discard invalid spans in external blocks

[bvanjoi]

Fixes #116203

This PR has discarded the invalid const_span, thereby making the format more neat.

r? @Nilstrieb

[Noratrieb]

Sorry for the late review, I forgot about this PR.
I don't like this fix, because it papers over the problem of assuming that the const keyword and the name will be adjacent.
For example, with

macro_rules! hello {
    ($name:ident) => {
        const $name: () = ();
    };
}

extern "C" {
    hello! { yes }
}
fn main() {}

the span is valid and accessible, but still very bad:

error: extern items cannot be `const`
 --> uwu.rs:8:14
  |
3 | /         const $name: () = ();
4 | |     };
5 | | }
6 | |
7 | | extern "C" {
8 | |     hello! { yes }
  | |             -^^^
  | |_____________|
  |               help: try using a static value: `static`
  |

The suggestion suggests replacing all the code between the macro definition and macro invocation with static, which is not good at all :D.
We just want the span of the const keyword, and nothing more. Maybe there's a helper somewhere for this, I'm not sure.

[Dylan-DPC]

@bvanjoi any updates on this?

[bvanjoi]

Apologies for the late response.

I have utilized can_be_used_for_suggestions to filter out invalid spans, which I think could help to clean up the output.

[bvanjoi]

Another solution to this issue could involve setting const_span = span.with_hi(span.lo + len(kw::Const)). However, I'm uncertain which approach would be more suitable for this situation.

[bvanjoi]

ci is green.

@rustbot ready

[Noratrieb]

I will approve this if CI is happy with my style improvement. Thank you for the patience of waiting for 3 weeks while I kept forgetting that this PR existed. Next time, feel free to ping me after 2 weeks :)

[Noratrieb]

@bors r+ rollup

[bors]

📌 Commit 4138702 has been approved by Nilstrieb

It is now in the queue for this repository.