Add guard checking for associated types before computing intercrate ambiguity of projections. Bless test with more specific notes on the ambiguity cause.

Author: gavinleroy

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -20,6 +20,7 @@ use crate::traits::{
 };
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::Diagnostic;
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::{util, TraitEngine};
@@ -1002,7 +1003,12 @@ impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a> {
         // and then prove the resulting predicate as a nested goal.
         let trait_ref = match predicate.kind().no_bound_vars() {
             Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(tr))) => tr.trait_ref,
-            Some(ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj))) => {
+            Some(ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj)))
+                if matches!(
+                    infcx.tcx.def_kind(proj.projection_ty.def_id),
+                    DefKind::AssocTy | DefKind::AssocConst
+                ) =>
+            {
                 proj.projection_ty.trait_ref(infcx.tcx)
             }
             _ => return ControlFlow::Continue(()),

tests/ui/impl-trait/auto-trait-coherence.next.stderr

@@ -6,6 +6,8 @@ LL | impl<T: Send> AnotherTrait for T {}
 ...
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
+   |
+   = note: upstream crates may add a new impl of trait `std::marker::Send` for type `OpaqueType` in future versions
 
 error: aborting due to previous error