How does Rust ensure memory safety without a garbage collector?

Rust guarantees memory safety at compile time using three key mechanisms: ownership, borrowing, and lifetimes. These ensure no memory leaks, data races, or dangling pointers without the need for a garbage collector.

The C/C++ Problem

C and C++ give developers complete control over memory, but this leads to critical safety issues:

Dangling Pointers:

char* get_string() {
    char buffer[100] = "hello"; // Stack allocated
    return buffer;              // Returns pointer to freed memory
} // ERROR: buffer is destroyed here

int* ptr = malloc(sizeof(int));
free(ptr);
*ptr = 42; // ERROR: Use after free

Memory Leaks:

void leak_memory() {
    int* data = new int[1000]; // Heap allocation
    if (some_condition) {
        return; // ERROR: Memory never freed
    }
    delete[] data; // Only freed on normal path
}

Double Free:

int* ptr = malloc(sizeof(int));
free(ptr);
free(ptr); // ERROR: Double free causes undefined behavior

Java's Garbage Collection Approach

Java solves these issues with automatic memory management:

✅ Pros:

• No dangling pointers (references become null when objects are collected)
• No memory leaks for reachable objects
• No double free errors

❌ Cons:

• Runtime overhead: GC pauses can cause unpredictable latency
• Memory overhead: Additional metadata for tracking objects
• No deterministic cleanup: Objects freed at GC's discretion, not immediately

// Java - memory managed automatically
String createString() {
    String s = new String("hello"); // Heap allocated
    return s; // Safe: GC will clean up when no longer referenced
} // No explicit cleanup needed

1. Ownership Rules

• Each value in Rust has a single owner.
• When the owner goes out of scope, the value is dropped (memory freed).
• Prevents double frees and memory leaks.

Example:

fn main() {
    let s = String::from("hello"); // `s` owns the string
    takes_ownership(s);            // Ownership moved → `s` is invalid here
    // println!("{}", s); // ERROR: borrow of moved value
}

fn takes_ownership(s: String) { 
    println!("{}", s); 
} // `s` is dropped here

2. Borrowing & References

• Allows immutable (&T) or mutable (&mut T) borrows.
• Enforced rules:
  • Either one mutable reference or multiple immutable references (no data races).
  • References must always be valid (no dangling pointers).

Example:

fn main() {
    let mut s = String::from("hello");
    let r1 = &s;     // OK: Immutable borrow
    let r2 = &s;     // OK: Another immutable borrow
    // let r3 = &mut s; // ERROR: Cannot borrow as mutable while borrowed as immutable
    println!("{}, {}", r1, r2);
}

3. Lifetimes

• Ensures references never outlive the data they point to.
• Prevents dangling references.

Example:

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

fn main() {
    let s1 = String::from("hello");
    let result;
    {
        let s2 = String::from("world");
        result = longest(&s1, &s2); // ERROR: `s2` doesn't live long enough
    }
    // println!("{}", result); // `result` would be invalid here
}

Why No Garbage Collector (GC)?

• Zero-cost abstractions: No runtime overhead.
• Predictable performance: Memory is freed deterministically.
• No runtime pauses: Unlike GC-based languages (Java, Go).

Key Takeaways

✅ Ownership: Prevents memory leaks.
✅ Borrowing: Prevents data races.
✅ Lifetimes: Prevents dangling pointers.

Rust's model ensures memory safety without runtime checks, making it both safe and fast.