fix(go): unused imports emitted for type unions (#3664)

When imported types are solely referenced by type unions, a go import
is emitted, but is never used (type unions end up represented as opaque
`interface{}` type). This causes compilation failures.

Added a test case for this scenario in particular, and adjusted go code
generation to not emit dependency imports for type unions.

These imports may be re-introduced soon, as we are working to add
dynamic type checking around type unions in go (at which point those
imports would no longer be unused).

Fixes #3399

Author: RomainMuller

Diff for: packages/jsii-pacmak/jest.config.mjs

@@ -2,4 +2,5 @@ import { overriddenConfig } from '../../jest.config.mjs';
 
 export default overriddenConfig({
   coveragePathIgnorePatterns: ['/node_modules/', '<rootDir>/test'],
+  watchPathIgnorePatterns: ['.*\\.tsx?$'],
 });

Diff for: packages/jsii-pacmak/lib/targets/go/types/go-type-reference.ts

@@ -137,9 +137,13 @@ export class GoTypeRef {
         break;
 
       case 'union':
-        for (const t of this.typeMap.value) {
-          ret.push(...t.dependencies);
-        }
+        // Unions ultimately result in `interface{}` being rendered, so no import is needed. We
+        // hence ignore them entirely here for now. In the future, we may want to inject specific
+        // runtime type checks around use of unions, which may result in imports being useful.
+
+        // for (const t of this.typeMap.value) {
+        //   ret.push(...t.dependencies);
+        // }
         break;
 
       case 'void':

Diff for: packages/jsii-pacmak/test/targets/fixtures/base.jsii.json

@@ -0,0 +1,42 @@
+{
+  "author": {
+    "email": "[email protected]",
+    "name": "Amazon Web Services, Inc.",
+    "organization": true,
+    "roles": [
+      "owner"
+    ]
+  },
+  "description": "A dummy test package",
+  "fingerprint": "PHONY",
+  "homepage": "https://aws.github.io/jsii",
+  "jsiiVersion": "0.0.0-dev",
+  "license": "Apache-2.0",
+  "name": "base",
+  "repository": {
+    "directory": "packages/jsii-pacmak/test/targets/fixtures",
+    "type": "git",
+    "url": "https://github.com/aws/jsii"
+  },
+  "schema": "jsii/0.10.0",
+  "targets": {
+    "go": {
+      "moduleName": "github.com/aws/jsii/packages/jsii-pacmak/test/targets/fixtures"
+    }
+  },
+  "types": {
+    "BaseA": {
+      "assembly": "base",
+      "fqn": "base.BaseA",
+      "kind": "interface",
+      "name": "BaseA"
+    },
+    "BaseB": {
+      "assembly": "base",
+      "fqn": "base.BaseB",
+      "kind": "interface",
+      "name": "BaseB"
+    }
+  },
+  "version": "1.2.3"
+}

Diff for: packages/jsii-pacmak/test/targets/fixtures/dependent.jsii.json

@@ -0,0 +1,58 @@
+{
+  "author": {
+    "email": "[email protected]",
+    "name": "Amazon Web Services, Inc.",
+    "organization": true,
+    "roles": [
+      "owner"
+    ]
+  },
+  "dependencies": {
+    "base": "1.2.3"
+  },
+  "description": "A dummy test package",
+  "fingerprint": "PHONY",
+  "homepage": "https://aws.github.io/jsii",
+  "jsiiVersion": "0.0.0-dev",
+  "license": "Apache-2.0",
+  "name": "dependent",
+  "repository": {
+    "directory": "packages/jsii-pacmak/test/targets/fixtures",
+    "type": "git",
+    "url": "https://github.com/aws/jsii"
+  },
+  "schema": "jsii/0.10.0",
+  "targets": {
+    "go": {
+      "moduleName": "github.com/aws/jsii/packages/jsii-pacmak/test/targets/fixtures"
+    }
+  },
+  "types": {
+    "Dependent": {
+      "assembly": "dependent",
+      "fqn": "dependent.Dependent",
+      "kind": "class",
+      "name": "dependent",
+      "methods": [
+        {
+          "name": "getBaseUnion",
+          "returns": {
+            "type": {
+              "union": {
+                "types": [
+                  {
+                    "fqn": "base.BaseA"
+                  },
+                  {
+                    "fqn": "base.BaseB"
+                  }
+                ]
+              }
+            }
+          }
+        }
+      ]
+    }
+  },
+  "version": "4.5.6"
+}

Diff for: packages/jsii-pacmak/test/targets/go.test.ts

@@ -0,0 +1,40 @@
+import { promises as fs } from 'fs';
+import { TypeSystem } from 'jsii-reflect';
+import { Rosetta } from 'jsii-rosetta';
+import { tmpdir } from 'os';
+import { join } from 'path';
+
+import { Golang } from '../../lib/targets/go';
+
+test('does not generate imports for unused types', async () => {
+  const outDir = await fs.mkdtemp(join(tmpdir(), 'pacmak-golang-'));
+  try {
+    const tarball = join(outDir, 'mock-tarball.tgz');
+    await fs.writeFile(tarball, 'Mock Tarball');
+
+    const typeSystem = new TypeSystem();
+    await typeSystem.load(require.resolve('./fixtures/base.jsii.json'));
+    const assembly = await typeSystem.load(
+      require.resolve('./fixtures/dependent.jsii.json'),
+    );
+
+    const rosetta = new Rosetta();
+    const subject = new Golang({
+      arguments: {},
+      assembly,
+      packageDir: '',
+      rosetta,
+      targetName: 'golang',
+    });
+
+    await subject.generateCode(outDir, tarball);
+
+    await expect(
+      fs.readFile(join(outDir, assembly.name, `${assembly.name}.go`), 'utf-8'),
+    ).resolves.not.toContain(
+      'github.com/aws/jsii/packages/jsii-pacmak/test/targets/fixtures/base',
+    );
+  } finally {
+    await fs.rm(outDir, { recursive: true });
+  }
+});