Auto merge of #80828 - SNCPlay42:opaque-projections, r=estebank

Fix expected/found order on impl trait projection mismatch error

fixes #68561

This PR adds a new `ObligationCauseCode` used when checking the concrete type of an impl trait satisfies its bounds, and checks for that cause code in the existing test to see if a projection's normalized type should be the "expected" or "found" type.

The second commit adds a `peel_derives` to that test, which appears to be necessary in some cases (see projection-mismatch-in-impl-where-clause.rs, which would still give expected/found in the wrong order otherwise). This caused some other changes in diagnostics not involving impl trait, but they look correct to me.

18 files changed, 109 insertions, 55 deletions

diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 922d14bc2f5..00dec3b355f 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -323,6 +323,9 @@ pub enum ObligationCauseCode<'tcx> {
 
     /// #[feature(trivial_bounds)] is not enabled
     TrivialBound,
+
+    /// If `X` is the concrete type of an opaque type `impl Y`, then `X` must implement `Y`
+    OpaqueType,
 }
 
 impl ObligationCauseCode<'_> {
diff --git a/compiler/rustc_middle/src/ty/error.rs b/compiler/rustc_middle/src/ty/error.rs
index 4767e5743c2..d295b17d902 100644
--- a/compiler/rustc_middle/src/ty/error.rs
+++ b/compiler/rustc_middle/src/ty/error.rs
@@ -511,13 +511,18 @@ impl<T> Trait<T> for X {
                             "consider constraining the associated type `{}` to `{}`",
                             values.found, values.expected,
                         );
-                        if !self.suggest_constraint(
+                        if !(self.suggest_constraining_opaque_associated_type(
+                            db,
+                            &msg,
+                            proj_ty,
+                            values.expected,
+                        ) || self.suggest_constraint(
                             db,
                             &msg,
                             body_owner_def_id,
                             proj_ty,
                             values.expected,
-                        ) {
+                        )) {
                             db.help(&msg);
                             db.note(
                                 "for more information, visit \
@@ -701,20 +706,7 @@ impl<T> Trait<T> for X {
             }
         }
 
-        if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
-            // When the expected `impl Trait` is not defined in the current item, it will come from
-            // a return type. This can occur when dealing with `TryStream` (#71035).
-            if self.constrain_associated_type_structured_suggestion(
-                db,
-                self.def_span(def_id),
-                &assoc,
-                proj_ty.trait_ref_and_own_substs(self).1,
-                values.found,
-                &msg,
-            ) {
-                return;
-            }
-        }
+        self.suggest_constraining_opaque_associated_type(db, &msg, proj_ty, values.found);
 
         if self.point_at_associated_type(db, body_owner_def_id, values.found) {
             return;
@@ -752,6 +744,30 @@ fn foo(&self) -> Self::T { String::new() }
         }
     }
 
+    /// When the expected `impl Trait` is not defined in the current item, it will come from
+    /// a return type. This can occur when dealing with `TryStream` (#71035).
+    fn suggest_constraining_opaque_associated_type(
+        self,
+        db: &mut DiagnosticBuilder<'_>,
+        msg: &str,
+        proj_ty: &ty::ProjectionTy<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> bool {
+        let assoc = self.associated_item(proj_ty.item_def_id);
+        if let ty::Opaque(def_id, _) = *proj_ty.self_ty().kind() {
+            self.constrain_associated_type_structured_suggestion(
+                db,
+                self.def_span(def_id),
+                &assoc,
+                proj_ty.trait_ref_and_own_substs(self).1,
+                ty,
+                &msg,
+            )
+        } else {
+            false
+        }
+    }
+
     fn point_at_methods_that_satisfy_associated_type(
         self,
         db: &mut DiagnosticBuilder<'_>,
diff --git a/compiler/rustc_trait_selection/src/opaque_types.rs b/compiler/rustc_trait_selection/src/opaque_types.rs
index b6c3768d887..fb4a8ce687c 100644
--- a/compiler/rustc_trait_selection/src/opaque_types.rs
+++ b/compiler/rustc_trait_selection/src/opaque_types.rs
@@ -1171,7 +1171,7 @@ impl<'a, 'tcx> Instantiator<'a, 'tcx> {
             // This also instantiates nested instances of `impl Trait`.
             let predicate = self.instantiate_opaque_types_in_map(predicate);
 
-            let cause = traits::ObligationCause::new(span, self.body_id, traits::MiscObligation);
+            let cause = traits::ObligationCause::new(span, self.body_id, traits::OpaqueType);
 
             // Require that the predicate holds for the concrete type.
             debug!("instantiate_opaque_types: predicate={:?}", predicate);
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 93a37bd4090..15a3d3ddd8d 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1226,10 +1226,11 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
                 );
 
                 let is_normalized_ty_expected = !matches!(
-                    obligation.cause.code,
+                    obligation.cause.code.peel_derives(),
                     ObligationCauseCode::ItemObligation(_)
                         | ObligationCauseCode::BindingObligation(_, _)
                         | ObligationCauseCode::ObjectCastObligation(_)
+                        | ObligationCauseCode::OpaqueType
                 );
 
                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 326b85e1013..0549db06806 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -1840,6 +1840,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             | ObligationCauseCode::MethodReceiver
             | ObligationCauseCode::ReturnNoExpression
             | ObligationCauseCode::UnifyReceiver(..)
+            | ObligationCauseCode::OpaqueType
             | ObligationCauseCode::MiscObligation => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");
diff --git a/src/test/ui/associated-types/impl-trait-return-missing-constraint.stderr b/src/test/ui/associated-types/impl-trait-return-missing-constraint.stderr
index b0e9e33a6c3..89e05b61fc9 100644
--- a/src/test/ui/associated-types/impl-trait-return-missing-constraint.stderr
+++ b/src/test/ui/associated-types/impl-trait-return-missing-constraint.stderr
@@ -2,13 +2,13 @@ error[E0271]: type mismatch resolving `<impl Bar as Foo>::Item == i32`
   --> $DIR/impl-trait-return-missing-constraint.rs:25:13
    |
 LL | fn bar() -> impl Bar {
-   |             -------- the expected opaque type
+   |             -------- the found opaque type
 ...
 LL | fn baz() -> impl Bar<Item = i32> {
-   |             ^^^^^^^^^^^^^^^^^^^^ expected associated type, found `i32`
+   |             ^^^^^^^^^^^^^^^^^^^^ expected `i32`, found associated type
    |
-   = note: expected associated type `<impl Bar as Foo>::Item`
-                         found type `i32`
+   = note:         expected type `i32`
+           found associated type `<impl Bar as Foo>::Item`
 help: consider constraining the associated type `<impl Bar as Foo>::Item` to `i32`
    |
 LL | fn bar() -> impl Bar<Item = i32> {
diff --git a/src/test/ui/associated-types/issue-44153.stderr b/src/test/ui/associated-types/issue-44153.stderr
index 8ef71087958..cafc8ec52ca 100644
--- a/src/test/ui/associated-types/issue-44153.stderr
+++ b/src/test/ui/associated-types/issue-44153.stderr
@@ -5,7 +5,7 @@ LL |     fn visit() {}
    |     ---------- required by `Visit::visit`
 ...
 LL |     <() as Visit>::visit();
-   |     ^^^^^^^^^^^^^^^^^^^^ expected `()`, found `&()`
+   |     ^^^^^^^^^^^^^^^^^^^^ expected `&()`, found `()`
    |
    = note: required because of the requirements on the impl of `Visit` for `()`
 
diff --git a/src/test/ui/generator/type-mismatch-signature-deduction.stderr b/src/test/ui/generator/type-mismatch-signature-deduction.stderr
index 30e23ea8f65..3f1f33a3b12 100644
--- a/src/test/ui/generator/type-mismatch-signature-deduction.stderr
+++ b/src/test/ui/generator/type-mismatch-signature-deduction.stderr
@@ -16,10 +16,10 @@ error[E0271]: type mismatch resolving `<[generator@$DIR/type-mismatch-signature-
   --> $DIR/type-mismatch-signature-deduction.rs:5:13
    |
 LL | fn foo() -> impl Generator<Return = i32> {
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `Result`, found `i32`
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found enum `Result`
    |
-   = note: expected enum `Result<{integer}, _>`
-              found type `i32`
+   = note: expected type `i32`
+              found enum `Result<{integer}, _>`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/impl-trait/bound-normalization-fail.stderr b/src/test/ui/impl-trait/bound-normalization-fail.stderr
index a7d06c71663..6958cd97a4a 100644
--- a/src/test/ui/impl-trait/bound-normalization-fail.stderr
+++ b/src/test/ui/impl-trait/bound-normalization-fail.stderr
@@ -11,10 +11,10 @@ error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as imp
   --> $DIR/bound-normalization-fail.rs:27:32
    |
 LL |     fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
-   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found associated type
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found `()`
    |
-   = note:         expected type `()`
-           found associated type `<T as impl_trait::Trait>::Assoc`
+   = note: expected associated type `<T as impl_trait::Trait>::Assoc`
+                         found type `()`
 help: consider constraining the associated type `<T as impl_trait::Trait>::Assoc` to `()`
    |
 LL |     fn foo_fail<T: Trait<Assoc = ()>>() -> impl FooLike<Output=T::Assoc> {
@@ -30,10 +30,10 @@ error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as lif
   --> $DIR/bound-normalization-fail.rs:43:41
    |
 LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
-   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found associated type
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found `()`
    |
-   = note:         expected type `()`
-           found associated type `<T as lifetimes::Trait<'static>>::Assoc`
+   = note: expected associated type `<T as lifetimes::Trait<'static>>::Assoc`
+                         found type `()`
 help: consider constraining the associated type `<T as lifetimes::Trait<'static>>::Assoc` to `()`
    |
 LL |     fn foo2_fail<'a, T: Trait<'a, Assoc = ()>>() -> impl FooLike<Output=T::Assoc> {
diff --git a/src/test/ui/impl-trait/equality2.stderr b/src/test/ui/impl-trait/equality2.stderr
index 1443b76048b..3318866c52c 100644
--- a/src/test/ui/impl-trait/equality2.stderr
+++ b/src/test/ui/impl-trait/equality2.stderr
@@ -35,8 +35,10 @@ LL |     let _: i32 = Leak::leak(hide(0_i32));
    |
    = note:         expected type `i32`
            found associated type `<impl Foo as Leak>::T`
-   = help: consider constraining the associated type `<impl Foo as Leak>::T` to `i32`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: consider constraining the associated type `<impl Foo as Leak>::T` to `i32`
+   |
+LL | fn hide<T: Foo>(x: T) -> impl Foo<T = i32> {
+   |                                  ^^^^^^^^^
 
 error[E0308]: mismatched types
   --> $DIR/equality2.rs:38:10
diff --git a/src/test/ui/impl-trait/issues/issue-70877.full_tait.stderr b/src/test/ui/impl-trait/issues/issue-70877.full_tait.stderr
index bd4d4fdf2a6..8e42b9d46db 100644
--- a/src/test/ui/impl-trait/issues/issue-70877.full_tait.stderr
+++ b/src/test/ui/impl-trait/issues/issue-70877.full_tait.stderr
@@ -2,13 +2,13 @@ error[E0271]: type mismatch resolving `<Bar as Iterator>::Item == Box<(dyn for<'
   --> $DIR/issue-70877.rs:11:12
    |
 LL | type FooRet = impl std::fmt::Debug;
-   |               -------------------- the expected opaque type
+   |               -------------------- the found opaque type
 ...
 LL | type Foo = impl Iterator<Item = FooItem>;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected opaque type, found enum `Option`
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `Option`, found opaque type
    |
-   = note: expected struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> impl Debug + 'static)>`
-              found struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> Option<String> + 'static)>`
+   = note: expected struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> Option<String> + 'static)>`
+              found struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> impl Debug + 'static)>`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/impl-trait/issues/issue-70877.min_tait.stderr b/src/test/ui/impl-trait/issues/issue-70877.min_tait.stderr
index bd4d4fdf2a6..8e42b9d46db 100644
--- a/src/test/ui/impl-trait/issues/issue-70877.min_tait.stderr
+++ b/src/test/ui/impl-trait/issues/issue-70877.min_tait.stderr
@@ -2,13 +2,13 @@ error[E0271]: type mismatch resolving `<Bar as Iterator>::Item == Box<(dyn for<'
   --> $DIR/issue-70877.rs:11:12
    |
 LL | type FooRet = impl std::fmt::Debug;
-   |               -------------------- the expected opaque type
+   |               -------------------- the found opaque type
 ...
 LL | type Foo = impl Iterator<Item = FooItem>;
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected opaque type, found enum `Option`
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `Option`, found opaque type
    |
-   = note: expected struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> impl Debug + 'static)>`
-              found struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> Option<String> + 'static)>`
+   = note: expected struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> Option<String> + 'static)>`
+              found struct `Box<(dyn for<'r> Fn(&'r (dyn ToString + 'r)) -> impl Debug + 'static)>`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/impl-trait/projection-mismatch-in-impl-where-clause.rs b/src/test/ui/impl-trait/projection-mismatch-in-impl-where-clause.rs
new file mode 100644
index 00000000000..b4fd6b3e743
--- /dev/null
+++ b/src/test/ui/impl-trait/projection-mismatch-in-impl-where-clause.rs
@@ -0,0 +1,20 @@
+pub trait Super {
+    type Assoc;
+}
+
+impl Super for () {
+    type Assoc = u8;
+}
+
+pub trait Test {}
+
+impl<T> Test for T where T: Super<Assoc = ()> {}
+
+fn test() -> impl Test {
+    //~^ERROR type mismatch resolving `<() as Super>::Assoc == ()`
+    ()
+}
+
+fn main() {
+    let a = test();
+}
diff --git a/src/test/ui/impl-trait/projection-mismatch-in-impl-where-clause.stderr b/src/test/ui/impl-trait/projection-mismatch-in-impl-where-clause.stderr
new file mode 100644
index 00000000000..d1956a9afb8
--- /dev/null
+++ b/src/test/ui/impl-trait/projection-mismatch-in-impl-where-clause.stderr
@@ -0,0 +1,11 @@
+error[E0271]: type mismatch resolving `<() as Super>::Assoc == ()`
+  --> $DIR/projection-mismatch-in-impl-where-clause.rs:13:14
+   |
+LL | fn test() -> impl Test {
+   |              ^^^^^^^^^ expected `()`, found `u8`
+   |
+   = note: required because of the requirements on the impl of `Test` for `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.
diff --git a/src/test/ui/issues/issue-33941.stderr b/src/test/ui/issues/issue-33941.stderr
index e91dae08b3a..043658c9508 100644
--- a/src/test/ui/issues/issue-33941.stderr
+++ b/src/test/ui/issues/issue-33941.stderr
@@ -11,10 +11,10 @@ error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _,
   --> $DIR/issue-33941.rs:4:14
    |
 LL |     for _ in HashMap::new().iter().cloned() {}
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected tuple, found reference
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected reference, found tuple
    |
-   = note:  expected tuple `(&_, &_)`
-           found reference `&_`
+   = note: expected reference `&_`
+                  found tuple `(&_, &_)`
    = note: required because of the requirements on the impl of `Iterator` for `Cloned<std::collections::hash_map::Iter<'_, _, _>>`
    = note: required because of the requirements on the impl of `IntoIterator` for `Cloned<std::collections::hash_map::Iter<'_, _, _>>`
    = note: required by `into_iter`
@@ -23,10 +23,10 @@ error[E0271]: type mismatch resolving `<std::collections::hash_map::Iter<'_, _,
   --> $DIR/issue-33941.rs:4:14
    |
 LL |     for _ in HashMap::new().iter().cloned() {}
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected tuple, found reference
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected reference, found tuple
    |
-   = note:  expected tuple `(&_, &_)`
-           found reference `&_`
+   = note: expected reference `&_`
+                  found tuple `(&_, &_)`
    = note: required because of the requirements on the impl of `Iterator` for `Cloned<std::collections::hash_map::Iter<'_, _, _>>`
    = note: required by `std::iter::Iterator::next`
 
diff --git a/src/test/ui/issues/issue-39970.stderr b/src/test/ui/issues/issue-39970.stderr
index 6f342b459c0..8ecde9d1e68 100644
--- a/src/test/ui/issues/issue-39970.stderr
+++ b/src/test/ui/issues/issue-39970.stderr
@@ -5,7 +5,7 @@ LL |     fn visit() {}
    |     ---------- required by `Visit::visit`
 ...
 LL |     <() as Visit>::visit();
-   |     ^^^^^^^^^^^^^^^^^^^^ expected `&()`, found `()`
+   |     ^^^^^^^^^^^^^^^^^^^^ expected `()`, found `&()`
    |
    = note: required because of the requirements on the impl of `Visit` for `()`
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-63279.full_tait.stderr b/src/test/ui/type-alias-impl-trait/issue-63279.full_tait.stderr
index f544f61df97..53a0016c08e 100644
--- a/src/test/ui/type-alias-impl-trait/issue-63279.full_tait.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-63279.full_tait.stderr
@@ -11,10 +11,10 @@ error[E0271]: type mismatch resolving `<[closure@$DIR/issue-63279.rs:11:5: 11:28
   --> $DIR/issue-63279.rs:8:16
    |
 LL | type Closure = impl FnOnce();
-   |                ^^^^^^^^^^^^^ expected opaque type, found `()`
+   |                ^^^^^^^^^^^^^ expected `()`, found opaque type
    |
-   = note: expected opaque type `impl FnOnce<()>`
-                found unit type `()`
+   = note: expected unit type `()`
+            found opaque type `impl FnOnce<()>`
 
 error: aborting due to previous error; 1 warning emitted
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-63279.min_tait.stderr b/src/test/ui/type-alias-impl-trait/issue-63279.min_tait.stderr
index bdf414d0bad..be386ab90ea 100644
--- a/src/test/ui/type-alias-impl-trait/issue-63279.min_tait.stderr
+++ b/src/test/ui/type-alias-impl-trait/issue-63279.min_tait.stderr
@@ -11,10 +11,10 @@ error[E0271]: type mismatch resolving `<[closure@$DIR/issue-63279.rs:11:5: 11:28
   --> $DIR/issue-63279.rs:8:16
    |
 LL | type Closure = impl FnOnce();
-   |                ^^^^^^^^^^^^^ expected opaque type, found `()`
+   |                ^^^^^^^^^^^^^ expected `()`, found opaque type
    |
-   = note: expected opaque type `impl FnOnce<()>`
-                found unit type `()`
+   = note: expected unit type `()`
+            found opaque type `impl FnOnce<()>`
 
 error: aborting due to 2 previous errors