rustfmt puts `where` on same line as `impl` for empty `impl` even if long

[joshtriplett]

Given the following code:

impl<T: ?Sized, Idx> IndexRanges<Idx> for T
where
    T: Index<Range<Idx>, Output = Self::Output>
        + Index<RangeTo<Idx>, Output = Self::Output>
        + Index<RangeFrom<Idx>, Output = Self::Output>
        + Index<RangeInclusive<Idx>, Output = Self::Output>
        + Index<RangeToInclusive<Idx>, Output = Self::Output>
        + Index<RangeFull>,
{
}

rustfmt moves the where onto the same line as the impl. However, if you put a fn func() {} inside, it won't.

This may need to wait for a style edition to fix.

[ytmimi]

I believe this is related #5990.

We could version gate the fix before we get style edition rolled out.