fire debug assertion when `expected` and `found` item look the same in diagnostic?

Sometimes rustc outputs silly error hints like

   = note: expected trait `for<'a> <for<'a> fn(&'a u32) -> &'a u32 {function2} as FnOnce<(&'a u32,)>>`
              found trait `for<'a> <for<'a> fn(&'a u32) -> &'a u32 {function2} as FnOnce<(&'a u32,)>>`

where "expected" and "found" look like they are the same which is quite unhelpful.
(example from https://github.com/rust-lang/rust/issues/114883 )

I wonder if we should have a debug assertion in these cases (rustc displaying both types being identical although they shouldn't be) that helps catch these kid of diagnostic mishaps, similar to the "span must not be empty and have no suggestion" and "span must not have overlapping parts" debug assertions.

Start by reviewing the diagnostic behavior shown in issue #114883 and tracing where rustc formats the expected and found traits. Determine where an assertion can check for identical displayed items without affecting valid diagnostics. Done means the assertion catches this diagnostic mishap and relevant diagnostic tests still pass.