Fix E0596: Cannot Assign to Captured Variable in `Fn` Closure

Mon, 18 May 2026 19:56:04 +0800

1. Symptoms

When compiling Rust code that attempts to modify a captured variable from within a closure that only captures by immutable reference, the compiler produces error E0596:

error[E0596]: cannot assign to data in a Fn closure –> src/main.rs:6:5 | 6 | counter += 1; | ^^^^^^^^^^^^^ cannot assign | help: consider changing this closure to accept arguments by mutable reference | = note: closures can be mut to capture variables by mutable reference

E0596 on ErrorVault — Developer Error Code Dictionary

Fix E0596: Cannot Assign to Captured Variable in `Fn` Closure

1. Symptoms

When compiling Rust code that attempts to modify a captured variable from within a closure that only captures by immutable reference, the compiler produces error E0596: