Clippy lint · Style · checked with Clippy 0.1.99

clippy::mut_mutex_lock: calling `&mut Mutex::lock` unnecessarily locks an exclusive (mutable) reference

What it meansChecks for &mut Mutex::lock calls It is in the style group and warns by default.

What you see

warning: calling `&mut Mutex::lock` unnecessarily locks an exclusive (mutable) reference
 --> main.rs:8:29
  |
8 | let mut value = value_mutex.lock().unwrap();
  |                             ^^^^ help: change this to: `get_mut`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#mut_mutex_lock
  = note: `#[warn(clippy::mut_mutex_lock)]` on by default

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

Mutex::lock is less efficient than calling Mutex::get_mut. In addition you also have a statically guarantee that the mutex isn’t locked, instead of just a runtime guarantee.

Code that works but is not idiomatic Rust. Warns by default.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

use std::sync::{Arc, Mutex};

let mut value_rc = Arc::new(Mutex::new(42_u8));
let value_mutex = Arc::get_mut(&mut value_rc).unwrap();

let mut value = value_mutex.lock().unwrap();
*value += 1;

Write this instead

use std::sync::{Arc, Mutex};

let mut value_rc = Arc::new(Mutex::new(42_u8));
let value_mutex = Arc::get_mut(&mut value_rc).unwrap();

let value = value_mutex.get_mut().unwrap();
*value += 1;

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::mut_mutex_lock)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
mut_mutex_lock = "allow"

One run

cargo clippy -- -A clippy::mut-mutex-lock

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Stylegroup

warndefault level

nofixed by cargo clippy --fix

1.49.0added in Rust

Clippy's own documentation

What it does

Checks for &mut Mutex::lock calls

Why is this bad?

Mutex::lock is less efficient than calling Mutex::get_mut. In addition you also have a statically guarantee that the mutex isn’t locked, instead of just a runtime guarantee.

Example

use std::sync::{Arc, Mutex};

let mut value_rc = Arc::new(Mutex::new(42_u8));
let value_mutex = Arc::get_mut(&mut value_rc).unwrap();

let mut value = value_mutex.lock().unwrap();
*value += 1;

Use instead:

use std::sync::{Arc, Mutex};

let mut value_rc = Arc::new(Mutex::new(42_u8));
let value_mutex = Arc::get_mut(&mut value_rc).unwrap();

let value = value_mutex.get_mut().unwrap();
*value += 1;

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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