trait_sel: re-order host effect evaluations

When evaluating a host effect predicate on an opaque in `addr2line`,
running `evaluate_host_effect_from_bounds` first would create an opaque
type which wouldn't be removed from opaque type storage and that would
cause a delayed bug - creating a small reproduction of this has been
tricky but if `builtin_impls` checking is moved below
`evaluate_host_effect_from_item_bounds` then it will trigger.

The correct fix is probably to work out why the opaque type is created
and then not used, but this was particularly tricky. Given that this root
cause wasn't identified, writing a test case proved difficult.

Author: davidtwco

compiler/rustc_trait_selection/src/traits/effects.rs

@@ -38,19 +38,25 @@ pub fn evaluate_host_effect_obligation<'tcx>(
         return Err(EvaluationFailure::Ambiguous);
     }
 
-    match evaluate_host_effect_from_bounds(selcx, obligation) {
+    // FIXME: during implementation of `#![feature(sized_hierarchy)]`, when evaluating a host
+    // effect predicate on an opaque in addr2line, running `evaluate_host_effect_from_bounds` first
+    // would create an opaque type which wouldn't be removed from opaque type storage and that
+    // would cause a delayed bug - creating a small reproduction of this has been tricky but if
+    // `builtin_impls` checking is moved below `evaluate_host_effect_from_item_bounds` then it will
+    // trigger.
+    match evaluate_host_effect_from_builtin_impls(selcx, obligation) {
         Ok(result) => return Ok(result),
         Err(EvaluationFailure::Ambiguous) => return Err(EvaluationFailure::Ambiguous),
         Err(EvaluationFailure::NoSolution) => {}
     }
 
-    match evaluate_host_effect_from_item_bounds(selcx, obligation) {
+    match evaluate_host_effect_from_bounds(selcx, obligation) {
         Ok(result) => return Ok(result),
         Err(EvaluationFailure::Ambiguous) => return Err(EvaluationFailure::Ambiguous),
         Err(EvaluationFailure::NoSolution) => {}
     }
 
-    match evaluate_host_effect_from_builtin_impls(selcx, obligation) {
+    match evaluate_host_effect_from_item_bounds(selcx, obligation) {
         Ok(result) => return Ok(result),
         Err(EvaluationFailure::Ambiguous) => return Err(EvaluationFailure::Ambiguous),
         Err(EvaluationFailure::NoSolution) => {}