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

Author: lapla-cogito

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -663,6 +663,10 @@ where
             return Err(NoSolution);
         }
 
+        if goal.predicate.has_non_region_infer() {
+            return ecx.forced_ambiguity(MaybeCause::Ambiguity);
+        }
+
         ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc).enter(|ecx| {
             let assume = ecx.structurally_normalize_const(
                 goal.param_env,

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

@@ -0,0 +1,19 @@
+error[E0747]: type provided when a constant was expected
+  --> $DIR/transmutability-coherence.rs:4:37
+   |
+LL | impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+   |                                     ^^
+
+error[E0119]: conflicting implementations of trait `OpaqueTrait` for type `&_`
+  --> $DIR/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 `&_`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0747.
+For more information about an error, try `rustc --explain E0119`.