rustfmt fails to format braceless closure that starts with a comment

[matthiaskrgr]

This code from where I was doing exercism rust exercises

pub fn brackets_are_balanced(string: &str) -> bool {
    let mut ret = None;
    let mut stack = Vec::new();
    string.chars().for_each(|c|
        // remember opened brackets
        if ['[','{','('].contains(&c) {
            stack.push(c);
    } else if 
        // if we encounter closing brackets
            [']','}',')'].contains(&c) {
                let pair = (stack.pop(), c) ;
          
                match pair {
                    // make sure we have a corresponding bracket at the stack 
                    (Some('['), ']')|   (Some('{'), '}') |       (Some('('), ')')  =>  {},
                    // if this is not the case, return false
                _ => {ret = Some(false);},
                }
        }
   
   
    );
    if let Some(ret) = ret {
        ret
    } else {
        stack.is_empty()
    }
}

rustfmt really did not like that code:

 --> /home/matthias/exercism/rust/matching-brackets/src/lib.rs:8:8:14
  |
8 |     } else if 
  |              ^
  |

error[internal]: left behind trailing whitespace
  --> /home/matthias/exercism/rust/matching-brackets/src/lib.rs:12:12:1
   |
12 |           
   | ^^^^^^^^^^
   |

error[internal]: left behind trailing whitespace
  --> /home/matthias/exercism/rust/matching-brackets/src/lib.rs:20:20:1
   |
20 |    
   | ^^^
   |

error[internal]: left behind trailing whitespace
  --> /home/matthias/exercism/rust/matching-brackets/src/lib.rs:21:21:1
   |
21 |    
   | ^^^
   |

warning: rustfmt has failed to format. See previous 4 errors.

rustfmt 1.4.37-nightly (f58631b 2021-05-28)

[calebcartwright]

It's because of the // remember opened brackets comment that exists between the closure decl and the start of the body since the body isn't already block wrapped.

Definitely something we need to fix, but in the interim you can work around this by block wrapping the closure body manually:

e.g.

string.chars().for_each(|c| {
        // remember opened brackets
        if ['[', '{', '('].contains(&c)
.............

[matthiaskrgr]

Interesting, seems like

    String::from("abc").chars().for_each(|c| {
        // hello world
        if ['a'].contains(&c) {

formats but

    String::from("abc").chars().for_each(|c|
        // hello world
        { if ['a'].contains(&c) {

does indeed not.

fn main() {
    String::from("abc").chars().for_each(|c|
        // hello world
        { if ['a'].contains(&c) {
        } else {
        }
    });
}

[calebcartwright]

Correct. To elaborate, rustfmt's not expecting/prepared to handle comments placed between the closures's fndecl and the body expression. In the case of your original snippet it's pretty obvious to the human eye that the comment is associated with the body if expr, but the comment isn't contained with the span and is sitting between those two.

Block wrapping the if expr (which needs to happen from a formatting POV anyway) works because the comment is then shifted within the body expression which is handled. The second snippet doesn't because the same problem of comment placement still exists; the closure body's expression kind has been changed, but the comment is still sitting between the decl and body expr.