Use of `Self::SOME_ASSOC_CONST` inside a function-local `const` calls `Self` a "generic parameter from outer function"

[thomcc]

Code

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=01d9dd114b740509a3538ce91aa2bf6a.

#[repr(u8)]
pub enum Foo { A, B, C }

impl Foo {
    pub const COUNT: usize = 3;
    pub fn blah() {
        const _: () = assert!(Self::COUNT < (u8::MAX as usize));
    }
}

A slightly more realistic (but also slightly larger) example is here: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=01d9dd114b740509a3538ce91aa2bf6a (this is more or less code that I tried to emit from a macro).

Current output

error[E0401]: can't use generic parameters from outer function
 --> src/lib.rs:7:31
  |
4 | impl Foo {
  | ---- `Self` type implicitly declared here, by this `impl`
...
7 |         const _: () = assert!(Self::COUNT < (u8::MAX as usize));
  |                               ^^^^^^^^^^^
  |                               |
  |                               use of generic parameter from outer function
  |                               use a type here instead

For more information about this error, try `rustc --explain E0401`.
error: could not compile `playground` due to previous error

Desired output

error[...]: can't use `Self` type inside function-local `const`.

Or something. I don't have strong feelings.

It should probably suggest replacing Self::COUNT with Foo::COUNT though, since that's enough to fix this.

Rationale and extra context

Self here is not a generic parameter from the outer function (it's the implementing type from the impl block, although I'm not really sure that's a particularly clear way to refer to it either). Either way, the current message is wrong and misleading.

Anything else?

Happens on nightly (rustc 1.70.0-nightly (8be3c2bda 2023-03-24)) and on stable (1.68.1).

[thomcc]

Alternatively, perhaps this should work. Self::SOME_ASSOC_CONST is usable from other associated consts at impl block level, for example. I don't have strong feelings, though.

[compiler-errors]

Alternatively, perhaps this should work. Self::SOME_ASSOC_CONST is usable from other associated consts at impl block level, for example. I don't have strong feelings, though.

That would require desugaring the const item into an associated const, which seems like a lot of complexity.

[thomcc]

Yeah, that seems like a lang decision anyway. And it's mostly irrelevant if the error message is improved (esp. to suggest a fix in the cases it's just "change Self to the type name").

[scottmcm]

I think it's expected that this is an error -- if the person wants to use Self, it needs to be const { … } instead of a const item.

So updating the message for extra clarity is the way to go.

[Ved-s]

got similar weird error today

struct Test {}

impl Test {
    pub fn static_fn() {}

    pub fn test() {
        fn nested_fn() {
            Self::static_fn();
        }
        nested_fn();
    }
}

error[E0401]: can't use generic parameters from outer function
   --> src/main.rs:985:13
    |
980 | impl Test {
    | ---- `Self` type implicitly declared here, by this `impl`
...
985 |             Self::static_fn();
    |             ^^^^^^^^^^^^^^^
    |             |
    |             use of generic parameter from outer function
    |             use a type here instead

I don't think Self is a generic parameter, should be like can't use Self from outer function