Relax a debug assertion in codegen

Author: compiler-errors

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -43,11 +43,11 @@ pub(crate) fn unsized_info<'tcx>(
                 // some reason, were to relax the `Unsize` trait, it could become
                 // unsound, so let's assert here that the trait refs are *equal*.
                 //
-                // We can use `assert_eq` because the binders should have been anonymized,
-                // and because higher-ranked equality now requires the binders are equal.
+                // FIXME: We could probably use real subtyping here, but that would require
+                // us to reimplement something like `relate_types` from `rustc_const_eval`.
                 debug_assert_eq!(
-                    data_a.principal(),
-                    data_b.principal(),
+                    data_a.principal().map(|p| fx.tcx().instantiate_bound_regions_with_erased(p)),
+                    data_b.principal().map(|p| fx.tcx().instantiate_bound_regions_with_erased(p)),
                     "NOP unsize vtable changed principal trait ref: {data_a} -> {data_b}"
                 );
                 return old_info;

compiler/rustc_codegen_ssa/src/base.rs

@@ -134,11 +134,11 @@ fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                 // some reason, were to relax the `Unsize` trait, it could become
                 // unsound, so let's assert here that the trait refs are *equal*.
                 //
-                // We can use `assert_eq` because the binders should have been anonymized,
-                // and because higher-ranked equality now requires the binders are equal.
+                // FIXME: We could probably use real subtyping here, but that would require
+                // us to reimplement something like `relate_types` from `rustc_const_eval`.
                 debug_assert_eq!(
-                    data_a.principal(),
-                    data_b.principal(),
+                    data_a.principal().map(|p| cx.tcx().instantiate_bound_regions_with_erased(p)),
+                    data_b.principal().map(|p| cx.tcx().instantiate_bound_regions_with_erased(p)),
                     "NOP unsize vtable changed principal trait ref: {data_a} -> {data_b}"
                 );
 

library/core/src/ops/unsize.rs

@@ -37,9 +37,6 @@ pub trait CoerceUnsized<T: ?Sized> {
     // Empty.
 }
 
-// &mut T -> &mut U
-#[unstable(feature = "coerce_unsized", issue = "18598")]
-impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a mut U> for &'a mut T {}
 // &mut T -> &U
 #[unstable(feature = "coerce_unsized", issue = "18598")]
 impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b mut T {}

tests/ui/sub-principals-in-codegen.rs

@@ -0,0 +1,8 @@
+//@ build-pass
+
+// Regression test for an overly aggressive assertion in #130855.
+
+fn main() {
+    let subtype: &(dyn for<'a> Fn(&'a i32) -> &'a i32) = &|x| x;
+    let supertype: &(dyn Fn(&'static i32) -> &'static i32) = subtype;
+}