Closure migration multispan suggestions

[m-ou-se]

This changes the rust_2021_incompatible_closure_captures lint to only suggest inserting the parts that need to be inserted, instead of suggesting to replace the entire closure by an almost identical closure with one statement added.

Before:

[...]
help: add a dummy let to cause `a` to be fully captured
   |
5  ~     let _ = || {
6  +         let _ = &a;
7  +         dbg!(a.0);
8  +         println!("1");
9  +         println!("2");
10 +         println!("3");
 ...
   |

[...]
help: add a dummy let to cause `b` to be fully captured
   |
14 |     let _ = || { let _ = &b; dbg!(b.0); };
   |                ~~~~~~~~~~~~~~~~~~~~~~~~~~

[...]
help: add a dummy let to cause `c` to be fully captured
   |
16 |     let _ = || { let _ = &c; dbg!(c.0) };
   |                ~~~~~~~~~~~~~~~~~~~~~~~~~

After:

[...]
help: add a dummy let to cause `a` to be fully captured
   |
5  ~     let _ = || {
6  +         let _ = &a;
   |

[...]
help: add a dummy let to cause `b` to be fully captured
   |
14 |     let _ = || { let _ = &b; dbg!(b.0); };
   |                  +++++++++++

[...]
help: add a dummy let to cause `c` to be fully captured
   |
16 |     let _ = || { let _ = &c; dbg!(c.0) };
   |                +++++++++++++           +

[rust-highfive]

r? @cjgillot

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[bors]

☔ The latest upstream changes (presumably #87980) made this pull request unmergeable. Please resolve the merge conflicts.

[m-ou-se]

r? @estebank

[estebank]

I feel it would be slightly easier to read if we had a single multpart_suggestion call and the branching logic got you the messages, applicability and necessary vector, but I'm not too bothered either way.

r=me with or without the suggested changes.

[m-ou-se]

I prefer calling it multiple times to avoid having to chase variables around, and to make it easier to make modifications to one of those cases later. E.g. using the _verbose or _short versions, or adding a note in a specific case, or adding a case in which there's no suggestion, etc.

@bors r=estebank

[bors]

📌 Commit 6431097 has been approved by estebank