A comment under struct field exceeds max width

Input

/// Contains data for specific types of import directives.
#[derive(Clone, Debug)]
pub enum ImportDirectiveSubclass<'a> {
    SingleImport {
        target: Ident,
        source: Ident,
        result: PerNS<Cell<Result<&'a NameBinding<'a>, Determinacy>>>,
        type_ns_only: bool,
    },
    GlobImport {
        is_prelude: bool,
        max_vis: Cell<ty::Visibility>, // The visibility of the greatest reexport.
        // n.b. `max_vis` is only used in `finalize_import` to check for reexport errors.
    },
    ExternCrate(Option<Name>),
    MacroUse,
}

Output

/// Contains data for specific types of import directives.
#[derive(Clone, Debug)]
pub enum ImportDirectiveSubclass<'a> {
    SingleImport {
        target: Ident,
        source: Ident,
        result: PerNS<Cell<Result<&'a NameBinding<'a>, Determinacy>>>,
        type_ns_only: bool,
    },
    GlobImport {
        is_prelude: bool,
        max_vis: Cell<ty::Visibility>, // The visibility of the greatest reexport.
                                       // n.b. `max_vis` is only used in `finalize_import` to check for reexport errors.
    },
    ExternCrate(Option<Name>),
    MacroUse,
}

Meta

0.2.16-nightly (4e04e82 2017-11-28)

The above example is taken from https://github.com/rust-lang/rust/blob/90f5cfdfbd3c9e39f897fd2271c7b1c7fdaae58e/src/librustc_resolve/resolve_imports.rs#L37-L53.

Reproduce the formatting difference with the Rust snippet in the issue and compare it with the referenced src/librustc_resolve/resolve_imports.rs example. Trace rustfmt's handling of comments under struct fields, then verify that the comment is aligned as shown in the expected output.