Use leaf obligation to create selection error code

Author: compiler-errors

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -196,8 +196,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
 
 fn fulfillment_error_for_no_solution<'tcx>(
     infcx: &InferCtxt<'tcx>,
-    obligation: PredicateObligation<'tcx>,
+    root_obligation: PredicateObligation<'tcx>,
 ) -> FulfillmentError<'tcx> {
+    let obligation = find_best_leaf_obligation(infcx, &root_obligation);
+
     let code = match obligation.predicate.kind().skip_binder() {
         ty::PredicateKind::Clause(ty::ClauseKind::Projection(_)) => {
             FulfillmentErrorCode::ProjectionError(
@@ -217,14 +219,14 @@ fn fulfillment_error_for_no_solution<'tcx>(
         }
         ty::PredicateKind::Subtype(pred) => {
             let (a, b) = infcx.enter_forall_and_leak_universe(
-                obligation.predicate.kind().rebind((pred.a, pred.b)),
+                root_obligation.predicate.kind().rebind((pred.a, pred.b)),
             );
             let expected_found = ExpectedFound::new(true, a, b);
             FulfillmentErrorCode::SubtypeError(expected_found, TypeError::Sorts(expected_found))
         }
         ty::PredicateKind::Coerce(pred) => {
             let (a, b) = infcx.enter_forall_and_leak_universe(
-                obligation.predicate.kind().rebind((pred.a, pred.b)),
+                root_obligation.predicate.kind().rebind((pred.a, pred.b)),
             );
             let expected_found = ExpectedFound::new(false, a, b);
             FulfillmentErrorCode::SubtypeError(expected_found, TypeError::Sorts(expected_found))
@@ -239,11 +241,7 @@ fn fulfillment_error_for_no_solution<'tcx>(
         }
     };
 
-    FulfillmentError {
-        obligation: find_best_leaf_obligation(infcx, &obligation),
-        code,
-        root_obligation: obligation,
-    }
+    FulfillmentError { obligation, code, root_obligation }
 }
 
 fn fulfillment_error_for_stalled<'tcx>(

tests/ui/traits/next-solver/env-shadows-impls/param-candidate-shadows-project.rs

@@ -25,7 +25,7 @@ fn foo<T: Foo>() {
     //
     //     https://github.com/rust-lang/trait-system-refactor-initiative/issues/76
     require_bar::<T>();
-    //~^ ERROR the trait bound `T: Bar` is not satisfied
+    //~^ ERROR type mismatch resolving `<T as Foo>::Assoc == i32`
 }
 
 fn main() {}

tests/ui/traits/next-solver/env-shadows-impls/param-candidate-shadows-project.stderr

@@ -1,19 +1,25 @@
-error[E0277]: the trait bound `T: Bar` is not satisfied
+error[E0271]: type mismatch resolving `<T as Foo>::Assoc == i32`
   --> $DIR/param-candidate-shadows-project.rs:27:19
    |
 LL |     require_bar::<T>();
-   |                   ^ the trait `Bar` is not implemented for `T`
+   |                   ^ type mismatch resolving `<T as Foo>::Assoc == i32`
    |
+note: types differ
+  --> $DIR/param-candidate-shadows-project.rs:10:18
+   |
+LL |     type Assoc = i32;
+   |                  ^^^
+note: required for `T` to implement `Bar`
+  --> $DIR/param-candidate-shadows-project.rs:13:9
+   |
+LL | impl<T> Bar for T where T: Foo<Assoc = i32> {}
+   |         ^^^     ^              ----------- unsatisfied trait bound introduced here
 note: required by a bound in `require_bar`
   --> $DIR/param-candidate-shadows-project.rs:15:19
    |
 LL | fn require_bar<T: Bar>() {}
    |                   ^^^ required by this bound in `require_bar`
-help: consider further restricting this bound
-   |
-LL | fn foo<T: Foo + Bar>() {
-   |               +++++
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0271`.

tests/ui/traits/next-solver/more-object-bound.rs

@@ -10,7 +10,7 @@ trait Trait: SuperTrait<A = <Self as SuperTrait>::B> {}
 
 fn transmute<A, B>(x: A) -> B {
     foo::<A, B, dyn Trait<A = A, B = B>>(x)
-    //~^ ERROR the trait bound `dyn Trait<A = A, B = B>: Trait` is not satisfied
+    //~^ ERROR type mismatch resolving `<dyn Trait<A = A, B = B> as SuperTrait>::A == B`
 }
 
 fn foo<A, B, T: ?Sized>(x: T::A) -> B

tests/ui/traits/next-solver/more-object-bound.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `dyn Trait<A = A, B = B>: Trait` is not satisfied
+error[E0271]: type mismatch resolving `<dyn Trait<A = A, B = B> as SuperTrait>::A == B`
   --> $DIR/more-object-bound.rs:12:5
    |
 LL |     foo::<A, B, dyn Trait<A = A, B = B>>(x)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `dyn Trait<A = A, B = B>`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
    |
 note: required by a bound in `foo`
   --> $DIR/more-object-bound.rs:18:8
@@ -12,11 +12,7 @@ LL | fn foo<A, B, T: ?Sized>(x: T::A) -> B
 LL | where
 LL |     T: Trait<B = B>,
    |        ^^^^^^^^^^^^ required by this bound in `foo`
-help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
-   |
-LL | fn transmute<A, B>(x: A) -> B where dyn Trait<A = A, B = B>: Trait {
-   |                               ++++++++++++++++++++++++++++++++++++
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0271`.