Cargo env vars aren't set

[alecmocatta]

Cargo's docs specify various env vars that are passed to binaries run with cargo run/test/bench. cargo miri does not currently pass them, so this fails under miri:

fn main() {
    println!("{}", std::env::var("CARGO_PKG_NAME").unwrap());
}

(Playground)

[bjorn3]

You need to pass -Zmiri-disable-isolation to miri. For example cargo miri run -- -Zmiri-disable-isolation. Otherwise things like env var access and filesystem access are disabled.

[alecmocatta]

Sorry, I thought I'd repro'd this with -Zmiri-disable-isolation but I was mistaken. Closing!

[RalfJung]

@alecmocatta Also you should be able to use env! I think, which is evaluated at build-time rather than at run-time. That's advisable anyway as it will mean that this works even if you distribute the binary and run it outside cargo. (For tests that seems unlikely, for the main binaries more likely.)

[alecmocatta]

Thanks @RalfJung! I bumped into this when I changed env::current_dir() to PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap()) to workaround #1514. env! could plausibly be outdated whereas env::var_os is in theory guaranteed to be correct. Regardless, looking forward to removing once #1540 lands!

[RalfJung]

Not sure what you mean by "outdated"? With env! the file should be rebuilt if the env var changes (that used to not happen but was fixed recently I think).

[alecmocatta]

e.g. on moving the manifest directory, but now I think about it you're probably right that would trigger a rebuild on the next cargo test invocation.

[alecmocatta]

Actually I've just confirmed that moving the manifest directory does not trigger a rebuild, thus env! and env::var can disagree. This seems dubious behaviour to me, I'll file an issue rust-lang/rust.

[alecmocatta]

https://github.com/rust-lang/rust/issues/76511