/js/src/frontend/Stencil.cpp has been updated (a87db3b6)

[github-actions[bot]]

Files

• /js/src/frontend/Stencil.cpp

Changesets

• https://hg.mozilla.org/mozilla-central/rev/a87db3b698e19b65bc001936fc7a8b80239a25dd Bug 1638309 - Better document the debugger-eval scope hack r=tcampbell DONTBUILD

Diffs

/js/src/frontend/Stencil.cpp

--- e081749eef4040050988375205004d666a6b018c/js/src/frontend/Stencil.cpp
+++ a87db3b698e19b65bc001936fc7a8b80239a25dd/js/src/frontend/Stencil.cpp
@@ -754,16 +754,39 @@ void ScopeContext::cacheEnclosingScope(c
       case ScopeKind::WasmFunction:
         MOZ_CRASH("No direct eval inside wasm functions");
     }
   }

   MOZ_CRASH("Malformed scope chain");
 }

+// Given an input scope, possibly refine this to a more precise scope.
+// This is used during eval in the debugger to provide the appropriate scope and
+// ThisBinding kind and environment, which is key to making private field eval
+// work correctly.
+//
+// The trick here is that an eval may have a non-syntatic scope but nevertheless
+// have an 'interesting' environment which can be traversed to find the
+// appropriate scope the the eval to function as desired. See the diagram below.
+//
+// Eval Scope    Eval Env         Frame Env    Frame Scope
+// ============  =============    =========    =============
+//
+// NonSyntactic
+//    |
+//    v
+//   null        DebugEnvProxy                 LexicalScope
+//                     |                            |
+//                     v                            v
+//               DebugEnvProxy --> CallObj --> FunctionScope
+//                     |              |             |
+//                     v              v             v
+//                    ...            ...           ...
+//
 InputScope ScopeContext::determineEffectiveScope(InputScope& scope,
                                                  JSObject* environment) {
   MOZ_ASSERT(effectiveScopeHops == 0);
   // If the scope-chain is non-syntactic, we may still determine a more precise
   // effective-scope to use instead.
   if (environment && scope.hasOnChain(ScopeKind::NonSyntactic)) {
     JSObject* env = environment;
     while (env) {