Fix formatting of commented-out empty generic bounds

[Marsman1996]

Fix #6837

Now for code

trait SpinGuardian {}

struct RwLockUpgradeableGuard<'a, T, G: SpinGuardian>(&'a T, G);

impl<T: /*?Sized*/, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}

The format result is

trait SpinGuardian {}

struct RwLockUpgradeableGuard<'a, T, G: SpinGuardian>(&'a T, G);

impl<T /*?Sized*/, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}

[ytmimi]

Can you help me understand how updating the span fixes the issue?

[Marsman1996]

Before the fix

The T: /*?Sized*/, is parsed into T and : /*?Sized*/,.
For : /*?Sized*/,, extract_post_comment removes the : but keeps the , here:

rustfmt/src/lists.rs

Lines 635 to 636 in bdab101

	let post_snippet_trimmed = if post_snippet.starts_with(|c| c == ',' || c == ':') {
	post_snippet[1..].trim_matches(white_space)

Since the remaining is start with the comment, so write_list adds an extra , here:

rustfmt/src/lists.rs

Line 423 in bdab101

result.push_str(formatting.separator);

After the fix

The T: /*?Sized*/, is parsed into T: and /*?Sized*/,.
So extract_post_comment removes the original , here:

rustfmt/src/lists.rs

Lines 644 to 647 in bdab101

	else if post_snippet.ends_with(separator)
	&& (!post_snippet.trim().starts_with("//") || post_snippet.trim().contains('\n'))
	{
	post_snippet[..(post_snippet.len() - 1)].trim_matches(white_space)

[ytmimi]

Thanks for the explanation. It makes sense now. My initial thought is that it seems like a simple fix, but it also feels a little weird to extend the T -> T:. I want to think a bit more on the proposed solution to this one, but I'll get back to you.

[Marsman1996]

I do not think it is too weird, because for an ordinary bound like G: SpinGuardian, the GenericParam span G: SpinGuardian already includes the :.

And I also tried to fix by adding the , removal logic in extract_post_comment, but it will remove the /*?Sized*/ comment:

impl<T: /*Sized*/, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}

is formatted to

impl<T, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}

[Marsman1996]

Is that comment removal expected?
When I format

impl<T: /*Size*/ SpinGuardian, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}

rustfmt also removes the inline comment and produces

impl<T: SpinGuardian, G: SpinGuardian> RwLockUpgradeableGuard<'_, T, G> {}