Add a rule that enum discriminants may not use generic parameters

[ehuss]

This is implemented in the compiler with the diagnostic ParamInEnumDiscriminant and NoConstantGenericsReason::IsEnumDiscriminant.

[DanielEScherzer]

from the enclosing enum.

What about from anywhere else? Playground says that you also can't use from an enclosing item either

fn demo<const N: u32>() {
    enum Foo {
        Bar = N,
    }
}

though it does also have an unhelpful suggestion to

help: try introducing a local generic parameter here
  |
2 |     enum Foo<N> {
  |             +++

which then leads to

• same "can't use generic parameters from outer item" error
• with a new suggestion

help: try introducing a local generic parameter here
  |
2 |     enum Foo<N, N> {
  |              ++

• and also an error type parameter `N` is never used

---

I suggest

Suggested change

	Explicit enum discriminants may not use generic parameters from the enclosing enum.
	Explicit enum discriminants may not use generic parameters.

[ehuss]

The restriction that generic parameters are not in scope within items in a function body is covered by items.generics.syntax.scope.

The reason it uses the "enclosing enum" terminology is because generic parameters defined within the enum discriminant initializer may be used, as in:

enum E {
    V1 = {
        const fn f<T>(x: T) -> T { x }
        f::<isize>(123)
    }
}

I didn't want to risk potential confusion around that.

[DanielEScherzer]

But the actual discriminant of f::<isize>(123) is not based on any generic parameter, just the invocation of a constant function that happens to be generic, same as

const fn f<T>(x: T) -> T { x }
enum E {
    V1 = {
        f::<isize>(123)
    }
}

what about

Explicit enum discriminants may not use generic parameters. Note that they may still invoke generic functions.

or something like that?