Redundant import leaks prelude import

[eopb]

Code

use std::convert::TryFrom;

pub trait Foo<T>: TryFrom<T> {}

Current output

Compiling playground v0.0.1 (/playground)
warning: the item `TryFrom` is imported redundantly
   --> src/lib.rs:1:5
    |
1   | use std::convert::TryFrom;
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
   ::: /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:129:13
    |
129 |     pub use core::prelude::rust_2021::*;
    |             ------------------------ the item `TryFrom` is already defined here
    |
    = note: `#[warn(unused_imports)]` on by default

Desired output

Compiling playground v0.0.1 (/playground)
warning: the item `TryFrom` is imported redundantly
   --> src/lib.rs:1:5
    |
1   | use std::convert::TryFrom;
    |     ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `TryFrom` is imported in the 2021 edition prelude
    = note: `#[warn(unused_imports)]` on by default

Rationale and extra context

The pub use core::prelude::rust_2021::*; import is a compiler detail. The user did not write that line so it's confusing to display it to them.

Other cases

No response

Rust Version

rustc 1.78.0-nightly (2bf78d12d 2024-02-18)
binary: rustc
commit-hash: 2bf78d12d33ae02d10010309a0d85dd04e7cff72
commit-date: 2024-02-18
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

Anything else?

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=cafa3c0983d012f7430097f0fbd32e4f

[petrochenkov]

This diagnostic is not specific to the redundant import lint, it's shown every time we refer to a declaration site of any prelude name.

Those declarations are not compiler details, they are public APIs - https://doc.rust-lang.org/stable/std/prelude/rust_2021/index.html - so such diagnostics are at least acceptable.

[Kixunil]

@petrochenkov that's fine but as opposed to the OP I have a different output (with -D warnings):

error: the item `Box` is imported redundantly
 --> io/src/error.rs:2:5
  |
2 | use alloc::boxed::Box;
  |     ^^^^^^^^^^^^^^^^^
 --> /rustc/2bf78d12d33ae02d10010309a0d85dd04e7cff72/library/std/src/prelude/mod.rs:125:13
  |
  = note: the item `Box` is already defined here
  |
  = note: `-D unused-imports` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(unused_imports)]`

So I was confused why our CI started breaking out of blue and since I didn't see where the other location is it looked like a bug to me.

[petrochenkov]

@Kixunil
I cannot reproduce the case with "Box is already defined here" pointing to nowhere.
It looks like it's not specific to this lint, and caused by standard library source code not being available for some reason.

[Kixunil]

@petrochenkov ah, OK, IIRC this was in CI using dtolnay's GH action. I guess it doesn't install std source for efficiency.