rust-lang / rust-lang/rust-clippy
New lint: vec.extend(&vec![elem; len]) is slow, vec.resize() should be used instead
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A-lint
C-enhancement
L-perf
L-suggestion
- Dominant language
- Rust
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Description
There are some very slow ways of extending a vector with len new elements:
vec.extend(&vec![elem; len]);
vec.extend_from_slice(&vec![elem; len]);
Instead it should be written like this:
vec.resize(vec.len() + len, elem);
This sounds like code that nobody would ever write, but it actually has real-world precedent: https://github.com/Frommi/miniz_oxide/pull/55
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reviewing existing rust-clippy lint implementations and the miniz_oxide pull request linked in the issue. Locate the lint entry point and related tests, then verify that both inefficient vector-extension forms are detected and the resize form is accepted.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- tooling
- Issue type
- Feature
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100