rust-lang/rust-clippy

Catch transmuting Option<&T> to Option<&mut T>

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#372 opened on Oct 8, 2015

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C-enhancementE-mediumT-middlegood first issue

Description

Rustc already has a lint (mutable_transmutes) that catches transmutes from &T to &mut T, which can cause undefined behaviour. However, it doesn't catch minor variations of this problem, like Option<&T> -> Option<&mut T>, which are presumably just as dangerous. I've seen this particular example in real code. It would be nice to have a lint that catches this case (and ideally, other instances of the same problem, although this could get quite complex in the fully general case).

Contributor guide

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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust programmingUnderstanding of transmute and undefined behaviorClippy lint architectureRust testing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Examine the existing `mutable transmutes` lint implementation in clippy, likely in `clippy lints/src/transmute.rs`. Understand how it detects transmutes from `&T` to `&mut T`. Then extend the logic to handle `Option<&T>` to `Option<&mut T>` by checking the inner types. Consider also other generic wrappers. Write tests to cover the new cases.