impl Trait in argument position  #44721

(Part of #34511)

As part of the latest impl Trait RFC, we decided to accept impl Trait in argument position:

fn foo(x: impl Iterator<Item = u32>) {
    ...
}

this is roughly equivalent to fn foo<T: Iterator<Item = u32>>(t: T), except that explicitly specifying the type is not permitted (i.e., foo::<u32> is an error).

Mentoring instructions can be found here.

Questions to resolve:

• Should we permit specifying types if some parameters are implicit and some are explicit? e.g., fn foo<T>(x: impl Iterator<Item = T>>)? I think yes, foo::<u32> would be accepted (thus binding T = u32 explicitly).
  • Answer: for now, just forbid explicit bindings altogether in the presence of impl Trait arguments.