wire: allow non-panic version of injector (google/go-cloud#91)

Fixes google/go-cloud#7

internal/wire/parse.go

@@ -578,6 +578,11 @@ func isInjector(info *types.Info, fn *ast.FuncDecl) *ast.CallExpr {
 			only = stmt
 		case *ast.EmptyStmt:
 			// Do nothing.
+		case *ast.ReturnStmt:
+			// Allow the function to end in a return.
+			if only == nil {
+				return nil
+			}
 		default:
 			return nil
 		}
@@ -585,29 +590,24 @@ func isInjector(info *types.Info, fn *ast.FuncDecl) *ast.CallExpr {
 	if only == nil {
 		return nil
 	}
-	panicCall, ok := only.X.(*ast.CallExpr)
+	call, ok := only.X.(*ast.CallExpr)
 	if !ok {
 		return nil
 	}
-	panicIdent, ok := panicCall.Fun.(*ast.Ident)
-	if !ok {
-		return nil
+	if qualifiedIdentObject(info, call.Fun) == types.Universe.Lookup("panic") {
+		if len(call.Args) != 1 {
+			return nil
+		}
+		call, ok = call.Args[0].(*ast.CallExpr)
+		if !ok {
+			return nil
+		}
 	}
-	if info.ObjectOf(panicIdent) != types.Universe.Lookup("panic") {
-		return nil
-	}
-	if len(panicCall.Args) != 1 {
-		return nil
-	}
-	buildCall, ok := panicCall.Args[0].(*ast.CallExpr)
-	if !ok {
-		return nil
-	}
-	buildObj := qualifiedIdentObject(info, buildCall.Fun)
+	buildObj := qualifiedIdentObject(info, call.Fun)
 	if !isWireImport(buildObj.Pkg().Path()) || buildObj.Name() != "Build" {
 		return nil
 	}
-	return buildCall
+	return call
 }
 
 func isWireImport(path string) bool {

internal/wire/testdata/InjectWithReturn/foo/foo.go

@@ -0,0 +1,26 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectedMessage())
+}
+
+// provideMessage provides a friendly user greeting.
+func provideMessage() string {
+	return "Hello, World!"
+}

internal/wire/testdata/InjectWithReturn/foo/wire.go

@@ -0,0 +1,26 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+)
+
+func injectedMessage() string {
+	wire.Build(provideMessage)
+	return ""
+}

internal/wire/testdata/InjectWithReturn/out.txt

@@ -0,0 +1 @@
+Hello, World!

internal/wire/testdata/InjectWithReturn/pkg

@@ -0,0 +1 @@
+foo