Rebuild after modifying environment variables that the C compiler reads

[madsmtm]

Cargo has the ability to specify that a build script depends on specific environment variables using cargo:rerun-if-env-changed=NAME, but no such mechanism exist for C compilers (that I know of).

This means that when you update an environment variable that your C compiler reads, you have to run cargo clean before the change takes effect. It would be nice if cc could prevent this by outputting a list of known variables that the C compiler it is about to invoke may read.

A list of the major ones I've found so far in Clang/LLVM (see also their limited docs about it):

• SDKROOT on Apple targets
• VCINSTALLDIR / VCToolsInstallDir on Windows
• INCLUDE / CPATH / C_INCLUDE_PATH / C_PLUS_INCLUDE_PATH / OBJC_INCLUDE_PATH / OBJCPLUS_INCLUDE_PATH
• MACOSX_DEPLOYMENT_TARGET / IPHONEOS_DEPLOYMENT_TARGET / TVOS_DEPLOYMENT_TARGET / WATCHOS_DEPLOYMENT_TARGET on Apple targets

Note that some of these may need to be handled by rustc/cargo itself.

Clang/LLVM read a bunch more, but they're mostly for debugging, or for stuff like setting up the terminal width, so I don't think those are applicable.

Additionally, C compilers usually rely on a few fundamental ones like PATH, PWD and PATHEXT on Windows that I would honestly expect cargo to handle, though I get why it may be difficult to do right.