CODEWITHSUNDEEP
rustsuppress-warnings
Matthieu M.
Matthieu M.

Reputation: 299890

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

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)]:

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.

Upvotes: 4

Views: 531

Answers (1)

llogiq
llogiq

Reputation: 14511

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 impls as it goes, you need to annotate the whole crate.

Upvotes: 2

Related Questions