rust lifetime specifier

Give the following example:

struct A {}

fn foo<'c>() -> &'c A {
    let a = A {};
    &a
}

fn main() {
    let b = foo();
}

That code will not compiler, because variable a does not live as long as function foo, since foo has a lifetime of c. After foo function is called, the variable a will go out of scope.

To correct it,

struct A {}

fn foo<'a>(c: &'a A) -> &'a A {
    c
}

fn main() {
    let d = &A {};
    let b = foo(d);
}

This will work because d’s lifetime is longer than function foo. and after function main is out, the free sequence is like: free b -> free d. so d’s lifetime is bigger than b.