Error message for accessing associated type on concrete type is misleading

[euclio]

I tried this code:

type Foo = u8::AssocTy;

trait Bar {
  type AssocTy;
}

impl Bar for u8 {
  type AssocTy = u8;
}

I expected to see this happen: Compiler emits an error reporting that I cannot access associated types from concrete types, and must use fully-qualified syntax to specify the desired trait.

Instead, this happened: Compiler emits an error reporting that AssocTy is ambiguous:

error[E0223]: ambiguous associated type
 --> src/main.rs:1:12
  |
1 | type Foo = u8::AssocTy;
  |            ^^^^^^^^^^^ help: use fully-qualified syntax: `<u8 as Trait>::AssocTy`

This made me wonder why it's ambiguous, since there's only one trait implemented for u8 that has that type name. The suggestion only added more confusion, because it only shows a placeholder trait name instead of listing the actual names of potential traits. I thought that maybe there was another trait implemented for u8 that used the same associated type name, until I realized that you must specify the trait name when accessing an associated type from a concrete type.

Note that this error message is also emitted for non-existent associated types, and even for types that are specified with invalid syntax. See more examples in this UI test.

Related to #59225.

Meta

rustc --version --verbose:

1.59.0-nightly

(2022-01-01 c145692254e86974941f)

[hadilq]

I have a similar problem that made me nut, because I need to wrap the errors to be able to track them back! This is my case if you ask:

use std::result::Result;
use std::result::Result::Err;

pub trait Service {
    type Error;

    fn run() -> Result<(), Self::Error>;
}

enum ErrorWrapper {
    ServiceError(Service::Error)
}

struct ServiceImpl;

struct ServiceImplError;

impl Service for ServiceImpl {
    type Error = ServiceImplError;

    fn run() -> Result<(), Self::Error> {
        Err(ServiceImplError)
    }
}

fn main() -> Result<(), ErrorWrapper> {
    ServiceImpl::run().map_err(|e| ErrorWrapper::ServiceError(e))
}

It raises

error[E0223]: ambiguous associated type
  --> src/main.rs:11:18
   |
11 |     ServiceError(Service::Error)
   |                  ^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<Type as Service>::Error`

I need the Service trait because I'm injecting the implementation later, but if you think there's a simple solution to this please let me know.
Anyway, I didn't touch the Rust compiler before, but if you think I can help to fix this, just let me know.

[tavianator]

@hadilq In your case I think you want

enum ErrorWrapper<T: Service> {
    ServiceError(T::Error)
}

[hadilq]

Thanks @tavianator, yes it solved mine! Now I think maybe I could ask it in StackOverflow!

[euclio]

@rustbot label A-diagnostics