Rustfmt incorrectly strips comments when removing empty where clause

[vallentin]

Empty where clauses are semantically valid Rust. However rustfmt incorrectly removes them and comments.

Before:

trait Foo {
    fn bar(&self)
    where
    //     Self: Bar
    // Some comment
    ;
}

fn foo<T>()
where
//     T: Bar,
// Some comment
{
    println!("foo");
}

After:

trait Foo {
    fn bar(&self);
}

fn foo<T>() {
    println!("foo");
}

---

rustfmt 1.4.25-stable (0f29ff6 2020-11-11)
I also tested it on master a97fd77

[ChinYing-Li]

Hi @calebcartwright is this a valid issue? I think I have an idea.

[calebcartwright]

Hi @calebcartwright is this a valid issue? I think I have an idea.

@ChinYing-Li - yes we should never drop comments. I don't want to take it to the other end of the spectrum just yet though (leaving an empty where clause in the absence of comments), but we can and should certainly handle the case of comments between the empty clause and the start of the block.

[ralpha]

This issue also occurs when the where clause is not empty.
For example:

pub trait MyTrait
where
    Self: Sized,
// some comment
{

}

Once this is saved it will have saved:

pub trait MyTrait
where
    Self: Sized,
{

}

[davidBar-On]

The original case with empty where is resolved by PR #5411.

The other reported case with non-empty where is a different issue and is probably the same issue as reported in #4672.