Bare type in `impl` parameter list causes non-intuitive parse error

[estebank]

Code

impl<TkCanvas<Inst>> Deref for ChartDisplay<Inst> { // Note the incorrect `<TkCanvas<Inst>>`
    type Target = TkCanvas<Inst>;

    fn deref(&self) -> &Self::Target {
        &self.hull
    }
}

Current output

error: expected `::`, found `Deref`
 --> src/lib.rs:6:22
  |
6 | impl<TkCanvas<Inst>> Deref for ChartDisplay<Inst> {
  |                      ^^^^^ expected `::`

Desired output

error: expected type parameter, found path `TkCanvas<Inst>`
 --> src/lib.rs:6:22
  |
6 | impl<TkCanvas<Inst>> Deref for ChartDisplay<Inst> {
  |      ^^^^^^^^^^^^^^ expected type parameter, found path
  |
help: you might have meant to bind a type parameter to a trait
  |
6 | impl<T: TkCanvas<Inst>> Deref for ChartDisplay<Inst> {
  |      ++

help: alternatively, you might have meant to introduce type parameter
  |
6 | impl<Inst> Deref for ChartDisplay<Inst> {
  |      ++++

Rationale and extra context

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=0f2e1d057720a06ecd77956f5b411416

Found in the wild at https://users.rust-lang.org/t/help-me-understand-this-compiler-error/134753

Other cases

Rust Version

1.90.0

Anything else?

No response

[maplethecolor]

@rustbot claim

I think the desired output given is mostly OK
The second hint should suggest Inst: TkCanvas instead IMO, but that can be changed later on :)

[maplethecolor]

Should this be extended to functions where such a mistake might happen too?
Playground

fn meow<Box<String>>() {}

[maplethecolor]

Sorry; I haven't found the time to work on Rust due to life. Releasing.
@rustbot release-assignment

[bb1yd]

@rustbot claim