Pass include_dirs to cargo using env

[jameshilliard]

Sometimes the cargo build script needs to know the include_dirs, lets pass this in the SETUPTOOLS_RUST_INCLUDE_DIRS env variable.

See #9132 for example usage.

[davidhewitt]

How do these get passed to C / C++ compilations? I wonder if we can expose them in the same way and then let downstream projects handle them in the same way as if they were a C/C++ compilation. This might lead to best automatic support.

(e.g. should we maybe set CFLAGS?)

[jameshilliard]

How do these get passed to C / C++ compilations?

I think they are just passed to the compiler command line.

I wonder if we can expose them in the same way and then let downstream projects handle them in the same way as if they were a C/C++ compilation.

I mean, that's more or less what I'm trying to do here. The cargo build script in my cryptography pull request passes the include_dirs from the env variable to the compiler using -I command line include arguments(by setting them using build.include(include_path)).

(e.g. should we maybe set CFLAGS?)

This seems like it could potentially cause conflicts if CFLAGS are being set in the env already.

[alex]

FWIW, we ended up solving this with pyca/cryptography#9129

[jameshilliard]

I'll close this for now then.