What are lifetimes and when must you annotate them?

Summary

Lifetimes describe how long references are valid so the compiler can prove they never outlive their data. Most are inferred; you annotate when a function returns a reference that could come from several inputs, or when a struct stores references.

Longer Answer

Annotations don't change how long values live; they express relationships (e.g. the result lives as long as both inputs).

Key Points

• Describe, don't extend, lifetimes.
• Needed for ambiguous returned references and structs with references.
• 'static lives for the whole program.

Deep dive: Rust Fundamentals: Lifetimes