Rustfmt deletes comments in first position of trait bound

[sffc]

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 #3669, #4666, #5059 but the examples are in different contexts so I thought I would file a new issue with this test case.

CC @Manishearth

[nnethercote]

I just hit this:

diff --git a/tests/rustdoc/rfc-2632-const-trait-impl.rs b/tests/rustdoc/rfc-2632-const-trait-impl.rs
index 6f264969e54..0fec059f951 100644
--- a/tests/rustdoc/rfc-2632-const-trait-impl.rs
+++ b/tests/rustdoc/rfc-2632-const-trait-impl.rs
@@ -24,9 +24,9 @@ pub trait Tr<T> {
     // @has - '//section[@id="method.a"]/h4[@class="code-header"]/a[@class="trait"]' 'Fn'
     // @!has - '//section[@id="method.a"]/h4[@class="code-header"]/span[@class="where"]' '~const'
     // @has - '//section[@id="method.a"]/h4[@class="code-header"]/div[@class="where"]' ': Fn'
-    fn a<A: /* ~const */ Fn() + ~const Destruct>()
+    fn a<A: Fn() + ~const Destruct>()
     where
-        Option<A>: /* ~const */ Fn() + ~const Destruct,
+        Option<A>: Fn() + ~const Destruct,
     {
     }
 }

I have a comment removed in both trait bounds: within the generic params, and in the where clause.

[ijackson]

Another repro:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f590ab120d35f6aa1d296e9da5ce9d0f

@rustbot label +C-bug
@rustbot label +p-high

Not sure Rustbot will let me set the p-high label, but I feel I should try. After all this is a case where rustfmt corrupts the code, lossily, in a way that is unlikely to be detected.

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[RReverser]

I think hitting the same issue, so not going to raise a new one, but happens for me even in non-first position.

Minimal repro passed via rustfmt --check:

-trait RegistrableDevice<DynTrait: ?Sized>: Sized /* + Unsize<DynTrait> */ {}
+trait RegistrableDevice<DynTrait: ?Sized>: Sized {}