E0384: cannot assign twice to immutable variable Rust Origin: Application & API OSI layer 7 · Application Last updated 5 Nov 2026

Rust: error[E0384]: cannot assign twice to immutable variable `x`

You reassigned a variable declared without mut.

Exact message you may see error[E0384]: cannot assign twice to immutable variable `x`cannot assign twice to immutable variableE0384 help: consider making this binding mutable: `mut x`cannot assign to `x`, as it is not declared as mutablecannot assign to immutable argument
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Seen on: Rust

Meaning

Declare it let mut x = ... if reassignment is intended, or shadow it (let x = x + 1;) which creates a new binding. Function parameters are immutable too (mut param). For loops with counters use let mut.

Common causes

  • Missing mut on the variable
  • Reassigning function parameters
  • Loop counter immutable
  • Assigning in a closure without mut capture
  • Struct field assignment through immutable binding

⚡ Quick fix

  1. let mut x = 5;
  2. Shadow: let x = x + 1;
  3. fn f(mut x: i32) for parameters

Detailed fix by platform

Rust

  1. let mut count = 0; count += 1;

How to diagnose

  1. Binding — mut?
  2. Intent — Reassign or shadow?
  3. Param — mut?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

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

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