Comment before where removed (traits only)

[vmx]

There are already some issues about comments being removed by rustfmt, most notable #2781 and #3669.

I dare to open yet another issue as it's interesting that the removal happens for traits, but not for structs.

trait SomeTrait<T>
// This comment will be removed by rustfmt
where
   T: Copy,
{
}


struct SomeStruct<T>
// This comment won't be touched
where
    T: Copy,
{
    data: T,
}