1. Symptoms
When you encounter Rust compiler error E0713, the compiler produces output similar to the following:
error[E0713]: borrowed data escapes beyond mutable reference
--> src/main.rs:12:9
|
12 | let value = do_something(&mut x);
| ^^^^^^^^^^^^^^^^^ borrowed data escapes beyond mutable reference
|
= note: this is a special case of borrowed pointer being returned where the lifetime of the pointer is tied to the mutable reference. This typically happens when a closure captures a reference that must outlive the containing function.
help: consider changing the return type to capture the data with a higher lifetime
|
= note: the lifetime from the mutable reference is only valid within the containing function. To fix this, ensure that the lifetime is tied to the output of the function, not the input.
The error message indicates that a borrowed value is escaping the scope of a mutable reference, which violates Rust’s ownership and borrowing rules. You may see this error when working with closures, higher-order functions, or custom data structures that manage references across different lifetimes.
Additional symptoms include:
- Functions that accept
&mutreferences and return values derived from those references - Closures that capture references from their surrounding context
- Custom iterator implementations or async operations where lifetimes become complex
- Error messages that reference “escaping” or “beyond mutable reference”
2. Root Cause
Rust’s borrowing rules enforce strict memory safety guarantees, and error E0713 surfaces when those guarantees would be violated at compile time. The core issue stems from a mismatch between the lifetime of a mutable reference and the lifetime of data borrowed from it.
When you accept a &mut reference in a function signature, the compiler guarantees exclusive access to that data for the duration of the function’s execution. However, if you attempt to return a reference to data that originated from that mutable reference, the compiler must ensure the returned reference does not outlive the mutable reference itself. The mutable reference’s lifetime is tied to the scope where it was created, and the compiler prevents any borrow from escaping beyond that scope to avoid dangling references.
The most common scenarios that trigger E0713 involve functions returning references derived from mutable parameters. For instance, when a function receives a &mut T and tries to return a &'a T where 'a does not properly constrain the output lifetime, the compiler cannot verify safety. The mutable reference grants temporary exclusive access, and returning something tied to that reference would extend the borrow beyond the function’s scope, potentially creating use-after-free scenarios.
Closures present another frequent trigger for this error. When a closure captures a mutable reference from its environment and attempts to return a value that borrows from that captured reference, the lifetime of the returned value cannot exceed the closure’s own scope, but the compiler cannot always track this correctly through type inference alone.
3. Step-by-Step Fix
Solution 1: Adjust the return type to use the correct lifetime
When a function accepts a mutable reference and should return a reference with the same lifetime, the function signature must explicitly tie the output lifetime to the input.
Before:
fn first_or_default<'a>(slice: &'a mut [i32]) -> &'a i32 {
if slice.is_empty() {
&0 // ERROR: `0` is a temporary that does not live long enough
} else {
&slice[0]
}
}
After:
fn first_or_default<'a>(slice: &'a mut [i32]) -> Option<&'a mut i32> {
if slice.is_empty() {
None
} else {
Some(&mut slice[0])
}
}
This solution changes the return type to include the lifetime parameter and properly returns a mutable reference to an element within the slice, which is guaranteed to be valid for the lifetime 'a.
Solution 2: Return owned data instead of references
When you cannot guarantee that a reference will outlive the function call, return an owned value instead of a borrowed one.
Before:
fn process(data: &mut String) -> &str {
data.push_str(" processed");
&data[..] // ERROR: borrowed data escapes
}
After:
fn process(data: &mut String) -> String {
data.push_str(" processed");
data.clone()
}
By returning String instead of &str, you transfer ownership of the modified data to the caller, avoiding any lifetime constraints.
Solution 3: Use an explicit lifetime relationship in the function signature
When multiple references are involved, you may need to explicitly declare how their lifetimes relate to each other.
Before:
fn manipulate_and_return(input: &mut Data, output: &mut Data) -> &str {
// Complex logic that mixes references
// ERROR: borrowed data escapes
}
After:
fn manipulate_and_return<'a>(input: &'a mut Data, output: &'a mut Data) -> &'a str {
// Logic with proper lifetime annotation
// Now the compiler understands the relationship
}
Solution 4: Restructure to avoid capturing in closures
When closures cause this error, restructuring the code to avoid capturing mutable references improperly often resolves the issue.
Before:
fn build_string<'a>(input: &'a mut String) -> impl Fn() -> &'a str + 'a {
move || &input[..] // ERROR: captured value cannot outlive the closure
}
After:
fn build_string(input: &mut String) -> impl Fn() -> String + '_ {
let owned = input.clone();
move || owned.clone()
}
This approach removes the lifetime complexity by working with owned data throughout the closure’s lifecycle.
4. Verification
To confirm that the fix resolves error E0713, follow these verification steps:
First, compile your project to ensure no errors remain:
cargo build 2>&1
A successful build produces output without any error[E0713] messages. Look for the absence of the “borrowed data escapes beyond mutable reference” error text in the compilation output.
Next, verify the logic is preserved by running your test suite:
cargo test
All tests should pass, confirming that the lifetime changes did not alter the program’s semantics.
For functions that return references, create a test that exercises the returned reference:
fn main() {
let mut v = vec![1, 2, 3];
let reference = first_or_default(&mut v);
*reference = 10;
assert_eq!(v[0], 10); // Confirm mutation affects original
}
Compile and run this verification code to ensure the returned reference behaves correctly and maintains the connection to the original data when appropriate.
5. Common Pitfalls
When fixing E0713, developers frequently encounter several recurring mistakes that can lead to further complications or recurring errors.
The first pitfall is attempting to use 'static lifetime as a band-aid solution. While this may compile in some cases, it fundamentally changes the semantics of your function and can lead to unexpected behavior or memory leaks if the data does not actually have 'static lifetime.
The second common mistake is returning references to local temporaries. Any attempt to return &local_variable will fail because the local variable is dropped at the end of the function scope. Always ensure that returned references point to data with a lifetime at least as long as the return type’s lifetime parameter.
A third pitfall involves confusion between &T and &mut T in function signatures. When you have a mutable reference, you can obtain shared references to parts of that data, but the lifetime of those shared references is tied to the mutable reference’s lifetime. Misunderstanding this relationship leads to incorrect function signatures.
Developers also frequently misinterpret the compiler’s lifetime inference. Rust’s lifetime elision rules cover many common cases, but when you encounter E0713, the elision rules are insufficient, and you must explicitly annotate lifetimes. Failing to add the appropriate lifetime parameters when the compiler requests them is a common source of continued errors.
Finally, avoid using unsafe blocks to silence this error unless you have thoroughly analyzed the memory safety implications and can prove the code is correct. The compiler generates E0713 specifically to prevent memory safety violations, and circumventing this check with unsafe code requires deep expertise in Rust’s memory model.
6. Related Errors
E0502: Cannot borrow x as immutable because it is also borrowed as mutable
This error occurs when you have conflicting borrows of the same data at the same time. While E0713 focuses on data escaping beyond references, E0502 deals with simultaneous borrow conflicts that violate Rust’s aliasing rules.
E0506: Cannot assign to x because it is also borrowed
Similar to E0502, this error arises when a mutable assignment would invalidate an existing borrow. The borrow checker tracks these relationships and prevents operations that could lead to invalid memory access.
E0515: Cannot return value referencing local variable x
This error specifically addresses the case of returning a reference to a local variable. While E0713 is more general about data escaping, E0515 is a specific instance where the escaping data is a local variable that will be dropped.