Why do I get “does not live long enough” in a return value?

by

As pointed out in Stargateur’s answer, the reason for this is the lifetime of temporaries. While Rust does not have a full specification yet, the language reference still is quite good and at least gives a hint to understand the behaviour. Here is the relevant part from the section about temporary lifetimes:

[T]he lifetime of temporary values is typically

  • the innermost enclosing statement; the tail expression of a block is considered part of the statement that encloses the block, or
  • the condition expression or the loop conditional expression if the temporary is created in the condition expression of an if or in the loop conditional expression of a while expression.

The innermost enclosing statement of body.lock().unwrap() in the second version of your function is the return expression. The spec above states that this expression is “considered part of the statement that encloses the block”, which doesn’t really exist in this case, but it still gives the right idea: All variables local to the function body are dropped before any temporaries in the return expression is dropped, so body is dropped before the MutexGuard that borrows body. The fix you found makes sure the temporary is dropped before body, since local variables are dropped roughly in reverse order of their creation.

More Related Contents:

  1. What are Rust’s exact auto-dereferencing rules?
  2. Traits and Rust
  3. Why do try!() and ? not compile when used in a function that doesn’t return Option or Result?
  4. Why do I need to import a trait to use the methods it defines for a type?
  5. Why does linking lifetimes matter only with mutable references?
  6. Why is adding a lifetime to a trait with the plus operator (Iterator + ‘a) needed?
  7. Should trait bounds be duplicated in struct and impl?
  8. How do I collect into an array?
  9. Implementing a trait for multiple types at once
  10. How do I get an owned value out of a `Box`?
  11. How do I borrow a RefCell, find a key, and return a reference to the result? [duplicate]
  12. How can I store a Chars iterator in the same struct as the String it is iterating on?
  13. How to disable unused code warnings in Rust?
  14. Is there a way other than traits to add methods to a type I don’t own?
  15. What is the difference between `e1` and `&e2` when used as the for-loop variable?
  16. How do you use parent module imports in Rust?
  17. Why is the `Sized` bound necessary in this trait?
  18. Is it possible to return either a borrowed or owned type in Rust?
  19. How do I overcome match arms with incompatible types for structs implementing same trait?
  20. Example of how to use Conditional Compilation Macros in Rust
  21. How can I call a raw address from Rust?
  22. Is there a way to perform an index access to an instance of a struct?
  23. Is there any trait that specifies numeric functionality?
  24. Why would I use divergent functions?
  25. How to generate statically linked executables?
  26. Why can I iterate over a slice twice, but not a vector?
  27. When is it necessary to circumvent Rust’s borrow checker?
  28. How to idiomatically alias a crate in Rust 2018?
  29. How do I cope with lazy iterators?
  30. Can a struct containing a raw pointer implement Send and be FFI safe?

Leave a Comment