explicit_generic_args_with_impl_trait

The tracking issue for this feature is: #83701

The explicit_generic_args_with_impl_trait feature gate lets you specify generic arguments even when impl Trait is used in argument position.

A simple example is:

#![feature(explicit_generic_args_with_impl_trait)]

fn foo<T: ?Sized>(_f: impl AsRef<T>) {}

fn main() {
    foo::<str>("".to_string());
}

This is currently rejected:

error[E0632]: cannot provide explicit generic arguments when `impl Trait` is used in argument position
 --> src/main.rs:6:11
  |
6 |     foo::<str>("".to_string());
  |           ^^^ explicit generic argument not allowed

However it would compile if explicit_generic_args_with_impl_trait is enabled.

Note that the synthetic type parameters from impl Trait are still implicit and you cannot explicitly specify these:

#![feature(explicit_generic_args_with_impl_trait)]

fn foo<T: ?Sized>(_f: impl AsRef<T>) {}
fn bar<T: ?Sized, F: AsRef<T>>(_f: F) {}

fn main() {
    bar::<str, _>("".to_string()); // Okay
    bar::<str, String>("".to_string()); // Okay

    foo::<str>("".to_string()); // Okay
    foo::<str, String>("".to_string()); // Error, you cannot specify `impl Trait` explicitly
}