1c6d7aae69
closure field capturing: don't depend on alignment of packed fields This fixes the closure field capture part of https://github.com/rust-lang/rust/issues/115305: field capturing always stops at projections into packed structs, no matter the alignment of the field. This means changing a private field type from `u8` to `u64` can never change how closures capture fields, which is probably what we want. Here's an example where, before this PR, changing the type of a private field in a repr(Rust) struct can change the output of a program: ```rust #![allow(dead_code)] mod m { // before patch #[derive(Default)] pub struct S1(u8); // after patch #[derive(Default)] pub struct S2(u64); } struct NoisyDrop; impl Drop for NoisyDrop { fn drop(&mut self) { eprintln!("dropped!"); } } #[repr(packed)] struct MyType { field: m::S1, // output changes when this becomes S2 other_field: NoisyDrop, third_field: Vec<()>, } fn test(r: MyType) { let c = || { let _val = std::ptr::addr_of!(r.field); let _val = r.third_field; }; drop(c); eprintln!("before dropping"); } fn main() { test(MyType { field: Default::default(), other_field: NoisyDrop, third_field: Vec::new(), }); } ``` Of course this is a breaking change for the same reason that doing field capturing in the first place was a breaking change. Packed fields are relatively rare and depending on drop order is relatively rare, so I don't expect this to have much impact, but it's hard to be sure and even a crater run will only tell us so much. Also see the [nomination comment](https://github.com/rust-lang/rust/pull/115315#issuecomment-1702807825). Cc `@rust-lang/wg-rfc-2229` `@ehuss` |
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