Improve the explanation of a context's extension slot.

src/contexts.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_CONTEXTS_H_
 #define V8_CONTEXTS_H_
 
 #include "src/heap/heap.h"
 #include "src/objects.h"
 
@@ skipping 280 matching lines @@
 //                contexts inside a function. It provides access to the
 //                incoming context (i.e., the outer context, which may
 //                or may not become the current function's context), and
 //                it provides access to the functions code and thus it's
 //                scope information, which in turn contains the names of
 //                statically allocated context slots. The names are needed
 //                for dynamic lookups in the presence of 'with' or 'eval'.
 //
 // [ previous  ]  A pointer to the previous context.
 //
-// [ extension ]  A pointer to a ContextExtension object, or "the hole". Used
-//                to implement 'with' statements and dynamic declarations
-//                (through 'eval'). The object in a 'with' statement is
-//                stored in the extension slot of a 'with' context.
-//                Dynamically declared variables/functions are also added
-//                to lazily allocated extension object. Context::Lookup
-//                searches the extension object for properties.
-//                For script and block contexts, contains the respective
-//                ScopeInfo. For block contexts representing sloppy declaration
-//                block scopes, it may also be a struct being a
-//                ContextExtension, pairing the ScopeInfo with an extension
-//                object.
+// [ extension ]  Additional data.
+//
+//                For script contexts, it contains the respective ScopeInfo.
+//
+//                For catch contexts, it contains a ContextExtension object
+//                consisting of the ScopeInfo and the name of the catch
+//                variable.
+//
+//                For module contexts, it contains the module object.
+//
+//                For block contexts, it contains either the respective
+//                ScopeInfo or a ContextExtension object consisting of the
+//                ScopeInfo and an "extension object" (see below).
+//
+//                For with contexts, it contains a ContextExtension object
+//                consisting of the ScopeInfo and an "extension object".
+//
+//                An "extension object" is used to dynamically extend a context
+//                with additional variables, namely in the implementation of the
+//                'with' construct and the 'eval' construct.  For instance,
+//                Context::Lookup also searches the extension object for
+//                properties.  (Storing the extension object is the original
+//                purpose of this context slot, hence the name.)
 //
 // [ native_context ]  A pointer to the native context.
 //
 // In addition, function contexts may have statically allocated context slots
 // to store local variables/functions that are accessed from inner functions
 // (via static context addresses) or through 'eval' (dynamic context lookups).
 // The native context contains additional slots for fast access to native
 // properties.
 //
 // Finally, with Harmony scoping, the JSFunction representing a top level
@@ skipping 204 matching lines @@
 #endif
 
   STATIC_ASSERT(kHeaderSize == Internals::kContextHeaderSize);
   STATIC_ASSERT(EMBEDDER_DATA_INDEX == Internals::kContextEmbedderDataIndex);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CONTEXTS_H_