Odd formatting of empty `where` clause

The following is rustfmt's formatting on 2025-02-17 4d91de4e48 (the version currently on the Rust Playground):

impl<'a, T, I> Ptr<'a, T, I>
where
    T: 'a + TransparentWrapper<I, UnsafeCellVariance = Covariant> + ?Sized,
    I: Invariants,
{
    pub(crate) fn transparent_wrapper_into_inner(
        self,
    ) -> Ptr<
        'a,
        T::Inner,
        (
            I::Aliasing,
            <T::AlignmentVariance as AlignmentVariance<I::Alignment>>::Applied,
            <T::ValidityVariance as ValidityVariance<I::Validity>>::Applied,
        ),
    >
where {
    }
}

The where clause should probably be indented by four spaces.

Also note that, when wrapped in a module, the where clause is still not indented at all:

mod foo {
    impl<'a, T, I> Ptr<'a, T, I>
    where
        T: 'a + TransparentWrapper<I, UnsafeCellVariance = Covariant> + ?Sized,
        I: Invariants,
    {
        pub(crate) fn transparent_wrapper_into_inner(
            self,
        ) -> Ptr<
            'a,
            T::Inner,
            (
                I::Aliasing,
                <T::AlignmentVariance as AlignmentVariance<I::Alignment>>::Applied,
                <T::ValidityVariance as ValidityVariance<I::Validity>>::Applied,
            ),
        >
where {
        }
    }
}

Start by reproducing the supplied Rust examples with the rustfmt version described in the issue, including the nested module case. Trace the formatting path for an empty where clause and add a regression test covering both examples. Done means the where clause is indented consistently with the surrounding impl, including inside a module.