Allow less than all the generic parameters to be specified

[strega-nil]

fn foo<T, U>(u: U) -> T
  where T: From<U>
{
  From::from(u)
}

fn main() {
  println!("{}", foo(0u16): u16); // works
  println!("{}", foo::<u16>(0u16)); // doesn't work
  println!("{}", foo::<u16, u16>(0u16)); // works
}

I want the middle one to work. There's no real reason for it not to, and it does work in C++.

[abonander]

Being a language change, this belongs in https://github.com/rust-lang/rfcs.

Otherwise, you can also do this: foo::<u16, _>(0u16)

[leonardo-m]

Partial type application?

[strega-nil]

opened at rust-lang/rfcs#1989