Formatting match arm with a comment between the expression and the comma causes invalid code

[nw-github]

Example code (playground link)

fn main() {
    match 100 {
        0 => println!("its zero")
            // eprintln!("its zero")
        ,
        _ => (),
    }
}

After format with rustfmt 1.5.2-stable (90c5418 2023-05-31):

fn main() {
    match 100 {
        0 => println!("its zero"),
        // eprintln!("its zero")
        ,
        _ => (),
    }
}

A comma is inserted after the println! call, causing this syntax error:

   Compiling playground v0.0.1 (/playground)
error: expected pattern, found `,`
 --> src/main.rs:5:9
  |
5 |         ,
  |         ^ expected pattern

error: could not compile `playground` (bin "playground") due to previous error

[ytmimi]

@nw-github thanks for reaching out!

Confirming that I'm able to reproduce the issue with rustfmt 1.5.1-stable (90743e72 2023-01-10)

This seems similar to #5425, but happening in a different context so it may be totally unique

[calebcartwright]

@fee1-dead - would you be up for working on this one when you have a chance?

[fee1-dead]

Yep. I will look into this when I have time

[fee1-dead]

This reliably triggers for all comma-separated lists:

input:

struct Struct {
    field1: String
    // 1
    ,
    // 2
    filed2: String
    // 3
    ,
    // 4
}

pub fn testing(
    a: i32
    // 5
    ,
    // 6
    b: u32
    // 7
    ,
    // 8
) {
    testing(
        1
        // 9
        ,
        // 10
        2
        // 11
        ,
        // 12
    )
}

Out:

struct Struct {
    field1: String, // 1
                    ,
                    // 2
    filed2: String, // 3
                    ,
                    // 4
}

pub fn testing(
    a: i32, // 5
            ,
            // 6
    b: u32, // 7
            ,
            // 8
) {
    testing(
        1, // 9
           ,
           // 10
        2, // 11
          ,
          // 12
    )
}

Block comments, however, only seem to trigger for the last item according to the comment in the linked issue.