transmutability: only short-circuit is_transmutable for non-region infers

compiler/rustc_transmute/src/lib.rs

@@ -89,7 +89,7 @@ pub enum Reason<T> {
 #[cfg(feature = "rustc")]
 mod rustc {
     use rustc_hir::lang_items::LangItem;
-    use rustc_middle::ty::{Const, Region, Ty, TyCtxt};
+    use rustc_middle::ty::{Const, Region, Ty, TyCtxt, TypeVisitableExt};
 
     use super::*;
 
@@ -116,6 +116,10 @@ mod rustc {
             types: Types<'tcx>,
             assume: crate::Assume,
         ) -> crate::Answer<Region<'tcx>, Ty<'tcx>> {
+            if types.src.has_non_region_infer() || types.dst.has_non_region_infer() {
+                return Answer::No(Reason::TypeError);
+            }
+
             crate::maybe_transmutable::MaybeTransmutableQuery::new(
                 types.src, types.dst, assume, self.tcx,
             )

tests/ui/transmutability/ice-141400-transmutability-coherence.rs

@@ -0,0 +1,10 @@
+#![feature(transmutability)]
+trait OpaqueTrait {}
+
+impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+//~^ ERROR: type provided when a constant was expected
+
+impl<T> OpaqueTrait for &T where T: OpaqueTrait {}
+//~^ ERROR: conflicting implementations of trait `OpaqueTrait` for type `&_`
+
+fn main() {}

tests/ui/transmutability/ice-141400-transmutability-coherence.stderr

@@ -0,0 +1,21 @@
+error[E0747]: type provided when a constant was expected
+  --> $DIR/ice-141400-transmutability-coherence.rs:4:37
+   |
+LL | impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+   |                                     ^^
+
+error[E0119]: conflicting implementations of trait `OpaqueTrait` for type `&_`
+  --> $DIR/ice-141400-transmutability-coherence.rs:7:1
+   |
+LL | impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+   | ---------------------------------------------------------- first implementation here
+...
+LL | impl<T> OpaqueTrait for &T where T: OpaqueTrait {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`
+   |
+   = note: downstream crates may implement trait `std::mem::TransmuteFrom<(), std::mem::Assume { alignment: false, lifetimes: false, safety: false, validity: false }>` for type `&_`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0747.
+For more information about an error, try `rustc --explain E0119`.