X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/351dcf54d4afe8ff5baa83672d36531c0d3e901a..16cfabef240051c7d839ed75e19fb34ce8aeffe8:/personal/_posts/2018-11-16-stacked-borrows-implementation.md?ds=inline

diff --git a/personal/_posts/2018-11-16-stacked-borrows-implementation.md b/personal/_posts/2018-11-16-stacked-borrows-implementation.md
index f8f4950..df60719 100644
--- a/personal/_posts/2018-11-16-stacked-borrows-implementation.md
+++ b/personal/_posts/2018-11-16-stacked-borrows-implementation.md
@@ -314,7 +314,7 @@ fn demo2() {
 
   // The `x` here really is a `&*x`, but we have already seen above what
   // happens: `Uniq(0)` must be in the stack, but we leave it unchanged.
-  let z = x as *const u8 as *mut u8; // tag irrelevant because raw
+  let z = x as *const u8 as *mut u8; // tag erased: `Shr(None)`
   // stack: [Uniq(0), Shr]; frozen since 1
 
   // A write access through a raw pointer: Unfreeze the location and make sure
@@ -352,8 +352,7 @@ The following checks are done on every pointer dereference, for every location
 covered by the pointer (`size_of_val` tells us how many bytes the pointer
 covers):
 
-1. If this is a raw pointer, do nothing and reset the tag used for the access to
-   `Shr(None)`.  Raw accesses are checked as little as possible.
+1. If this is a raw pointer, do nothing.  Raw accesses are checked as little as possible.
 2. If this is a unique reference and the tag is `Shr(Some(_))`, that's an error.
 3. If the tag is `Uniq`, make sure there is a matching `Uniq` item with the same
    ID on the stack.
@@ -370,15 +369,15 @@ covers):
 [section 2.3]: #23-accessing-memory
 
 On an actual memory access, we know the tag of the pointer that was used to
-access (unless it was a raw pointer, in which case the tag we see is
-`Shr(None)`), and we know whether we are reading from or writing to the current
-location.  We perform the following operations on all locations affected by the
-access:
+access (we always use the actual tag and disregard the type of the pointer), and
+we know whether we are reading from or writing to the current location.  We
+perform the following operations on all locations affected by the access:
 
 1. If the location is frozen and this is a read access, nothing happens (even
    if the tag is `Uniq`).
-2. Unfreeze the location (set `frozen_since` to `None`).  Either the location is
-   already unfrozen, or this is a write.
+2. Otherwise, if this is a write access, unfreeze the location (set
+   `frozen_since` to `None`).  (If this is a read access and we come here, the
+   location is already unfrozen.)
 3. Pop the stack until the top item matches the tag of the pointer.
     - A `Uniq` item matches a `Uniq` tag with the same ID.
     - A `Shr` item matches any `Shr` tag (with or without timestamp).
@@ -450,9 +449,10 @@ reference type get retagged.  On every assignment, if the assigned value is of
 reference type, it gets retagged.  Moreover, we do this even when the reference
 value is inside the field of a `struct` or `enum`, to make sure we really cover
 all references.  (This recursive descent is already implemented, but the
-implementation has not landed yet.)  However, we do *not* descend recursively
-through references: Retagging a `&mut &mut u8` will only retag the *outer*
-reference.
+implementation has not landed yet.)  Finally, `Box` is treated like a mutable
+reference, to encode that it asserts unique access.  However, we do *not*
+descend recursively through references: Retagging a `&mut &mut u8` will only
+retag the *outer* reference.
 
 Retagging is the *only* operation that generates fresh tags.  Taking a reference
 simply forwards the tag of the pointer we are basing this reference on.
@@ -482,12 +482,12 @@ fn demo0() {
 }
 {% endhighlight %}
 
-For each reference, `Retag` does the following (we will slightly refine these
-instructions later) on all locations covered by the reference (again, according
-to `size_of_val`):
+For each reference and `Box`, `Retag` does the following (we will slightly
+refine these instructions later) on all locations covered by the reference
+(again, according to `size_of_val`):
 
-1. Compute a fresh tag, `Uniq(_)` for a mutable reference and `Shr(Some(_))` for
-   a shared reference.
+1. Compute a fresh tag: `Uniq(_)` for mutable references and `Box`, and
+   `Shr(Some(_))` for shared references.
 2. Perform the checks that would also happen when we dereference this reference.
 3. Perform the actions that would also happen when an actual access happens
    through this reference (for shared references a read access, for mutable
@@ -528,7 +528,7 @@ fn demo4() {
   EscapeToRaw(x);
   // stack: [Uniq(0), Shr]; not frozen
   
-  let y1 = x as *mut u8;
+  let y1 = x as *mut u8; // tag erased: `Shr(None)`
   let y2 = y1;
   unsafe {
     // All of these first dereference a raw pointer (no checks, tag gets
@@ -556,6 +556,11 @@ pointer": The steps are the same as for `Retag` above, except that the new
 pointer's tag is `Shr(None)` and we do not freeze (i.e., we behave as if the
 entire pointee was inside an `UnsafeCell`).
 
+This is needed, because when casting a reference to a raw pointer (and actually,
+on any cast involving pointers), the tag gets erased, meaning it gets reset to
+`Shr(None)`.  Thanks to `EscapeToRaw`, there is a matching `Shr` on the stack,
+making sure the raw pointer can actually be used.
+
 Knowing about both `Retag` and `EscapeToRaw`, you can now go back to `demo2` and
 should be able to fully explain why the stack changes the way it does in that
 example.
@@ -648,8 +653,8 @@ With this extension, the instructions for retagging and `EscapeToRaw` now look
 as follows (again executed on all locations covered by the reference, according
 to `size_of_val`):
 
-1. Compute a fresh tag: `Uniq(_)` for a mutable reference, `Shr(Some(_))` for a
-   shared reference, `Shr(None)` if this is `EscapeToRaw`.
+1. Compute a fresh tag: `Uniq(_)` for mutable references and `Box`,
+   `Shr(Some(_))` for shared references, `Shr(None)` if this is `EscapeToRaw`.
 2. Perform the checks that would also happen when we dereference this reference.
    Remember the position of the item matching the tag in the stack.
 3. Redundancy check: If the new tag passes the checks performed on a
@@ -916,3 +921,9 @@ If you want to
 help or report results of your experiments, if you have any questions or
 comments, please join the
 [discussion in the forums](https://internals.rust-lang.org/t/stacked-borrows-implemented/8847).
+
+## Changelog
+
+**2018-11-21:** Dereferencing a pointer now always preserves the tag, but
+casting to a raw pointer resets the tag to `Shr(None)`.  `Box` is treated like a
+mutable reference.