feat: use go.mod when no version inputs specified

.github/workflows/versions.yml

@@ -142,6 +142,28 @@ jobs:
         run: __tests__/verify-go.sh 1.21
         shell: bash
 
+  auto-detect-go-mod:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest, macos-latest-large]
+    steps:
+      - uses: actions/checkout@v6
+      - name: Create go.mod in workspace root
+        run: |
+          cat > go.mod << 'EOF'
+          module github.com/actions/setup-go
+
+          go 1.21
+          EOF
+        shell: bash
+      - name: Setup Go with auto-detection
+        uses: ./
+      - name: Verify go version
+        run: __tests__/verify-go.sh 1.21
+        shell: bash
+
   go-version-file-with-tool-versions:
     runs-on: ${{ matrix.os }}
     strategy:

__tests__/setup-go.test.ts

@@ -1112,6 +1112,31 @@ use .
     expect(process.env).toHaveProperty('GOTOOLCHAIN', 'local');
   });
 
+  describe('auto-detect go.mod', () => {
+    it('uses go.mod from workspace root when no inputs provided', async () => {
+      existsSpy.mockImplementation((filePath: string) => {
+        return filePath === 'go.mod';
+      });
+      readFileSpy.mockImplementation(() =>
+        Buffer.from('module test\n\ngo 1.20')
+      );
+
+      await main.run();
+
+      expect(logSpy).toHaveBeenCalledWith('Setup go version spec 1.20');
+    });
+
+    it('uses pre-installed Go when no inputs and no go.mod exists', async () => {
+      existsSpy.mockImplementation(() => false);
+
+      await main.run();
+
+      expect(logSpy).toHaveBeenCalledWith(
+        '[warning]go-version input was not specified. The action will try to use pre-installed version.'
+      );
+    });
+  });
+
   describe('go-download-base-url', () => {
     it('downloads a version from custom base URL using version listing', async () => {
       os.platform = 'linux';

action.yml

@@ -5,7 +5,7 @@ inputs:
   go-version:
     description: 'The Go version to download (if necessary) and use. Supports semver spec and ranges. Be sure to enclose this option in single quotation marks.'
   go-version-file:
-    description: 'Path to the go.mod, go.work, .go-version, or .tool-versions file.'
+    description: 'Path to the go.mod, go.work, .go-version, or .tool-versions file. If not specified, the action will automatically use go.mod from the workspace root if it exists.'
   check-latest:
     description: 'Set this option to true if you want the action to always check for the latest available version that satisfies the version spec'
     default: false

dist/setup/index.js

@@ -77741,6 +77741,9 @@ function resolveVersionInput() {
         }
         version = installer.parseGoVersionFile(versionFilePath);
     }
+    if (!version && fs_1.default.existsSync('go.mod')) {
+        version = installer.parseGoVersionFile('go.mod');
+    }
     return version;
 }
 function setGoToolchain() {

docs/advanced-usage.md

@@ -168,6 +168,18 @@ jobs:
 
 If both the `go-version` and the `go-version-file` inputs are provided then the `go-version` input is used. The `.tool-versions` file supports version specifications in accordance with asdf standards, adhering to Semantic Versioning ([semver](https://semver.org)).
 
+### Automatic go.mod detection
+
+If neither `go-version` nor `go-version-file` is specified, the action will automatically look for a `go.mod` file in the repository root and use the Go version specified in it. This simplifies workflows for projects that already have a `go.mod` file.
+
+```yaml
+steps:
+  - uses: actions/checkout@v6
+  - uses: actions/setup-go@v6
+    # Automatically uses go.mod from repository root
+  - run: go version
+```
+
 ```yaml
 steps:
   - uses: actions/checkout@v6

src/main.ts

@@ -170,6 +170,10 @@ function resolveVersionInput(): string {
     version = installer.parseGoVersionFile(versionFilePath);
   }
 
+  if (!version && fs.existsSync('go.mod')) {
+    version = installer.parseGoVersionFile('go.mod');
+  }
+
   return version;
 }