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Lifetimes in Data Structures

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

// Copyright 2024 Google LLC
// SPDX-License-Identifier: Apache-2.0

#[derive(Debug)]
enum HighlightColor {
    Pink,
    Yellow,
}

#[derive(Debug)]
struct Highlight<'document> {
    slice: &'document str,
    color: HighlightColor,
}

fn main() {
    let doc = String::from("The quick brown fox jumps over the lazy dog.");
    let noun = Highlight { slice: &doc[16..19], color: HighlightColor::Yellow };
    let verb = Highlight { slice: &doc[20..25], color: HighlightColor::Pink };
    // drop(doc);
    dbg!(noun);
    dbg!(verb);
}
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. A struct cannot live longer than the data it references.
  • If doc is dropped before the end of the lifetime of noun or verb, 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.