Recipe

Write out the lifetimes a signature leaves out

Use when: you want to know what a signature without lifetime names really promises, or why it is E0106

Goal: the signature with every lifetime written in, or the knowledge that you must choose them yourself

Taught in Elision rules

1. 1

   Give each & in the parameters that has no name its own fresh name: 'a, 'b, and so on. Count the ones inside tuples and structs too.

   Example

   fn first_field(line: &str, sep: &str) -> &str becomes fn first_field<'a, 'b>(line: &'a str, sep: &'b str) -> &str.

   Why: rule 1 treats every elided input lifetime as a separate parameter.

2. 2

   If there is exactly one input lifetime, write it on every output reference.

   Why: rule 2; with one source, the result can only borrow from it.

3. 3

   If there are several and one is &self or &mut self, write self's lifetime on every output reference.

   Why: rule 3; a method's result usually borrows from the value it was called on.

4. 4

   If the output has no lifetime yet, the signature is E0106. Choose the inputs the result really borrows from, as in section 2.

   • the result can come from several inputs — give them one shared name.
   • the result comes from one input — name only that input and the output.

   Why: the rules never read the body, so only you can say where the result comes from.