Rust Error E0515 — Cannot Return Reference to Local Variable

What Does This Error Mean?

You tried to return a reference to a value that is created inside the function. When the function ends, that value is dropped — leaving the reference dangling. Return the owned value instead, or store it somewhere with a longer lifetime.

Affected Models

Rust stable
Rust nightly
Cargo build
Rust 2021 edition

Common Causes

Returning a reference to a String created inside the function
Returning a reference to a Vec or other heap value created locally
Returning a reference to a local variable from a block or loop
Creating a temporary value inside the function and returning a reference to it
Building a value with a method chain and returning a reference to an intermediate result

How to Fix It

Return the owned value directly instead of a reference to it.

Change fn get_name() -> &String to fn get_name() -> String and return the owned String.
If you must return a reference, accept the data as a parameter and return a reference to it.

fn first_word(s: &str) -> &str — the reference returned borrows from the input, which lives long enough.
For string data, return &'static str if the value is a string literal known at compile time.

fn greeting() -> &'static str { 'Hello' } — string literals live for the entire program duration.
Use Rc or Arc to share ownership of the value across caller and callee.

Return Rc<String> instead of &String — the caller and function both hold a reference-counted pointer to the data.
Restructure so the value is created by the caller and passed into the function.

Instead of the function creating and returning the value, have the caller provide a buffer that the function fills.

When to Call a Professional

E0515 is a critical safety error that Rust catches at compile time. In C or C++, this compiles but causes undefined behaviour — a serious security vulnerability. Fix it yourself by returning the owned value instead of a reference.

Frequently Asked Questions

Why does Rust prevent returning references to local variables?

When a function returns, all its local variables are dropped and their memory is freed. A reference to that memory would be a dangling pointer — pointing to freed memory. In C, this compiles silently and causes undefined behaviour or security vulnerabilities. Rust prevents it entirely.

What does a dangling reference mean in practice?

A dangling reference points to memory that has been freed. Reading it returns garbage data or crashes the program. Writing to it corrupts memory — a common source of security vulnerabilities in C programs. Rust makes it impossible to create dangling references.

What is the 'static lifetime and when should I use it?

The 'static lifetime means the data lives for the entire duration of the program. String literals like 'hello' have 'static lifetime — they are baked into the binary. Use 'static only for data that genuinely outlives all functions — not as a workaround for lifetime errors.