But before I delve into my latest proposal, I want to briefly discuss a key difference between my previous model and this one:
"Types as Contracts" was a fully "validity"-based model, while "Stacked Borrows" is (to some extent) "access"-based.
+**Update:**
+Since publishing this post, I have written [another]({% post_url 2018-11-16-stacked-borrows-implementation %}) blog post about a slightly adjusted version of Stacked Borrows (the first version that actually got implemented).
+That other post is self-contained, so if you are just interested in the current state of Stacked Borrows, I suggest you go there.
+Only go on reading here if you want some additional historic context.
+**/Update**
+
## 1 Validity-based vs. Access-based
An "access"-based model is one where certain properties -- in this case, mutable references being unique and shared references pointing to read-only memory -- are only enforced when the reference is actually used to *access* memory.
let y = unsafe { & *raw }; // Now memory gets frozen (recording the timestamp)
let _val = *y; // Okay because memory was frozen since `y` was created
*x = 3; // This "reactivates" `x` by unfreezing and popping the stack.
- let z = unsafe { & *raw }; // Now memory gets frozen *again*
+ let z = &*x; // Now memory gets frozen *again*
*y // This is UB! Memory has been frozen strictly after `y` got created.
}
{% endhighlight %}