Allow a field's type to be different than the value parser type

[epage]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

3.2.8

Describe your use case

Some value parsers (Count, SetTrue, SetFalse) only support one specific data type but the user might want to support a different user type.

For example, the following parse(from_flag) test cannot be ported to value_parser

#[test]
fn non_bool_type_flag() {
    fn parse_from_flag(b: bool) -> std::sync::atomic::AtomicBool {
        std::sync::atomic::AtomicBool::new(b)
    }

    #[derive(Parser, Debug)]
    struct Opt {
        #[clap(short, long, parse(from_flag = parse_from_flag))]
        alice: std::sync::atomic::AtomicBool,
        #[clap(short, long, parse(from_flag))]
        bob: std::sync::atomic::AtomicBool,
    }

    let falsey = Opt::try_parse_from(&["test"]).unwrap();
    assert!(!falsey.alice.load(std::sync::atomic::Ordering::Relaxed));
    assert!(!falsey.bob.load(std::sync::atomic::Ordering::Relaxed));
}

(a SetConst might also work to a degree)

Others might want a different count type than u8.

Someone even brought up processing a Vec<String> into a String, see #3905 (reply in thread)

Describe the solution you'd like

The user should be able to

• Specify the ValueParser output type to use with get_one / get_many
  • Defaults to the field's type
• Optionally a conversion function from value parser type to field type
  • Defaults to TryInto (or Into?)

This will only be called when extracting the fields and will only be called once. We will not do a proper validation step for this.

Alternatives, if applicable

No response

Additional Context

No response

[epage]

What would we call these new attributes?

[chipsenkbeil]

If we use serde as an example, it has into, from, and try_from as attributes at the struct level for converting to a different type before serializing and deserializing. So my thought would be to use similar naming for the conversion function that you can place on an individual field. Heck, you could just have it use the corresponding trait implementations if you wanted something simpler than an arbitrary function.

In terms of output type, would just having #[clap(type = "...")] be too simple?

[epage]

Some value parsers (Count, SetTrue, SetFalse) only support one specific data type but the user might want to support a different user type.

As an update, we've loosened this restriction for SetTrue/SetFalse in the 4.0 release, see #4095 for the latest version of it