Allow visible aliases for PossibleValue

[dbrgn]

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Clap Version

4.0.18

Describe your use case

Tealdeer has a parameter -p, --platform <PLATFORM> to select the platform. For historic reasons, this list must include "osx", but should also include the alias "macos". This alias should not be hidden, but should be visible.

With Clap 3, this was implemented using FromStr:

#[derive(Debug, Eq, PartialEq, Copy, Clone, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
#[allow(dead_code)]
pub enum PlatformType {
    Linux,
    OsX,
    SunOs,
    Windows,
    Android,
}

impl str::FromStr for PlatformType {
    type Err = anyhow::Error;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "linux" => Ok(Self::Linux),
            "osx" | "macos" => Ok(Self::OsX),
            "sunos" => Ok(Self::SunOs),
            "windows" => Ok(Self::Windows),
            "android" => Ok(Self::Android),
            other => Err(anyhow!(
                "Unknown OS: {}. Possible values: linux, macos, osx, sunos, windows, android",
                other
            )),
        }
    }
}

    /// Override the operating system
    #[clap(
        short = 'p',
        long = "platform",
        possible_values = ["linux", "macos", "windows", "sunos", "osx", "android"],
    )]
    pub platform: Option<PlatformType>,

With Clap 4, I tried to migrate to EnumValueParser:

impl clap::ValueEnum for PlatformType {
    fn value_variants<'a>() -> &'a [Self] {
        &[Self::Linux, Self::OsX, Self::Windows, Self::SunOs, Self::Android]
    }

    fn to_possible_value<'a>(&self) -> Option<clap::builder::PossibleValue> {
        match self {
            Self::Linux => Some(clap::builder::PossibleValue::new("linux")),
            Self::OsX => Some(clap::builder::PossibleValue::new("osx").alias("macos")),
            Self::Windows => Some(clap::builder::PossibleValue::new("windows")),
            Self::SunOs => Some(clap::builder::PossibleValue::new("sunos")),
            Self::Android => Some(clap::builder::PossibleValue::new("android")),
        }
    }
}

This works, but the alias is not visible in help output or error messages.

Describe the solution you'd like

Since Command has both .alias(...) and .visible_alias(...), would this be possible for PossibleValue as well?

Alternatives, if applicable

No response

Additional Context

No response

[dbrgn]

Note: This comment by @epage might be related / an alternative API to the suggested .visible_alias method:

Kind of like how clap switched from possible values being a &str to PossibleValue, I wonder if we should have an Alias<T> which has a hide method on it.

[epage]

I broke that comment out into #4421. I think I'd prefer not going that route for PossibleValue until we do it for everything as it'd be weird to have such a divergent API.