Add release note and update docs

Author: ldionne

libcxx/docs/Hardening.rst

@@ -341,6 +341,12 @@ Vendors can use the following ABI options to enable additional hardening checks:
 
   ABI impact: changes the iterator type of ``vector`` (except ``vector<bool>``).
 
+- ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY`` -- changes the iterator type of ``std::array`` to a
+  bounded iterator that keeps track of whether it's within the bounds of the container and asserts it
+  on every dereference and when performing iterator arithmetic.
+
+  ABI impact: changes the iterator type of ``std::array``, its size and its layout.
+
 ABI tags
 --------
 

libcxx/docs/ReleaseNotes/20.rst

@@ -60,6 +60,9 @@ Improvements and New Features
   compile times and smaller debug information as well as better code generation if optimizations are disabled.
   The Chromium project measured a 5% reduction in object file and debug information size.
 
+- The ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY`` ABI configuration was added, which allows storing valid bounds
+  in ``std::array::iterator`` and detecting OOB accesses when the appropriate hardening mode is enabled.
+
 Deprecations and Removals
 -------------------------
 

libcxx/include/__configuration/abi.h

@@ -179,7 +179,7 @@
 // Changes the iterator type of `array` to a bounded iterator that keeps track of whether it's within the bounds of the
 // container and asserts it on every dereference and when performing iterator arithmetic.
 //
-// ABI impact: changes the iterator type of `array`.
+// ABI impact: changes the iterator type of `array`, its size and its layout.
 // #define _LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY
 
 // [[msvc::no_unique_address]] seems to mostly affect empty classes, so the padding scheme for Itanium doesn't work.