rust-lang/rust-clippy

Catch use of PhantomData that requires T: Sized

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#2,308 opened on Dec 27, 2017

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T-middlegood first issue

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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

Research direction
Look at existing Clippy lints for examples, understand the lint registration and visitor pattern, and implement a lint that detects `impl<..., T, ...> Trait<...> for PhantomData<T>` without `?Sized` bound on T.
Tech stack
rust
Domain
tooling
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Rust
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70