UB post: clarify more

[web.git] / personal / _posts / 2017-07-17-types-as-contracts.md

diff --git a/personal/_posts/2017-07-17-types-as-contracts.md b/personal/_posts/2017-07-17-types-as-contracts.md
index 12276415b34b2d43dc20217d52d36de7717a1deb..bb867e78bcd9060690bafd94b568d903a397ce3a 100644 (file)
--- a/personal/_posts/2017-07-17-types-as-contracts.md
+++ b/personal/_posts/2017-07-17-types-as-contracts.md
@@ -283,11 +283,11 @@ For compound types like tuples, `struct` or `enum`, validation proceeds recursiv
 In particular, the enum discriminant is checked to be in-range (in particular, nothing can pass validation at an empty enum type).
 However, because the recursive validation will only lock memory that's actually covered by a field, we also have to acquire the appropriate lock for padding bytes and the enum discriminant here.
 
-Finally, at a reference type, two things happen.
-First, the reference itself is stored in memory somewhere; this memory has to be locked just like the validation of `i32` locks the memory used to store the integer.
-The reference is also checked to be non-NULL and properly aligned for the type it points to.
-Furthermore, validation proceeds recursively after *dereferencing* the reference.
-Crucially, the `mutbl` and `lft` for this recursive call are taking the reference type into account:
+Finally, when encountering a box or reference type, two things happen.
+First, the pointer itself is stored in memory somewhere; this memory has to be locked just like the validation of `i32` locks the memory used to store the integer.
+The pointer is also checked to be non-NULL and properly aligned for the type it points to.
+Furthermore, validation proceeds recursively after *dereferencing* the pointer.
+Crucially, for the case of validating a reference, the `mutbl` and `lft` for this recursive call are taking the type into account:
 If `lft` was `None` and this reference's lifetime ends within the function (i.e., there is a corresponding `EndRegion` somewhere), it is now set to the reference's lifetime.
 If `mutbl` was mutable, it becomes immutable when following a shared reference.
 
@@ -321,8 +321,7 @@ If a function takes an `x: &Cell<i32>`, following the rules above, it will acqui
 Clearly, we do not want to do that -- calling `x.set` *will* actually mutate `*x`, and mutating through a shared reference is exactly the point of using `Cell`!
 
 Lucky enough, the compiler *already* says that interior mutability is only allowed via [`UnsafeCell`](https://doc.rust-lang.org/beta/core/cell/struct.UnsafeCell.html).
-We can use this for our purposes: To adjust validation for interior mutability, we *stop* our recursive descent and do not do anything when reaching an `UnsafeCell` *while `mutbl` indicates we are in immutable mode*.
-(`&mut UnsafeCell` is not affected.)
+We can use this for our purposes: To adjust validation for interior mutability, we *stop* our recursive descent and do not do anything when reaching an `UnsafeCell` *while `mutbl` indicates we are in immutable mode* -- `&mut UnsafeCell` is not affected.
 In particular, no locks are acquired.
 This justifies calling `set` on a shared reference and having the value changed.
 Of course, it also means we cannot do some of the optimizations we discussed above -- but that's actually exactly what we want!