incompatible_msrv false negative: tests

[bryceberger]

Summary

I disagree with the resolution of #12257, and think that incompatible_msrv should apply to tests. Given the conflicting desires, I think it would make sense to provide a config option for this lint.

I would like something akin to the following:

# clippy.toml
check-incompatible-msrv-in-tests = true

Background: jj currently has an MSRV of 1.76. However, it's encouraged to use newer / nightly compilers during development, to take advantage of speed improvements / parallel frontend / etc. As such, they do not force a version using rust-toolchain.toml. A PRs that added a rust-toolchain.toml pin was closed under the assumption that incompatible_msrv would help with this (jj-vcs/jj#1913 (comment)).

#12257 makes the argument:

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.

However, tests are compiled when the tests are ran in CI. We would like to keep the CI pinned to MSRV, and do not want to have separate Build (MSRV) and Test (latest stable) CI jobs.

Lint Name

incompatible_msrv

Reproducer

# Cargo.toml
[package]
name = "repro"
edition = "2021"
rust-version = "1.76"

// src/lib.rs
#[test]
fn repro() {
    assert!(std::fs::exists("/").unwrap());
}

$ cargo clippy --tests
# no output...

Version

rustc 1.86.0-nightly (854f22563 2025-01-31)
binary: rustc
commit-hash: 854f22563c8daf92709fae18ee6aed52953835cd
commit-date: 2025-01-31
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7