Curriculum Ownership & borrowing Lifetime intuition exercise 2 · mcq-explain

Lifetime intuition

rustc rejects this function with error[E0515]: cannot return reference to local variables``. Pick the explanation that names the real problem.

&s is &String — &String does not auto-deref to &str in return position; replace with &s[..].

The signature needs a lifetime annotation: fn first<'a>(s: String) -> &'a str — that tells the compiler the lifetime relationship.

String slicing must use s.as_str() rather than &s; the explicit method bypasses the borrow checker.

s is OWNED by the function (moved in by value). When the function returns, s is dropped — the returned reference would point at freed memory. The fix is to change the signature: either accept &str and return &str (caller keeps the value alive), or return String (transfer ownership out).

About this theme

Lifetimes are not a separate runtime feature; they describe how long references are valid. This theme avoids advanced annotation syntax at first and focuses on why Rust rejects dangling references. Planned exercises: 1. choose the value that lives long enough. 2. recognise a dangling reference. 3. choose owned return instead of borrowed return. 4. fill a scope change that extends an owner. 5. fill a returned String. 6. write a function that returns one of two borrowed inputs with an explicit lifetime. 7. write code that avoids returning a local reference. 8. translate a TS closure over local data. 9. fix a lifetime error by changing ownership rather than adding random annotations.