What is "impl const" in Rust?

Question

I see this a lot in Rust code. Here are a few examples from the standard library:

impl<T> const Default for Option<T> {...}

impl const From<char> for u64 {...}

What is impl const?

Answer 1

If you run the code:

trait Example {}
impl const Example for u8 {}

You get an error message that points you in the right direction:

error[E0658]: const trait impls are experimental
 --> src/lib.rs:3:6
  |
3 | impl const Example for u8 {}
  |      ^^^^^
  |
  = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
  = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable

Issue 67792 is the tracking issue for RFC 2632 — Calling methods on generic parameters of const fns.

This is an unstable syntax that allows defining traits that can be used in a const context. It's paired with the equally unstable ~const syntax that can be used in trait bounds:

// 1.59.0-nightly (2021-12-20 23f69235ad2eb9b44ac1)
#![feature(const_fn_trait_bound)]
#![feature(const_trait_impl)]

trait Example {
    fn usage(&self) -> Self;
}

impl const Example for u8 {
    fn usage(&self) -> Self {
        42
    }
}

const fn demo<T: ~const Example>(v: &T) -> T {
    v.usage()
}

const VALUE: u8 = demo(&1);

fn main() {
    dbg!(VALUE);
}