Break lines for attributes in otherwise vertical display (argument / type attributes)

This seems like the sort of thing that may have an issue that's just difficult to find, so please point me in the right direction if so.

Currently rustfmt seems to not break lines for attributes in the arguments or generics position:

pub struct Arc<
    T: ?Sized,
    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
> {
    ptr: NonNull<ArcInner<T>>,
    phantom: PhantomData<ArcInner<T>>,
    alloc: A,
}

It seems more consistent to break the line after the attribute. This keeps the generics in line, and is more similar to current behavior for block-level attributes.

pub struct Arc<
    T: ?Sized,
    #[unstable(feature = "allocator_api", issue = "32838")]
    A: Allocator = Global,
> {
    // ...
}

Reproduce the formatting with the Rust snippet, then trace rustfmt's handling of attributes in generic arguments and type parameters. No source file or test is named; done means the formatter breaks after the attribute while preserving the surrounding vertical generic layout, with coverage in the relevant formatter tests.