Rust 2018 allows you to explicitly mark where a lifetime is elided, for types where this elision might otherwise be unclear. To do this, you can use the special lifetime '_ much like you can explicitly mark that a type is inferred with the syntax let x: _ = ..;.

Let's say, for whatever reason, that we have a simple wrapper around &'a str:

# #![allow(unused_variables)]
#fn main() {
struct StrWrap<'a>(&'a str);
#}

In Rust 2015, you might have written:

# #![allow(unused_variables)]
#fn main() {
// Rust 2015

use std::fmt;

# struct StrWrap<'a>(&'a str);

fn make_wrapper(string: &str) -> StrWrap {
    StrWrap(string)
}

impl<'a> fmt::Debug for StrWrap<'a> {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.write_str(self.0)
    }
}
#}

In Rust 2018, you can instead write:

# #![allow(unused_variables)]
#fn main() {
# use std::fmt;
# struct StrWrap<'a>(&'a str);

// Rust 2018

fn make_wrapper(string: &str) -> StrWrap<'_> {
    StrWrap(string)
}

impl fmt::Debug for StrWrap<'_> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.write_str(self.0)
    }
}
#}

In the Rust 2015 snippet above, we've used -> StrWrap. However, unless you take a look at the definition of StrWrap, it is not clear that the returned value is actually borrowing something. Therefore, starting with Rust 2018, it is deprecated to leave off the lifetime parameters for non-reference-types (types other than & and &mut). Instead, where you previously wrote -> StrWrap, you should now write -> StrWrap<'_>, making clear that borrowing is occurring.

What exactly does '_ mean? It depends on the context! In output contexts, as in the return type of make_wrapper, it refers to a single lifetime for all "output" locations. In input contexts, a fresh lifetime is generated for each "input location". More concretely, to understand input contexts, consider the following example:

# #![allow(unused_variables)]
#fn main() {
// Rust 2015

struct Foo<'a, 'b: 'a> {
    field: &'a &'b str,
}

impl<'a, 'b: 'a> Foo<'a, 'b> {
    // some methods...
}
#}

We can rewrite this as:

# #![allow(unused_variables)]
#fn main() {
# struct Foo<'a, 'b: 'a> {
#     field: &'a &'b str,
# }

// Rust 2018

impl Foo<'_, '_> {
    // some methods...
}
#}

This is the same, because for each '_, a fresh lifetime is generated. Finally, the relationship 'a: 'b which the struct requires must be upheld.

For more details, see the tracking issue on In-band lifetime bindings.