Catch use of PhantomData that requires T: Sized · rust-lang/rust-clippy#2308

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T-middlegood first issue

説明

In Serde the traits we implement for PhantomData should have worked for T: ?Sized, but as a bug they required T: Sized -- fixed in https://github.com/serde-rs/serde/commit/4751627f1cd14cacdf216188ccbb9ab0831e2b3f. Could Clippy have caught this? I struggle to think of any use of PhantomData<T> where T is an unconstrained type parameter where you would not want to allow ?Sized. As a starting point, maybe we could catch impls of the form:

impl<..., T, ...> Trait<...> for PhantomData<T> { /* ... */ }

where there are no trait bounds on T. In this case you pretty much always want T: ?Sized right?

コントリビューターガイド

技術スタック: rust
領域: developer experiencetooling
Issue 種別: feature
難度: 3
推定時間: 1-2 days
活動状況: stale
明確さ: clear
前提条件: Rust programmingUnderstanding of Sized traitClippy lint development
初心者向け度: 35
調査方針: Review the Serde commit mentioned (4751627f1cd14cacdf216188ccbb9ab0831e2b3f) to understand the fix. Examine existing Clippy lints that check trait implementations on PhantomData, such as those in the clippy lints crate. The new lint should detect impl blocks for PhantomData<T> where T is unconstrained and suggest adding T: ?Sized. Check comments for any prior attempts or maintainer feedback.