Redundant imports from std prelude in maybe-no_std crate

[mumbleskates]

Summary

This appears to be a regression; it does not happen in nightly-2024-02-18, but happens in nightly-2024-02-19 and later (the rustc -Vv version below)

Lint Name

unused_imports

Reproducer

I tried this code, which (in the real code) has std-dependent functionality gated behind a feature:

#![cfg_attr(not(feature = "std"), no_std)]

extern crate alloc;

use alloc::boxed::Box;

pub fn make() -> Box<usize> {
    Box::new(1)
}

As of the nightly-2024-04-19 toolchain, it now produces these errors when the std feature is enabled, complaining that the alloc imports are redundant:

$ cargo clippy
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
$ cargo clippy --all-features
warning: the item `Box` is imported redundantly
   --> src/lib.rs:5:5
    |
5   | use alloc::boxed::Box;
    |     ^^^^^^^^^^^^^^^^^
    |
   ::: /home/widders/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:125:13
    |
125 |     pub use super::v1::*;
    |             --------- the item `Box` is already defined here
    |
    = note: `#[warn(unused_imports)]` on by default

warning: `clippytest` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.08s

I expected to see this happen:

No warning with the feature either enabled or disabled :)

The imports it is complaining about are redundant, in theory... we could put each and every import for something that is missing in the no_std prelude but present in the std one behind a #[cfg(feature = "std")]. However: this feels bad, this looks bad, and until less than a week ago it was not requested by clippy. Therefore I suspect this is a regression.

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

Additional Labels

No response

[y21]

unused_imports is a rustc lint (implemented in the compiler, not in clippy), so there's not much we can do about it here. We just inherit rustc lints (including its bugs); you should also see these warnings with a regular cargo check.

That said, this looks like the same issue described in rust-lang/rust#121362 ?

[mumbleskates]

Thank you! You're very right, I think I was about to create an issue there and ended up making it in the wrong repo.

And thank you for finding that issue, that's the one! I wasn't having luck digging it up, but many hands make light work 🤝