Lint `let mut x = x;` where `x` is a function argument

Description

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.

mostly clear

Prerequisites

basic Rustclippy lint structure

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

40

Research direction

The issue proposes a new lint to detect `let mut x = x;` where `x` is a function parameter. The goal is to suggest making the parameter mutable directly in the function signature. To implement, study existing clippy lints like `mut mut` or similar patterns in the clippy source code. Look at the clippy documentation for adding a new lint, and consider potential false positives, e.g., when the function argument is used elsewhere or the rebinding is intentional. The implementation should check for function parameter bindings followed by an immediate `let mut` rebinding with the same name.