Support default field values

[epage]

Default field values would provide an alternative to default_value_t

From the RFC (why is an RFC in 2024 using structopt?)

#[derive(Debug, StructOpt)]
#[structopt(name = "example", about = "An example of StructOpt usage.")]
struct Opt {
    /// Set speed
    #[structopt(short = "s", long = "speed", default_value_t = 42)]
    speed: f64,
    ...
}

default_value_t should have higher precedence

Open questions

• How do we handle this while its nightly only?
  • Put it behind a nightly feature?
• How do users opt-out of using the default field value?

[jhpratt]

why is an RFC in 2024 using structopt?

The RFC work started in 2018 😅

[jalil-salame]

I don't see a use for different default_value_t and a struct default's value, so I would fire a warning/deny by default lint ideally (I'd love a compiler_warning!() macro)