Update trait static methods using Self to use UFCS once available in language

[oko]

Traits using Self to in static method parameters are called through the trait rather than the implementer. Type hinting currently requires addition of a throwaway typed parameter:

impl RData for A {
    ...
    fn from_rdata(rdata: &[u8], _ignored: Option<A>) -> Result<A, &'static str> {
        let rlen = rdata.len();
        ...
    }
}

Once UFCS is completed hopefully the call syntax will change.

Requires completion of rust-lang/rust#16293

[ghost]

It's possible get this to work without requiring a dummy method parameter:

trait Is<A> {}
impl<A> Is<A> for A {}

trait RData {
    fn from_rdata<X: Is<Self>>(_rdata: &[u8]) -> Result<Self, &'static str>
        // adding the bound on Self here is optional but allows the X
        // parameter to be omitted given an outer annotation; see the
        // `l` binding below in `main`
        where Self: Is<X>;
}

impl RData for u16
{
    fn from_rdata<X>(_rdata: &[u8]) -> Result<Self, &'static str> {
        Ok(0u16)
    }
}

impl RData for bool
{
    fn from_rdata<X>(_rdata: &[u8]) -> Result<Self, &'static str> {
        Ok(true)
    }
}

fn main() {
    let x: &[u8] = &[];
    let l: Result<u16, _> = RData::from_rdata(x);
    let r = RData::from_rdata::<bool>(x);
    println!("{} {}", l, r);
}

[oko]

That's an improvement. May go with this if there's still no movement on UFCS in the near future.

[ghost]

Yeah… also if equality constraints for where clauses land sometime soon it should allow this same trick without having to define the Is trait.