bind_mut() ownership error

[miruji]

In my project, I had a code example where I did this:

class RGlobal -> rustFunc() -> godotLoadEntity() -> use something from RGlobal

This leads to an ownership error with RGlobal.

Then I started moving the logic into Rust and got:

class RGlobal -> rustFunc() -> rustLoadEntity() -> use something from RGlobal

In the following variations, it still resulted in the same error:

fn loadEntity(&mut selfl) -> ()
{
    let global: &mut Gd<RGlobal> = &mut RGlobal::getGlobal(self.base().to_godot().upcast());
    let globalBind: GdMut<RGlobal> = global.bind_mut(); // <-- error

fn loadEntity(&mut self, mut rglobal: Gd<RGlobal>) -> ()
{
    let globalBind: GdMut<RGlobal> = global.bind_mut(); // <-- error

This made me wonder why this was happening, so I decided to share it here.

I'll say that I fixed the issue in only one way. In issue #1157, there was an interesting example with:

let _lock_self = self.base_mut();

It turned out this also works in my case:

fn loadEntity(&mut self, rglobal: &mut RGlobal) -> ()
{
  let rglobalBase: BaseMut<RGlobal> = rglobal.base_mut();
  let mut rglobal: Gd<RGlobal> = rglobalBase.to_godot().cast();
  let rglobalBind: GdMut<RGlobal> = rglobal.bind_mut(); // <-- ok

[TitanNano]

Rusts borrowing rules do not allow more than a single mutable borrow. Unfortunately, you can run into scenarios like yours where you call an engine API, which then causes a call back into the same rust class / struct. To enable multiple borrows of the same struct in the same call stack, you can use self.base_mut() as you discovered.
This is documented here: https://godot-rust.github.io/docs/gdext/master/godot/obj/trait.WithBaseField.html#method.base_mut

[Bromeon]

Yes, we should probably dedicate a whole page in the book to elaborate these topics.

I created godot-rust/book#100 for it.