The reference outlives the data
You write a helper function to parse a configuration string. Inside the function, you create a temporary buffer, process the input, and try to return a slice of the result. The compiler rejects the code with E0597: "borrowed value does not live long enough." You stare at the variable. It is declared right there. It has a value. Why does Rust think it is gone?
The error means exactly what it says. You are holding a reference to a value that is about to be destroyed. The reference is a pointer to memory. The memory is being freed. The reference becomes a dangling pointer. Rust stops this at compile time. You cannot run code that dereferences freed memory.
What E0597 actually means
E0597 is a lifetime error. Lifetimes are the compiler's way of tracking how long values stay alive. Every value in Rust has an owner. When the owner goes out of scope, the value is dropped. If you create a reference to that value, the reference is allowed to exist only while the owner is alive.
E0597 triggers when the compiler proves that a reference will be used after the owner has been dropped. The reference outlives the data. That is impossible in safe Rust. The compiler enforces this rule to prevent use-after-free bugs.
The error message usually points to the variable that is dying too soon. It tells you the borrowed value does not live long enough. Sometimes it points to the place where the reference is used. The compiler is showing you the gap between the data's lifetime and the reference's required lifetime.
The mental model: Views, not claims
Coming from Python or JavaScript, this error feels backwards. In those languages, a reference keeps the object alive. If you have a variable pointing to an object, the garbage collector sees that reference and knows the object is still in use. The reference is a claim. It asserts, "This object must stay alive because I am using it."
Rust references are not claims. They are views. A reference is a passive pointer. It does not keep the data alive. It just points to data that someone else owns. If the owner drops the data, the reference points to nothing. The reference cannot save the data.
This distinction is the core of Rust's memory safety. In JavaScript, references form a graph, and the garbage collector walks the graph to find live objects. In Rust, ownership is a tree. The root owns the data. References are branches that point back to the trunk. If the trunk is cut down, the branches fall. References cannot become roots. They cannot resurrect the data.
E0597 is the compiler enforcing this rule. It sees a branch trying to survive after the trunk is gone. It rejects the code. You must restructure the ownership so the trunk lives as long as the branch needs it.
Minimal example: Returning a local reference
The most common trigger for E0597 is a function that tries to return a reference to a local variable.
fn get_greeting() -> &str {
let message = String::from("Hello, world!");
// message is owned by this function.
// It will be dropped when the function returns.
&message
// Returning &message creates a reference that escapes the function.
// The compiler sees that message dies before the reference is used.
}
fn main() {
let greeting = get_greeting();
println!("{}", greeting);
}
This code fails with E0597. The variable message is created inside get_greeting. It is owned by the function. When the function returns, message is dropped. The memory is freed. You are trying to return a reference to that memory. The reference would point to garbage.
The compiler walks through the scopes. It sees message is born at the start of the function and dies at the end. It sees the return value is a reference to message. That reference is used in main, which is outside the function. The reference lives longer than message. The compiler rejects the code.
The fix is to return the owned value instead of a reference.
fn get_greeting() -> String {
let message = String::from("Hello, world!");
message
// Returning message moves ownership to the caller.
// The data stays alive as long as the caller needs it.
}
fn main() {
let greeting = get_greeting();
println!("{}", greeting);
}
Now message is moved out of the function. The caller owns the String. The data lives as long as greeting lives. No dangling pointer. No error.
Realistic example: The inner scope trap
E0597 also appears when you create a variable inside a block and try to use a reference to it outside the block.
fn main() {
let reference;
{
let data = String::from("temporary");
reference = &data;
// data is created here.
// reference points to data.
}
// data is dropped here. The block ends.
println!("{}", reference);
// E0597: borrowed value does not live long enough.
// reference is used after data is dropped.
}
The variable data is owned by the inner block. When the block ends, data is dropped. The reference reference tries to live past the block. The compiler sees the mismatch. data dies at the closing brace. reference is used after the brace. E0597.
The fix is to move the owner out of the inner scope.
fn main() {
let data = String::from("temporary");
// data is now owned by main.
// It lives until the end of main.
let reference = &data;
// reference points to data.
// Both live until the end of main.
println!("{}", reference);
}
Now data lives for the entire duration of main. The reference is valid for the entire duration. The compiler is happy.
Convention: Returning owned values is cheap
When you fix E0597 by returning an owned value, you might worry about performance. In many languages, returning a large object copies the data. That is expensive.
Rust uses move semantics. Returning a String does not copy the data. It moves the ownership. The pointer, length, and capacity are transferred to the caller. The heap allocation stays where it is. No bytes are copied. The operation is as cheap as passing a pointer.
Returning owned values is idiomatic Rust. If a function creates data that needs to survive, return the owned type. The caller gets full control. They can choose to borrow it, move it, or drop it. This flexibility is better than returning a reference with restrictive lifetimes.
There is a convention for cloning Rc and Arc. When you clone a reference-counted pointer, write Rc::clone(&value) instead of value.clone(). This makes it clear that you are cloning the pointer, not the data. For plain owned types, just return the value. The move is implicit and efficient.
Pitfalls and related errors
E0597 often appears alongside E0515. E0515 says "returns a value referencing data owned by the current function." The root cause is the same. The data dies too soon. The error code differs based on the exact phrasing of the diagnostic, but the fix is the same. Extend the lifetime of the data or return the owned value.
Do not try to bypass E0597 with unsafe. You can cast a reference to a raw pointer and dereference it in an unsafe block. That suppresses the error. It also guarantees a crash or memory corruption when you run the code. The compiler is not guessing. It knows exactly when memory is freed. If it says the value is dead, it is dead. unsafe does not bring it back.
Do not try to fix E0597 by adding &'static. A &'static reference promises that the data lives for the entire program. Local variables do not live that long. Adding &'static lies to the compiler. The error remains. The only way to get a &'static reference is from a string literal or a static variable. Local data cannot be static.
Sometimes you see advice to use Cow. Cow stands for "Clone on Write." It is an enum that can hold either a borrowed reference or an owned value. Cow is useful when you want to avoid allocation if the data does not change. If the data is already borrowed, you store the reference. If you need to modify it, you clone into an owned value. Cow helps with E0597 when you have a function that sometimes returns a slice of input and sometimes returns a new string. The return type can be Cow<str>. This avoids allocation in the borrow case. It is an optimization, not a general fix. Start with owned returns. Reach for Cow only when profiling shows allocation is the bottleneck.
Decision matrix
Use owned returns when the function creates data that needs to survive past the function boundary. Return String instead of &str. Return Vec<T> instead of &[T]. Move semantics make this cheap and idiomatic.
Reach for input references when the data exists elsewhere and the function only needs to read or modify it. Pass &str or &[T] as arguments. The caller owns the data. The function borrows it. The reference cannot outlive the input.
Pick Cow when the function sometimes returns a slice of the input and sometimes returns a newly allocated value. Cow lets you return a reference when possible and an owned value when necessary. This avoids allocation in the common case.
Use Rc when multiple parts of the program need to share ownership of the same data. Rc keeps the data alive as long as any Rc exists. This solves E0597 by extending the lifetime through reference counting. Use Rc for single-threaded code. Use Arc for multi-threaded code.