Chaining Arc<RefCell<..>> confuses borrowck.

[dpc]

http://is.gd/rNjThX

use std::sync::Arc;
use std::cell::RefCell;

struct A {
    a : Arc<RefCell<i32>>
}

struct B {
    b : Arc<RefCell<A>>
}

impl B {
    fn is_foo(&self) -> bool {
        *self.b.borrow().a.borrow() == 2
    }
}

This should compile, but it does not. Breaking the chain is a workaround.

[Aatch]

cc @pnkfelix is this an issue with destruction scopes specifically, or related to rvalue lifetimes in general?

[arielb1]

This is intentional well-known fallout of dropck - our current definition of strictly-outlives is not fine-grained enough.

[arielb1]

@Aatch

The issue here is that the first borrow must be drop-valid during the destruction of the second borrow, but the current destruction-scope implementation requires it to be drop-valid for the entire is_foo block.

[pnkfelix]

cc #22321

[pnkfelix]

this seems like a probable dupe of #22323

[alexcrichton]

Yeah I think this is just a dupe of #22323 as well.