`cfg!(test)` is always false in `build.rs`

[elldritch]

Problem

cfg!(test) is always false in build.rs. This makes it hard for me to write build script logic that should only run on test builds. I would have expected that cfg!(test) is true in build.rs when I run cargo test, because this is what the cfg! documentation here suggests.

Steps

1. Clone https://github.com/elldritch/cargo-cfg-test-release-repro
2. Run cargo run, cargo run --release, cargo test -- --nocapture, and cargo test --release -- --nocapture.
3. Observe that TEST_MODE=no is always outputted, even though cfg!(test) is true in the actual program itself.

Possible Solution(s)

My guess is that what's happening here is that the build script itself is not being built in "test mode", and therefore cfg!(test) is false in the build script. That... sort of makes sense? After all, we're not testing the build script. But if this is the intended behavior, then it surprised me, and I think it would be worth noting in the cfg.test documentation here. If this is intended behavior, I'm also not sure how I would write build script logic that should only run in test mode.

Notes

No response

Version

cargo 1.89.0 (c24e10642 2025-06-23)
release: 1.89.0
commit-hash: c24e1064277fe51ab72011e2612e556ac56addf7
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
libgit2: 1.9.0 (sys:0.20.2 vendored)
libcurl: 8.12.1-DEV (sys:0.4.80+curl-8.12.1 vendored ssl:OpenSSL/3.5.0)
ssl: OpenSSL 3.5.0 8 Apr 2025
os: Arch Linux Rolling Release [64-bit]

[epage]

Build scripts are only built and ran once for the entire package, generate content for any of the involved crates.

As the crates a build script might be manipulating could have a different build environment than the build script, cfgs will only apply for building of the build script itself. For the environment the build script is generating against, you need to read the cfgs through environment variables, see also build_rs::input.

test in particular is a crate-specific cfg: it is only set on test binaries and not on libraries other other crates and so it is not available as an environment variable as noted in the docs

[elldritch]

Got it! I somehow missed all of this documentation while writing the script. I just used cfg! because I saw our crate code using cfg!(test). Thanks! Happy to leave this open if you want to track any documentation changes, or close as fixed.

[elldritch]

Actually, I just took a look at the build script docs you linked. So is there a way for me to write a build script that behaves differently for test binaries? It looks like the answer is no.

[epage]

There is not. Finally found the canonical issue for this: #4789. #13294 tracked improving the documentation. Closing in favor of those. If there is a reason for us to re-open this, let us know.