Windows environment variables are case preserving (but case-insensitive)

[ChrisDenton]

I recently submitted a PR to stop Rust ASCII upper casing environment variables: rust-lang/rust#85270. This causes the following test to fail.

cargo/tests/testsuite/tool_paths.rs

Lines 360 to 373 in 3691752

	let target = rustc_host();
	let env_key = format!(
	"CARGO_TARGET_{}_LINKER",
	target.to_lowercase().replace('-', "_")
	);
	let mut execs = p.cargo("build -v --target");
	execs.arg(target).env(&env_key, "nonexistent-linker");
	if cfg!(windows) {
	// Windows env keys are case insensitive, so no warning, but it will
	// fail due to the missing linker.
	execs
	.with_stderr_does_not_contain("warning:[..]")
	.with_status(101);

I think it's because something relies on a new process upper casing key names? Which only happened before because Rust manually ASCII upper cased keys when starting a new process.

---

Hm, that seems to be the assumption here:

cargo/src/cargo/util/config/mod.rs

Lines 698 to 703 in 3691752

	fn check_environment_key_case_mismatch(&self, key: &ConfigKey) {
	if cfg!(windows) {
	// In the case of windows the check for case mismatch in keys can be skipped
	// as windows already converts its environment keys into the desired format.
	return;
	}

Whereas Windows itself does no such conversion.

---

cc @ehuss ? (only because you most recently touched the relevant test 🙂)

[ChrisDenton]

Here's a quick summary of the issue the linked PR seeks to address (using a few simple Rust programs you can try):

Details

listenv.rs:

fn main() {
    for (key, _) in std::env::vars() {
        if key != key.to_ascii_uppercase() {
            println!("{}", key);
        }
    }
}

spawn.rs:

fn main() -> std::io::Result<()> {
    std::process::Command::new(r".\listenv.exe").spawn()?.wait()?;
    Ok(())
}

withenv.rs

fn main() ->  std::io::Result<()> {
    std::process::Command::new(r".\listenv.exe").env("hello", "world").spawn()?.wait()?;
    Ok(())
}

Compile all these with rustc:

Z:\test> rustc listenv.rs
Z:\test> rustc spawn.rs
Z:\test> rustc withenv.rs

If you run .\listenv.exe you should see environment keys with lower case names (e.g. "windir", "Path", "ProgramFiles", etc). The same goes for .\spawn.exe. However .\withenv.exe invokes Rust's ASCII upper casing behaviour and therefore all keys are upper cased (or were, if my PR gets merged).

Note that in any case, std::env::var(key) will work case-insensitively.

[ehuss]

I posted #9646 to disable the test on Windows for now. Once that is merged, I'll update rust-lang/rust, and then re-approve your PR. Afterwards, I'll work on fixing Cargo to work with the preserved casing.

[ChrisDenton]

Thanks! I've added a comment to my PR with a (hopefully accurate) summary of the issue.