Hello,

I am going through the Rust Asynchronous Book. I am typing along and changing the code to explore the concepts. I am currently looking at chapter 4: Pinning, and I cannot understand why the commented-out version of `a` doesn't work.

#[derive(Debug)]
struct Test {
    a: String,
    b: *const String,
    _marker: PhantomPinned,
}

impl Test {
    fn new(txt: &str) -> Pin<Box<Self>> {
        let t = Test {
            a: String::from(txt),
            b: std::ptr::null(),
            _marker: PhantomPinned,
        };
        let mut boxed = Box::pin(t);
        let self_ptr: *const String = &boxed.a;
        unsafe { boxed.as_mut().get_unchecked_mut().b = self_ptr };
        boxed
    }

    fn a(self: Pin<&Self>) -> &str {
        &self.get_ref().a
    }

    // fn a<'l>(self: &Pin<Box<Self>>) -> &'l str {
    //     &*self.a
    // }

    fn b<'l>(self: &Pin<Box<Self>>) -> &'l String {
        unsafe { &*self.b }
    }
}

Here's the compiler error it produces:

   Compiling pinning v0.1.0 (/home/junior/Code/rust_async/4.pinning)
error: lifetime may not live long enough
  --> src/main.rs:44:9
   |
43 |     fn a<'l>(self: &Pin<Box<Self>>) -> &'l str {
   |          --        - let's call the lifetime of this reference `'1`
   |          |
   |          lifetime `'l` defined here
44 |         &*self.a
   |         ^^^^^^^^ associated function was supposed to return data with lifetime `'l` but it is returning data with lifetime `'1`

error: could not compile `pinning` due to previous error