sink in subscripts don't move arguments

[seanbaxter]

public type Foo : Deinitializable {

  public memberwise init
  public var data : Int

  public subscript(_ arg: sink Int): Int {
    inout {
      yield &data
    }
  }
}

public fun f(_ arg: sink Int) { }

public fun main() {
  var obj = Foo(data:1)

  let x = 1;
  let y = 2;

  // This should relocate x.
  inout d = obj[x]
  &d = 50;

  // Can still print x. Why no error?
  print(x);

  // This should relocate y.
  f(y);

  // Error!
  print(y); 
}

I can use x after passing it to a subscript taking a sink parameter.
I cannot use y after passing it to a normal function taking a sink parameter.

[kyouko-taiga]

Minimal reproducer:

subscript p(_ arg: sink Int): Int {
  let { yield arg }
}

public fun use(_ x: Int) { }

public fun main() {
  var x = 42
  let d = p[x]
  use(d)
  use(x)
}

Problem seems to be that the abstract interpreter doesn't properly update the sate of x after the end d's projection. See corresponding the IR:

external fun FunctionDecl(2424834)() -> {} {
b0(%b0#0 : &{}):
  // var x = 42
  %i0.0: &Int = alloc_stack Int
  %i0.1: &word = subfield_view %i0.0, 0
  %i0.2: &word = access [set] %i0.1
  store i64(0x2a), %i0.2
  end_access %i0.2

  // let d = p[x]
  %i0.4: &Int = access [sink] %i0.0
  %i0.5: &Int = project SubscriptImpl(458754), %i0.4
  %i0.6: &Int = access [let] %i0.5

  // use(d)
  %i0.7: &{} = alloc_stack {}
  %i0.9: &{} = access [set] %i0.7
  call @FunctionDecl(851970)(%i0.6) to %i0.9
  end_access %i0.6
  end_project %i0.5      // <- %i0.0 should be considered moved here
  end_access %i0.4
  end_access %i0.9
  mark_state deinitialized %i0.7
  
  // use(x)
  %i0.13: &{} = alloc_stack {}
  %i0.14: &Int = access [let] %i0.0
  %i0.15: &{} = access [set] %i0.13
  call @FunctionDecl(851970)(%i0.14) to %i0.15
  end_access %i0.14
  end_access %i0.15
  mark_state deinitialized %i0.13
  dealloc_stack %i0.13
  dealloc_stack %i0.7
  mark_state deinitialized %i0.0
  dealloc_stack %i0.0
  mark_state initialized %b0#0
  return
}