Rust has rules that allow lifetimes to be elided in various places where the compiler can infer a sensible default choice.

In order to make common patterns more ergonomic, lifetime arguments can be elided in function item, function pointer and closure trait signatures. The following rules are used to infer lifetime parameters for elided lifetimes. It is an error to elide lifetime parameters that cannot be inferred. The placeholder lifetime, '_, can also be used to have a lifetime inferred in the same way. For lifetimes in paths, using '_ is preferred. Trait object lifetimes follow different rules discussed below.

• Each elided lifetime in the parameters becomes a distinct lifetime parameter.
• If there is exactly one lifetime used in the parameters (elided or not), that lifetime is assigned to all elided output lifetimes.

In method signatures there is another rule

• If the receiver has type &Self or &mut Self, then the lifetime of that reference to Self is assigned to all elided output lifetime parameters.

Examples:

fn print(s: &str);                                      // elided
fn print(s: &'_ str);                                   // also 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 - preferred
fn new(buf: &mut [u8]) -> BufWriter;                    // elided
fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a>;         // expanded

type FunPtr = fn(&str) -> &str;                         // elided
type FunPtr = for<'a> fn(&'a str) -> &'a str;           // expanded

type FunTrait = dyn Fn(&str) -> &str;                   // elided
type FunTrait = dyn for<'a> Fn(&'a str) -> &'a str;     // expanded

The assumed lifetime of references held by a trait object is called its default object lifetime bound. These were defined in RFC 599 and amended in RFC 1156.

These default object lifetime bounds are used instead of the lifetime parameter elision rules defined above when the lifetime bound is omitted entirely. If '_ is used as the lifetime bound then the bound follows the usual elision rules.

If the trait object is used as a type argument of a generic type then the containing type is first used to try to infer a bound.

• If there is a unique bound from the containing type then that is the default
• If there is more than one bound from the containing type then an explicit bound must be specified

If neither of those rules apply, then the bounds on the trait are used:

• If the trait is defined with a single lifetime bound then that bound is used.
• If 'static is used for any lifetime bound then 'static is used.
• If the trait has no lifetime bounds, then the lifetime is inferred in expressions and is 'static outside of expressions.

// For the following trait...
trait Foo { }

// These two are the same as Box<T> has no lifetime bound on T
Box<dyn Foo>
Box<dyn Foo + 'static>

// ...and so are these:
impl dyn Foo {}
impl dyn Foo + 'static {}

// ...so are these, because &'a T requires T: 'a
&'a dyn Foo
&'a (dyn Foo + 'a)

// std::cell::Ref<'a, T> also requires T: 'a, so these are the same
std::cell::Ref<'a, dyn Foo>
std::cell::Ref<'a, dyn Foo + 'a>

// This is an error:
struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b>
TwoBounds<'a, 'b, dyn Foo> // Error: the lifetime bound for this object type
                           // cannot be deduced from context

Note that the innermost object sets the bound, so &'a Box<dyn Foo> is still &'a Box<dyn Foo + 'static>.

// For the following trait...
trait Bar<'a>: 'a { }

// ...these two are the same:
Box<dyn Bar<'a>>
Box<dyn Bar<'a> + 'a>

// ...and so are these:
impl<'a> dyn Foo<'a> {}
impl<'a> dyn Foo<'a> + 'a {}

// This is still an error:
struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b>
TwoBounds<'a, 'b, dyn Foo<'c>>

Both constant and static declarations of reference types have implicit 'static lifetimes unless an explicit lifetime is specified. As such, the constant declarations involving 'static above may be written without the lifetimes.

# #![allow(unused_variables)]
#fn main() {
// STRING: &'static str
const STRING: &str = "bitstring";

struct BitsNStrings<'a> {
    mybits: [u32; 2],
    mystring: &'a str,
}

// BITS_N_STRINGS: BitsNStrings<'static>
const BITS_N_STRINGS: BitsNStrings<'_> = BitsNStrings {
    mybits: [1, 2],
    mystring: STRING,
};
#}

Note that if the static or const items include function or closure references, which themselves include references, the compiler will first try the standard elision rules. If it is unable to resolve the lifetimes by its usual rules, then it will error. By way of example:

// Resolved as `fn<'a>(&'a str) -> &'a str`.
const RESOLVED_SINGLE: fn(&str) -> &str = ..

// Resolved as `Fn<'a, 'b, 'c>(&'a Foo, &'b Bar, &'c Baz) -> usize`.
const RESOLVED_MULTIPLE: &dyn Fn(&Foo, &Bar, &Baz) -> usize = ..

// There is insufficient information to bound the return reference lifetime
// relative to the argument lifetimes, so this is an error.
const RESOLVED_STATIC: &dyn Fn(&Foo, &Bar) -> &Baz = ..