rust-lang/rust-clippy

unused_lifetime false positive

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#2678 opened on Apr 17, 2018

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 (12 comments) (0 reactions) (1 assignee)Rust (10,406 stars) (1,391 forks)batch import
C-enhancementgood first issue

Description

I'm using this type of stuff in a -sys-crate:

unsafe extern "C" fn callback<'a>(data: *mut c_void) {
    let _s = Box::from_raw(data as *mut S<'a>);
}

and this triggers unused_lifetime.

Playground with more context (also shows the #2677 issue): https://play.rust-lang.org/?gist=21a412537258ede9cbda42350501b90a&version=nightly

Contributor guide

Tech stack
rust
Domain
developer experience
Issue type
bug
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.
half day
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 programming language basicsUnderstanding of Rust lifetimes
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
The issue is a false positive for the `unused lifetime` lint in Clippy when a lifetime is used inside an `unsafe extern "C"` function via `Box::from raw`. Investigate the lint's logic in `clippy lints/src/unused lifetime.rs` (or similar path) to see why it incorrectly marks the lifetime as unused. Check existing tests and comments, especially referencing issue #2677. Consider whether the lint should account for lifetimes used in unsafe code that are not directly visible in the function signature. The playground link provides a minimal reproduction.