rust-lang/rust-clippy

New lint: wildcard pattern in let

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#4090 opened on May 13, 2019

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 (9 comments) (0 reactions) (0 assignees)Rust (10,406 stars) (1,391 forks)batch import
A-lintL-restrictiongood first issue

Description

I think there should be a warning about the wildcard pattern in let. I can't imagine any situation where this makes sense, and there are cases where it's a mistake caused by taking _ for identifier though it's not.

Wrong:

{
    let _ = my scope guard or whatever
    // dropped right away

    do stuff
}

Correct:

{
    let _g = my scope guard or whatever

    do stuff

    // dropped at the end of the scope
}

Contributor guide

Tech stack
rust
Domain
toolingdeveloper experience
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.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rust programmingClippy contribution basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
The issue requests a new lint to warn against using the wildcard pattern ` ` in a `let` statement, which causes immediate drop of the bound value. To implement, study existing lint implementations in the `clippy lints` crate, e.g., the `let underscore` lint. Create a new lint file following the pattern, register it in the lint list, and add tests. The lint should detect `let = expr` and suggest using a named binding. See comments in the issue for further discussion on edge cases.