Migration guide and comment

Author: wks

docs/userguide/src/migration/prefix.md

@@ -55,6 +55,9 @@ API changes:
         *   `is_movable`
         *   `is_reachable`
 -   module `memory_manager`
+    +   `is_mmtk_object`: It now requires the address parameter to be non-zero and word-aligned.
+        *   Otherwise it will not be a legal `ObjectReference` in the first place.  The user should
+            filter out such illegal values.
     +   The following functions no longer have the `<VM>` type argument.
         *   `find_object_from_internal_pointer`
         *   `is_in_mmtk_space`
@@ -68,6 +71,13 @@ API changes:
 -   trait `ObjectModel`
     +   `IN_OBJECT_ADDRESS_OFFSET`: removed because it is no longer needed.
 
+See also:
+
+-   PR: <https://github.com/mmtk/mmtk-core/issues/1170>
+-   Examples:
+    +   https://github.com/mmtk/mmtk-openjdk/pull/286: a simple case
+    +   https://github.com/mmtk/mmtk-jikesrvm/issues/178: a VM that needs much change for this
+
 ## 0.27.0
 
 ### `is_mmtk_object` returns `Option<ObjectReference>

src/util/linear_scan.rs

@@ -125,7 +125,7 @@ pub trait Region: Copy + PartialEq + PartialOrd {
         debug_assert!(self.start().as_usize() < usize::MAX - (n << Self::LOG_BYTES));
         Self::from_aligned_address(self.start() + (n << Self::LOG_BYTES))
     }
-    /// Return the region that contains the object (by its cell address).
+    /// Return the region that contains the object.
     fn containing(object: ObjectReference) -> Self {
         Self::from_unaligned_address(object.to_raw_address())
     }