New upstream version 1.62.1+dfsg1

[rustc.git] / src / doc / unstable-book / src / language-features / explicit-generic-args-with-impl-trait.md

# `explicit_generic_args_with_impl_trait`

The tracking issue for this feature is: [#83701]

[#83701]: https://github.com/rust-lang/rust/issues/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:

```rust
#![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:

```text
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:

```rust,compile_fail
#![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
}
```