1.97 beta regression: Rust claims import is unused but crate doesn't compile with it removed

I don't have a minimal reproducer, but I do have a reproducer.

Check out polars at commit 133a52633ba7b0dc08c70a2662abf5d7547df06f.
Run make build. Rust will give the following warning (among others):

warning: unused import: `crate::prelude`
 --> crates/polars-core/src/chunked_array/ops/sort/options.rs:5:5
  |
5 | use crate::prelude::*;
  |     ^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

However, if we remove this line, the project will fail to compile.

Meta

rustc --version --verbose:

rustc 1.98.0-nightly (d595fce01 2026-06-02)

I have a more minimal reproduction that only touches the polars-core crate without having to do the entire make build setup. It goes as follows (still on pola-rs/polars@133a526):

cargo check -p polars-core 2>&1 | rg -i 'crate::prelude::\*' -C 4

This will output:

warning: unused import: `crate::prelude`
 --> crates/polars-core/src/chunked_array/ops/sort/options.rs:5:5
  |
5 | use crate::prelude::*;
  |     ^^^^^^^^^^^^^^

(There are a lot more warnings due to polars-core not being meant as a stand-alone crate, we don't bother getting rid of other unused warnings that aren't unused in the larger project / with full features.)

However, removing that line will cause cargo check -p polars-core to fail to compile, so it's a false-positive warning.

@petrochenkov

I've added #[deny(unused_imports)] to crates/polars-core/src/chunked_array/ops/sort/options.rs:5:5. This let me use cargo bisect-rustc --start=2026-04-01 --end=2026-06-03 -- check -p polars-core:

searched toolchains nightly-2026-04-01 through nightly-2026-06-03

********************************************************************************
Regression in nightly-2026-05-01
********************************************************************************

searched toolchains c935696dd07ca51e6fba2f6579919eea2a50863b through f53b654a8882fd5fc036c4ca7a4ff41ce32497a6

********************************************************************************
Regression in a021a7796f66600f46013d6c8d1dfc9e8d7f4a92
********************************************************************************

It seems a021a77 is the issue.

From that roll-up commit I think it's #154149.

@petrochenkov Assuming the above bisect blame isn't wrong, this would be the second regression introduced to Polars on nightly in that chain of commits. That PR is a revert of #152498 which also broke Polars (#152004). Perhaps it would be possible to try to build Polars with a potential new fix PR and see if it introduces new spurious warnings?

I've also confirmed now that cargo +beta check -p polars-core fails with #[deny(unused_imports)] added before crates/polars-core/src/chunked_array/ops/sort/options.rs:5:5 on rustc 1.97.0-beta.2 (0417c2586 2026-05-30), so I've updated the title.

@petrochenkov

cc @petrochenkov @mu001999 (author and reviewer of #154149)

I've assigned myself and will look at this in the next few days, but having a minimized reproducer until then would still be nice.

I'm still trying to figure out what's going on.

What is clear:

the crate::prelude::* import gets into maybe_unused_trait_imports, that's why it's reported as unused after type checking (compiler\rustc_hir_analysis\src\check_unused.rs), the imported trait is indeed not used
import gets into maybe_unused_trait_imports because /// [A::f] forces collection of traits in scope (and imports to them), and their addition to the potentially unused trait import set

What is not clear:

why the crate::prelude::* import is not removed from the maybe_unused_trait_imports set, which is typically happens with imports that are used early, before type checking
- basically early unused import checking considers the import neither unused (otherwise it'd report it), nor used (otherwise it'd remove it from maybe_unused_trait_imports)

Fixed in #157713.

Rollup merge of rust-lang#157713 - petrochenkov:regr197unused, r=oli-obk

Unrolled build for #157713

Rollup merge of #157713 - petrochenkov:regr197unused, r=oli-obk

Navigation Menu

Repository navigation

1.97 beta regression: Rust claims import is unused but crate doesn't compile with it removed #157420

Meta

9 remaining items

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

1.97 beta regression: Rust claims import is unused but crate doesn't compile with it removed #157420

Description

Meta

Activity

orlp commented on Jun 4, 2026

orlp commented on Jun 4, 2026

orlp commented on Jun 4, 2026

theemathas commented on Jun 7, 2026

petrochenkov commented on Jun 7, 2026

9 remaining items

petrochenkov commented on Jun 10, 2026

petrochenkov commented on Jun 10, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions