fix(assertions): nested stacks inside non-root stages don't resolve t…

…emplates

Templates are placed inside a path described by their closest assembly. Using this assembly lets nested stack templates resolve, regardless of their stage depth.

I took the opportunity to reduce the scope of synth to only the containing template, regardless of the stack being nested or not. This should decrease load on user machines when synthesizing templates for apps with multiple stages.

Closes #24004.

packages/@aws-cdk/assertions/lib/template.ts

@@ -296,12 +296,13 @@ export interface TemplateParsingOptions {
 }
 
 function toTemplate(stack: Stack): any {
-  const root = stack.node.root;
-  if (!Stage.isStage(root)) {
+  // get the nearest stage to the stack in the construct tree
+  const containingStage = stack.node.scopes.reverse().find(Stage.isStage);
+  if (!containingStage) {
     throw new Error('unexpected: all stacks must be part of a Stage or an App');
   }
 
-  const assembly = root.synth();
+  const assembly = containingStage.synth();
   if (stack.nestedStackParent) {
     // if this is a nested stack (it has a parent), then just read the template as a string
     return JSON.parse(fs.readFileSync(path.join(assembly.directory, stack.templateFile)).toString('utf-8'));

packages/@aws-cdk/assertions/test/template.test.ts

@@ -1,4 +1,4 @@
-import { App, CfnCondition, CfnMapping, CfnOutput, CfnParameter, CfnResource, Fn, LegacyStackSynthesizer, NestedStack, Stack } from '@aws-cdk/core';
+import { App, CfnCondition, CfnMapping, CfnOutput, CfnParameter, CfnResource, Fn, LegacyStackSynthesizer, NestedStack, Stack, Stage } from '@aws-cdk/core';
 import { Construct } from 'constructs';
 import { Capture, Match, Template } from '../lib';
 
@@ -74,6 +74,29 @@ describe('Template', () => {
         },
       });
     });
+
+    test('nested stack inside a Stage in an App', () => {
+      const app = new App();
+      const stage = new Stage(app, 'Stage');
+      const stack = new Stack(stage);
+      const nested = new NestedStack(stack, 'MyNestedStack');
+      new CfnResource(nested, 'Bar', {
+        type: 'Bar::Baz',
+        properties: {
+          Qux: 'Foo',
+        },
+      });
+      const template = Template.fromStack(nested);
+
+      expect(template.toJSON()).toEqual({
+        Resources: {
+          Bar: {
+            Type: 'Bar::Baz',
+            Properties: { Qux: 'Foo' },
+          },
+        },
+      });
+    });
   });
 
   describe('fromString', () => {