Improve diagnostics regarding how static_in_const doesn't work for associated constants

[clarfonthey]

Example code:

#![feature(associated_consts, static_in_const)]
struct Test;
impl Test {
    const CONST: &str = "Hello, world!";
}

Would default CONST to &'static str if this actually worked for associated constants. The original RFC decided to not do this, so, it makes sense to document the behaviour.

EDIT: the error message should be better, not the docs.

[petrochenkov]

This is intentional, but it seems like the RFC text wasn't updated to accommodate this detail.

[clarfonthey]

@petrochenkov I feel that if that's the case, then the error message should be improved and the RFC text should be updated.

[steveklabnik]

Doc team triage: tagging as p-low because this is for an unstable feature

[frewsxcv]

Would extending static_in_const to work with associated constants be a controversial move? Is there any reason it shouldn't be done other than not explicitly being spelled out in the RFC?

[clarfonthey]

I personally would be careful to only default to 'static if the impl doesn't have any lifetime parameters. But otherwise it'd make sense imho.

[clarfonthey]

Should this still be P-low considering how this feature got stabilised? @steveklabnik