E0596: cannot borrow as mutable, as it is not declared as mutable Rust Origin: Application & API OSI layer 7 · Application Last updated 5 Nov 2026

Rust: error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable

You called a &mut method (or took &mut) on a binding/reference that isn't mutable.

Exact message you may see error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutablecannot borrow `*self` as mutable, as it is behind a `&` referencecannot borrow data in a `&` reference as mutableerror[E0596]: cannot borrow `x` as mutableE0596 help: consider changing this to be mutable: `mut v`
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Seen on: Rust

Meaning

Add mut to the binding (let mut v = Vec::new();), take &mut parameters (fn f(v: &mut Vec<i32>)), and make self &mut self for mutating methods. Through &T you can't mutate — use &mut T, RefCell, or Cell.

Common causes

  • let v = Vec::new(); v.push(1);
  • Method takes &self but mutates
  • Parameter is &T not &mut T
  • Closure capturing a variable mutably without mut
  • Iterating with iter() then mutating (use iter_mut())

⚡ Quick fix

  1. Declare let mut
  2. Change signature to &mut self/&mut T
  3. Use iter_mut(); consider RefCell for interior mutability

Detailed fix by platform

Rust

  1. fn add(&mut self, x: i32) { self.items.push(x); }

How to diagnose

  1. Binding — mut?
  2. Signature — &mut?
  3. Iterator — iter_mut?

🔧 Still not fixed?

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Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

Blocking waiting for file lock on package cache / build directory — Cargo: Blocking waiting for file lock on package cache / on build directory (Rust)Blocking waiting for …cargo: failed to get from registry (spurious network error) — Cargo: warning: spurious network error (2 tries remaining): [35] SSL connect error / error: failed to download from `https://index.crates.io/...` (Rust)cargo: failed to get …E0384: cannot assign twice to immutable variable — Rust: error[E0384]: cannot assign twice to immutable variable `x` (Rust)E0384: cannot assign …E0499: cannot borrow as mutable more than once at a time — Rust: error[E0499]: cannot borrow `v` as mutable more than once at a time (Rust)E0499: cannot borrow …E0502 — Rust: error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable (Rust)E0502E0505: cannot move out of value because it is borrowed — Rust: error[E0505]: cannot move out of `s` because it is borrowed (Rust)E0505: cannot move ou…E0507: cannot move out of a shared reference / index — Rust: error[E0507]: cannot move out of `self.name` which is behind a shared reference (Rust)E0507: cannot move ou…E0597: borrowed value does not live long enough — Rust: error[E0597]: `x` does not live long enough — borrowed value does not live long enough (Rust)E0597: borrowed value…E0596: ca…

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