Why does this static life time doesn't live long enough?

Question

I have the following code which I would expect to work.

let mut some_str: &'static str = "Hello world";
let _borrow: &'static mut &'static str = &mut some_str;

However when I compile this I get:

  --> src/lib.rs:26:50
   |
25 |         let mut some_str: &'static str = "Hello world";
   |             ------------ binding `some_str` declared here
26 |         let _borrow: &'static mut &'static str = &mut some_str;
   |                      -------------------------   ^^^^^^^^^^^^^ borrowed value does not live long enough
   |                      |
   |                      type annotation requires that `some_str` is borrowed for `'static`
27 |     }
   |     - `some_str` dropped here while still borrowed

This tells me that some_str is being dopped before borrow however, both references are static so I would expect both references to have the same lifetime, which is not the case. Why does this happens? Is a static life time shorter than another or am I misunderstanding the error code?

Answer 1

The lifetime annotation always describes the lifetime of the pointee, not the pointer. The data some_str is pointing to has static lifetime. The pointer itself is a local variable, and as all local variables it is dropped at the end of the function.