Rust: error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable
You tried to mutate a value while an immutable reference to it is still in use.
Exact message you may see
cannot borrow as mutable because it is also borrowed as immutableE0499 cannot borrow as mutable more than once at a time
Seen on:
Rust
Meaning
Rust allows many readers or one writer at a time. Holding a reference into a Vec (e.g. from .first() or an index) and then pushing to it is the most common case.
Common causes
- Holding a reference (iterator, slice, &v[0]) while mutating
- Calling a &mut self method while a borrow is alive
- Long-lived borrows in loops
⚡ Quick fix
- Finish using the immutable reference before mutating
- Copy/clone the needed value out first
- Collect changes and apply them after the loop
Detailed fix by platform
Rust
- rust
let first = v[0]; // copy the value (i32 is Copy) v.push(4); println!("{first}");
How to diagnose
- Borrows — Where is the immutable borrow used later?
- Loop — Mutating what you iterate?
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Last updated 14 Oct 2026