rust-lang/rust-clippy

rc_buffer lint is dangerous as it changes runtime behaviour

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#6170 opened on Oct 13, 2020

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 (7 comments) (1 reaction) (0 assignees)Rust (10,406 stars) (1,391 forks)batch import
C-bugL-suggestiongood first issue

Description

clippy warns if one tries to use a type like Rc<Vec<T>>, e.g.

Arc<Vec<u8>>
^^^^^^^^^^^^ help: try: `Arc<[u8]>`

This is dangerous as creating a Rc<[T]> causes a memcpy of the whole memory area with all the elements, while creating an Rc<Vec<T>> simply copies over the 3 pointers inside the Vec without copying all the elements.

A better suggestion would probably be Rc<Box<[T]>> as that has the same runtime behaviour, but even independent of that it can be useful to have a Vec inside an Rc / Arc in combination with the make_mut() / get_mut() API.

The lint should probably be at least disabled by default.

The same also applies to Rc<String> vs Rc<str>.

Contributor guide

Tech stack
rust
Domain
developer experienceperformancetooling
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.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
needs maintainer response
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust programming basicsClippy lint structureMemory semantics of Rc and Vec
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
The issue requests disabling the `rc buffer` lint by default due to its dangerous runtime behavior change. To resolve, find the lint definition in the clippy source, likely in `clippy lints/src/rc buffer.rs`, and change its default level from `warn` to `allow` or adjust the lint's configuration. The comments may contain additional context about the specific trade offs.