Formatting an impl with a trailing comment is impossible

[dplassgit]

Describe the bug

If there is a comment between the end of the last function and the end of an impl, the impl cannot be formatted.

To Reproduce
Steps to reproduce the behavior:

1. Format

struct MyStruct{}

impl MyStruct {
    fn my_function() -> u32 { u32:3 }
    // Format
}

2. See error

Formatting failed: Error: INTERNAL: Formatting was skipped because a comment at (filename) would be deleted by the formatter: // Format
This is probably due to a bug (which may not have been reported yet). To complete formatting, try moving the comment to a different line.

Expected behavior
It formats it