Lifetimes in Data Structures

If a data type stores borrowed data, it must be annotated with a lifetime:

  1. #[derive(Debug)]
  2. struct Highlight<'doc>(&'doc str);
  4. fn erase(text: String) {
  5.     println!("Bye {text}!");
  6. }
  8. fn main() {
  9.     let text = String::from("The quick brown fox jumps over the lazy dog.");
  10.     let fox = Highlight(&text[4..19]);
  11.     let dog = Highlight(&text[35..43]);
  12.     // erase(text);
  13.     println!("{fox:?}");
  14.     println!("{dog:?}");
  15. }

This slide should take about 5 minutes.

  • In the above example, the annotation on Highlight enforces that the data underlying the contained &str lives at least as long as any instance of Highlight that uses that data.
  • If text is consumed before the end of the lifetime of fox (or dog), the borrow checker throws an error.
  • Types with borrowed data force users to hold on to the original data. This can be useful for creating lightweight views, but it generally makes them somewhat harder to use.
  • When possible, make data structures own their data directly.
  • Some structs with multiple references inside can have more than one lifetime annotation. This can be necessary if there is a need to describe lifetime relationships between the references themselves, in addition to the lifetime of the struct itself. Those are very advanced use cases.