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

Fixes google/go-cloud#7

README.md

@@ -18,11 +18,13 @@ produce a value. These functions are ordinary Go code.
 ```go
 package foobarbaz
 
-type Foo int
+type Foo struct {
+	X int
+}
 
 // ProvideFoo returns a Foo.
 func ProvideFoo() Foo {
-	return 42
+	return Foo{X: 42}
 }
 ```
 
@@ -33,11 +35,13 @@ package foobarbaz
 
 // ...
 
-type Bar int
+type Bar struct {
+	X int
+}
 
 // ProvideBar returns a Bar: a negative Foo.
 func ProvideBar(foo Foo) Bar {
-	return Bar(-foo)
+	return Bar{X: -foo.X}
 }
 ```
 
@@ -53,14 +57,16 @@ import (
 
 // ...
 
-type Baz int
+type Baz struct {
+	X int
+}
 
 // ProvideBaz returns a value if Bar is not zero.
 func ProvideBaz(ctx context.Context, bar Bar) (Baz, error) {
 	if bar == 0 {
 		return 0, errors.New("cannot provide baz when bar is zero")
 	}
-	return Baz(bar), nil
+	return Baz{X: bar.X}, nil
 }
 ```
 
@@ -102,9 +108,11 @@ calls providers in dependency order. With Wire, you write the injector's
 signature, then Wire generates the function's body.
 
 An injector is declared by writing a function declaration whose body is a call
-to `panic()` with a call to `wire.Build` as its argument. Let's say that the
+to `wire.Build`. The return values don't matter as long as they are of the
-above providers were defined in a package called `example.com/foobarbaz`. The
+correct type. The values themselves will be ignored in the generated code. Let's
-following would declare an injector to obtain a `Baz`:
+say that the above providers were defined in a package called
+`example.com/foobarbaz`. The following would declare an injector to obtain a
+`Baz`:
 
 ```go
 // +build wireinject
@@ -121,7 +129,8 @@ import (
 )
 
 func initializeApp(ctx context.Context) (foobarbaz.Baz, error) {
-	panic(wire.Build(foobarbaz.MegaSet))
+	wire.Build(foobarbaz.MegaSet)
+	return foobarbaz.Baz{}, nil
 }
 ```
 
@@ -311,10 +320,13 @@ Instead of having injectors depend on a throwaway provider function, you can
 add a value expression to a provider set.
 
 ```go
-type Foo int
+type Foo struct {
+	X int
+}
 
 func injectFoo() Foo {
-	panic(wire.Build(wire.Value(Foo(42))))
+	wire.Build(wire.Value(Foo{X: 42}))
+	return Foo{}
 }
 ```
 
@@ -322,7 +334,7 @@ The generated injector would look like this:
 
 ```go
 func injectFoo() Foo {
-	foo := Foo(42)
+	foo := Foo{X: 42}
 	return foo
 }
 ```
@@ -355,3 +367,15 @@ func provideFile(log Logger, path Path) (*os.File, func(), error) {
 
 A cleanup function is guaranteed to be called before the cleanup function of any
 of the provider's inputs and must have the signature `func()`.
+
+### Alternate Injector Syntax
+
+If you grow weary of writing `return foobarbaz.Foo{}, nil` at the end of your
+injector function declaration, you can instead write it more concisely with a
+`panic`:
+
+```go
+func injectFoo() Foo {
+	panic(wire.Build(/* ... */))
+}
+```

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 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 {
+	buildObj := qualifiedIdentObject(info, call.Fun)
-		return nil
-	}
-	buildCall, ok := panicCall.Args[0].(*ast.CallExpr)
-	if !ok {
-		return nil
-	}
-	buildObj := qualifiedIdentObject(info, buildCall.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