Cargo should not prepend `$HOME/.cargo/bin` to `PATH` for subcommands when it's already in `PATH`, to avoid unnecessary recompilations

[Boscop]

cargo is prepending $HOME/.cargo/bin/%HOME%\.cargo\bin to PATH, even if PATH already contains this dir.
Cargo should only prepend it if it's not already in PATH!
Currently this behavior causes unnecessarily recompilations when any dependency uses rerun-if-env-changed=PATH, such as libsqlite3-sys.
(I have to wait several minutes every time when switching between cargo check in my editor and having cargo watch -x run running in cmd.exe, because cargo passes an unnecessarily modified PATH to cargo-watch, even though my PATH already contains %HOME%\.cargo\bin.)

[jonhoo]

I've been doing some digging, and I can't actually find where this is added. From what I can tell, nothing in Cargo sets PATH. The closest I can find is

cargo/src/bin/cargo/main.rs

Lines 206 to 210 in 65c8266

	fn search_directories(config: &Config) -> Vec<PathBuf> {
	let mut dirs = vec![config.home().clone().into_path_unlocked().join("bin")];
	if let Some(val) = env::var_os("PATH") {
	dirs.extend(env::split_paths(&val));
	}

which does search $CARGO_HOME/bin first and then the rest of PATH, but it only ever seems to be iterated over and not actually used to set anything that would affect PATH.

Instead, I wonder if what's going on is that something in the call hierarchy is invoking sh or bash, which in turn sources ~/.profile, which tends to contain

export PATH="$HOME/.cargo/bin:$PATH"

or

source "$HOME/.cargo/env"

which in turn contains

#!/bin/sh
# rustup shell setup
# affix colons on either side of $PATH to simplify matching
case ":${PATH}:" in
  *:"$HOME/.cargo/bin":*)
    ;;
  *)
        # Prepending path in case a system-installed rustc needs to be overridden
        export PATH="$HOME/.cargo/bin:$PATH"
        ;;
esac

And that's what ends up adding to $PATH, not Cargo itself. In your linked example for instance, the subcommand has #!/usr/bin/sh, which will end up round-tripping through the configuration above.

[jonhoo]

Hmm, no, that doesn't seem right. Even when commenting out all those, and using a subcommand to print the env implemented purely in Rust, I still see PATH being prepended to 🤔

[jonhoo]

Aha! Rustup is the culprit, not Cargo:
https://github.com/rust-lang/rustup/blob/641f498159f7480716cfebfd1eec464770ece4b6/src/toolchain.rs#L459-L466

[jonhoo]

See also rust-lang/rustup#2922 and rust-lang/rustup#2848.

[jonhoo]

Filed a fix for this in rust-lang/rustup#2978.

[weihanglo]

Closed as rustup patch was released in 1.25.0. Thank you all involving in this issue.