cargo test does not export CARGO_CFG_TEST for build.rs scripts

[nagisa]

e here is fairly simple – I need to build a shared library to be used for tests, however it is also important to me that this library is not built when people are compiling the library for regular use.

rustc --test --print=cfg does output the test configuration variable, however when cargo test is run, the build script does not receive this variable in form of CARGO_CFG_TEST.

Caveat is that rerunning the build script for both test and non-test would potentially re-run expensive build.rs operations by default, however that seems easily remedied by requiring people to print out a cargo:rerun-if-env-changed=CARGO_CFG_TEST (it would become the first exception to the list of environment variables set by cargo and tracked by default).

[alexcrichton]

This is currently intentional behavior because build scripts don't have a "I'm running tests" mode. That would require a test build script most likely and it's an unimplemented feature so far.

[stale]

As there hasn't been any activity here in over 6 months I've marked this as stale and if no further activity happens for 7 days I will close it.

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[stale]

As I didn't see any updates in 30 days I'm going to close this. Please see the previous comment for more information!

[nvzqz]

@alexcrichton why would a separate test build script be needed and not just allow for #[cfg(test)] to work?

[Supreeeme]

I'm not sure how this has changed over the last few years, but I don't see why build scripts would need a specific "tests mode". My use case is I have a C++ dependency that I build and link in my application, but I fake it for unit tests so there's no point in building and linking it.

A workaround for this for anyone interested is using the self depends trick to enable a feature in dev-dependencies:

[package]
name = "my-crate"

[features]
unittest = []

[dev-dependencies]
my-crate = { path = ".", features = ["unittest"] }

You can now use #[cfg(feature = "unittest")] in the build script where you would use #[cfg(test)].

[michaelmior]

What if my crate is doing code generation and I want to test the behavior of the generated code? It seems a reasonably logical way to do this is to generate and compile the code in the build script, but this is only needed for tests.