Can't `impl const From<T> for U` anymore

[nazar-pc]

I tried this code:

#![feature(const_trait_impl)]
struct S;

impl const From<()> for S {
    fn from(value: ()) -> Self {
        Self
    }
}

I expected to see this happen: Code must compile

Instead, this happened:

Code doesn't compile in latest Nightly rustc 1.71.0-nightly (f9a6b7158 2023-05-05), but worked fine (with corresponding features enabled) in rustc 1.70.0-nightly (0599b6b93 2023-04-01)

error: const `impl` for trait `From` which is not marked with `#[const_trait]`
 --> src/lib.rs:4:12
  |
4 | impl const From<()> for S {
  |            ^^^^^^^^
  |
  = note: marking a trait with `#[const_trait]` ensures all default method bodies are `const`
  = note: adding a non-const method body in the future would be a breaking change

Error message is also confusing to me because From does have #[const_trait] attribute and various types in standard library have const implementation, like impl const From<u8> for usize.

Does this mean it is no longer possible to have const implementations in user code?

Meta

rustc --version --verbose:

rustc 1.71.0-nightly (f9a6b7158 2023-05-05)

[nazar-pc]

Hm... looks like it is gone from latest nightly. I was using it with const generics alongside with Ord to determine size of arrays in a generic way.

The reason I prefer usize::from(x) over x as usize is that it'll fail to compile when there is no compile-time guarantee that it can be converted without data loss.

[CraftSpider]

I believe the types / const team is ripping out the current implementation of const traits entirely, pending a rework of implementation and possibly syntax. So this isn't a bug, simply a side effect of using an (I believe incomplete, so particularly likely to change) nightly feature.

[nazar-pc]

Yes, it was an incomplete nightly feature indeed. It is a bit unfortunate though that it is going away. Do you happen to have a link about this discussion (not the generic tracking issue, there is not many updates there)?

[SkiFire13]

#110393 is the PR that removed them.

[Noratrieb]

See the tracking issue for work on const traits. Closing it as it's not a bug.