Rust: cannot borrow `x` as mutable more than once at a time

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Root Cause

This is the classic Rust borrow checker error. To guarantee memory safety without a garbage collector, Rust enforces strict rules: at any given time, you can have either one mutable reference to data, OR any number of immutable references, but never both, and never more than one mutable reference. This prevents data races at compile time. The error occurs when you create a mutable reference to a variable, and then try to create a second mutable reference (or use the original variable) before the first reference's scope ends.

Fix / Solution

You must restructure your code to comply with the borrow checker. This often involves reducing the scope of the mutable reference by wrapping it in a block `{}` so the reference is dropped sooner. Alternatively, reconsider your data architecture: can you pass ownership instead of borrowing? Or can you clone the data if it's small enough? Interior mutability patterns (`RefCell`, `Mutex`) can be used for complex scenarios.

Code Snippet

// ❌ Borrow checker prevents multiple mutable borrows
let mut s = String::from("hello");
let r1 = &mut s;
let r2 = &mut s; // ERROR: cannot borrow twice
println!("{}, {}", r1, r2);

// ✅ Scope the borrows so they don't overlap
let mut s = String::from("hello");
{
    let r1 = &mut s;
    r1.push_str(", world");
} // r1 goes out of scope here
let r2 = &mut s; // Now this is perfectly valid
r2.push('!');