src/test/ui/underscore-lifetimes.rs

// run-pass

#![allow(dead_code)]
struct Foo<'a>(&'a u8);

fn foo(x: &u8) -> Foo<'_> {
    Foo(x)
}

fn foo2(x: &'_ u8) -> Foo<'_> {
    Foo(x)
}

fn foo3(x: &'_ u8) -> Foo {
    Foo(x)
}

fn foo4(_: Foo<'_>) {}

struct Foo2<'a, 'b> {
    a: &'a u8,
    b: &'b u8,
}
fn foo5<'b>(foo: Foo2<'_, 'b>) -> &'b u8 {
    foo.b
}

fn main() {
    let x = &5;
    let _ = foo(x);
    let _ = foo2(x);
    let _ = foo3(x);
    foo4(Foo(x));
    let _ = foo5(Foo2 {
        a: x,
        b: &6,
    });
}