Confusing E0308 (one type is more general) with GATs points to call site instead of the mismatched impl block

When a type implements a trait with a Generic Associated Type (GAT) incorrectly (e.g., omitting a required reference), passing that type to a function with a Higher-Rank Trait Bound (for<'a>) generates a confusing E0308 error that does not mention the location of the actual mistake in the impl block.

Playground

Code

pub trait PipelineSink {
    type InputItem<'a>;
}

pub struct DynamicPipelineBuilder<OutputItem>(std::marker::PhantomData<OutputItem>);

impl<OutputItem> DynamicPipelineBuilder<OutputItem> {
    pub fn new() -> Self { Self(std::marker::PhantomData) }
    pub fn sink(self, _pipeline_sink: impl for<'a> PipelineSink<InputItem<'a> = &'a OutputItem>) {}
}

pub struct PrintingSink<O>(std::marker::PhantomData<O>);

impl<O> PrintingSink<O> {
    pub fn new() -> Self { Self(std::marker::PhantomData) }
}

impl<O: 'static> PipelineSink for PrintingSink<O> {
    // BUG: Missing reference. Should be `&'a O`
    // Error should mention this.
    type InputItem<'a> = O;
}

fn main() {
    let pipeline_builder = DynamicPipelineBuilder::<usize>::new();
    let sink = PrintingSink::new();
    
    // ERROR points here
    let _dyn_pipeline = pipeline_builder.sink(sink);
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/main.rs:43:25
   |
43 |     let _dyn_pipeline = pipeline_builder.sink(sink);
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected reference `&_`
              found reference `&'a _`
note: the lifetime requirement is introduced here
  --> src/main.rs:16:65
   |
16 |     pub fn sink(self, _pipeline_sink: impl for<'a> PipelineSink<InputItem<'a> = &'a OutputItem>) {}
   |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

error[E0271]: type mismatch resolving `<PrintingSink<usize> as PipelineSink>::InputItem<'a> == &'a usize`
  --> src/main.rs:43:43
   |
43 |     let _dyn_pipeline = pipeline_builder.sink(sink);
   |                                          ^^^^ expected `&usize`, found `usize`
   |
note: the associated type is defined here
  --> src/main.rs:35:5
   |
35 |     type InputItem<'a> = O;
   |     ^^^^^^^^^^^^^^^^^^^^^^^ resolves to `usize`
   |
note: required by a bound in `DynamicPipelineBuilder::<OutputItem>::sink`
  --> src/main.rs:16:65
   |
16 |     pub fn sink(self, _pipeline_sink: impl for<'a> PipelineSink<InputItem<'a> = &'a OutputItem>) {}
   |                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound
   |
help: consider updating the associated type to match the expected reference type
   |
35 |     type InputItem<'a> = &'a O;
   |                          +++

Rationale and extra context

Suggesting adding a reference like that would be super cool if possible.
Don't know if the desired output is correct/achievable.
I just made something up that would have helped me.
At least mentioning type InputItem<'a> = O; in the error would already help a ton since that is where the issue arises.

Other cases

Rust Version

rustc 1.94.0 (4a4ef493e 2026-03-02)
binary: rustc
commit-hash: 4a4ef493e3a1488c6e321570238084b38948f6db
commit-date: 2026-03-02
host: x86_64-unknown-linux-gnu
release: 1.94.0
LLVM version: 21.1.8

Anything else?

No response

Reproduce the provided GAT and higher-ranked trait bound example in the linked Rust Playground and inspect how rustc constructs the E0308 diagnostic. Trace the diagnostic from the call site toward the mismatched associated type in the impl block. Done means the error identifies or points to type InputItem<'a> = O;, while retaining the relevant bound information.