make effect infer variables suggestable in diagnostics

it works when a non-const context that does not enable effects
calls into a const effects-enabled trait. We'd simply suggest the
non-const trait bound in this case consistent to its fallback.

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -98,6 +98,7 @@ impl<'tcx, T> IsSuggestable<'tcx> for T
 where
     T: TypeVisitable<TyCtxt<'tcx>> + TypeFoldable<TyCtxt<'tcx>>,
 {
+    #[tracing::instrument(level = "debug", skip(tcx))]
     fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool {
         self.visit_with(&mut IsSuggestableVisitor { tcx, infer_suggestable }).is_continue()
     }
@@ -533,6 +534,9 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IsSuggestableVisitor<'tcx> {
         match c.kind() {
             ConstKind::Infer(InferConst::Var(_)) if self.infer_suggestable => {}
 
+            // effect variables are always suggestable, because they are not visible
+            ConstKind::Infer(InferConst::EffectVar(_)) => {}
+
             ConstKind::Infer(..)
             | ConstKind::Bound(..)
             | ConstKind::Placeholder(..)