rust-lang/rust-clippy

Catch use of PhantomData that requires T: Sized

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#2308 opened on Dec 27, 2017

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T-middlegood first issue

Description

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?

Contributor guide

Tech stack
rust
Domain
developer experiencetooling
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust programmingUnderstanding of Sized traitClippy lint development
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
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.