Rust borrow checker error: cannot borrow as mutable because it is also borrowed as immutable

Rust compiler rejects code that borrows a value as mutable while an immutable borrow is still active. Common when iterating over a collection and trying to modify it.

You cannot have a mutable and immutable reference to the same data simultaneously. Fixes: (1) Clone the data to break the borrow: let keys: Vec<_> = map.keys().cloned().collect(); then mutate. (2) Use interior mutability with RefCell or Mutex. (3) Restructure to separate the read and write phases. (4) Use entry API for HashMap: map.entry(key).or_insert(value). (5) Use indices instead of references when iterating and mutating.

Rust's ownership system prevents data races at compile time. Allowing mutable + immutable borrows simultaneously could lead to iterator invalidation or reading partially-written data.