Add better diagnostic for const-generic `impl Foo for S<N>`

[JulianKnodt]

Adding better diagnostics when we resolve to a res::Err for N.

Original comment:
Ideally this would suggest "you meant to add a const N here and write this as A<{ N }>".

Originally posted by @estebank in #82055 (comment)

[estebank]

Given

struct A<const N: u8>;
impl Foo for A<N> {}

we should suggest

impl<const N: u8> Foo for A<{ N }> {}

[slightlyoutofphase]

It's worth noting that the impl part is specifically the error there... the rest is fine. I don't think the "const braces" are required anywhere at all currently (except maybe for particularly complex stuff, not quite sure). For example:

trait Foo<const N1: u8, const N2: u8, const N3: u8> {}

struct A<const N1: u8, const N2: u8, const N3: u8>;

type AA = A<1, 2, 3>;

// The compiler actively tells you that the braces on the next line are unnecessary, currently.
type AAA = A<{ 1 }, { 2 }, { 3 }>;

// The braces on the next line are equally unnecessary, but not warned against the way they are
// above for some reason.
impl<const N1: u8, const N2: u8, const N3: u8> Foo<N1, N2, N3> for A<{ N1 }, { N2 }, { N3 }> {}

[JulianKnodt]

they are required for anything more than plain ints such as { [0u8, 1u8] }, of { foo(N) }. For the general type I think it'd be nicer to suggest braces if we don't have any prior info.

[slightlyoutofphase]

I tried out a few different things later on yesterday, and it seems like the compiler does aleady suggest adding "const braces" in situations where they might actually help.

However, in the scenarios above, they wouldn't help, and as far as I can tell the compiler is fully aware of that since the only place they could be added is not a place that has anything to do with what's causing the syntax error in the first place.

Ultimately either way of displaying the new suggestion is fine though of course, as they're both equally valid code, so it doesn't really make a difference either way.

[estebank]

You're right. Single ident paths are accepted without braces and have always been meant to be so. suggesting just the addition of the const arguments to the impl block would be enough for this case.