Positional arguments redo

[tertsdiepraam]

I'm not happy with positional arguments at the moment. They do not fit nicely with the rest of the library and have a weird API. We need to reassess how they should work.

The problems are:

1. The positional arguments do not really fit in an enum.
2. The distribution of positional arguments cannot be dependent on options (see for instance what cp requires).
3. The ranges are unintuitive and have no "obvious" default value.

[tertsdiepraam]

I'll post several comments exploring some designs.

The simplest alternative is to change the implementation a bit by removing the ranges and instead encoding the number of arguments in the specification:

#[derive(Arguments)]
enum Arg {
    #[arg("VAL")]      // 1
    Foo,
    #[arg("[VAL]")]    // 0 or 1
    Bar,
    #[arg("VAL...")]   // 1 or more
    Baz,
    #[arg("[VAL]...")] // 0 or more
    Qux,
}

This brings it more in line with the option arguments in how it is defined. However this only really solves the third problem.

[tertsdiepraam]

A second option is to take the positional arguments out of the enum and pass them as Vec to the Settings:

#[derive(Arguments)]
#[arguments(positional = "VAL1 [VAL2]...")]
enum Arg {}

struct Settings {
    a: PathBuf,
    b: Vec<PathBuf>,
}

impl Options<Arg> for Settings {
    fn apply(&mut self, arg: Arg) {
        ...
    }
    
    fn apply_positional(&mut sef, args: Vec<PathBuf>) {
        ...
    }
}

This "solves" all the problems, but is also verbose. It also makes error handling for positional arguments more difficult.

[tertsdiepraam]

A third option is to not try to really integrate the positional arguments at all:

#[derive(Arguments)]
enum Arg { ... }

#[derive(Default)]
struct Settings { ... }

impl Options<Arg> for Settings { ... }

fn main() {
    // `operands` is a `Vec<OsString>` with the positional arguments:
    let (settings, operands) = Settings::default().parse();
}

Instead of just having a Vec, it could also be a wrapper type around a Vec that could provide better error messages.