`Pin` can't borrow multiple fields mutable (even if type implements `Unpin`).

[zvavybir]

Fields of Pin<&mut T> can't be independently borrowed from each other.

I tried this code:

struct TestStruct
{
    a: u8,
    b: u8,
}

fn t(mut ts: Pin<&mut TestStruct>)
{
    let x = &mut ts.a;
    let y = &mut ts.b;

    *y = *x;
}

I expected to see this happen: Nothing, it compiles fine.

Instead, this happened: I get the following error message:

 error[E0499]: cannot borrow `ts` as mutable more than once at a time
  --> src/main.rs:12:18
   |
11 |     let x = &mut ts.a;
   |                  -- first mutable borrow occurs here
12 |     let y = &mut ts.b;
   |                  ^^ second mutable borrow occurs here
13 | 
14 |     *y = *x;
   |          -- first borrow later used here

For more information about this error, try `rustc --explain E0499`.

I also think that the mut directly behind fn t is wrong, because I don't change the reference itself but that to which it points, but if I remove it rustc prints it's usual cannot borrow … as mutable, as it is not declared as mutable.

Meta

rustc --version --verbose:

rustc 1.57.0-nightly (4e89811b4 2021-10-16)
binary: rustc
commit-hash: 4e89811b46323f432544f9c4006e40d5e5d7663f
commit-date: 2021-10-16
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0

It also doesn't work on current stable.

[veber-alex]

To get from the Pin to the inner TestStruct you go through the DerefMut trait.
The error is more clear if you write the code like this:

let x = &mut ts.deref_mut().a;
let y = &mut ts.deref_mut().b;

You can see that the second call to deref_mut happens while a mutable borrow from ts is still alive.
The solution here is to use DerefMut only once.

fn t(mut ts: Pin<&mut TestStruct>) {
    let d = &mut *ts;
    let x = &mut d.a;
    let y = &mut d.b;

    *y = *x;
}

Perhaps a better diagnostic message will help in situations like this.

[zvavybir]

Perhaps a better diagnostic message will help in situations like this.

That would be a good idea. Thank you!