Support cargo term variables.

[Alexhuszagh]

Use the CARGO_TERM_VERBOSE, CARGO_TERM_QUIET, and CARGO_TERM_COLOR environment variables for cross terminal output. This enables us to detect term settings via environment variables, which are overridden by command-line flags, which is our own behavior.

Cargo has CLI flags override environment variables, and verbose and quiet cannot be set for the same level. So if CARGO_TERM_VERBOSE=true and --quiet are used, then we use the quiet setting, but if --verbose and --quiet are used, then we print a fatal error message.

Closes #919.

[Emilgardis]

lgtm!

the clap Option<String> should be converted to ColorChoice though, with default auto

[Alexhuszagh]

Should I change everything to an enumerated values? I had it a list of options with a default earlier, but removed it because of envvars, but I can restore it.

https://github.com/clap-rs/clap/blob/v3.1.18/examples/tutorial_derive/README.md#enumerated-values

[Emilgardis]

Was thinking this one: https://docs.rs/clap/3.2.8/clap/builder/struct.EnumValueParser.html or https://docs.rs/clap/3.2.8/clap/trait.ValueEnum.html

[Alexhuszagh]

Was thinking this one: https://docs.rs/clap/3.2.8/clap/builder/struct.EnumValueParser.html or https://docs.rs/clap/3.2.8/clap/trait.ValueEnum.html

That's the same I believe, just using the derive API.

[Alexhuszagh]

It creates really bad help messages when we change those, so I've just made sure the error messages for invalid colors are nicer.

Using Clap EnumParser

Using Custom Validation

I even tried the following, but it doesn't do enough:

    /// Coloring: auto, always, never
    #[clap(
        long,
        arg_enum,
        env = "CARGO_TERM_COLOR",
        hide_possible_values = true,
        hide_env_values = true,
        hide_default_value = true,
    )]
    pub color: ColorChoice,

I've made sure the output is similar to clap however, identical even on small terms.

Generic Clap

Cross Error

[Emilgardis]

bors r+

[bors]

Build succeeded:

• conclusion