Type System 7 @Nilstrieb

Warning: this quiz is still "work-in-progress", some questions might not have good explanations (or any at all), formatting/structure/titles/etc are not final, and so on. You might want to return here on a later date.

trait Cat {}

// meow!
impl<T> Cat for T {}

fn main() {
    let r = &0;
    require_box(Box::new(r));

    let local = 0;
    let r = &local;
    require_box(Box::new(r));
}

// Cats love boxes.
fn require_box(_a: Box<dyn Cat>) {}
Solution 
error[E0597]: `local` does not live long enough
  --> examples/trait_solver_7.rs:11:13
   |
10 |     let local = 0;
   |         ----- binding `local` declared here
11 |     let r = &local;
   |             ^^^^^^ borrowed value does not live long enough
12 |     require_box(Box::new(r));
   |                 ----------- cast requires that `local` is borrowed for `'static`
13 | }
   | - `local` dropped here while still borrowed
   |
   = note: due to object lifetime defaults, `Box<dyn Cat>` actually means `Box<(dyn Cat + 'static)>`

For more information about this error, try `rustc --explain E0597`.
error: could not compile `code` (example "trait_solver_7") due to 1 previous error

Box<dyn Cat> means Box<dyn Cat + 'static>, and the local variable is not 'static. &0 does produce a &'static i32 though, because it is implicitly static-promoted.