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Confusing mismatched types error with the same expected/found types when universes are involved #124471

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@WaffleLapkin

Description

@WaffleLapkin

Code

fn main() {
    consume_fn(identity);
}

fn identity(x: &u32) -> &u32 { x }

fn consume_fn<F>(_: impl FnOnce(&u32) -> F) {}

Current output

error[E0308]: mismatched types
 --> ./t.rs:4:5
  |
4 |     consume_fn(identity);
  |     ^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected reference `&_`
             found reference `&_`
note: the lifetime requirement is introduced here
 --> ./t.rs:9:42
  |
9 | fn consume_fn<F>(_: impl FnOnce(&u32) -> F) {}
  |                                          ^

Desired output

error[E0308]: mismatched types
 --> ./t.rs:4:5
  |
4 |     consume_fn(identity);
  |     ^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
  |
  = note: expected reference `&'1 _`
             found reference `for<'2> &'2 _`
note: the lifetime requirement is introduced here
 --> ./t.rs:9:42
  |
9 | fn consume_fn<F>(_: impl FnOnce(&u32) -> F) {}
  |                                          ^

Rationale and extra context

Emitting expected/found with the same thing is not very helpful. Ideally we would also mention why the types can't be actually equal here (F being a concrete type and not being able to mention the lifetime in FnOnce), but I'm not sure how to best fit it in an error message.

Other cases

No response

Rust Version

rustc 1.79.0-nightly (ef8b9dcf2 2024-04-24)
binary: rustc
commit-hash: ef8b9dcf23700f2e2265317611460d3a65c19eff
commit-date: 2024-04-24
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.4

Anything else?

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    A-diagnosticsArea: Messages for errors, warnings, and lintsA-lifetimesArea: Lifetimes / regionsD-confusingDiagnostics: Confusing error or lint that should be reworked.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.T-typesRelevant to the types team, which will review and decide on the PR/issue.

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