Hello, I want to have State that I know will never borrow from View, but I can't convince the borrow checker. Adding 'static bound, which should make it clear that State cannot borrow, doesn't help.

I remember similar issue but with opaque types: `impl Trait + 'static` is not `'static` if returned from a generic function · Issue #76882 · rust-lang/rust · GitHub.

Edit: I replaced &mut with &, error still holds. I hoped that maybe using interior mutability as a workaround.

Code (playground):

trait View {
    type State: 'static;

    // initialize the state
    fn build_state(&self) -> Self::State;
    // `view` uses `self` and `state` to do computations. Return type omitted.
    fn render_tree(&self, state: &mut Self::State);
}

impl View for () {
    type State = ();

    fn build_state(&self) -> Self::State {
        ()
    }
    fn render_tree(&self, _state: &mut Self::State) {}
}

fn view_fn<'a>(_: &'a ()) -> impl View + use<'a> {
    ()
}

fn main() {
    let view = view_fn(&());
    let mut state = view.build_state();
    for _ in 0..10 {
        let unit = ();
        let view = view_fn(&unit);
        view.render_tree(&mut state);
    }
}

Error:

 1  error[E0597]: `unit` does not live long enough
   --> src/main.rs:26:28
    |
 25 |         let unit = ();
    |             ---- binding `unit` declared here
 26 |         let view = view_fn(&unit);
    |                            ^^^^^ borrowed value does not live long enough
 27 |         view.render_tree(&mut state);
 28 |     }
    |     - `unit` dropped here while still borrowed
 29 | }
    | - borrow might be used here, when `view` is dropped and runs the destructor for type `impl View`
    |
    = note: values in a scope are dropped in the opposite order they are defined

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

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