Do not trim paths in MIR validator

compiler/rustc_mir_transform/src/validate.rs

@@ -12,6 +12,7 @@ use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::mir::visit::{NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::adjustment::PointerCoercion;
+use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{
     self, CoroutineArgsExt, InstanceKind, ScalarInt, Ty, TyCtxt, TypeVisitableExt, Upcast, Variance,
 };
@@ -543,7 +544,13 @@ pub(super) fn validate_types<'tcx>(
     caller_body: &Body<'tcx>,
 ) -> Vec<(Location, String)> {
     let mut type_checker = TypeChecker { body, caller_body, tcx, typing_env, failures: Vec::new() };
-    type_checker.visit_body(body);
+    // The type checker formats a bunch of strings with type names in it, but these strings
+    // are not always going to be encountered on the error path since the inliner also uses
+    // the validator, and there are certain kinds of inlining (even for valid code) that
+    // can cause validation errors (mostly around where clauses and rigid projections).
+    with_no_trimmed_paths!({
+        type_checker.visit_body(body);
+    });
     type_checker.failures
 }
 

tests/ui/mir/inline-causes-trimmed-paths.rs

@@ -0,0 +1,36 @@
+//@ build-pass
+//@ compile-flags: -Zinline-mir
+
+trait Storage {
+    type Buffer: ?Sized;
+}
+
+struct Array<const N: usize>;
+impl<const N: usize> Storage for Array<N> {
+    type Buffer = [(); N];
+}
+
+struct Slice;
+impl Storage for Slice {
+    type Buffer = [()];
+}
+
+struct Wrap<S: Storage> {
+    _b: S::Buffer,
+}
+
+fn coerce<const N: usize>(this: &Wrap<Array<N>>) -> &Wrap<Slice>
+where
+    Array<N>: Storage,
+{
+    coerce_again(this)
+}
+
+fn coerce_again<const N: usize>(this: &Wrap<Array<N>>) -> &Wrap<Slice> {
+    this
+}
+
+fn main() {
+    let inner: Wrap<Array<1>> = Wrap { _b: [(); 1] };
+    let _: &Wrap<Slice> = coerce(&inner);
+}