Suggest `Self::CONST` when trying to use an associated constant lexically

[kpreid]

Code

struct Foo;
// const A_CONST: usize = 1;
impl Foo {
    const A_CONST: usize = 1;
    fn foo() -> usize {
        // const A_CONST: usize = 1;
        A_CONST
    }
}

Current output

error[E0425]: cannot find value `A_CONST` in this scope
 --> src/lib.rs:7:9
  |
7 |         A_CONST
  |         ^^^^^^^ not found in this scope

Desired output

help: try referring to the associated constant: `Self::A_CONST`

Rationale and extra context

Associated constants work entirely differently from constants at a higher scope (module) or lower (function) which are looked up lexically — if either of the other two declarations in the sample code are uncommented, compilation succeeds. This may be surprising to beginners (example that inspired this post), and the “what they meant” is fairly obvious, so this should be a good opportunity to help in the error message.

Additionally, if the suggestion is machine-applicable then it can be used to help refactor code when a constant was intentionally converted into an associated constant.

Other cases

No response

Anything else?

No response

[kpreid]

Further precedent: the compiler already provides this help for calling associated functions.

struct Foo;
impl Foo {
    fn bar() {}
    fn foo() {
        bar();
    }
}

error[E0425]: cannot find function `bar` in this scope
 --> src/lib.rs:5:9
  |
5 |         bar();
  |         ^^^ not found in this scope
  |
help: consider using the associated function
  |
5 |         Self::bar();
  |         ~~~~~~~~~

[euclio]

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