Auto merge of #131483 - compiler-errors:coercion-tgt, r=<try>

Always structurally resolve coercion target

This is morally a no-op. We have a few callsites that manually call `resolve_vars_with_obligations` before coercion, so let's not do that and just always structurally resolve in the coerce function itself.

r? lcnr

Author: bors

Diff for: compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -208,11 +208,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             );
         }
 
+        // Guide inference on the formal output ty if necessary.
+        self.select_obligations_where_possible(|_| {});
+
         // First, let's unify the formal method signature with the expectation eagerly.
         // We use this to guide coercion inference; it's output is "fudged" which means
         // any remaining type variables are assigned to new, unrelated variables. This
         // is because the inference guidance here is only speculative.
-        let formal_output = self.resolve_vars_with_obligations(formal_output);
         let expected_input_tys: Option<Vec<_>> = expectation
             .only_has_type(self)
             .and_then(|expected_output| {

Diff for: compiler/rustc_hir_typeck/src/op.rs

@@ -231,7 +231,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 self.check_expr(lhs_expr)
             }
         };
-        let lhs_ty = self.resolve_vars_with_obligations(lhs_ty);
+
+        // Guide inference on the lhs ty if necessary.
+        self.select_obligations_where_possible(|_| {});
+        let lhs_ty = self.resolve_vars_if_possible(lhs_ty);
 
         // N.B., as we have not yet type-checked the RHS, we don't have the
         // type at hand. Make a variable to represent it. The whole reason

Diff for: tests/ui/delegation/correct_body_owner_parent_found_in_diagnostics.stderr

@@ -52,6 +52,14 @@ LL | pub struct InvariantRef<'a, T: ?Sized>(&'a T, PhantomData<&'a mut &'a T>);
 LL |     pub const NEW: Self = InvariantRef::new(&());
    |                                         ^^^ function or associated item not found in `InvariantRef<'_, _>`
 
+error[E0277]: the trait bound `u8: Trait` is not satisfied
+  --> $DIR/correct_body_owner_parent_found_in_diagnostics.rs:22:12
+   |
+LL |     reuse <u8 as Trait>::{foo, bar, meh} { &const { InvariantRef::<'a>::NEW } }
+   |            ^^ the trait `Trait` is not implemented for `u8`
+   |
+   = help: the trait `Trait` is implemented for `Z`
+
 error[E0308]: mismatched types
   --> $DIR/correct_body_owner_parent_found_in_diagnostics.rs:22:53
    |
@@ -68,6 +76,7 @@ LL |     reuse <u8 as Trait>::{foo, bar, meh} { &const { InvariantRef::<'a>::NEW
    |            ^^ the trait `Trait` is not implemented for `u8`
    |
    = help: the trait `Trait` is implemented for `Z`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0308]: mismatched types
   --> $DIR/correct_body_owner_parent_found_in_diagnostics.rs:22:53
@@ -98,15 +107,6 @@ LL |     reuse <u8 as Trait>::{foo, bar, meh} { &const { InvariantRef::<'a>::NEW
             found struct `InvariantRef<'_, ()>`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error[E0277]: the trait bound `u8: Trait` is not satisfied
-  --> $DIR/correct_body_owner_parent_found_in_diagnostics.rs:22:12
-   |
-LL |     reuse <u8 as Trait>::{foo, bar, meh} { &const { InvariantRef::<'a>::NEW } }
-   |            ^^ the trait `Trait` is not implemented for `u8`
-   |
-   = help: the trait `Trait` is implemented for `Z`
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-
 error: aborting due to 10 previous errors
 
 Some errors have detailed explanations: E0261, E0277, E0308, E0599.

Diff for: tests/ui/issues/issue-50781.stderr

@@ -15,10 +15,10 @@ LL |     fn foo(&self) where Self: Trait;
    = help: only type `()` implements the trait, consider using it directly instead
 
 error[E0038]: the trait `X` cannot be made into an object
-  --> $DIR/issue-50781.rs:16:23
+  --> $DIR/issue-50781.rs:16:6
    |
 LL |     <dyn X as X>::foo(&());
-   |                       ^^^ `X` cannot be made into an object
+   |      ^^^^^ `X` cannot be made into an object
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $DIR/issue-50781.rs:4:8
@@ -29,13 +29,12 @@ LL |     fn foo(&self) where Self: Trait;
    |        ^^^ ...because method `foo` references the `Self` type in its `where` clause
    = help: consider moving `foo` to another trait
    = help: only type `()` implements the trait, consider using it directly instead
-   = note: required for the cast from `&()` to `&dyn X`
 
 error[E0038]: the trait `X` cannot be made into an object
-  --> $DIR/issue-50781.rs:16:6
+  --> $DIR/issue-50781.rs:16:23
    |
 LL |     <dyn X as X>::foo(&());
-   |      ^^^^^ `X` cannot be made into an object
+   |                       ^^^ `X` cannot be made into an object
    |
 note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> $DIR/issue-50781.rs:4:8
@@ -46,6 +45,7 @@ LL |     fn foo(&self) where Self: Trait;
    |        ^^^ ...because method `foo` references the `Self` type in its `where` clause
    = help: consider moving `foo` to another trait
    = help: only type `()` implements the trait, consider using it directly instead
+   = note: required for the cast from `&()` to `&dyn X`
 
 error: aborting due to 3 previous errors
 

Diff for: tests/ui/overloaded/overloaded-calls-nontuple.stderr

@@ -39,17 +39,17 @@ LL |         self.call_mut(z)
 note: required by a bound in `call_mut`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
 
-error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
+error[E0277]: `isize` is not a tuple
   --> $DIR/overloaded-calls-nontuple.rs:29:10
    |
 LL |     drop(s(3))
-   |          ^^^^
+   |          ^^^^ the trait `Tuple` is not implemented for `isize`
 
-error[E0277]: `isize` is not a tuple
+error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
   --> $DIR/overloaded-calls-nontuple.rs:29:10
    |
 LL |     drop(s(3))
-   |          ^^^^ the trait `Tuple` is not implemented for `isize`
+   |          ^^^^
 
 error: aborting due to 7 previous errors
 

Diff for: tests/ui/suggestions/fn-trait-notation.stderr

@@ -37,17 +37,17 @@ LL |     F: Fn<i32, Output = i32>,
 note: required by a bound in `Fn`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
 
-error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
+error[E0277]: `i32` is not a tuple
   --> $DIR/fn-trait-notation.rs:9:5
    |
 LL |     f(3);
-   |     ^^^^
+   |     ^^^^ the trait `Tuple` is not implemented for `i32`
 
-error[E0277]: `i32` is not a tuple
+error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
   --> $DIR/fn-trait-notation.rs:9:5
    |
 LL |     f(3);
-   |     ^^^^ the trait `Tuple` is not implemented for `i32`
+   |     ^^^^
 
 error[E0308]: mismatched types
   --> $DIR/fn-trait-notation.rs:17:5

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

@@ -1,5 +1,5 @@
 error[E0271]: type mismatch resolving `<dyn Trait<A = A, B = B> as SuperTrait>::A == B`
-  --> $DIR/more-object-bound.rs:12:5
+  --> $DIR/more-object-bound.rs:12:17
    |
 LL | fn transmute<A, B>(x: A) -> B {
    |              -  -
@@ -9,7 +9,7 @@ LL | fn transmute<A, B>(x: A) -> B {
    |              found type parameter
    |              expected type parameter
 LL |     foo::<A, B, dyn Trait<A = A, B = B>>(x)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `A`, found type parameter `B`
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `A`, found type parameter `B`
    |
    = note: expected type parameter `A`
               found type parameter `B`