我们如何返回 static 引用到我们在 rust 中的 function 中创建的变量？

Question

struct Foo<'a>(&'a str);
impl<'a> Foo<'a> {
    fn get(&self) -> &'static str {self.0}
}
fn main() {
    let value: &str;
    {
        let foo = Foo("Test Value");
        value = foo.get();
    }
    println!("{}", value);
}

In this test code, I need to get the value stored in foo but as str is not sized, I can't just copy it's value. I even tried cloning the str. In a real life scenario, I could just use String or make a wrapper or encapsulate it in a box and return it but is there no way I can return a static reference to a variable I create in a function? If so, how does the as_str function work in rust?

----Edit----

In this example, Neither putting 'static before str in Foo nor returning value with lifetime 'a works. What to do then?

struct Foo<'a>(&'a [i32]);
impl<'a> Foo<'a> {
    fn get(&self) -> &'static [i32] {self.0}
}
fn main() {
    let value: &[i32];
    {
        let test_value = [0, 1, 2, 3];
        let foo = Foo(&test_value);
        value = foo.get();
    }
    println!("{:?}", value);
}

Answer 1

You can do it like this:

struct Foo<'a>(&'a str);
impl<'a> Foo<'a> {
    fn get(&self) -> &'a str {self.0}
}
fn main() {
    let value: &str;
    {
        let foo = Foo("Test Value");
        value = foo.get();
    }
    println!("{}", value);
}

Or like this:

struct Foo(&'static str);
impl Foo {
    fn get(&self) -> &'static str {self.0}
}
fn main() {
    let value: &str;
    {
        let foo = Foo("Test Value");
        value = foo.get();
    }
    println!("{}", value);
}

The reason your code doesn't compile is that you're declaring you are returning a reference with a (potentially) longer lifetime. Compare the lifetime of static with that of a ( self.0 has a lifetime of a ).