clippy::map_flatten multiline suggestion is missing closing tokens

[CBenoit]

Summary

Suggestion is incorrect Rust code.

Reproducer

I tried this code:

Some(5)
    .map(|some_long_variable_name| {
        if some_long_variable_name <= 5 {
            Some(some_long_variable_name)
        } else {
            None
        }
    })
    .flatten();

I expected to see this happen:

warning: called `map(..).flatten()` on an `Option`
  --> picky-signtool/src/main.rs:14:12
   |
14 |       Some(5)
   |  ____________^
15 | |         .map(|some_long_variable_name| {
16 | |             if some_long_variable_name <= 5 {
17 | |                 Some(some_long_variable_name)
...  |
21 | |         })
22 | |         .flatten();
   | |__________________^
   |
   = note: `#[warn(clippy::map_flatten)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten
help: try using `and_then` instead
   |
14 ~     Some(5).and_then(|some_long_variable_name| {
15 +             if some_long_variable_name <= 5 {
16 +                 Some(some_long_variable_name)
17 +             } else {
18 +                 None
19 +             }
20 +     });
 ...

Instead, this happened:

warning: called `map(..).flatten()` on an `Option`
  --> picky-signtool/src/main.rs:14:12
   |
14 |       Some(5)
   |  ____________^
15 | |         .map(|some_long_variable_name| {
16 | |             if some_long_variable_name <= 5 {
17 | |                 Some(some_long_variable_name)
...  |
21 | |         })
22 | |         .flatten();
   | |__________________^
   |
   = note: `#[warn(clippy::map_flatten)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten
help: try using `and_then` instead
   |
14 ~     Some(5).and_then(|some_long_variable_name| {
15 +             if some_long_variable_name <= 5 {
16 +                 Some(some_long_variable_name)
17 +             } else {
18 +                 None
19 +             }
 ...

Version

rustc 1.59.0 (9d1b2106e 2022-02-23)
binary: rustc
commit-hash: 9d1b2106e23b1abd32fce1f17267604a5102f57a
commit-date: 2022-02-23
host: x86_64-unknown-linux-gnu
release: 1.59.0
LLVM version: 13.0.0

Additional Labels

No response

[J-ZhengLi]

This reminds me of issue #8239, I'll see what I can do. @rustbot claim

[J-ZhengLi]

@xFrednet Sorry for the pin, but I'm confused about how should I implement this. Since this seems to be a design flaw of rustc_errors::emitter, until someone has find better solution, should I try to hide the suggestion code while displaying it as part of help message using span_lint_and_then?

[xFrednet]

Hey @J-ZhengLi, no worries you're always welcome to ping me 🙃 This is a "design flaw" at one pint it might be good to add a new display option to rustc to say that the edges of the marked span are important. For now, we can check if the suggestion is larger than MAX_SUGGESTION_HIGHLIGHT_LINES and if so adjust the suggestion to only show the important snippets. In this case, you can create a suggestion like this:

warning: called `map(..).flatten()` on an `Option`
  --> picky-signtool/src/main.rs:14:12
   |
14 |       Some(5)
   |  ____________^
15 | |         .map(|some_long_variable_name| {
16 | |             if some_long_variable_name <= 5 {
17 | |                 Some(some_long_variable_name)
...  |
21 | |         })
22 | |         .flatten();
   | |__________________^
   |
   = note: `#[warn(clippy::map_flatten)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten
help: try using `and_then` instead
   |
14 ~     Some(5).and_then(|some_long_variable_name| {
15 +             if some_long_variable_name <= 5 {
16 +                 Some(some_long_variable_name)
   |
help: and remove the final `.flatten()`
   |
18 +                 None
19 +             }
20 ~         });
   |

You can also create a helper method that can be shared between lints if you want to be fancy. The suggested method of formatting will be a good solution in the meantime 🙃

Feel free to ping me if you want to implement either solution and want help or just a review 🙃