incompatible_msrv false positive: tests

[dtolnay]

Summary

I believe the warn-by-default incompatible_msrv lint should not apply to tests.

The "Why is this bad?" description of the lint says:

It would prevent the crate to be actually used with the specified MSRV.

However, test-only code does not prevent the crate from being used with the specified MSRV, as a library. Tests are not compiled when a crate is used as a dependency.

#![cfg_attr(test, allow(clippy::incompatible_msrv))] is possible as a workaround but I believe this should not be necessary: the default behavior of the warn-by-default lint should be that tests are not considered. If MSRV enforcement for tests is valuable to some users, that should be a separate restriction lint.

Lint Name

incompatible_msrv

Reproducer

# Cargo.toml

[package]
name = "repro"
edition = "2018"
rust-version = "1.31"

// src/lib.rs

#[test]
fn repro() {
    assert_ne!(f32::NAN.copysign(1.0), f32::NAN);
}

$ cargo clippy --tests

warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.35.0`
 --> src/lib.rs:5:25
  |
5 |     assert_ne!(f32::NAN.copysign(1.0), f32::NAN);
  |                         ^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
  = note: `#[warn(clippy::incompatible_msrv)]` on by default

Version

rustc 1.78.0-nightly (d44e3b95c 2024-02-09)
binary: rustc
commit-hash: d44e3b95cb9d410d89cb8ab3233906a33f43756a
commit-date: 2024-02-09
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 17.0.6

Additional Labels

No response

[ctz]

I think maybe the fix for this is incomplete: test code isn't uniquely characterised by a #[test] attribute on the immediate function. In our case we have false positives on this for a benchmark using bencher, but also I guess this will trigger on test helper functions inside a '#[cfg(test)] mod tests { }`?