Improve support for the synthetic scope above the top module.

Author: whitequark

example/.vscode/settings.json

@@ -4,7 +4,7 @@
     ],
     "rtlDebugger.watchList": [
         {
-            "id": "data"
+            "id": "top data"
         }
     ],
     "rtlDebugger.variableOptions": {

example/design_sim.cc

@@ -13,7 +13,7 @@ int main(int argc, char **argv) {
   }
 
   cxxrtl_design::p_top top;
-  cxxrtl::agent<cxxrtl_design::p_top> agent(cxxrtl::spool("spool.bin"), top);
+  cxxrtl::agent<cxxrtl_design::p_top> agent(cxxrtl::spool("spool.bin"), top, "top ");
 
   std::string uri = agent.start_debugging(cxxrtl::tcp_link());
   fprintf(stderr, "Simulation started on %s\n", uri.c_str());

src/debug/session.ts

@@ -108,6 +108,23 @@ export class Session {
                 scope: scopeName,
             });
             this.rootScopeDesc = response.scopes[scopeName];
+            if (this.rootScopeDesc === undefined) {
+                // This can happen if the root scope has never been defined anywhere, i.e. if it
+                // is synthesized for the simulation, e.g. by passing `"top "` as the last argument
+                // to the CXXRTL agent constructor.
+                this.rootScopeDesc = {
+                    type: 'module',
+                    definition: {
+                        src: null,
+                        name: null,
+                        attributes: {}
+                    },
+                    instantiation: {
+                        src: null,
+                        attributes: {}
+                    },
+                };
+            }
         }
         return Scope.fromCXXRTL(
             scopeName,

src/ui/sidebar.ts

@@ -161,6 +161,7 @@ class ScopeTreeItem extends TreeItem {
     constructor(
         provider: TreeDataProvider,
         readonly scope: Scope,
+        readonly hasSiblings: boolean,
     ) {
         super(provider);
     }
@@ -183,19 +184,23 @@ class ScopeTreeItem extends TreeItem {
                 }
                 treeItem.command = this.scope.location.asOpenCommand();
             }
-            if ((await this.scope.scopes).length > 0 || (await this.scope.variables).length > 0) {
-                treeItem.collapsibleState = vscode.TreeItemCollapsibleState.Collapsed;
+            const [subScopes, variables] = await Promise.all([this.scope.scopes, this.scope.variables]);
+            if (subScopes.length > 0 || variables.length > 0) {
+                treeItem.collapsibleState = this.hasSiblings ?
+                    vscode.TreeItemCollapsibleState.Collapsed :
+                    vscode.TreeItemCollapsibleState.Expanded;
             }
             return treeItem;
         }
     }
 
     override async getChildren(): Promise<TreeItem[]> {
+        const [subScopes, variables] = await Promise.all([this.scope.scopes, this.scope.variables]);
         const children = [];
-        for (const scope of await this.scope.scopes) {
-            children.push(new ScopeTreeItem(this.provider, scope));
+        for (const scope of subScopes) {
+            children.push(new ScopeTreeItem(this.provider, scope, subScopes.length > 1));
         }
-        for (const variable of await this.scope.variables) {
+        for (const variable of variables) {
             if (variable instanceof ScalarVariable) {
                 children.push(new ScalarTreeItem(this.provider, variable.designation(),
                     variable.width > 1 ? 'canWatch|canSetRadix' : 'canWatch'));
@@ -281,7 +286,7 @@ export class TreeDataProvider implements vscode.TreeDataProvider<TreeItem> {
             if (session !== null) {
                 this.observer = new Observer(session, 'sidebar');
                 this.watchTreeItem = new WatchTreeItem(this);
-                this.scopeTreeItem = new ScopeTreeItem(this, await session.getRootScope());
+                this.scopeTreeItem = new ScopeTreeItem(this, await session.getRootScope(), false);
             } else {
                 this.observer?.dispose();
                 this.observer = null;