clippy::mut_mut: multiple successive mutable borrows

What you see

warning: multiple successive mutable borrows
 --> main.rs:3:9
  |
3 | let x = &mut &mut 1;
  |         ^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#mut_mut
  = note: `-W clippy::mut-mut` implied by `-W clippy::pedantic`
help: make only a single borrow
  |
3 - let x = &mut &mut 1;
3 + let x = &mut 1;
  |

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?

This is usually just a typo or a misunderstanding of how references work.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

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

let x = &mut &mut 1;

let mut x = &mut 1;
let y = &mut x;

fn foo(x: &mut &mut u32) {}

Write this instead

let x = &mut 1;

let mut x = &mut 1;
let y = &mut *x; // reborrow

fn foo(x: &mut u32) {}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

[lints.clippy]
mut_mut = "warn"

#![warn(clippy::mut_mut)]

cargo clippy -- -W clippy::mut-mut

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.