Rust E0502: Cannot borrow as immutable while mutable borrow active

error[E0502]: cannot borrow `v` as immutable because it is also borrowed as mutable --> src/main.rs:10:15 = help: use `iter` to access elements without re-borrowing

Immediate Remediation

bash

cat > src/main.rs <<'EOF'
fn main() {
    let mut v = vec![1, 2, 3];
    let first = v[0]; // copy before mutation
    for item in v.iter_mut() {
        *item += first;
    }
}
EOF

Root Cause Analysis

The borrow checker rejected the code because an immutable borrow of `v` was attempted while a mutable borrow (from `iter_mut()`) was still active, violating Rust's ownership and borrowing rules.

Verification & Guardrails

This fix assumes the element type implements Copy; if not, use `v.first().cloned()` or restructure to avoid borrowing the whole vec.
If you need to read and mutate the same collection during iteration, consider using `RefCell` or `Cell` for interior mutability.
Verify the fix with `cargo check` and ensure no other borrows conflict in the surrounding scope.

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