Rebuild if deployment target changes

[madsmtm]

rustc reads the MACOSX_DEPLOYMENT_TARGET environment variable (and similar other *_DEPLOYMENT_TARGET variables), and uses that value to determine (probably amongst other things) certain options for LLVM and the linker.

However, it seems like this environment variable is not tracked as an external dependency, meaning that changing it does not cause rustc/cargo to rebuild the target, as is otherwise expected when changing environment variables that modify compilation.

E.g. running the following code:

cargo clean
MACOSX_DEPLOYMENT_TARGET=12.0 cargo build
MACOSX_DEPLOYMENT_TARGET=13.0 cargo build

I expected to see this happen: The project was built twice.

Instead, this happened: The project was built once, and then the cache was used the second time.

Meta

rustc --version --verbose:

rustc 1.76.0-nightly (2f8d81f9d 2023-11-21)
binary: rustc
commit-hash: 2f8d81f9dbac6b8df982199f69da04a4c8357227
commit-date: 2023-11-21
host: aarch64-apple-darwin
release: 1.76.0-nightly
LLVM version: 17.0.5

[madsmtm]

I know that a crate can emit cargo:rerun-if-env-changed=MACOSX_DEPLOYMENT_TARGET in their build.rs to opt in to this behaviour, but I'm not sure that's enough if there are other cached crates in the dependency graph that may have been built with a different deployment target?

[Noratrieb]

This also affects other apple targets with their *_DEPLOYMENT_TARGETs. Looks like the relevant code should somehow add the env var to sess.parse_sess.env_depinfo. I'm not sure what the nicest way to ensure that would be.
cc @BlackHoleFox

[madsmtm]

I'll add that this also affects the environment variable SDKROOT