Rustfmt duplicates commas in struct declarations

[xTibor]

Describe the bug

There's a weird interaction between comments and commas in struct declarations that leads to duplication of commas and breaking the syntax of the formatted code (error: expected identifier, found ',')

To Reproduce

Input:

struct Something {
    field: u32/*a*/,/*b*/
}

Formatting result:

struct Something {
    field: u32, /*a*/,/*b*/
}

Expected behavior

Do not duplicate commas.

Meta

• rustfmt version: 1.4.36-nightly (2021-02-07 7de6968)
• From where did you install rustfmt?: playground
• How do you run rustfmt: playground

[xTibor]

Just noticed there's a similar issue at #3876. My repro is in a different context than discussed there, however if the root cause is the same this issue can be closed as a dupe.

[whizsid]

@xTibor Yes it was a duplicate and #4520 will fix it.

[calebcartwright]

Thanks for the report @xTibor and @whizsid for pulling this in as a test case on #4520 👍

[calebcartwright]

Resolved via 74768b7