rust-lang/rust-clippy

BufReader Stdin or StdinLock

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#6,755 opened on Feb 17, 2021

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 (12 comments) (2 reactions) (1 assignee)Rust (10,406 stars) (1,391 forks)batch import
A-lintL-perfT-middlegood first issue

Description

What it does

Suggest using StdinLock rather than BufReader::new(io::stdin()) since it is already buffered. We might also want to take care of let stdin = io::stdin(); BufReader::new(stdin) and also BufReader::new(stdinlock) (there could be people doing this).

Categories (optional)

  • Kind: perf

Locking stdin also provide benefit for not having to re-lock it on every read.

std::io::stdin() already is buffered. Re-buffering it only increase the memcpy calls.

Source https://users.rust-lang.org/t/correct-way-for-taking-large-input-from-keyboard/55304/11 Related to https://github.com/rust-lang/rust-clippy/issues/1805

For example:

  • Recommend locking of stdin to reduce re-locking
  • Suggest not re-buffering stdin lock which already implements BufRead

Drawbacks

None.

Example

let reader = BufReader::new(io::stdin());  // this could a separate variable or StdinLock

Could be written as:

let stdin = io::stdin();
let reader = stdin.lock();

Contributor guide

Tech stack
rust
Domain
performance
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.
half day
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
Basic Rust knowledgeFamiliarity with clippy lint structure
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
The issue proposes a new lint to detect unnecessary BufReader wrapping of stdin. Look at existing lints in clippy lints/src for similar performance lints. The implementation should detect `BufReader::new(io::stdin())` and suggest using `stdin.lock()`. Check the assignee's work (user yawara) to avoid duplication. The related issue #1805 provides additional context. Start by examining the clippy development guide and the structure of a lint pass.