Rollup merge of #120549 - lcnr:errs-showcase, r=compiler-errors

modify alias-relate to also normalize ambiguous opaques

allows a bunch of further cleanups and generally simplifies the type system. To handle https://github.com/rust-lang/trait-system-refactor-initiative/issues/8 we'll have to add a some additional complexity to the `(Alias, Infer)` branches in alias-relate, so removing the opaque type special case here is really valuable.

It does worsen `deduce_closure_signature` and friends even more as they now receive an inference variable which is only constrained via an `AliasRelate` goal. These probably have to look into alias relate goals somehow. Leaving that for a future PR as this is something we'll have to tackle regardless.

r? `@compiler-errors`

4 files changed, 29 insertions, 4 deletions

diff --git a/tests/ui/impl-trait/recursive-coroutine-boxed.next.stderr b/tests/ui/impl-trait/recursive-coroutine-boxed.next.stderr
new file mode 100644
index 00000000000..fee3b86034a
--- /dev/null
+++ b/tests/ui/impl-trait/recursive-coroutine-boxed.next.stderr
@@ -0,0 +1,23 @@
+error[E0282]: type annotations needed
+  --> $DIR/recursive-coroutine-boxed.rs:10:23
+   |
+LL |         let mut gen = Box::pin(foo());
+   |                       ^^^^^^^^ cannot infer type of the type parameter `T` declared on the struct `Box`
+...
+LL |         let mut r = gen.as_mut().resume(());
+   |                         ------ type must be known at this point
+   |
+help: consider specifying the generic argument
+   |
+LL |         let mut gen = Box::<T>::pin(foo());
+   |                          +++++
+
+error[E0282]: type annotations needed
+  --> $DIR/recursive-coroutine-boxed.rs:10:32
+   |
+LL |         let mut gen = Box::pin(foo());
+   |                                ^^^^^ cannot infer type for opaque type `impl Coroutine<Yield = (), Return = ()>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0282`.
diff --git a/tests/ui/impl-trait/recursive-coroutine-boxed.rs b/tests/ui/impl-trait/recursive-coroutine-boxed.rs
index b9291f07e21..3f677986c13 100644
--- a/tests/ui/impl-trait/recursive-coroutine-boxed.rs
+++ b/tests/ui/impl-trait/recursive-coroutine-boxed.rs
@@ -1,5 +1,5 @@
-// check-pass
 // revisions: current next
+//[current] check-pass
 //[next] compile-flags: -Znext-solver
 #![feature(coroutines, coroutine_trait)]
 
@@ -8,6 +8,8 @@ use std::ops::{Coroutine, CoroutineState};
 fn foo() -> impl Coroutine<Yield = (), Return = ()> {
     || {
         let mut gen = Box::pin(foo());
+        //[next]~^ ERROR type annotations needed
+        //[next]~| ERROR type annotations needed
         let mut r = gen.as_mut().resume(());
         while let CoroutineState::Yielded(v) = r {
             yield v;
diff --git a/tests/ui/impl-trait/two_tait_defining_each_other2.next.stderr b/tests/ui/impl-trait/two_tait_defining_each_other2.next.stderr
index e49d1d18b0c..69328e20583 100644
--- a/tests/ui/impl-trait/two_tait_defining_each_other2.next.stderr
+++ b/tests/ui/impl-trait/two_tait_defining_each_other2.next.stderr
@@ -1,8 +1,8 @@
-error[E0284]: type annotations needed: cannot satisfy `A <: B`
+error[E0284]: type annotations needed: cannot satisfy `A == B`
   --> $DIR/two_tait_defining_each_other2.rs:11:5
    |
 LL |     x // B's hidden type is A (opaquely)
-   |     ^ cannot satisfy `A <: B`
+   |     ^ cannot satisfy `A == B`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/impl-trait/two_tait_defining_each_other2.rs b/tests/ui/impl-trait/two_tait_defining_each_other2.rs
index 8a79af19776..b2f768f4dcd 100644
--- a/tests/ui/impl-trait/two_tait_defining_each_other2.rs
+++ b/tests/ui/impl-trait/two_tait_defining_each_other2.rs
@@ -10,7 +10,7 @@ trait Foo {}
 fn muh(x: A) -> B {
     x // B's hidden type is A (opaquely)
     //[current]~^ ERROR opaque type's hidden type cannot be another opaque type
-    //[next]~^^ ERROR type annotations needed: cannot satisfy `A <: B`
+    //[next]~^^ ERROR type annotations needed: cannot satisfy `A == B`
 }
 
 struct Bar;