Understanding dependent lifetimes

Question

How do I tell the compiler that one lifetime must outlive another?

use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize, Debug)]
pub struct Tokens<'a> {
    buffer: String,
    list: Vec<Token<'a>>,
}

#[derive(Serialize, Deserialize, Debug)]
pub struct Token<'a> {
    term: &'a str,
}

yields

error: lifetime may not live long enough
 --> src/pipeline/tokenizers/test.rs:6:5
  |
3 | #[derive(Serialize, Deserialize, Debug)]
  |                     ----------- lifetime `'de` defined here
4 | pub struct Tokens<'a> {
  |                   -- lifetime `'a` defined here
5 |     buffer: String,
6 |     list: Vec<Token<'a>>,
  |     ^^^^ requires that `'de` must outlive `'a`
  |
  = help: consider adding the following bound: `'de: 'a`

In the code above, the token.term: &str will always refer to a slice of tokens.buffer. I'm not sure how to specify that relationship. I'm also not sure how to add the requested bound.

Will this even work? If so, what's the magic syntax?

Answer 1

You can use the attribute, #[serde(borrow)], which makes the generated Deserialize implementation borrow the data.

use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize, Debug)]
pub struct Tokens<'a> {
    buffer: String,
    #[serde(borrow)]
    list: Vec<Token<'a>>,
}

#[derive(Serialize, Deserialize, Debug)]
pub struct Token<'a> {
    term: &'a str,
}

See:

• Borrowing Data in a Derived impl
• Serde field attributes (borrow)