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

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

@rustbot

@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 :)

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

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

@rustbot

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

@rustbot

@rustbot claim

Rollup merge of rust-lang#157466 - bb1yd:issue-147889, r=davidtwco

Unrolled build for #157466

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Bare type in `impl` parameter list causes non-intuitive parse error #147889

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Bare type in impl parameter list causes non-intuitive parse error #147889

Description

Code

Current output

Desired output

Rationale and extra context

Other cases

Rust Version

Anything else?

Activity

maplethecolor commented on Oct 19, 2025

maplethecolor commented on Oct 19, 2025

maplethecolor commented on Nov 29, 2025

bb1yd commented on Jun 2, 2026

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Bare type in `impl` parameter list causes non-intuitive parse error #147889