Fix E0301: Mutable Immutable Borrow in Rust

Mon, 13 Jul 2026 05:32:04 +0800

1. Symptoms

The Rust compiler emits error E0301 when you attempt to mutate a value while an immutable borrow is active. This violation occurs when your code tries to reassign, modify, or take a mutable reference to a variable that is currently borrowed immutably.

Typical compiler output for this error looks like:

error[E0301]: cannot assign to data in a borrowed reference
  --> src/main.rs:5:5
   |
4  |     let y = &x;
   |             - borrow of `x` occurs here
5  |     x = 10;
   |     ^^^^^^ assignment to borrowed `x`
   |
   = note: this error occurs in the generation of the MIR for this function

The error message explicitly identifies where the borrow occurs and where the invalid assignment is attempted. You may also encounter variations such as “cannot use mutably borrowed variable as mutable” or “cannot assign to x because it is also borrowed”. The compiler always marks the exact line of the problematic mutation and points to the borrow site that prevents the operation.

E0301 on ErrorVault — Developer Error Code Dictionary

Fix E0301: Mutable Immutable Borrow in Rust

1. Symptoms