[StackOverflow/rust] Modify both inner and outer part of a nested structure in a loop

### ROOT CAUSE The issue arises from Rust's strict borrowing rules, which prevent simultaneous mutable borrows of overlapping data. The original code likely attempts to borrow both the outer and inner parts of a nested structure mutably at the same time, which is unsafe and results in a compile-time error. ### CODE FIX To safely modify both levels of a nested structure in a loop, use a mutable reference to the outer struct and then borrow the inner part mutably within the loop. This ensures only one mutable borrow exists at a time. ```rust struct Inner { value: i32, } struct Outer { inner: Inner, count: i32, } fn main() { let mut outer = Outer { inner: Inner { value: 0 }, count: 0, }; for i in 0..10 { // Borrow the inner part mutably let inner = &mut outer.inner; // Modify inner and outer using the mutable references inner.value = i; outer.count = i; } } ``` **Explanation:** - The outer struct (`Outer`) holds a field (`inner`) of type `Inner`. - In each loop iteration, `&mut outer.inner` creates a temporary mutable reference to `inner`. - Rust guarantees that while `inner` is borrowed, no other borrows (even of the outer struct) conflict, allowing modifications to both levels. - The loop body can safely modify `inner.value` and `outer.count` because the borrows are properly scoped.