How to fix Rust E0502 cannot borrow as mutable

The E0502 error occurs because you are trying to borrow a value mutably while an immutable borrow of the same value is still active. You must ensure the immutable borrow ends before starting the mutable one, often by restructuring the code or using indices instead of references.

let mut s = String::from("hello");
let r1 = &s; // Immutable borrow starts
// let r2 = &mut s; // ERROR: E0502 - cannot borrow as mutable

// Correct approach: ensure r1 is no longer used
let r1 = &s;
println!("r1: {}", r1); // r1 used here
// r1 scope ends implicitly if not used further, or explicitly via block
{
    let r2 = &mut s; // Mutable borrow starts after r1 is done
    r2.push_str(" world");
}

The Rust E0502 cannot borrow as mutable error means you are trying to change data while someone else is still reading it. Rust prevents this to avoid crashes or corrupted data, similar to how you can't edit a document while someone else is currently viewing it. You must wait until the reading is finished before making changes.