How to suppress the warning for “drop_with_repr_extern” at a fine granularity?

Question

I am currently experimenting with multi-threading code, and its performance is affected by whether two data members share the same cache line or not.

In order to avoid false-sharing, I need to specify the layout of the struct without the Rust compiler interfering, and thus I use repr(C) . However, this same struct also implements Drop , and therefore the compiler warns about the "incompatibility" of repr(C) and Drop , which I care naught for.

However, attempting to silence this futile warning has proven beyond me.

Here is a reduced example :

#[repr(C)]
#[derive(Default, Debug)]
struct Simple<T> {
    item: T,
}

impl<T> Drop for Simple<T> {
    fn drop(&mut self) {}
}

fn main() {
    println!("{:?}", Simple::<u32>::default());
}

which emits #[warn(drop_with_repr_extern)] .

I have tried specifying #[allow(drop_with_repr_extern)] :

• at struct
• at impl Drop
• at mod

and neither worked. Only the crate-level suppression worked, which is rather heavy-handed.

Which leads us to: is there a more granular way of suppressing this warning?

Note: remarks on a better way to ensure that two data members are spread over different cache lines are welcome; however they will not constitute answers on their own.

Answer 1

The reason is near the end of rustc_lint/builtin.rs :

The lint does not walk the crate, instead using ctx.tcx.lang_items.drop_trait() to look up all Drop trait implementations within the crate. The annotations are only picked up while walking the crate. I've stumbled upon the same problem in this question . So unless someone changes the lint to actually walk the crate and pick up Drop impl s as it goes, you need to annotate the whole crate.