Rollup merge of #131112 - jswrenn:fix-130413, r=compiler-errors

TransmuteFrom: Gracefully handle unnormalized types and normalization errors

~~Refactor to share code between `TransmuteFrom`'s trait selection and error reporting code paths. Additionally normalizes the source and destination types, and gracefully handles normalization errors.~~

Fixes #130413

r​? `@compiler-errors`

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -245,6 +245,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                                         span, "silent safe transmute error"
                                     );
                                 }
+                                GetSafeTransmuteErrorAndReason::Default => {
+                                    (err_msg, None)
+                                }
                                 GetSafeTransmuteErrorAndReason::Error {
                                     err_msg,
                                     safe_transmute_explanation,
@@ -2226,6 +2229,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
     ) -> GetSafeTransmuteErrorAndReason {
         use rustc_transmute::Answer;
 
+        // We don't assemble a transmutability candidate for types that are generic
+        // and we should have ambiguity for types that still have non-region infer.
+        if obligation.predicate.has_non_region_param() || obligation.has_non_region_infer() {
+            return GetSafeTransmuteErrorAndReason::Default;
+        }
+
         // Erase regions because layout code doesn't particularly care about regions.
         let trait_ref =
             self.tcx.erase_regions(self.tcx.instantiate_bound_regions_with_erased(trait_ref));
@@ -2248,6 +2257,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
         let dst = trait_ref.args.type_at(0);
         let src = trait_ref.args.type_at(1);
+
         let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
 
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(

compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs

@@ -43,6 +43,7 @@ pub struct ImplCandidate<'tcx> {
 
 enum GetSafeTransmuteErrorAndReason {
     Silent,
+    Default,
     Error { err_msg: String, safe_transmute_explanation: Option<String> },
 }