Cargo config does not expand env vars for target-dir or CARGO_TARGET_DIR

[apiraino]

I'm not 100% sure of what I am looking at, so feel free to close.
Does cargo support expanding env vars when setting --target-dir or CARGO_TARGET_DIR?

I am using

$ cargo --version
cargo 1.87.0 (99624be96 2025-05-06)

and this is my ~/.cargo/config.toml

[build]
target-dir = "$HOME/.cargo/target"
# or 
# target-dir = "~/.cargo/target"

Both variants seems to be ignored, same if I use the CLI option --target-dir or the corresponding CARGO_TARGET_DIR env var.

I've added a small test to cargo to verify my assumption:

Details

#[cargo_test]
fn cargo_target_dir_env_var() {
    write_config_toml(
        "\
[build]
target-dir = \"$HOME/.cargo/target\"
",
    );

    let gctx = new_gctx();

    assert_eq!(
        gctx.target_dir().unwrap(),
        Some(Filesystem::new("/home/my-user/.cargo/target".into()))
    );
}

and it fails with this error:

assertion `left == right` failed
  left: Some(Filesystem { root: "/home/my-user/$HOME/.cargo/target/tmp/cit/t0/$HOME/.cargo/target" })
 right: Some(Filesystem { root: "/home/my-user/.cargo/target" })

Am I doing something wrong?

Thanks!

[epage]

Cargo does not support environment variable or ~ expansion.

In #14125, we're tracking the creation of a build-dir (all intermediate artifacts) and have some built-in variables that can be used, like {cargo-cache-home}. See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#build-dir

Would that fit your needs?

[weihanglo]

Variable expansions are also discussed here #10789

[apiraino]

ah great thanks for the superquick reply :) For some reason I didn't find the relevant issue

Closing this as duplicate. I'll subscribe to (and maybe also test) #14125