Rustfmt deletes comments in first position of trait bound

Example code:

    pub fn try_new_with_date_length_unstable<P>(
        provider: &P,
        locale: &DataLocale,
        length: length::Date,
    ) -> Result<Self, Error>
    where
        P: // Explanatory comment about the bounds
            ?Sized
            + DataProvider<BuddhistDatePatternV1Marker>
            + DataProvider<BuddhistYearNamesV1Marker>
            + DataProvider<BuddhistMonthNamesV1Marker>
    { ... }

Running this through rustfmt deletes the // Explanatory comment about the bounds.

Workaround: move the ?Sized into the first position. If your bound is not ?Sized, you can use Sized. The comment is retained:

    pub fn try_new_with_date_length_unstable<P>(
        provider: &P,
        locale: &DataLocale,
        length: length::Date,
    ) -> Result<Self, Error>
    where
        P: ?Sized
            // Explanatory comment about the bounds
            + DataProvider<BuddhistDatePatternV1Marker>
            + DataProvider<BuddhistYearNamesV1Marker>
            + DataProvider<BuddhistMonthNamesV1Marker>
    { ... }

Possibly related to https://github.com/rust-lang/rustfmt/issues/3669, https://github.com/rust-lang/rustfmt/pull/4666, https://github.com/rust-lang/rustfmt/pull/5059 but the examples are in different contexts so I thought I would file a new issue with this test case.

CC @Manishearth

Reproduce the reported trait-bound example with rustfmt, then inspect the related issues and pull requests linked in the report for context. The work is done when the explanatory comment is preserved in this form and a regression test covers the case.