Rollup merge of #124690 - compiler-errors:only-ambig-if-ambig, r=lcnr

Only consider ambiguous goals when finding best obligation for ambiguities

We don't care about ambiguous goals when reporting true errors, and vice versa for ambiguities.

r? lcnr

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -137,7 +137,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
  .collect();
 
  errors.extend(self.obligations.overflowed.drain(..).map(|obligation| FulfillmentError {
- obligation: find_best_leaf_obligation(infcx, &obligation),
+ obligation: find_best_leaf_obligation(infcx, &obligation, true),
  code: FulfillmentErrorCode::Ambiguity { overflow: Some(true) },
  root_obligation: obligation,
  }));
@@ -198,7 +198,7 @@ fn fulfillment_error_for_no_solution<'tcx>(
  infcx: &InferCtxt<'tcx>,
  root_obligation: PredicateObligation<'tcx>,
 ) -> FulfillmentError<'tcx> {
- let obligation = find_best_leaf_obligation(infcx, &root_obligation);
+ let obligation = find_best_leaf_obligation(infcx, &root_obligation, false);
 
  let code = match obligation.predicate.kind().skip_binder() {
  ty::PredicateKind::Clause(ty::ClauseKind::Projection(_)) => {
@@ -266,7 +266,7 @@ fn fulfillment_error_for_stalled<'tcx>(
  });
 
  FulfillmentError {
- obligation: find_best_leaf_obligation(infcx, &obligation),
+ obligation: find_best_leaf_obligation(infcx, &obligation, true),
  code,
  root_obligation: obligation,
  }
@@ -275,19 +275,21 @@ fn fulfillment_error_for_stalled<'tcx>(
 fn find_best_leaf_obligation<'tcx>(
  infcx: &InferCtxt<'tcx>,
  obligation: &PredicateObligation<'tcx>,
+ consider_ambiguities: bool,
 ) -> PredicateObligation<'tcx> {
  let obligation = infcx.resolve_vars_if_possible(obligation.clone());
  infcx
  .visit_proof_tree(
  obligation.clone().into(),
- &mut BestObligation { obligation: obligation.clone() },
+ &mut BestObligation { obligation: obligation.clone(), consider_ambiguities },
  )
  .break_value()
  .unwrap_or(obligation)
 }
 
 struct BestObligation<'tcx> {
  obligation: PredicateObligation<'tcx>,
+ consider_ambiguities: bool,
 }
 
 impl<'tcx> BestObligation<'tcx> {
@@ -355,11 +357,11 @@ impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
  }
  }
 
- // Skip nested goals that hold.
- //FIXME: We should change the max allowed certainty based on if we're
- // visiting an ambiguity or error obligation.
- if matches!(nested_goal.result(), Ok(Certainty::Yes)) {
- continue;
+ // Skip nested goals that aren't the *reason* for our goal's failure.
+ match self.consider_ambiguities {
+  true if matches!(nested_goal.result(), Ok(Certainty::Maybe(_))) => {}
+  false if matches!(nested_goal.result(), Err(_)) => {}
+ _ => continue,
  }
 
  self.with_derived_obligation(obligation, |this| nested_goal.visit_with(this))?;

tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs

@@ -61,7 +61,7 @@ where
  // entering the cycle from `A` fails, but would work if we were to use the cache
  // result of `B<X>`.
  impls_trait::<A<X>, _, _, _>();
- //~^ ERROR the trait bound `X: IncompleteGuidance<_, _>` is not satisfied
+ //~^ ERROR the trait bound `A<X>: Trait<i32, u8, u8>` is not satisfied
 }
 
 fn main() {

tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.stderr

@@ -1,26 +1,28 @@
-error[E0277]: the trait bound `X: IncompleteGuidance<_, _>` is not satisfied
+error[E0277]: the trait bound `A<X>: Trait<i32, u8, u8>` is not satisfied
  --> $DIR/incompleteness-unstable-result.rs:63:19
  |
 LL | impls_trait::<A<X>, _, _, _>();
- | ^^^^ the trait `IncompleteGuidance<_, _>` is not implemented for `X`, which is required by `A<X>: Trait<_, _, _>`
+ | ^^^^ the trait `Trait<i32, u8, u8>` is not implemented for `A<X>`, which is required by `A<X>: Trait<_, _, _>`
  |
- = help: the following other types implement trait `IncompleteGuidance<T, V>`:
- <T as IncompleteGuidance<U, i16>>
- <T as IncompleteGuidance<U, i8>>
- <T as IncompleteGuidance<U, u8>>
-note: required for `A<X>` to implement `Trait<_, _, u8>`
+note: required for `A<X>` to implement `Trait<i32, u8, u8>`
  --> $DIR/incompleteness-unstable-result.rs:32:50
  |
 LL | impl<T: ?Sized, U: ?Sized, V: ?Sized, D: ?Sized> Trait<U, V, D> for A<T>
  | ^^^^^^^^^^^^^^ ^^^^
-LL | where
-LL | T: IncompleteGuidance<U, V>,
- | ------------------------ unsatisfied trait bound introduced here
+...
+LL | A<T>: Trait<U, D, V>,
+ | -------------- unsatisfied trait bound introduced here
+ = note: 8 redundant requirements hidden
+ = note: required for `A<X>` to implement `Trait<i32, u8, u8>`
 note: required by a bound in `impls_trait`
  --> $DIR/incompleteness-unstable-result.rs:51:28
  |
 LL | fn impls_trait<T: ?Sized + Trait<U, V, D>, U: ?Sized, V: ?Sized, D: ?Sized>() {}
  | ^^^^^^^^^^^^^^ required by this bound in `impls_trait`
+help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
+ |
+LL | X: IncompleteGuidance<u32, i16>, A<X>: Trait<i32, u8, u8>
+ | ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to 1 previous error