Cfg block containing only a comment duplicates the previous lines.

[ExPixel]

Describe the bug

Formatting an empty cfg block with a comment causes rustfmt to duplicate the cfg line.

pub fn main() {
    #[cfg(feature = "std")]
    {
        // Comment
    }
}

gets formatted into

pub fn main() {
    #[cfg(feature = "std")]
    {
        [cfg(feature = "std")]
    { // Comment
    }
}

Everything works as expected if there is actual code in the block or if the block is empty so the issue isn't too bad. Just thought it was worth mentioning.

To Reproduce
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=161085030374989f3243ad8eeb0328f5

Running rustfmt on the playground should display the weird behavior.

Expected behavior
I expect the output to just be

pub fn main() {
    #[cfg(feature = "std")]
    {
        // Comment
    }
}

Meta

• rustfmt version: rustfmt 1.4.20-stable (48f6c32 2020-08-09)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: rustfmt in vim

[calebcartwright]

Thanks for the report @ExPixel

This is due to an upstream rustc bug that's subsequently been resolved, previously reported and discussed on the thread linked below. Closing as a duplicate, will go away after our next update.
#4452 (comment)
#4452 (comment)