Highlight conflicting param-env candidates

compiler/rustc_middle/src/traits/mod.rs

@@ -546,9 +546,17 @@ pub enum SelectionError<'tcx> {
     ErrorReporting,
     /// Multiple applicable `impl`s where found. The `DefId`s correspond to
     /// all the `impl`s' Items.
-    Ambiguous(Vec<DefId>),
+    Ambiguous(Vec<AmbiguousSelection>),
 }
 
+#[derive(Copy, Clone, Debug)]
+pub enum AmbiguousSelection {
+    Impl(DefId),
+    ParamEnv(Span),
+}
+
+TrivialTypeTraversalAndLiftImpls! { AmbiguousSelection, }
+
 /// When performing resolution, it is typically the case that there
 /// can be one of three outcomes:
 ///