`Fn(a: u8)` should get a clear diagnostic

[cyrgani]

Code

fn f(_: impl Fn(a: u8)) {}

Current output

error: expected one of `!`, `(`, `)`, `+`, `,`, `::`, or `<`, found `:`
 --> src/lib.rs:1:18
  |
1 | fn f(_: impl Fn(a: u8)) {}
  |                  ^ expected one of 7 possible tokens

Desired output

error: `Fn`-trait syntax does not allow named arguments

Rationale and extra context

fn f(_: fn(a: u8)) {} is valid syntax, so the difference should get better diagnostics

Other cases

Rust Version

rustc 1.88.0-nightly (d6c1e454a 2025-04-21)
binary: rustc
commit-hash: d6c1e454aa8af5e7e59fbf5c4e7d3128d2f99582
commit-date: 2025-04-21
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

Anything else?

No response

[petrochenkov]

There's an RFC issue for actually making this legal (rust-lang/rfcs#2812), but implementing parsing recovery for this would be a good first step.

[xizheyin]

@rustbot claim