Error diagnostics for associated items are different with `-Z unstable-options`

[jyn514]

I tried this code: https://github.com/rust-lang/rust/blob/573a697a6197abaad5a2d7208dbf1bbc77f4dcf3/src/test/ui/associated-types/defaults-cyclic-fail-1.rs

I expected to see this happen: The error output is the same with and without -Z unstable-options

Instead, this happened: -Z unstable-options reports more errors than without.

diff --git a/src/test/ui/associated-types/defaults-cyclic-fail-1.stderr b/src/test/ui/associated-types/defaults-cyclic-fail-1.stderr
index 5e98520b411..c2887ccac74 100644
--- a/src/test/ui/associated-types/defaults-cyclic-fail-1.stderr
+++ b/src/test/ui/associated-types/defaults-cyclic-fail-1.stderr
@@ -4,12 +4,6 @@ error[E0275]: overflow evaluating the requirement `<bool as Tr>::B == _`
 LL |     type A = Box<Self::B>;
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0275]: overflow evaluating the requirement `<usize as Tr>::A == _`
-  --> $DIR/defaults-cyclic-fail-1.rs:32:5
-   |
-LL |     type B = &'static Self::A;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error

There are similar issues with other errors related to associated items; see 3205303 for details.

cc @matthewjasper - do you have any idea what's going on here?

[jyn514]

cc @estebank, this also affects the "required by this bound" diagnostic I think you recently added.

diff --git a/src/test/ui/associated-type-bounds/bounds-on-assoc-in-trait.stderr b/src/test/ui/associated-type-bounds/bounds-on-assoc-in-trait.stderr
index 919b18632e6..ba5391dd803 100644
--- a/src/test/ui/associated-type-bounds/bounds-on-assoc-in-trait.stderr
+++ b/src/test/ui/associated-type-bounds/bounds-on-assoc-in-trait.stderr
@@ -1,13 +1,8 @@
 error[E0277]: `<<Self as Case1>::A as Iterator>::Item` doesn't implement `Debug`
   --> $DIR/bounds-on-assoc-in-trait.rs:18:28
    |
-LL |     type A: Iterator<Item: Debug>;
-   |                            ^^^^^ `<<Self as Case1>::A as Iterator>::Item` cannot be formatted using `{:?}` because it doesn't implement `Debug`
-   |
-  ::: $SRC_DIR/core/src/fmt/mod.rs:LL:COL
-   |
-LL | pub trait Debug {
-   | --------------- required by this bound in `Debug`
+LL |       type A: Iterator<Item: Debug>;
+   |                              ^^^^^ `<<Self as Case1>::A as Iterator>::Item` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
    = help: the trait `Debug` is not implemented for `<<Self as Case1>::A as Iterator>::Item`
 help: consider further restricting the associated type