Rust / Editorial solution guide

Resolve E0515: a reference escapes local data

Identify the owner that ends at function return, choose owned output or a genuine input borrow, and understand why lifetime annotations cannot resurrect storage.

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Recognize the symptoms

E0515 reports a returned value referencing data owned by the current function. The function may return &str, a slice, an iterator, or another object containing a reference to local storage. The diagnostic identifies the local owner or temporary and the escaping return. There is no valid runtime output for the rejected program.

Adding a named lifetime or changing the return to &'static str does not make a local String survive. Similar ownership defects can produce related lifetime diagnostics in larger programs, so use the complete compiler spans instead of classifying every lifetime failure by one code alone.

Understand the likely causes

  • A reference borrows storage rather than taking ownership. When a function returns, its ordinary local owners are dropped unless their values are transferred out. A slice into that local data would point to expired storage if allowed to escape. Rust rejects this safe-code relationship instead of letting callers discover a dangling reference during execution.
  • Lifetime annotations express relationships among valid borrows and do not allocate or extend an owner scope. A function can return a borrow from a caller-owned input when that input supports the output lifetime. A function creating new data ordinarily needs an owned return type. Leaking memory is a specialized ownership decision, not a default lifetime repair.

Find the owner behind the returned view

Read through the return expression to the actual allocation. The rejected example formats a new String and returns its slice. The owner belongs to label and is dropped at return, so the reference cannot satisfy a caller lifetime. The explicit annotation illustrates the mistake: it describes a promise the implementation cannot fulfill. Keep the snippet non-runnable and do not attempt to test which text remains in freed storage.

rust Declaration or intentionally rejected example 
fn label<'a>(id: u32) -> &'a str {
    let text = format!("item-{id}");
    &text
}
fn main() { println!("{}", label(3)); }

Intentionally rejected with E0515; annotations do not extend text.

Transfer newly created data to the caller

Return String when creating a label. The caller becomes the owner and can retain it independently of the function frame. This correction does not require an extra clone of the local value; it transfers ownership through the return. Rust can optimize representation movement, but correctness does not depend on counting machine-level copies. The complete program below has a defined line because the returned owner remains alive during printing and is dropped afterward.

rust A returned owner with an independent lifetime 
fn label(id: u32) -> String { format!("item-{id}") }
fn main() {
    let owned = label(3);
    println!("{owned}");
}
Expected output
item-3

owned belongs to main after label returns. No reference to a dropped local is exposed.

Borrow only data whose owner actually survives

If the desired result is a view into caller input, accept &str and return an appropriately related slice without first creating a temporary owned buffer. For mixed unchanged and transformed results, Cow can represent borrowing or ownership explicitly. A struct containing a borrowed field must carry the same owner dependency. A stored closure returning a local borrow has the same fundamental problem as a direct function return. Do not convert to raw pointers to hide the relationship; that removes a check while leaving the storage obligation unsatisfied.

Verify the fix

Compile and run only the corrected complete example with the stated edition; expect item-3 and a newline. Add a caller that stores the returned String and uses it after label has returned, preserving the ownership requirement rather than testing only an immediate expression.

For an intentionally borrowed API, compile consumers with nested input scopes to confirm that use cannot escape the actual owner. Read rustc --explain E0515 for the official classification. Compile-only rejected cases are useful; executing unsafe substitutes is not a verification of the original safe design.

Keep it from recurring

  • Choose ownership at the signature before writing the body. Return owned values for newly created data, document the source owner of every escaping reference, and distinguish a static type bound from an actual static storage reference.

Official references