rustfmt removes import if it only contains comments

[ytmimi]

Formatting deletes the import if all of the items are commented out.

Reproducible using rustfmt 1.6.0-nightly (16db2a4b 2023-08-18) and rustfmt 1.5.3-nightly (f4b80cac 2023-06-30). Likely other versions as well.

input

use std::fs::{
    // File
};

output

expect

use std::fs::{
    // File
};

Originally posted by @okaneco in #5852 (comment)

[ytmimi]

@rsammelson any interest in helping out with this one since you also worked on #5853?

[ytmimi]

I also noticed that comments aren't necessary to trigger this.

input

use a::b::c::{};

output

[rsammelson]

With the comment I would agree that this is definitely a bug, but without the comment isn't this desirable (just like removing the braces when they are unused; once the braces are gone it would be an invalid statement so it's just removed)?

[ytmimi]

I found this comment that suggests that the case without any comments is intentional:

rustfmt/src/imports.rs

Lines 537 to 543 in b636723

	// Remove foo::{} or self without attributes.
	match last.kind {
	_ if self.attrs.is_some() => (),
	UseSegmentKind::List(ref list) if list.is_empty() => {
	self.path = vec![];
	return self;
	}

[okaneco]

I found this comment that suggests that the case without any comments is intentional:

That's how I found the originally posted issue, I was trying to work around that behavior which I do not find desirable and would like if there was an option to disable.

I would prefer that an unused import is preserved rather than deleted. This crops up for me while refactoring and prototyping with format-on-save, which results in having to retype it or go back in the undo-stack to find the deleted import (or disable formatting temporarily which is also inconvenient).

[ytmimi]

@okaneco thanks for providing that additional context!