E0603 "note: the trait … is defined here" not showing any detail

[kpreid]

Given the following code:

type Foo = Box<dyn std::pin::Unpin>;

The current output is:

error[E0603]: trait `Unpin` is private
 --> src/lib.rs:1:30
  |
1 | type Foo = Box<dyn std::pin::Unpin>;
  |                              ^^^^^ private trait
  |
note: the trait `Unpin` is defined here

The note is useless and confusing since it doesn't say what “here” is. Better output would be any of:

• A suggestion to use the public path (std::marker::Unpin in this case) would be most helpful to anyone using an unfamiliar type (this could occur in third-party libraries as well as std).
• Include file and code snippet (this might depend on having rust-src installed in this case?)
• Hide the note entirely if nothing better can be said

Occurs on stable 1.63 and current nightly, in the Rust Playground.

Previous similar issue: #89159

[compiler-errors]

Probably just need to check the span of the note with rustc_span::DUMMY_SP (there's Span::is_dummy to do this)

[compiler-errors]

We also probably have the visible_parent_map and could use that to suggest the correct path here.... I think? At least for foreign traits that we have the cmeta for.

[jieyouxu]

This seems to have been fixed on stable (1.70.0):

error[E0603]: trait `Unpin` is private
   --> test.rs:1:30
    |
1   | type Foo = Box<dyn std::pin::Unpin>;
    |                              ^^^^^ private trait
    |
note: the trait `Unpin` is defined here
   --> /home/jieyouxu/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/pin.rs:387:28
    |
387 | use crate::marker::{Sized, Unpin};
    |                            ^^^^^

error: aborting due to previous error

rustc 1.70.0 (90c541806 2023-05-31)
binary: rustc
commit-hash: 90c541806f23a127002de5b4038be731ba1458ca
commit-date: 2023-05-31
host: x86_64-unknown-linux-gnu
release: 1.70.0
LLVM version: 16.0.2

[compiler-errors]

👍