Rust Error E0515: cannot return reference to local variable

What does E0515 mean?

Rust error E0515 occurs when a function tries to return a reference to a value created inside the function. This is a compile-time error that prevents potentially unsafe or incorrect code from running.

Broken Code

// This will cause error E0515
fn get_string() -> &str {
    let s = String::from("hello");
    &s
}

Why This Happens

The Rust compiler performs strict checks on ownership, borrowing, lifetimes, and types at compile time. When it reports E0515, it is preventing code that could lead to:

• Use-after-free bugs (dangling references)
• Data races (concurrent unsynchronized access)
• Type confusion (mixing incompatible types)
• Undefined behavior (violating Rust's safety guarantees)

How to Fix E0515

The fix is to return an owned value instead of a reference.

// Fixed version
fn get_string() -> String {
    String::from("hello")
}

Step-by-Step Debugging

1. Read the full error message — the compiler usually points to the exact line

2. Check the "note" lines — they often explain the root cause

3. Trace ownership flow — find where the value is created, moved, and used

4. Decide on the fix — borrow, clone, restructure, or change types

FAQ

Is E0515 a runtime error?

No. E0515 is a compile-time error. The program will not run until the issue is fixed.

Should I always use .clone() to fix this?

Not necessarily. While clone() can work, it may not be the most efficient solution. Consider borrowing, restructuring ownership, or using references instead.

Why is Rust so strict about this?

Rust guarantees memory safety without a garbage collector. The strict rules prevent entire classes of bugs at compile time, making your code more reliable.

Related Errors

See other common Rust errors in the error reference.