Cargo doesn't respect `--color` or `CARGO_TERM_COLOR` for help text and errors from clap

[mchernyavsky]

Problem

^title

I expect that with --color=always option (or CARGO_TERM_COLOR=always environment variable) the output will contain expected ANSI escape codes.

Steps

1. Run cargo build --color=always --unknown &> out.txt
2. Theout.txt will contain the following output (without ANSI escape codes):

error: Found argument '--unknown' which wasn't expected, or isn't valid in this context

USAGE:
    cargo build --color <WHEN>

For more information try --help

Notes

Output of cargo version:

cargo 1.50.0-nightly (a3c2627fb 2020-12-14)

[epage]

The main problem here is the circular nature of this.

• To read --color=always, we need to parse the CLI but the error is coming from our CLI parser
• To read CARGO_TERM_COLOR, we need the Config configured which requires the CLI to be parsed.

We can't workaround it for --color=always but we can for CARGO_TERM_COLOR by directly looking up the value and passing that to clap. We'd probably want to extract the value parser out of set_color_choice into a std::str::FromStr impl on ColorChoice.

This would make it so fn cli() would have something like

    let color_choice = std::env::var_os("CARGO_TERM_COLOR");
    let color_choice = color_choice.and_then(|c| c.parse::<ColorChoice>().ok());
    let color_choice = match color_choice {
        Some(ColorChoice::CargoAuto) | None => clap::ColorChoice::Auto,
        Some(ColorChoice::Always) => clap::ColorChoice::Always,
        Some(ColorChoice::Never) => clap::ColorChoice::Never,
    };

This intentionally ignores the errors since it isn't worth reporting at this stage.

The main question is if this is worth it or not.