The rust plugin ignores environment variables

[jdx]

Discussed in #4022

^{Originally posted by hqnna January 9, 2025}
Unlike other plugins such as the Go plugin which respects environment variables, the Rust plugin for Mise seems to completely ignore if the CARGO_HOME and RUSTUP_HOME environment variables are set in the shell's environment, the code for this can be seen here.

fn rustup_home() -> PathBuf {
    SETTINGS
        .rust
        .rustup_home
        .clone()
        .unwrap_or(dirs::HOME.join(".rustup"))
}

fn cargo_home() -> PathBuf {
    SETTINGS
        .rust
        .cargo_home
        .clone()
        .unwrap_or(dirs::HOME.join(".cargo"))
}

While the ability to set these paths via the Mise configuration file is nice, having it blatantly ignore the environment seems like a poor choice. A good example of why this would be bad is environment configurations that respect the XDG standard, such as my own.

# XDG Standard Directories
export XDG_DATA_HOME="$HOME/.local/share"
export XDG_STATE_HOME="$HOME/.local/state"
export XDG_CONFIG_HOME="$HOME/.config"
export XDG_CACHE_HOME="$HOME/.cache"

# Software Directory Paths
export NPM_CONFIG_USERCONFIG="$XDG_CONFIG_HOME/npm/npmrc"
export NODE_REPL_HISTORY="$XDG_STATE_HOME/node_history"
export SQLITE_HISTORY="$XDG_STATE_HOME/sqlite_history"
export LESSHISTFILE="$XDG_STATE_HOME/less_history"
export MISE_CONFIG_DIR="$XDG_CONFIG_HOME/mise"
export CUDA_CACHE_PATH="$XDG_CACHE_HOME/nv"
export CARGO_HOME="$XDG_DATA_HOME/cargo"
export GNUPGHOME="$XDG_DATA_HOME/gnupg"
export GOPATH="$XDG_DATA_HOME/golang"

I'm proposing to by default detect the environment variables RUSTUP_HOME and CARGO_HOME from the shell environment, and if not set in the environment fallback to Mise's own configuration file allowing both to be possible and respect shell environment setups.