rust lifetime_error ai_generated true

error: lifetime may not live long enough -- returning this value requires that `'1` must outlive `'2`

ID: rust/e0700-async-closure-lifetime

Also available as: JSON · Markdown
82%Fix Rate
85%Confidence
50Evidence
2019-11-01First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
1 active — — —

Root Cause

Async closures capture references that may not outlive the future they produce. The solution depends on context: usually adding 'move' to take ownership, or using named lifetimes on the function signature.

generic

Workarounds

  1. 85% success Add 'move' to async closures to take ownership of captured variables 
    Change 'tokio::spawn(async { ... })' to 'tokio::spawn(async move { ... })'. The 'move' keyword transfers ownership of all captured variables into the closure, satisfying the 'static bound that spawn requires.

    Sources: https://doc.rust-lang.org/book/ch13-01-closures.html#moving-captured-values-out-of-closures-and-the-fn-traits

  2. 80% success Use named lifetimes on the function signature 
    Instead of 'async fn foo(s: &str) -> &str', use 'async fn foo<'a>(s: &'a str) -> &'a str'. Named lifetimes show the compiler the relationship between input and output references.

    Sources: https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html

  3. 78% success Use Pin<Box<dyn Future + Send + 'static>> for trait-object futures 
    When returning futures from trait methods or storing them in collections, the compiler cannot infer lifetimes. Boxing with explicit bounds resolves it: fn foo() -> Pin<Box<dyn Future<Output = T> + Send + 'static>>.

Dead Ends

Common approaches that don't work:

  1. Adding 'static lifetime bound to everything 70% fail

    'static means the value lives for the entire program. This is overly restrictive: it prevents borrowing local data, forces all data to be owned or leaked, and leads to excessive .clone() calls. Most async code does not need 'static — it needs clear ownership.

  2. Using Box::leak to make references 'static 90% fail

    Box::leak deliberately leaks memory. The value is never freed. This 'works' at the cost of a memory leak every time the code runs. It is a code smell, not a solution.

  3. Wrapping everything in Arc<Mutex<T>> 60% fail

    Arc<Mutex<T>> adds unnecessary synchronization overhead and complexity. Often the issue is simply that the closure needs 'move' to take ownership, or the function needs a named lifetime — not interior mutability.

Error Chain

Leads to:
Frequently confused with: