Erroneous warning about packed struct not implementing Copy #71740
Description
I tried this code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7d186291bfe30222d01ed0af2fa28c4c
use serde::{Serialize, Deserialize}; #[repr(C)] #[repr(packed)] #[derive(Copy, Clone, Serialize, Deserialize, PartialEq, Debug)] struct Packed { a: u8, b: u16, c: u8 } fn main() {}I expected to see this happen: Compile without any warning or error.
Instead, this happened: The following warning was emitted:
Compiling playground v0.0.1 (/playground) warning: `#[derive]` can't be used on a `#[repr(packed)]` struct that does not derive Copy (error E0133) --> src/main.rs:8:23 | 8 | #[derive(Copy, Clone, Serialize, Deserialize, PartialEq, Debug)] | ^^^^^^^^^ | = note: `#[warn(safe_packed_borrows)]` on by default = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043> = note: this warning originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info) Finished dev [unoptimized + debuginfo] target(s) in 0.65s Running `target/debug/playground` Meta
rustc --version --verbose:
<version> Backtrace
<backtrace>