rust-lang/rust-clippy

rc_buffer lint is dangerous as it changes runtime behaviour

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#6,170 opened on Oct 13, 2020

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C-bugL-suggestiongood first issue

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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

Research direction
Examine the rc buffer lint implementation and understand the performance differences between Rc<Vec<T>> and Rc<[T]>. Consider adjusting the lint to suggest Rc<Box<[T]>> or disabling it by default.
Tech stack
rust
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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.
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.
60