Custom attributes sometimes not formatted

rustfmt 1.4.5-nightly (2019-08-19 9792ff0)

enum E {
    #[error(display = "invalid max keyframe interval {} (expected > 0, < {})", _0, i32::max_value() as u64)]
    InvalidMaxKeyFrameInterval(u64),
    #[error(display = "invalid max keyframe interval {} (expected > 0, < {})", _0, i32::max_value())]
    InvalidMaxKeyFrameInterval(u64),
}

gets reformatted as

enum E {
    #[error(display = "invalid max keyframe interval {} (expected > 0, < {})", _0, i32::max_value() as u64)]
    InvalidMaxKeyFrameInterval(u64),
    #[error(
        display = "invalid max keyframe interval {} (expected > 0, < {})",
        _0,
        i32::max_value()
    )]
    InvalidMaxKeyFrameInterval(u64),
}

even though this also fits fine

enum E {
    #[error(
        display = "invalid max keyframe interval {} (expected > 0, < {})",
        _0,
        i32::max_value() as u64
    )]
    InvalidMaxKeyFrameInterval(u64),
    #[error(
        display = "invalid max keyframe interval {} (expected > 0, < {})",
        _0,
        i32::max_value()
    )]
    InvalidMaxKeyFrameInterval(u64),
}

Reproduce the issue with the Rust enum snippets and compare rustfmt's handling of the two custom attributes. Start by tracing custom attribute formatting in the rustfmt codebase; done means equivalent attributes are formatted consistently when either expression fits within the available width.