rust-lang/rust-clippy

Detect creation of new collection when it could be emptied

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#16520 opened on Feb 5, 2026

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A-lintgood first issue

Description

Description

See https://github.com/uutils/coreutils/pull/10695

When a variable holding a collection (such as Vec/VecDeque/HashMap/BTreeMap) is replaced by a new empty instance, it can be more efficient to instead clear the existing one to be able to reuse the allocated memory.

For example, x = Vec::new() can be replaced by x.clear(): x will be an empty vector, except that the storage (and capacity) will be inherited from the previous instance.

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Contributor guide

Tech stack
rust
Domain
performancetooling
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.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
RustClippy development setup
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Review the referenced PR #10695 to understand the pattern where a collection is replaced by a new empty instance. Examine existing Clippy lints that suggest using `.clear()` for similar optimizations to understand the lint structure. The implementation involves creating a new lint that matches assignment expressions like `x = Vec::new()` and suggests replacing with `x.clear()`. Ensure the lint includes proper tests and documentation.