantIndex { .. }
-                    | ProjectionElem::Subslice { .. } => (),
+                    | ProjectionElem::Subslice { .. }
+                    | ProjectionElem::UnwrapUnsafeBinder(_) => (),
                 }
             }
         }

compiler/rustc_mir_build/src/builder/expr/as_rvalue.rs

@@ -508,6 +508,19 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.and(Rvalue::Use(Operand::Constant(Box::new(constant))))
             }
 
+            ExprKind::WrapUnsafeBinder { source } => {
+                let source = unpack!(
+                    block = this.as_operand(
+                        block,
+                        scope,
+                        source,
+                        LocalInfo::Boring,
+                        NeedsTemporary::Maybe
+                    )
+                );
+                block.and(Rvalue::WrapUnsafeBinder(source, expr.ty))
+            }
+
             ExprKind::Yield { .. }
             | ExprKind::Block { .. }
             | ExprKind::Match { .. }
@@ -532,7 +545,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Become { .. }
             | ExprKind::InlineAsm { .. }
             | ExprKind::PlaceTypeAscription { .. }
-            | ExprKind::ValueTypeAscription { .. } => {
+            | ExprKind::ValueTypeAscription { .. }
+            | ExprKind::PlaceUnwrapUnsafeBinder { .. }
+            | ExprKind::ValueUnwrapUnsafeBinder { .. } => {
                 // these do not have corresponding `Rvalue` variants,
                 // so make an operand and then return that
                 debug_assert!(!matches!(

compiler/rustc_mir_build/src/builder/expr/category.rs

@@ -41,7 +41,9 @@ impl Category {
             | ExprKind::UpvarRef { .. }
             | ExprKind::VarRef { .. }
             | ExprKind::PlaceTypeAscription { .. }
-            | ExprKind::ValueTypeAscription { .. } => Some(Category::Place),
+            | ExprKind::ValueTypeAscription { .. }
+            | ExprKind::PlaceUnwrapUnsafeBinder { .. }
+            | ExprKind::ValueUnwrapUnsafeBinder { .. } => Some(Category::Place),
 
             ExprKind::LogicalOp { .. }
             | ExprKind::Match { .. }
@@ -68,7 +70,8 @@ impl Category {
             | ExprKind::Assign { .. }
             | ExprKind::AssignOp { .. }
             | ExprKind::ThreadLocalRef(_)
-            | ExprKind::OffsetOf { .. } => Some(Category::Rvalue(RvalueFunc::AsRvalue)),
+            | ExprKind::OffsetOf { .. }
+            | ExprKind::WrapUnsafeBinder { .. } => Some(Category::Rvalue(RvalueFunc::AsRvalue)),
 
             ExprKind::ConstBlock { .. }
             | ExprKind::Literal { .. }

compiler/rustc_mir_build/src/builder/expr/into.rs

@@ -554,7 +554,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             ExprKind::VarRef { .. }
             | ExprKind::UpvarRef { .. }
             | ExprKind::PlaceTypeAscription { .. }
-            | ExprKind::ValueTypeAscription { .. } => {
+            | ExprKind::ValueTypeAscription { .. }
+            | ExprKind::PlaceUnwrapUnsafeBinder { .. }
+            | ExprKind::ValueUnwrapUnsafeBinder { .. } => {
                 debug_assert!(Category::of(&expr.kind) == Some(Category::Place));
 
                 let place = unpack!(block = this.as_place(block, expr_id));
@@ -613,7 +615,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::ConstParam { .. }
             | ExprKind::ThreadLocalRef(_)
             | ExprKind::StaticRef { .. }
-            | ExprKind::OffsetOf { .. } => {
+            | ExprKind::OffsetOf { .. }
+            | ExprKind::WrapUnsafeBinder { .. } => {
                 debug_assert!(match Category::of(&expr.kind).unwrap() {
                     // should be handled above
                     Category::Rvalue(RvalueFunc::Into) => false,

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -439,6 +439,9 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
             | ExprKind::NeverToAny { .. }
             | ExprKind::PlaceTypeAscription { .. }
             | ExprKind::ValueTypeAscription { .. }
+            | ExprKind::PlaceUnwrapUnsafeBinder { .. }
+            | ExprKind::ValueUnwrapUnsafeBinder { .. }
+            | ExprKind::WrapUnsafeBinder { .. }
             | ExprKind::PointerCoercion { .. }
             | ExprKind::Repeat { .. }
             | ExprKind::StaticRef { .. }
@@ -680,6 +683,11 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                     }
                 }
             }
+            ExprKind::PlaceUnwrapUnsafeBinder { .. }
+            | ExprKind::ValueUnwrapUnsafeBinder { .. }
+            | ExprKind::WrapUnsafeBinder { .. } => {
+                self.requires_unsafe(expr.span, UnsafeBinderCast);
+            }
             _ => {}
         }
         visit::walk_expr(self, expr);
@@ -728,6 +736,7 @@ enum UnsafeOpKind {
         /// (e.g., with `-C target-feature`).
         build_enabled: Vec<Symbol>,
     },
+    UnsafeBinderCast,
 }
 
 use UnsafeOpKind::*;
@@ -891,6 +900,15 @@ impl UnsafeOpKind {
                     unsafe_not_inherited_note,
                 },
             ),
+            UnsafeBinderCast => tcx.emit_node_span_lint(
+                UNSAFE_OP_IN_UNSAFE_FN,
+                hir_id,
+                span,
+                UnsafeOpInUnsafeFnUnsafeBinderCastRequiresUnsafe {
+                    span,
+                    unsafe_not_inherited_note,
+                },
+            ),
         }
     }
 
@@ -1099,6 +1117,15 @@ impl UnsafeOpKind {
                     function: tcx.def_path_str(*function),
                 });
             }
+            UnsafeBinderCast if unsafe_op_in_unsafe_fn_allowed => {
+                dcx.emit_err(UnsafeBinderCastRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
+                    span,
+                    unsafe_not_inherited_note,
+                });
+            }
+            UnsafeBinderCast => {
+                dcx.emit_err(UnsafeBinderCastRequiresUnsafe { span, unsafe_not_inherited_note });
+            }
         }
     }
 }

compiler/rustc_mir_build/src/errors.rs

@@ -160,6 +160,18 @@ pub(crate) struct UnsafeOpInUnsafeFnBorrowOfLayoutConstrainedFieldRequiresUnsafe
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedLintNote>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(
+    mir_build_unsafe_binder_cast_requires_unsafe,
+    code = E0133,
+)]
+pub(crate) struct UnsafeOpInUnsafeFnUnsafeBinderCastRequiresUnsafe {
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedLintNote>,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(mir_build_unsafe_op_in_unsafe_fn_call_to_fn_with_requires_unsafe, code = E0133)]
 #[help]
@@ -494,6 +506,32 @@ pub(crate) struct CallToFunctionWithRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
 }
 
+#[derive(Diagnostic)]
+#[diag(
+    mir_build_unsafe_binder_cast_requires_unsafe,
+    code = E0133,
+)]
+pub(crate) struct UnsafeBinderCastRequiresUnsafe {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
+#[derive(Diagnostic)]
+#[diag(
+    mir_build_unsafe_binder_cast_requires_unsafe_unsafe_op_in_unsafe_fn_allowed,
+    code = E0133,
+)]
+pub(crate) struct UnsafeBinderCastRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
 #[derive(Subdiagnostic)]
 #[label(mir_build_unsafe_not_inherited)]
 pub(crate) struct UnsafeNotInheritedNote {

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -1,5 +1,6 @@
 use itertools::Itertools;
 use rustc_abi::{FIRST_VARIANT, FieldIdx};
+use rustc_ast::UnsafeBinderCastKind;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
@@ -910,8 +911,19 @@ impl<'tcx> Cx<'tcx> {
                 }
             }
 
-            hir::ExprKind::UnsafeBinderCast(_kind, _source, _ty) => {
+            hir::ExprKind::UnsafeBinderCast(UnsafeBinderCastKind::Unwrap, source, _ty) => {
-                unreachable!("unsafe binders are not yet implemented")
+                // FIXME(unsafe_binders): Take into account the ascribed type, too.
+                let mirrored = self.mirror_expr(source);
+                if source.is_syntactic_place_expr() {
+                    ExprKind::PlaceUnwrapUnsafeBinder { source: mirrored }
+                } else {
+                    ExprKind::ValueUnwrapUnsafeBinder { source: mirrored }
+                }
+            }
+            hir::ExprKind::UnsafeBinderCast(UnsafeBinderCastKind::Wrap, source, _ty) => {
+                // FIXME(unsafe_binders): Take into account the ascribed type, too.
+                let mirrored = self.mirror_expr(source);
+                ExprKind::WrapUnsafeBinder { source: mirrored }
             }
 
             hir::ExprKind::DropTemps(source) => ExprKind::Use { source: self.mirror_expr(source) },

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -326,9 +326,10 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             | Use { source }
             | PointerCoercion { source, .. }
             | PlaceTypeAscription { source, .. }
-            | ValueTypeAscription { source, .. } => {
+            | ValueTypeAscription { source, .. }
-                self.is_known_valid_scrutinee(&self.thir()[*source])
+            | PlaceUnwrapUnsafeBinder { source }
-            }
+            | ValueUnwrapUnsafeBinder { source }
+            | WrapUnsafeBinder { source } => self.is_known_valid_scrutinee(&self.thir()[*source]),
 
             // These diverge.
             Become { .. } | Break { .. } | Continue { .. } | Return { .. } => true,

compiler/rustc_mir_build/src/thir/print.rs

@@ -477,6 +477,24 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
                 self.print_expr(*source, depth_lvl + 2);
                 print_indented!(self, "}", depth_lvl);
             }
+            PlaceUnwrapUnsafeBinder { source } => {
+                print_indented!(self, "PlaceUnwrapUnsafeBinder {", depth_lvl);
+                print_indented!(self, "source:", depth_lvl + 1);
+                self.print_expr(*source, depth_lvl + 2);
+                print_indented!(self, "}", depth_lvl);
+            }
+            ValueUnwrapUnsafeBinder { source } => {
+                print_indented!(self, "ValueUnwrapUnsafeBinder {", depth_lvl);
+                print_indented!(self, "source:", depth_lvl + 1);
+                self.print_expr(*source, depth_lvl + 2);
+                print_indented!(self, "}", depth_lvl);
+            }
+            WrapUnsafeBinder { source } => {
+                print_indented!(self, "WrapUnsafeBinder {", depth_lvl);
+                print_indented!(self, "source:", depth_lvl + 1);
+                self.print_expr(*source, depth_lvl + 2);
+                print_indented!(self, "}", depth_lvl);
+            }
             Closure(closure_expr) => {
                 print_indented!(self, "Closure {", depth_lvl);
                 print_indented!(self, "closure_expr:", depth_lvl + 1);

compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs

@@ -97,7 +97,8 @@ where
             | Rvalue::UnaryOp(..)
             | Rvalue::Discriminant(..)
             | Rvalue::Aggregate(..)
-            | Rvalue::CopyForDeref(..) => {}
+            | Rvalue::CopyForDeref(..)
+            | Rvalue::WrapUnsafeBinder(..) => {}
         }
     }
 

compiler/rustc_mir_dataflow/src/move_paths/abs_domain.rs

@@ -32,6 +32,7 @@ impl<'tcx> Lift for PlaceElem<'tcx> {
             }
             ProjectionElem::Downcast(a, u) => ProjectionElem::Downcast(a, u),
             ProjectionElem::Subtype(_ty) => ProjectionElem::Subtype(()),
+            ProjectionElem::UnwrapUnsafeBinder(_ty) => ProjectionElem::UnwrapUnsafeBinder(()),
         }
     }
 }

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -208,7 +208,7 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
                     | ty::Infer(_)
                     | ty::Error(_)
                     | ty::Placeholder(_) => bug!(
-                        "When Place contains ProjectionElem::Field it's type shouldn't be {place_ty:#?}"
+                        "When Place contains ProjectionElem::Field its type shouldn't be {place_ty:#?}"
                     ),
                 },
                 ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } => {
@@ -226,6 +226,7 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
                     }
                     _ => bug!("Unexpected type {place_ty:#?}"),
                 },
+                ProjectionElem::UnwrapUnsafeBinder(_) => {}
                 // `OpaqueCast`:Only transmutes the type, so no moves there.
                 // `Downcast`  :Only changes information about a `Place` without moving.
                 // `Subtype`   :Only transmutes the type, so moves.
@@ -399,7 +400,8 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
             | Rvalue::Repeat(ref operand, _)
             | Rvalue::Cast(_, ref operand, _)
             | Rvalue::ShallowInitBox(ref operand, _)
-            | Rvalue::UnaryOp(_, ref operand) => self.gather_operand(operand),
+            | Rvalue::UnaryOp(_, ref operand)
+            | Rvalue::WrapUnsafeBinder(ref operand, _) => self.gather_operand(operand),
             Rvalue::BinaryOp(ref _binop, box (ref lhs, ref rhs)) => {
                 self.gather_operand(lhs);
                 self.gather_operand(rhs);

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -504,7 +504,8 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             | Rvalue::Cast(..)
             | Rvalue::BinaryOp(..)
             | Rvalue::Aggregate(..)
-            | Rvalue::ShallowInitBox(..) => {
+            | Rvalue::ShallowInitBox(..)
+            | Rvalue::WrapUnsafeBinder(..) => {
                 // No modification is possible through these r-values.
                 return ValueOrPlace::TOP;
             }

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -575,6 +575,9 @@ impl WriteInfo {
                             self.add_operand(op);
                         }
                     }
+                    Rvalue::WrapUnsafeBinder(op, _) => {
+                        self.add_operand(op);
+                    }
                     Rvalue::ThreadLocalRef(_)
                     | Rvalue::NullaryOp(_, _)
                     | Rvalue::Ref(_, _, _)

compiler/rustc_mir_transform/src/gvn.rs

@@ -476,6 +476,9 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     }
                     ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(ty),
                     ProjectionElem::Subtype(ty) => ProjectionElem::Subtype(ty),
+                    ProjectionElem::UnwrapUnsafeBinder(ty) => {
+                        ProjectionElem::UnwrapUnsafeBinder(ty)
+                    }
                     // This should have been replaced by a `ConstantIndex` earlier.
                     ProjectionElem::Index(_) => return None,
                 };
@@ -713,6 +716,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
             ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(ty),
             ProjectionElem::Subtype(ty) => ProjectionElem::Subtype(ty),
+            ProjectionElem::UnwrapUnsafeBinder(ty) => ProjectionElem::UnwrapUnsafeBinder(ty),
         };
 
         Some(self.insert(Value::Projection(value, proj)))
@@ -867,6 +871,9 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                 self.simplify_place_projection(place, location);
                 return self.new_pointer(*place, AddressKind::Address(mutbl));
             }
+            Rvalue::WrapUnsafeBinder(ref mut op, _) => {
+                return self.simplify_operand(op, location);
+            }
 
             // Operations.
             Rvalue::Len(ref mut place) => return self.simplify_len(place, location),
@@ -931,6 +938,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             ProjectionElem::Downcast(symbol, idx) => ProjectionElem::Downcast(symbol, idx),
             ProjectionElem::OpaqueCast(idx) => ProjectionElem::OpaqueCast(idx),
             ProjectionElem::Subtype(idx) => ProjectionElem::Subtype(idx),
+            ProjectionElem::UnwrapUnsafeBinder(ty) => ProjectionElem::UnwrapUnsafeBinder(ty),
         })
     }
 

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -444,7 +444,8 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             | Rvalue::Cast(..)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::Discriminant(..)
-            | Rvalue::NullaryOp(..) => {}
+            | Rvalue::NullaryOp(..)
+            | Rvalue::WrapUnsafeBinder(..) => {}
         }
 
         // FIXME we need to revisit this for #67176
@@ -546,7 +547,9 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         let val: Value<'_> = match *rvalue {
             ThreadLocalRef(_) => return None,
 
-            Use(ref operand) => self.eval_operand(operand)?.into(),
+            Use(ref operand) | WrapUnsafeBinder(ref operand, _) => {
+                self.eval_operand(operand)?.into()
+            }
 
             CopyForDeref(place) => self.eval_place(place)?.into(),
 

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -293,7 +293,8 @@ impl<'tcx> Validator<'_, 'tcx> {
             // Recurse directly.
             ProjectionElem::ConstantIndex { .. }
             | ProjectionElem::Subtype(_)
-            | ProjectionElem::Subslice { .. } => {}
+            | ProjectionElem::Subslice { .. }
+            | ProjectionElem::UnwrapUnsafeBinder(_) => {}
 
             // Never recurse.
             ProjectionElem::OpaqueCast(..) | ProjectionElem::Downcast(..) => {
@@ -426,7 +427,9 @@ impl<'tcx> Validator<'_, 'tcx> {
 
     fn validate_rvalue(&mut self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
         match rvalue {
-            Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
+            Rvalue::Use(operand)
+            | Rvalue::Repeat(operand, _)
+            | Rvalue::WrapUnsafeBinder(operand, _) => {
                 self.validate_operand(operand)?;
             }
             Rvalue::CopyForDeref(place) => {

compiler/rustc_mir_transform/src/validate.rs

@@ -807,6 +807,25 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     )
                 }
             }
+            ProjectionElem::UnwrapUnsafeBinder(unwrapped_ty) => {
+                let binder_ty = place_ref.ty(&self.body.local_decls, self.tcx);
+                let ty::UnsafeBinder(binder_ty) = *binder_ty.ty.kind() else {
+                    self.fail(
+                        location,
+                        format!("WrapUnsafeBinder does not produce a ty::UnsafeBinder"),
+                    );
+                    return;
+                };
+                let binder_inner_ty = self.tcx.instantiate_bound_regions_with_erased(*binder_ty);
+                if !self.mir_assign_valid_types(unwrapped_ty, binder_inner_ty) {
+                    self.fail(
+                        location,
+                        format!(
+                            "Cannot unwrap unsafe binder {binder_ty:?} into type {unwrapped_ty:?}"
+                        ),
+                    );
+                }
+            }
             _ => {}
         }
         self.super_projection_elem(place_ref, elem, context, location);
@@ -1362,6 +1381,24 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             | Rvalue::RawPtr(_, _)
             | Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf | NullOp::UbChecks, _)
             | Rvalue::Discriminant(_) => {}
+
+            Rvalue::WrapUnsafeBinder(op, ty) => {
+                let unwrapped_ty = op.ty(self.body, self.tcx);
+                let ty::UnsafeBinder(binder_ty) = *ty.kind() else {
+                    self.fail(
+                        location,
+                        format!("WrapUnsafeBinder does not produce a ty::UnsafeBinder"),
+                    );
+                    return;
+                };
+                let binder_inner_ty = self.tcx.instantiate_bound_regions_with_erased(*binder_ty);
+                if !self.mir_assign_valid_types(unwrapped_ty, binder_inner_ty) {
+                    self.fail(
+                        location,
+                        format!("Cannot wrap {unwrapped_ty:?} into unsafe binder {binder_ty:?}"),
+                    );
+                }
+            }
         }
         self.super_rvalue(rvalue, location);
     }

compiler/rustc_smir/src/rustc_smir/convert/mir.rs

@@ -217,6 +217,7 @@ impl<'tcx> Stable<'tcx> for mir::Rvalue<'tcx> {
                 stable_mir::mir::Rvalue::ShallowInitBox(op.stable(tables), ty.stable(tables))
             }
             CopyForDeref(place) => stable_mir::mir::Rvalue::CopyForDeref(place.stable(tables)),
+            WrapUnsafeBinder(..) => todo!("FIXME(unsafe_binders):"),
         }
     }
 }
@@ -395,6 +396,7 @@ impl<'tcx> Stable<'tcx> for mir::PlaceElem<'tcx> {
             Downcast(_, idx) => stable_mir::mir::ProjectionElem::Downcast(idx.stable(tables)),
             OpaqueCast(ty) => stable_mir::mir::ProjectionElem::OpaqueCast(ty.stable(tables)),
             Subtype(ty) => stable_mir::mir::ProjectionElem::Subtype(ty.stable(tables)),
+            UnwrapUnsafeBinder(..) => todo!("FIXME(unsafe_binders):"),
         }
     }
 }

compiler/rustc_span/src/symbol.rs

@@ -2152,7 +2152,6 @@ symbols! {
         unwrap,
         unwrap_binder,
         unwrap_or,
-        unwrap_unsafe_binder,
         use_extern_macros,
         use_nested_groups,
         used,

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -830,8 +830,25 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                 // Let the visitor iterate into the argument/return
                 // types appearing in the fn signature.
             }
-            ty::UnsafeBinder(_) => {
+            ty::UnsafeBinder(ty) => {
-                // FIXME(unsafe_binders): We should also recurse into the binder here.
+                // FIXME(unsafe_binders): For now, we have no way to express
+                // that a type must be `ManuallyDrop` OR `Copy` (or a pointer).
+                if !ty.has_escaping_bound_vars() {
+                    self.out.push(traits::Obligation::new(
+                        self.tcx(),
+                        self.cause(ObligationCauseCode::Misc),
+                        self.param_env,
+                        ty.map_bound(|ty| {
+                            ty::TraitRef::new(
+                                self.tcx(),
+                                self.tcx().require_lang_item(LangItem::Copy, Some(self.span)),
+                                [ty],
+                            )
+                        }),
+                    ));
+                }
+
+                // We recurse into the binder below.
             }
 
             ty::Dynamic(data, r, _) => {

compiler/rustc_ty_utils/src/consts.rs

@@ -116,6 +116,11 @@ fn recurse_build<'tcx>(
         | &ExprKind::ValueTypeAscription { source, .. } => {
             recurse_build(tcx, body, source, root_span)?
         }
+        &ExprKind::PlaceUnwrapUnsafeBinder { .. }
+        | &ExprKind::ValueUnwrapUnsafeBinder { .. }
+        | &ExprKind::WrapUnsafeBinder { .. } => {
+            todo!("FIXME(unsafe_binders)")
+        }
         &ExprKind::Literal { lit, neg } => {
             let sp = node.span;
             tcx.at(sp).lit_to_const(LitToConstInput { lit: &lit.node, ty: node.ty, neg })
@@ -347,6 +352,9 @@ impl<'a, 'tcx> IsThirPolymorphic<'a, 'tcx> {
             | thir::ExprKind::Adt(_)
             | thir::ExprKind::PlaceTypeAscription { .. }
             | thir::ExprKind::ValueTypeAscription { .. }
+            | thir::ExprKind::PlaceUnwrapUnsafeBinder { .. }
+            | thir::ExprKind::ValueUnwrapUnsafeBinder { .. }
+            | thir::ExprKind::WrapUnsafeBinder { .. }
             | thir::ExprKind::Closure(_)
             | thir::ExprKind::Literal { .. }
             | thir::ExprKind::NonHirLiteral { .. }

library/core/src/cmp.rs

@@ -973,6 +973,24 @@ pub trait Ord: Eq + PartialOrd<Self> {
     /// assert_eq!(1.max(2), 2);
     /// assert_eq!(2.max(2), 2);
     /// ```
+    /// ```
+    /// use std::cmp::Ordering;
+    ///
+    /// #[derive(Eq)]
+    /// struct Equal(&'static str);
+    ///
+    /// impl PartialEq for Equal {
+    ///     fn eq(&self, other: &Self) -> bool { true }
+    /// }
+    /// impl PartialOrd for Equal {
+    ///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> { Some(Ordering::Equal) }
+    /// }
+    /// impl Ord for Equal {
+    ///     fn cmp(&self, other: &Self) -> Ordering { Ordering::Equal }
+    /// }
+    ///
+    /// assert_eq!(Equal("self").max(Equal("other")).0, "other");
+    /// ```
     #[stable(feature = "ord_max_min", since = "1.21.0")]
     #[inline]
     #[must_use]
@@ -981,7 +999,7 @@ pub trait Ord: Eq + PartialOrd<Self> {
     where
         Self: Sized,
     {
-        max_by(self, other, Ord::cmp)
+        if other < self { self } else { other }
     }
 
     /// Compares and returns the minimum of two values.
@@ -994,6 +1012,24 @@ pub trait Ord: Eq + PartialOrd<Self> {
     /// assert_eq!(1.min(2), 1);
     /// assert_eq!(2.min(2), 2);
     /// ```
+    /// ```
+    /// use std::cmp::Ordering;
+    ///
+    /// #[derive(Eq)]
+    /// struct Equal(&'static str);
+    ///
+    /// impl PartialEq for Equal {
+    ///     fn eq(&self, other: &Self) -> bool { true }
+    /// }
+    /// impl PartialOrd for Equal {
+    ///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> { Some(Ordering::Equal) }
+    /// }
+    /// impl Ord for Equal {
+    ///     fn cmp(&self, other: &Self) -> Ordering { Ordering::Equal }
+    /// }
+    ///
+    /// assert_eq!(Equal("self").min(Equal("other")).0, "self");
+    /// ```
     #[stable(feature = "ord_max_min", since = "1.21.0")]
     #[inline]
     #[must_use]
@@ -1002,7 +1038,7 @@ pub trait Ord: Eq + PartialOrd<Self> {
     where
         Self: Sized,
     {
-        min_by(self, other, Ord::cmp)
+        if other < self { other } else { self }
     }
 
     /// Restrict a value to a certain interval.
@@ -1414,6 +1450,24 @@ pub macro PartialOrd($item:item) {
 /// assert_eq!(cmp::min(1, 2), 1);
 /// assert_eq!(cmp::min(2, 2), 2);
 /// ```
+/// ```
+/// use std::cmp::{self, Ordering};
+///
+/// #[derive(Eq)]
+/// struct Equal(&'static str);
+///
+/// impl PartialEq for Equal {
+///     fn eq(&self, other: &Self) -> bool { true }
+/// }
+/// impl PartialOrd for Equal {
+///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> { Some(Ordering::Equal) }
+/// }
+/// impl Ord for Equal {
+///     fn cmp(&self, other: &Self) -> Ordering { Ordering::Equal }
+/// }
+///
+/// assert_eq!(cmp::min(Equal("v1"), Equal("v2")).0, "v1");
+/// ```
 #[inline]
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1431,20 +1485,22 @@ pub fn min<T: Ord>(v1: T, v2: T) -> T {
 /// ```
 /// use std::cmp;
 ///
-/// let result = cmp::min_by(-2, 1, |x: &i32, y: &i32| x.abs().cmp(&y.abs()));
+/// let abs_cmp = |x: &i32, y: &i32| x.abs().cmp(&y.abs());
-/// assert_eq!(result, 1);
 ///
-/// let result = cmp::min_by(-2, 3, |x: &i32, y: &i32| x.abs().cmp(&y.abs()));
+/// let result = cmp::min_by(2, -1, abs_cmp);
-/// assert_eq!(result, -2);
+/// assert_eq!(result, -1);
+///
+/// let result = cmp::min_by(2, -3, abs_cmp);
+/// assert_eq!(result, 2);
+///
+/// let result = cmp::min_by(1, -1, abs_cmp);
+/// assert_eq!(result, 1);
 /// ```
 #[inline]
 #[must_use]
 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
 pub fn min_by<T, F: FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T {
-    match compare(&v1, &v2) {
+    if compare(&v2, &v1).is_lt() { v2 } else { v1 }
-        Ordering::Less | Ordering::Equal => v1,
-        Ordering::Greater => v2,
-    }
 }
 
 /// Returns the element that gives the minimum value from the specified function.
@@ -1456,17 +1512,20 @@ pub fn min_by<T, F: FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T {
 /// ```
 /// use std::cmp;
 ///
-/// let result = cmp::min_by_key(-2, 1, |x: &i32| x.abs());
+/// let result = cmp::min_by_key(2, -1, |x: &i32| x.abs());
-/// assert_eq!(result, 1);
+/// assert_eq!(result, -1);
 ///
-/// let result = cmp::min_by_key(-2, 2, |x: &i32| x.abs());
+/// let result = cmp::min_by_key(2, -3, |x: &i32| x.abs());
-/// assert_eq!(result, -2);
+/// assert_eq!(result, 2);
+///
+/// let result = cmp::min_by_key(1, -1, |x: &i32| x.abs());
+/// assert_eq!(result, 1);
 /// ```
 #[inline]
 #[must_use]
 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
 pub fn min_by_key<T, F: FnMut(&T) -> K, K: Ord>(v1: T, v2: T, mut f: F) -> T {
-    min_by(v1, v2, |v1, v2| f(v1).cmp(&f(v2)))
+    if f(&v2) < f(&v1) { v2 } else { v1 }
 }
 
 /// Compares and returns the maximum of two values.
@@ -1483,6 +1542,24 @@ pub fn min_by_key<T, F: FnMut(&T) -> K, K: Ord>(v1: T, v2: T, mut f: F) -> T {
 /// assert_eq!(cmp::max(1, 2), 2);
 /// assert_eq!(cmp::max(2, 2), 2);
 /// ```
+/// ```
+/// use std::cmp::{self, Ordering};
+///
+/// #[derive(Eq)]
+/// struct Equal(&'static str);
+///
+/// impl PartialEq for Equal {
+///     fn eq(&self, other: &Self) -> bool { true }
+/// }
+/// impl PartialOrd for Equal {
+///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> { Some(Ordering::Equal) }
+/// }
+/// impl Ord for Equal {
+///     fn cmp(&self, other: &Self) -> Ordering { Ordering::Equal }
+/// }
+///
+/// assert_eq!(cmp::max(Equal("v1"), Equal("v2")).0, "v2");
+/// ```
 #[inline]
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1500,20 +1577,22 @@ pub fn max<T: Ord>(v1: T, v2: T) -> T {
 /// ```
 /// use std::cmp;
 ///
-/// let result = cmp::max_by(-2, 1, |x: &i32, y: &i32| x.abs().cmp(&y.abs()));
+/// let abs_cmp = |x: &i32, y: &i32| x.abs().cmp(&y.abs());
+///
+/// let result = cmp::max_by(3, -2, abs_cmp) ;
+/// assert_eq!(result, 3);
+///
+/// let result = cmp::max_by(1, -2, abs_cmp);
 /// assert_eq!(result, -2);
 ///
-/// let result = cmp::max_by(-2, 2, |x: &i32, y: &i32| x.abs().cmp(&y.abs())) ;
+/// let result = cmp::max_by(1, -1, abs_cmp);
-/// assert_eq!(result, 2);
+/// assert_eq!(result, -1);
 /// ```
 #[inline]
 #[must_use]
 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
 pub fn max_by<T, F: FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T {
-    match compare(&v1, &v2) {
+    if compare(&v2, &v1).is_lt() { v1 } else { v2 }
-        Ordering::Less | Ordering::Equal => v2,
-        Ordering::Greater => v1,
-    }
 }
 
 /// Returns the element that gives the maximum value from the specified function.
@@ -1525,17 +1604,20 @@ pub fn max_by<T, F: FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T {
 /// ```
 /// use std::cmp;
 ///
-/// let result = cmp::max_by_key(-2, 1, |x: &i32| x.abs());
+/// let result = cmp::max_by_key(3, -2, |x: &i32| x.abs());
+/// assert_eq!(result, 3);
+///
+/// let result = cmp::max_by_key(1, -2, |x: &i32| x.abs());
 /// assert_eq!(result, -2);
 ///
-/// let result = cmp::max_by_key(-2, 2, |x: &i32| x.abs());
+/// let result = cmp::max_by_key(1, -1, |x: &i32| x.abs());
-/// assert_eq!(result, 2);
+/// assert_eq!(result, -1);
 /// ```
 #[inline]
 #[must_use]
 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
 pub fn max_by_key<T, F: FnMut(&T) -> K, K: Ord>(v1: T, v2: T, mut f: F) -> T {
-    max_by(v1, v2, |v1, v2| f(v1).cmp(&f(v2)))
+    if f(&v2) < f(&v1) { v1 } else { v2 }
 }
 
 /// Compares and sorts two values, returning minimum and maximum.
@@ -1549,13 +1631,32 @@ pub fn max_by_key<T, F: FnMut(&T) -> K, K: Ord>(v1: T, v2: T, mut f: F) -> T {
 /// use std::cmp;
 ///
 /// assert_eq!(cmp::minmax(1, 2), [1, 2]);
-/// assert_eq!(cmp::minmax(2, 2), [2, 2]);
+/// assert_eq!(cmp::minmax(2, 1), [1, 2]);
 ///
 /// // You can destructure the result using array patterns
 /// let [min, max] = cmp::minmax(42, 17);
 /// assert_eq!(min, 17);
 /// assert_eq!(max, 42);
 /// ```
+/// ```
+/// #![feature(cmp_minmax)]
+/// use std::cmp::{self, Ordering};
+///
+/// #[derive(Eq)]
+/// struct Equal(&'static str);
+///
+/// impl PartialEq for Equal {
+///     fn eq(&self, other: &Self) -> bool { true }
+/// }
+/// impl PartialOrd for Equal {
+///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> { Some(Ordering::Equal) }
+/// }
+/// impl Ord for Equal {
+///     fn cmp(&self, other: &Self) -> Ordering { Ordering::Equal }
+/// }
+///
+/// assert_eq!(cmp::minmax(Equal("v1"), Equal("v2")).map(|v| v.0), ["v1", "v2"]);
+/// ```
 #[inline]
 #[must_use]
 #[unstable(feature = "cmp_minmax", issue = "115939")]
@@ -1563,7 +1664,7 @@ pub fn minmax<T>(v1: T, v2: T) -> [T; 2]
 where
     T: Ord,
 {
-    if v1 <= v2 { [v1, v2] } else { [v2, v1] }
+    if v2 < v1 { [v2, v1] } else { [v1, v2] }
 }
 
 /// Returns minimum and maximum values with respect to the specified comparison function.
@@ -1576,11 +1677,14 @@ where
 /// #![feature(cmp_minmax)]
 /// use std::cmp;
 ///
-/// assert_eq!(cmp::minmax_by(-2, 1, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), [1, -2]);
+/// let abs_cmp = |x: &i32, y: &i32| x.abs().cmp(&y.abs());
-/// assert_eq!(cmp::minmax_by(-2, 2, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), [-2, 2]);
+///
+/// assert_eq!(cmp::minmax_by(-2, 1, abs_cmp), [1, -2]);
+/// assert_eq!(cmp::minmax_by(-1, 2, abs_cmp), [-1, 2]);
+/// assert_eq!(cmp::minmax_by(-2, 2, abs_cmp), [-2, 2]);
 ///
 /// // You can destructure the result using array patterns
-/// let [min, max] = cmp::minmax_by(-42, 17, |x: &i32, y: &i32| x.abs().cmp(&y.abs()));
+/// let [min, max] = cmp::minmax_by(-42, 17, abs_cmp);
 /// assert_eq!(min, 17);
 /// assert_eq!(max, -42);
 /// ```
@@ -1591,7 +1695,7 @@ pub fn minmax_by<T, F>(v1: T, v2: T, compare: F) -> [T; 2]
 where
     F: FnOnce(&T, &T) -> Ordering,
 {
-    if compare(&v1, &v2).is_le() { [v1, v2] } else { [v2, v1] }
+    if compare(&v2, &v1).is_lt() { [v2, v1] } else { [v1, v2] }
 }
 
 /// Returns minimum and maximum values with respect to the specified key function.
@@ -1620,7 +1724,7 @@ where
     F: FnMut(&T) -> K,
     K: Ord,
 {
-    minmax_by(v1, v2, |v1, v2| f(v1).cmp(&f(v2)))
+    if f(&v2) < f(&v1) { [v2, v1] } else { [v1, v2] }
 }
 
 // Implementation of PartialEq, Eq, PartialOrd and Ord for primitive types

library/core/src/ptr/const_ptr.rs

@@ -1681,7 +1681,7 @@ impl<T, const N: usize> *const [T; N] {
     }
 }
 
-// Equality for pointers
+/// Pointer equality is by address, as produced by the [`<*const T>::addr`](pointer::addr) method.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> PartialEq for *const T {
     #[inline]
@@ -1691,10 +1691,11 @@ impl<T: ?Sized> PartialEq for *const T {
     }
 }
 
+/// Pointer equality is an equivalence relation.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Eq for *const T {}
 
-// Comparison for pointers
+/// Pointer comparison is by address, as produced by the `[`<*const T>::addr`](pointer::addr)` method.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Ord for *const T {
     #[inline]
@@ -1710,6 +1711,7 @@ impl<T: ?Sized> Ord for *const T {
     }
 }
 
+/// Pointer comparison is by address, as produced by the `[`<*const T>::addr`](pointer::addr)` method.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> PartialOrd for *const T {
     #[inline]

library/core/src/ptr/mut_ptr.rs

@@ -2097,7 +2097,7 @@ impl<T, const N: usize> *mut [T; N] {
     }
 }
 
-// Equality for pointers
+/// Pointer equality is by address, as produced by the [`<*mut T>::addr`](pointer::addr) method.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> PartialEq for *mut T {
     #[inline(always)]
@@ -2107,9 +2107,11 @@ impl<T: ?Sized> PartialEq for *mut T {
     }
 }
 
+/// Pointer equality is an equivalence relation.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Eq for *mut T {}
 
+/// Pointer comparison is by address, as produced by the [`<*mut T>::addr`](pointer::addr) method.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Ord for *mut T {
     #[inline]
@@ -2125,6 +2127,7 @@ impl<T: ?Sized> Ord for *mut T {
     }
 }
 
+/// Pointer comparison is by address, as produced by the [`<*mut T>::addr`](pointer::addr) method.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> PartialOrd for *mut T {
     #[inline(always)]

library/std/src/sync/once_lock.rs

@@ -174,8 +174,6 @@ impl<T> OnceLock<T> {
     ///
     /// Waiting for a computation on another thread to finish:
     /// ```rust
-    /// #![feature(once_wait)]
-    ///
     /// use std::thread;
     /// use std::sync::OnceLock;
     ///
@@ -189,7 +187,7 @@ impl<T> OnceLock<T> {
     /// })
     /// ```
     #[inline]
-    #[unstable(feature = "once_wait", issue = "127527")]
+    #[stable(feature = "once_wait", since = "CURRENT_RUSTC_VERSION")]
     pub fn wait(&self) -> &T {
         self.once.wait_force();
 

library/std/src/sync/poison/mutex.rs

@@ -181,10 +181,29 @@ pub struct Mutex<T: ?Sized> {
     data: UnsafeCell<T>,
 }
 
-// these are the only places where `T: Send` matters; all other
+/// `T` must be `Send` for a [`Mutex`] to be `Send` because it is possible to acquire
-// functionality works fine on a single thread.
+/// the owned `T` from the `Mutex` via [`into_inner`].
+///
+/// [`into_inner`]: Mutex::into_inner
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
+
+/// `T` must be `Send` for [`Mutex`] to be `Sync`.
+/// This ensures that the protected data can be accessed safely from multiple threads
+/// without causing data races or other unsafe behavior.
+///
+/// [`Mutex<T>`] provides mutable access to `T` to one thread at a time. However, it's essential
+/// for `T` to be `Send` because it's not safe for non-`Send` structures to be accessed in
+/// this manner. For instance, consider [`Rc`], a non-atomic reference counted smart pointer,
+/// which is not `Send`. With `Rc`, we can have multiple copies pointing to the same heap
+/// allocation with a non-atomic reference count. If we were to use `Mutex<Rc<_>>`, it would
+/// only protect one instance of `Rc` from shared access, leaving other copies vulnerable
+/// to potential data races.
+///
+/// Also note that it is not necessary for `T` to be `Sync` as `&T` is only made available
+/// to one thread at a time if `T` is not `Sync`.
+///
+/// [`Rc`]: crate::rc::Rc
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
 
@@ -211,8 +230,17 @@ pub struct MutexGuard<'a, T: ?Sized + 'a> {
     poison: poison::Guard,
 }
 
+/// A [`MutexGuard`] is not `Send` to maximize platform portablity.
+///
+/// On platforms that use POSIX threads (commonly referred to as pthreads) there is a requirement to
+/// release mutex locks on the same thread they were acquired.
+/// For this reason, [`MutexGuard`] must not implement `Send` to prevent it being dropped from
+/// another thread.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
+
+/// `T` must be `Sync` for a [`MutexGuard<T>`] to be `Sync`
+/// because it is possible to get a `&T` from `&MutexGuard` (via `Deref`).
 #[stable(feature = "mutexguard", since = "1.19.0")]
 unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
 

library/std/src/sync/poison/once.rs

@@ -269,8 +269,6 @@ impl Once {
     /// # Example
     ///
     /// ```rust
-    /// #![feature(once_wait)]
-    ///
     /// use std::sync::Once;
     /// use std::thread;
     ///
@@ -289,7 +287,7 @@ impl Once {
     /// If this [`Once`] has been poisoned because an initialization closure has
     /// panicked, this method will also panic. Use [`wait_force`](Self::wait_force)
     /// if this behavior is not desired.
-    #[unstable(feature = "once_wait", issue = "127527")]
+    #[stable(feature = "once_wait", since = "CURRENT_RUSTC_VERSION")]
     pub fn wait(&self) {
         if !self.inner.is_completed() {
             self.inner.wait(false);
@@ -298,7 +296,7 @@ impl Once {
 
     /// Blocks the current thread until initialization has completed, ignoring
     /// poisoning.
-    #[unstable(feature = "once_wait", issue = "127527")]
+    #[stable(feature = "once_wait", since = "CURRENT_RUSTC_VERSION")]
     pub fn wait_force(&self) {
         if !self.inner.is_completed() {
             self.inner.wait(true);

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -116,6 +116,7 @@ fn check_rvalue<'tcx>(
         Rvalue::CopyForDeref(place) => check_place(tcx, *place, span, body, msrv),
         Rvalue::Repeat(operand, _)
         | Rvalue::Use(operand)
+        | Rvalue::WrapUnsafeBinder(operand, _)
         | Rvalue::Cast(
             CastKind::PointerWithExposedProvenance
             | CastKind::IntToInt
@@ -289,7 +290,8 @@ fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &B
             | ProjectionElem::Downcast(..)
             | ProjectionElem::Subslice { .. }
             | ProjectionElem::Subtype(_)
-            | ProjectionElem::Index(_) => {},
+            | ProjectionElem::Index(_)
+            | ProjectionElem::UnwrapUnsafeBinder(_) => {},
         }
     }
 

tests/ui/unsafe-binders/expr.rs

@@ -1,3 +1,5 @@
+//@ check-pass
+
 #![feature(unsafe_binders)]
 //~^ WARN the feature `unsafe_binders` is incomplete
 
@@ -7,8 +9,6 @@ fn main() {
     unsafe {
     let x = 1;
         let binder: unsafe<'a> &'a i32 = wrap_binder!(&x);
-        //~^ ERROR unsafe binder casts are not fully implemented
         let rx = *unwrap_binder!(binder);
-        //~^ ERROR unsafe binder casts are not fully implemented
     }
 }

tests/ui/unsafe-binders/expr.stderr

@@ -1,5 +1,5 @@
 warning: the feature `unsafe_binders` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/expr.rs:1:12
+  --> $DIR/expr.rs:3:12
    |
 LL | #![feature(unsafe_binders)]
    |            ^^^^^^^^^^^^^^
@@ -7,17 +7,5 @@ LL | #![feature(unsafe_binders)]
    = note: see issue #130516 <https://github.com/rust-lang/rust/issues/130516> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error: unsafe binder casts are not fully implemented
+warning: 1 warning emitted
-  --> $DIR/expr.rs:9:55
-   |
-LL |         let binder: unsafe<'a> &'a i32 = wrap_binder!(&x);
-   |                                                       ^^
-
-error: unsafe binder casts are not fully implemented
-  --> $DIR/expr.rs:11:34
-   |
-LL |         let rx = *unwrap_binder!(binder);
-   |                                  ^^^^^^
-
-error: aborting due to 2 previous errors; 1 warning emitted
 

tests/ui/unsafe-binders/mismatch.rs

@@ -5,38 +5,31 @@ use std::unsafe_binder::{wrap_binder, unwrap_binder};
 
 fn a() {
     let _: unsafe<'a> &'a i32 = wrap_binder!(&());
-    //~^ ERROR unsafe binder casts are not fully implemented
+    //~^ ERROR mismatched types
-    //~| ERROR mismatched types
 }
 
 fn b() {
     let _: i32 = wrap_binder!(&());
-    //~^ ERROR unsafe binder casts are not fully implemented
+    //~^ ERROR `wrap_binder!()` can only wrap into unsafe binder
-    //~| ERROR `wrap_binder!()` can only wrap into unsafe binder
 }
 
 fn c() {
     let y = 1;
     unwrap_binder!(y);
-    //~^ ERROR unsafe binder casts are not fully implemented
+    //~^ ERROR expected unsafe binder, found integer as input
-    //~| ERROR expected unsafe binder, found integer as input
 }
 
 fn d() {
     let unknown = Default::default();
+    //~^ ERROR type annotations needed
     unwrap_binder!(unknown);
-    //~^ ERROR unsafe binder casts are not fully implemented
-    // FIXME(unsafe_binders): This should report ambiguity once we've removed
-    // the error above which taints the infcx.
 }
 
 fn e() {
     let x = wrap_binder!(&42);
-    //~^ ERROR unsafe binder casts are not fully implemented
+    //~^ ERROR type annotations needed
     // Currently, type inference doesn't flow backwards for unsafe binders.
     // It could, perhaps, but that may cause even more surprising corners.
-    // FIXME(unsafe_binders): This should report ambiguity once we've removed
-    // the error above which taints the infcx.
     let _: unsafe<'a> &'a i32 = x;
 }
 

tests/ui/unsafe-binders/mismatch.stderr

@@ -7,12 +7,6 @@ LL | #![feature(unsafe_binders)]
    = note: see issue #130516 <https://github.com/rust-lang/rust/issues/130516> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error: unsafe binder casts are not fully implemented
-  --> $DIR/mismatch.rs:7:46
-   |
-LL |     let _: unsafe<'a> &'a i32 = wrap_binder!(&());
-   |                                              ^^^
-
 error[E0308]: mismatched types
   --> $DIR/mismatch.rs:7:46
    |
@@ -22,14 +16,8 @@ LL |     let _: unsafe<'a> &'a i32 = wrap_binder!(&());
    = note: expected reference `&i32`
               found reference `&()`
 
-error: unsafe binder casts are not fully implemented
-  --> $DIR/mismatch.rs:13:31
-   |
-LL |     let _: i32 = wrap_binder!(&());
-   |                               ^^^
-
 error: `wrap_binder!()` can only wrap into unsafe binder, not `i32`
-  --> $DIR/mismatch.rs:13:18
+  --> $DIR/mismatch.rs:12:18
    |
 LL |     let _: i32 = wrap_binder!(&());
    |                  ^^^^^^^^^^^^^^^^^
@@ -37,32 +25,35 @@ LL |     let _: i32 = wrap_binder!(&());
    = note: unsafe binders are the only valid output of wrap
    = note: this error originates in the macro `wrap_binder` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: unsafe binder casts are not fully implemented
-  --> $DIR/mismatch.rs:20:20
-   |
-LL |     unwrap_binder!(y);
-   |                    ^
-
 error: expected unsafe binder, found integer as input of `unwrap_binder!()`
-  --> $DIR/mismatch.rs:20:20
+  --> $DIR/mismatch.rs:18:20
    |
 LL |     unwrap_binder!(y);
    |                    ^
    |
    = note: only an unsafe binder type can be unwrapped
 
-error: unsafe binder casts are not fully implemented
+error[E0282]: type annotations needed
-  --> $DIR/mismatch.rs:27:20
+  --> $DIR/mismatch.rs:23:9
    |
-LL |     unwrap_binder!(unknown);
+LL |     let unknown = Default::default();
    |         ^^^^^^^
+LL |
+LL |     unwrap_binder!(unknown);
+   |                    ------- type must be known at this point
+   |
+help: consider giving `unknown` an explicit type
+   |
+LL |     let unknown: /* Type */ = Default::default();
+   |                ++++++++++++
 
-error: unsafe binder casts are not fully implemented
+error[E0282]: type annotations needed
-  --> $DIR/mismatch.rs:34:26
+  --> $DIR/mismatch.rs:29:26
    |
 LL |     let x = wrap_binder!(&42);
-   |                          ^^^
+   |                          ^^^ cannot infer type
 
-error: aborting due to 8 previous errors; 1 warning emitted
+error: aborting due to 5 previous errors; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0282, E0308.
+For more information about an error, try `rustc --explain E0282`.

tests/ui/unsafe-binders/moves.rs

@@ -0,0 +1,41 @@
+//@ known-bug: unknown
+
+#![feature(unsafe_binders)]
+// FIXME(unsafe_binders) ~^ WARN the feature `unsafe_binders` is incomplete
+
+use std::unsafe_binder::{wrap_binder, unwrap_binder};
+use std::mem::{drop, ManuallyDrop};
+
+struct NotCopyInner;
+type NotCopy = ManuallyDrop<NotCopyInner>;
+
+fn use_after_wrap() {
+    unsafe {
+        let base = NotCopy;
+        let binder: unsafe<> NotCopy = wrap_binder!(base);
+        drop(base);
+        // FIXME(unsafe_binders) ~^ ERROR use of moved value: `base`
+    }
+}
+
+fn move_out_of_wrap() {
+    unsafe {
+        let binder: unsafe<> NotCopy = wrap_binder!(NotCopy);
+        drop(unwrap_binder!(binder));
+        drop(unwrap_binder!(binder));
+        // FIXME(unsafe_binders) ~^ ERROR use of moved value: `binder`
+    }
+}
+
+fn not_conflicting() {
+    unsafe {
+        let binder: unsafe<> (NotCopy, NotCopy) = wrap_binder!((NotCopy, NotCopy));
+        drop(unwrap_binder!(binder).0);
+        drop(unwrap_binder!(binder).1);
+        // ^ NOT a problem.
+        drop(unwrap_binder!(binder).0);
+        // FIXME(unsafe_binders) ~^ ERROR use of moved value: `binder.0`
+    }
+}
+
+fn main() {}

tests/ui/unsafe-binders/moves.stderr

@@ -0,0 +1,85 @@
+error[E0423]: expected value, found type alias `NotCopy`
+  --> $DIR/moves.rs:14:20
+   |
+LL |         let base = NotCopy;
+   |                    ^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0423]: expected value, found type alias `NotCopy`
+  --> $DIR/moves.rs:23:53
+   |
+LL |         let binder: unsafe<> NotCopy = wrap_binder!(NotCopy);
+   |                                                     ^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0423]: expected value, found type alias `NotCopy`
+  --> $DIR/moves.rs:32:65
+   |
+LL |         let binder: unsafe<> (NotCopy, NotCopy) = wrap_binder!((NotCopy, NotCopy));
+   |                                                                 ^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0423]: expected value, found type alias `NotCopy`
+  --> $DIR/moves.rs:32:74
+   |
+LL |         let binder: unsafe<> (NotCopy, NotCopy) = wrap_binder!((NotCopy, NotCopy));
+   |                                                                          ^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+warning: the feature `unsafe_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/moves.rs:3:12
+   |
+LL | #![feature(unsafe_binders)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: see issue #130516 <https://github.com/rust-lang/rust/issues/130516> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0277]: the trait bound `NotCopyInner: Copy` is not satisfied
+  --> $DIR/moves.rs:15:21
+   |
+LL |         let binder: unsafe<> NotCopy = wrap_binder!(base);
+   |                     ^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `NotCopyInner`
+   |
+   = note: required for `ManuallyDrop<NotCopyInner>` to implement `Copy`
+help: consider annotating `NotCopyInner` with `#[derive(Copy)]`
+   |
+LL + #[derive(Copy)]
+LL | struct NotCopyInner;
+   |
+
+error[E0277]: the trait bound `NotCopyInner: Copy` is not satisfied
+  --> $DIR/moves.rs:23:21
+   |
+LL |         let binder: unsafe<> NotCopy = wrap_binder!(NotCopy);
+   |                     ^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `NotCopyInner`
+   |
+   = note: required for `ManuallyDrop<NotCopyInner>` to implement `Copy`
+help: consider annotating `NotCopyInner` with `#[derive(Copy)]`
+   |
+LL + #[derive(Copy)]
+LL | struct NotCopyInner;
+   |
+
+error[E0277]: the trait bound `NotCopyInner: Copy` is not satisfied
+  --> $DIR/moves.rs:32:21
+   |
+LL |         let binder: unsafe<> (NotCopy, NotCopy) = wrap_binder!((NotCopy, NotCopy));
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `NotCopyInner`
+   |
+   = note: required for `ManuallyDrop<NotCopyInner>` to implement `Copy`
+   = note: required because it appears within the type `(ManuallyDrop<NotCopyInner>, ManuallyDrop<NotCopyInner>)`
+help: consider annotating `NotCopyInner` with `#[derive(Copy)]`
+   |
+LL + #[derive(Copy)]
+LL | struct NotCopyInner;
+   |
+
+error: aborting due to 7 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0277, E0423.
+For more information about an error, try `rustc --explain E0277`.