CARGO_CFG_TARGET_FAMILY is not defined for build-script in no_std

[lukaslueg]

Problem

There is an issue reported downstream where the CARGO_CFG_TARGET_FAMILY environment variable is not defined when executing a build-script for a no_std binary. The said environment variable seems to be indeed not defined at all. AFAICS this is contrary to what is documented about the environment variables set by cargo, e.g. here or here.

Steps

• Set main.rs to be [no_std]
• Add a build.rs
• Try to observe CARGO_CFG_TARGET_FAMILY in build.rs while building the no_std-binary.
• While most variables are defined as documented, CARGO_CFG_TARGET_FAMILY is not present in the build-script's environment.

Possible Solution(s)

The way I read the documentation, the listed environment variables (including CARGO_CFG_TARGET_FAMILY, even in a no_std setting) should always be defined. If they have no value, possibly because they have no meaning in the given setting, they could be set to the empty string / NULL. The exact behavior of subprocesses launched by a build-script (if any) seems to vary with respect to empty environment variables, though.

If the full set of environment variables mentioned in the documentation can't be made present in all settings, this should be documented.

Notes

Notice that CARGO_CFG_TARGET_OS: none is defined in the downstream bug report, while CARGO_CFG_TARGET_FAMILY is not.

Version

cargo 1.72.0 (103a7ff2e 2023-08-15)
release: 1.72.0
commit-hash: 103a7ff2ee7678d34f34d778614c5eb2525ae9de
commit-date: 2023-08-15
host: x86_64-apple-darwin
libgit2: 1.6.4 (sys:0.17.2 vendored)
libcurl: 8.1.2 (sys:0.4.63+curl-8.1.2 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 1.1.1u  30 May 2023
os: Mac OS 13.5.2 [64-bit]

[epage]

Looking at the code, it seems we are generically processing whatever rustc gives us with the only special case being debug_assertions.

[cavivie]

I had a similar issue happening with the cross-rs i686_pc_windows_msvc target, where cross was not passing CARGO_CFG_TARGET_FAMILY to the docker runner container launched by cross-rs docker. plz note that cross-rs uses xargo to build the target i686_pc_windows_msvc.

[ehuss]

I'm marking this as a documentation issue. Some targets just don't have families, in which case it won't be set. The documentation at /~https://github.com/rust-lang/cargo/blob/master/src/doc/src/reference/environment-variables.md#environment-variables-cargo-sets-for-build-scripts could be a little more clear to explicitly mention that some variables won't be set if the target does not have the corresponding setting.