Fix closure migration suggestion when the body is a macro.

[m-ou-se]

Fixes #87955

Before:

warning: changes to closure capture in Rust 2021 will affect drop order
 --> src/main.rs:5:13
  |
5 |     let _ = || panic!(a.0);
  |             ^^^^^^^^^^---^
  |                       |
  |                       in Rust 2018, closure captures all of `a`, but in Rust 2021, it only captures `a.0`
6 | }
  | - in Rust 2018, `a` would be dropped here, but in Rust 2021, only `a.0` would be dropped here alongside the closure
  |

help: add a dummy let to cause `a` to be fully captured
  |
20~     ($msg:expr $(,)?) => ({ let _ = &a; 
21+         $crate::rt::begin_panic($msg)
22~     }),
  |

After:

warning: changes to closure capture in Rust 2021 will affect drop order
 --> src/main.rs:5:13
  |
5 |     let _ = || panic!(a.0);
  |             ^^^^^^^^^^---^
  |                       |
  |                       in Rust 2018, closure captures all of `a`, but in Rust 2021, it only captures `a.0`
6 | }
  | - in Rust 2018, `a` would be dropped here, but in Rust 2021, only `a.0` would be dropped here alongside the closure
  |
help: add a dummy let to cause `a` to be fully captured
  |
5 |     let _ = || { let _ = &a; panic!(a.0) };
  |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~

[rust-highfive]

r? @LeSeulArtichaut

(rust-highfive has picked a reviewer for you, use r? to override)

[LeSeulArtichaut]

r? @Aaron1011 do you want to take this?

[Aaron1011]

If we end with a dummy span, then the overall diagnostic will end up worse than it currently is. Can you restore closure_body_span to its original value if the loop ends up setting it to DUMMY_SP?

[m-ou-se]

I kept the dummy span for the suggestion part. Otherwise .span_to_snippet is going to return the wrong snippet and the suggestion will be wrong and confusing. The fallback span now uses the entire closure, which chas the highest chance of pointing to the place where modifications need to be made.

[Aaron1011]

Pre-existing: I think saying 'as part as the closure' would make more sense than 'alongside the closure'.

[m-ou-se]

#87984

[Aaron1011]

r=me, unless you want to change the note I mentioned in this PR as well.

[m-ou-se]

@bors r=Aaron1011

[bors]

📌 Commit 26c590d has been approved by Aaron1011

[bors]

⌛ Testing commit 26c590d with merge 2fc3c69...

[bors]

☀️ Test successful - checks-actions
Approved by: Aaron1011
Pushing 2fc3c69 to master...