nll: correctly deal with bivariance

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -23,7 +23,7 @@ use super::{
     MemPlaceMeta, Memory, MemoryKind, Operand, Place, PlaceTy, PointerArithmetic, Provenance,
     Scalar, StackPopJump,
 };
-use crate::transform::validate::equal_up_to_regions;
+use crate::transform::validate;
 
 pub struct InterpCx<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
     /// Stores the `Machine` instance.
@@ -354,8 +354,8 @@ pub(super) fn mir_assign_valid_types<'tcx>(
     // Type-changing assignments can happen when subtyping is used. While
     // all normal lifetimes are erased, higher-ranked types with their
     // late-bound lifetimes are still around and can lead to type
-    // differences. So we compare ignoring lifetimes.
-    if equal_up_to_regions(tcx, param_env, src.ty, dest.ty) {
+    // differences.
+    if validate::is_subtype(tcx, param_env, src.ty, dest.ty) {
         // Make sure the layout is equal, too -- just to be safe. Miri really
         // needs layout equality. For performance reason we skip this check when
         // the types are equal. Equal types *can* have different layouts when

compiler/rustc_const_eval/src/transform/validate.rs

@@ -2,7 +2,8 @@
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::bit_set::BitSet;
-use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::infer::{DefiningAnchor, TyCtxtInferExt};
+use rustc_infer::traits::ObligationCause;
 use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::mir::visit::NonUseContext::VarDebugInfo;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
@@ -12,12 +13,12 @@ use rustc_middle::mir::{
     ProjectionElem, RuntimePhase, Rvalue, SourceScope, Statement, StatementKind, Terminator,
     TerminatorKind, UnOp, START_BLOCK,
 };
-use rustc_middle::ty::fold::BottomUpFolder;
-use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeFoldable, TypeVisitable};
+use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeVisitable};
 use rustc_mir_dataflow::impls::MaybeStorageLive;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
 use rustc_target::abi::{Size, VariantIdx};
+use rustc_trait_selection::traits::ObligationCtxt;
 
 #[derive(Copy, Clone, Debug)]
 enum EdgeKind {
@@ -70,13 +71,11 @@ impl<'tcx> MirPass<'tcx> for Validator {
     }
 }
 
-/// Returns whether the two types are equal up to lifetimes.
-/// All lifetimes, including higher-ranked ones, get ignored for this comparison.
-/// (This is unlike the `erasing_regions` methods, which keep higher-ranked lifetimes for soundness reasons.)
+/// Returns whether the two types are equal up to subtyping.
 ///
-/// The point of this function is to approximate "equal up to subtyping".  However,
-/// the approximation is incorrect as variance is ignored.
-pub fn equal_up_to_regions<'tcx>(
+/// This is used in case we don't know the expected subtyping direction
+/// and still want to check whether anything is broken.
+pub fn is_equal_up_to_subtyping<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ParamEnv<'tcx>,
     src: Ty<'tcx>,
@@ -87,27 +86,40 @@ pub fn equal_up_to_regions<'tcx>(
         return true;
     }
 
-    // Normalize lifetimes away on both sides, then compare.
-    let normalize = |ty: Ty<'tcx>| {
-        tcx.try_normalize_erasing_regions(param_env, ty).unwrap_or(ty).fold_with(
-            &mut BottomUpFolder {
-                tcx,
-                // FIXME: We erase all late-bound lifetimes, but this is not fully correct.
-                // If you have a type like `<for<'a> fn(&'a u32) as SomeTrait>::Assoc`,
-                // this is not necessarily equivalent to `<fn(&'static u32) as SomeTrait>::Assoc`,
-                // since one may have an `impl SomeTrait for fn(&32)` and
-                // `impl SomeTrait for fn(&'static u32)` at the same time which
-                // specify distinct values for Assoc. (See also #56105)
-                lt_op: |_| tcx.lifetimes.re_erased,
-                // Leave consts and types unchanged.
-                ct_op: |ct| ct,
-                ty_op: |ty| ty,
-            },
-        )
-    };
-    tcx.infer_ctxt().build().can_eq(param_env, normalize(src), normalize(dest)).is_ok()
+    // Check for subtyping in either direction.
+    is_subtype(tcx, param_env, src, dest) || is_subtype(tcx, param_env, dest, src)
 }
 
+pub fn is_subtype<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    src: Ty<'tcx>,
+    dest: Ty<'tcx>,
+) -> bool {
+    if src == dest {
+        return true;
+    }
+
+    let mut builder =
+        tcx.infer_ctxt().ignoring_regions().with_opaque_type_inference(DefiningAnchor::Bubble);
+    let infcx = builder.build();
+    let ocx = ObligationCtxt::new(&infcx);
+    let cause = ObligationCause::dummy();
+    let src = ocx.normalize(cause.clone(), param_env, src);
+    let dest = ocx.normalize(cause.clone(), param_env, dest);
+    let Ok(infer_ok) = infcx.at(&cause, param_env).sub(src, dest) else {
+        return false;
+    };
+    let () = ocx.register_infer_ok_obligations(infer_ok);
+    let errors = ocx.select_all_or_error();
+    // With `Reveal::All`, opaque types get normalized away, with `Reveal::UserFacing`
+    // we would get unification errors because we're unable to look into opaque types,
+    // even if they're constrained in our current function.
+    //
+    // It seems very unlikely that this hides any bugs.
+    let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
+    errors.is_empty()
+}
 struct TypeChecker<'a, 'tcx> {
     when: &'a str,
     body: &'a Body<'tcx>,
@@ -183,22 +195,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             return true;
         }
 
-        // Normalize projections and things like that.
-        // Type-changing assignments can happen when subtyping is used. While
-        // all normal lifetimes are erased, higher-ranked types with their
-        // late-bound lifetimes are still around and can lead to type
-        // differences. So we compare ignoring lifetimes.
-
-        // First, try with reveal_all. This might not work in some cases, as the predicates
-        // can be cleared in reveal_all mode. We try the reveal first anyways as it is used
-        // by some other passes like inlining as well.
-        let param_env = self.param_env.with_reveal_all_normalized(self.tcx);
-        if equal_up_to_regions(self.tcx, param_env, src, dest) {
-            return true;
-        }
-
-        // If this fails, we can try it without the reveal.
-        equal_up_to_regions(self.tcx, self.param_env, src, dest)
+        is_subtype(self.tcx, self.param_env, src, dest)
     }
 }
 

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -556,8 +556,12 @@ where
         self.ambient_variance_info = self.ambient_variance_info.xform(info);
 
         debug!(?self.ambient_variance);
-
-        let r = self.relate(a, b)?;
+        // In a bivariant context this always succeeds.
+        let r = if self.ambient_variance == ty::Variance::Bivariant {
+            a
+        } else {
+            self.relate(a, b)?
+        };
 
         self.ambient_variance = old_ambient_variance;
 

compiler/rustc_mir_transform/src/inline.rs

@@ -1,7 +1,6 @@
 //! Inlining pass for MIR functions
 use crate::deref_separator::deref_finder;
 use rustc_attr::InlineAttr;
-use rustc_const_eval::transform::validate::equal_up_to_regions;
 use rustc_index::bit_set::BitSet;
 use rustc_index::vec::Idx;
 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
@@ -14,7 +13,8 @@ use rustc_span::{hygiene::ExpnKind, ExpnData, LocalExpnId, Span};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi::Abi;
 
-use super::simplify::{remove_dead_blocks, CfgSimplifier};
+use crate::simplify::{remove_dead_blocks, CfgSimplifier};
+use crate::validate;
 use crate::MirPass;
 use std::iter;
 use std::ops::{Range, RangeFrom};
@@ -180,7 +180,7 @@ impl<'tcx> Inliner<'tcx> {
         let TerminatorKind::Call { args, destination, .. } = &terminator.kind else { bug!() };
         let destination_ty = destination.ty(&caller_body.local_decls, self.tcx).ty;
         let output_type = callee_body.return_ty();
-        if !equal_up_to_regions(self.tcx, self.param_env, output_type, destination_ty) {
+        if !validate::is_subtype(self.tcx, self.param_env, output_type, destination_ty) {
             trace!(?output_type, ?destination_ty);
             return Err("failed to normalize return type");
         }
@@ -200,7 +200,7 @@ impl<'tcx> Inliner<'tcx> {
                 arg_tuple_tys.iter().zip(callee_body.args_iter().skip(skipped_args))
             {
                 let input_type = callee_body.local_decls[input].ty;
-                if !equal_up_to_regions(self.tcx, self.param_env, arg_ty, input_type) {
+                if !validate::is_subtype(self.tcx, self.param_env, input_type, arg_ty) {
                     trace!(?arg_ty, ?input_type);
                     return Err("failed to normalize tuple argument type");
                 }
@@ -209,7 +209,7 @@ impl<'tcx> Inliner<'tcx> {
             for (arg, input) in args.iter().zip(callee_body.args_iter()) {
                 let input_type = callee_body.local_decls[input].ty;
                 let arg_ty = arg.ty(&caller_body.local_decls, self.tcx);
-                if !equal_up_to_regions(self.tcx, self.param_env, arg_ty, input_type) {
+                if !validate::is_subtype(self.tcx, self.param_env, input_type, arg_ty) {
                     trace!(?arg_ty, ?input_type);
                     return Err("failed to normalize argument type");
                 }
@@ -847,7 +847,7 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
             let parent = Place { local, projection: self.tcx.intern_place_elems(proj_base) };
             let parent_ty = parent.ty(&self.callee_body.local_decls, self.tcx);
             let check_equal = |this: &mut Self, f_ty| {
-                if !equal_up_to_regions(this.tcx, this.param_env, ty, f_ty) {
+                if !validate::is_equal_up_to_subtyping(this.tcx, this.param_env, ty, f_ty) {
                     trace!(?ty, ?f_ty);
                     this.validation = Err("failed to normalize projection type");
                     return;

src/test/ui/mir/important-higher-ranked-regions.rs

@@ -0,0 +1,26 @@
+// check-pass
+// compile-flags: -Zvalidate-mir
+
+// This test checks that bivariant parameters are handled correctly
+// in the mir.
+#![allow(coherence_leak_check)]
+trait Trait {
+    type Assoc;
+}
+
+struct Foo<T, U>(T)
+where
+    T: Trait<Assoc = U>;
+
+impl Trait for for<'a> fn(&'a ()) {
+    type Assoc = u32;
+}
+impl Trait for fn(&'static ()) {
+    type Assoc = String;
+}
+
+fn foo(x: Foo<for<'a> fn(&'a ()), u32>) -> Foo<fn(&'static ()), String> {
+    x
+}
+
+fn main() {}