nomicon/src/lifetime-elision.md

Lifetime Elision

In order to make common patterns more ergonomic, Rust allows lifetimes to be elided in function signatures.

A lifetime position is anywhere you can write a lifetime in a type:

&'a T
&'a mut T
T<'a>

Lifetime positions can appear as either "input" or "output":

• For fn definitions, fn types, and the traits Fn, FnMut, and FnOnce, input refers to the types of the formal arguments, while output refers to result types. So fn foo(s: &str) -> (&str, &str) has elided one lifetime in input position and two lifetimes in output position. Note that the input positions of a fn method definition do not include the lifetimes that occur in the method's impl header (nor lifetimes that occur in the trait header, for a default method).

• For impl headers, all types are input. So impl Trait<&T> for Struct<&T> has elided two lifetimes in input position, while impl Struct<&T> has elided one.

Elision rules are as follows:

• Each elided lifetime in input position becomes a distinct lifetime parameter.

• If there is exactly one input lifetime position (elided or not), that lifetime is assigned to all elided output lifetimes.

• If there are multiple input lifetime positions, but one of them is &self or &mut self, the lifetime of self is assigned to all elided output lifetimes.

• Otherwise, it is an error to elide an output lifetime.

Examples:

fn print(s: &str);                                      // elided
fn print<'a>(s: &'a str);                               // expanded

fn debug(lvl: usize, s: &str);                          // elided
fn debug<'a>(lvl: usize, s: &'a str);                   // expanded

fn substr(s: &str, until: usize) -> &str;               // elided
fn substr<'a>(s: &'a str, until: usize) -> &'a str;     // expanded

fn get_str() -> &str;                                   // ILLEGAL

fn frob(s: &str, t: &str) -> &str;                      // ILLEGAL

fn get_mut(&mut self) -> &mut T;                        // elided
fn get_mut<'a>(&'a mut self) -> &'a mut T;              // expanded

fn args<T: ToCStr>(&mut self, args: &[T]) -> &mut Command                  // elided
fn args<'a, 'b, T: ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command // expanded

fn new(buf: &mut [u8]) -> BufWriter;                    // elided
fn new(buf: &mut [u8]) -> BufWriter<'_>;                // elided (with `rust_2018_idioms`)
fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a>          // expanded