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.

6 files changed, 49 insertions, 16 deletions

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`.