clippy::needless_maybe_sized emits a false positive on derived impls with Sized bounds #13360
Description
Summary
If there is a type with a ?Sized type parameter which has a derived impl (e.g. via serde) that requires a Sized bound, a needless_maybe_sized warning will be emitted. This is a false positive because the Sized bound then only applies to that specific impl, not to the type in general.
As far as I can tell this happens on 1.81 and on nightly. I couldn't find any issues about this yet, but if there is one already, I'm sorry for the noise :)
Lint Name
clippy::needless_maybe_sized
Reproducer
I tried this code:
use serde;
use std::marker::PhantomData
#[derive(serde::Serialize)]
#[serde(bound = "T: Bar")]
struct Foo<T: ?Sized> {
t: PhantomData<T>,
}
trait Bar: Sized {}I saw this happen:
warning: `?Sized` bound is ignored because of a `Sized` requirement
--> src/main.rs:6:15
|
6 | struct Foo<T: ?Sized> {
| ^^^^^^
|
note: `T` cannot be unsized because of the bound
--> src/main.rs:5:17
|
5 | #[serde(bound = "T: Bar")]
| ^^^^^^^^
= note: ...because `Bar` has the bound `Sized`
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_maybe_sized
= note: `#[warn(clippy::needless_maybe_sized)]` on by default
help: change the bounds that require `Sized`, or remove the `?Sized` bound
|
6 - struct Foo<T: ?Sized> {
6 + struct Foo<T> {
|
I expected to see no warning about the ?Sized bound.
Version
rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7
It also happens on nightly still.
Additional Labels
No response