wire: wire.FieldsOf should not provide pointer to field type for non-pointer structs

docs/guide.md

@@ -423,7 +423,8 @@ func injectedMessage() string {
 ```
 
 You can add as many field names to a `wire.FieldsOf` function as you like.
-For a given field type `T`, `FieldsOf` provides both `T` and `*T`.
+For a given field type `T`, `FieldsOf` provides at least `T`; if the struct
+argument is a pointer to a struct, then `FieldsOf` also provides `*T`.
 
 ### Cleanup functions
 

internal/wire/analyze.go

@@ -232,9 +232,9 @@ dfs:
 			}
 			// Use args[0] to store the position of the parent struct.
 			args := []int{v.(int)}
-			// len(f.Out) is always 2; if curr.t is the 2nd one, then the call must
+			// If f.Out has 2 elements and curr.t is the 2nd one, then the call must
 			// provide a pointer to the field.
-			ptrToField := types.Identical(curr.t, f.Out[1])
+			ptrToField := len(f.Out) == 2 && types.Identical(curr.t, f.Out[1])
 			calls = append(calls, call{
 				kind:       selectorExpr,
 				pkg:        f.Pkg,

internal/wire/parse.go

@@ -232,7 +232,8 @@ type Field struct {
 	// defining these fields.
 	Pos token.Pos
 	// Out is the field's provided types. The first element provides the
-	// field type, the second element provides a pointer to it.
+	// field type. If the field is coming from a pointer to a struct,
+	// there will be a second element providing a pointer to the field.
 	Out []types.Type
 }
 
@@ -1006,13 +1007,15 @@ func processFieldsOf(fset *token.FileSet, info *types.Info, call *ast.CallExpr)
 	}
 
 	var struc *types.Struct
+	isPtrToStruct := false
 	switch t := structPtr.Elem().Underlying().(type) {
 	case *types.Pointer:
 		struc, ok = t.Elem().Underlying().(*types.Struct)
 		if !ok {
 			return nil, notePosition(fset.Position(call.Pos()),
 				fmt.Errorf(firstArgReqFormat, types.TypeString(struc, nil)))
 		}
+		isPtrToStruct = true
 	case *types.Struct:
 		struc = t
 	default:
@@ -1030,12 +1033,18 @@ func processFieldsOf(fset *token.FileSet, info *types.Info, call *ast.CallExpr)
 		if err != nil {
 			return nil, notePosition(fset.Position(call.Pos()), err)
 		}
+		out := []types.Type{v.Type()}
+		if isPtrToStruct {
+			// If the field is from a pointer to a struct, then
+			// wire.Fields also provides a pointer to the field.
+			out = append(out, types.NewPointer(v.Type()))
+		}
 		fields = append(fields, &Field{
 			Parent: structPtr.Elem(),
 			Name:   v.Name(),
 			Pkg:    v.Pkg(),
 			Pos:    v.Pos(),
-			Out:    []types.Type{v.Type(), types.NewPointer(v.Type())},
+			Out:    out,
 		})
 	}
 	return fields, nil

internal/wire/testdata/FieldsOfStruct/foo/foo.go

@@ -26,5 +26,4 @@ func provideS() S {
 
 func main() {
 	fmt.Println(injectedMessage())
-	fmt.Println("pointer to " + *injectedMessagePtr())
 }

internal/wire/testdata/FieldsOfStruct/foo/wire.go

@@ -26,10 +26,3 @@ func injectedMessage() string {
 		wire.FieldsOf(new(S), "Foo"))
 	return ""
 }
-
-func injectedMessagePtr() *string {
-	wire.Build(
-		provideS,
-		wire.FieldsOf(new(S), "Foo"))
-	return nil
-}

internal/wire/testdata/FieldsOfStruct/want/program_out.txt

@@ -1,2 +1 @@
 Hello, World!
-pointer to Hello, World!

internal/wire/testdata/FieldsOfStructDoNotProvidePtrToField/foo/foo.go

@@ -0,0 +1,29 @@
+// Copyright 2018 The Wire Authors
+//
+// 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"
+
+type S struct {
+	Foo string
+}
+
+func provideS() S {
+	return S{Foo: "Hello, World!"}
+}
+
+func main() {
+	fmt.Println("pointer to " + *injectedMessagePtr())
+}

internal/wire/testdata/FieldsOfStructDoNotProvidePtrToField/foo/wire.go

@@ -0,0 +1,32 @@
+// Copyright 2018 The Wire Authors
+//
+// 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/wire"
+)
+
+func injectedMessagePtr() *string {
+	// This shouldn't work; FieldsOf provides a pointer to the
+	// field only when the struct type is a pointer to a struct.
+	// See FieldsOfStructPointer for a working example using
+	// a pointer to a struct.
+	wire.Build(
+		provideS,
+		wire.FieldsOf(new(S), "Foo"))
+	return nil
+}

internal/wire/testdata/FieldsOfStructDoNotProvidePtrToField/pkg

@@ -0,0 +1 @@
+example.com/foo

internal/wire/testdata/FieldsOfStructDoNotProvidePtrToField/want/wire_errs.txt

@@ -0,0 +1 @@
+example.com/foo/wire.go:x:y: inject injectedMessagePtr: no provider found for *string, output of injector

internal/wire/testdata/FieldsOfStructPointer/foo/foo.go

@@ -26,4 +26,5 @@ func provideS() *S {
 
 func main() {
 	fmt.Println(injectedMessage())
+	fmt.Println("pointer to " + *injectedMessagePtr())
 }

internal/wire/testdata/FieldsOfStructPointer/foo/wire.go

@@ -26,3 +26,10 @@ func injectedMessage() string {
 		wire.FieldsOf(new(*S), "Foo"))
 	return ""
 }
+
+func injectedMessagePtr() *string {
+	wire.Build(
+		provideS,
+		wire.FieldsOf(new(*S), "Foo"))
+	return nil
+}

internal/wire/testdata/FieldsOfStructPointer/want/program_out.txt

@@ -1 +1,2 @@
 Hello, World!
+pointer to Hello, World!

wire.go

@@ -187,6 +187,10 @@ type StructFields struct{}
 //
 //  func NewStruct() *S { /* ... */ }
 //  var Set = wire.NewSet(wire.FieldsOf(new(*S), "MyFoo", "MyBar"))
+//
+//  If the structType argument is a pointer to a pointer to a struct, then FieldsOf
+//  additionally provides a pointer to each field type (e.g., **Foo and **Bar in the
+//  example above).
 func FieldsOf(structType interface{}, fieldNames ...string) StructFields {
 	return StructFields{}
 }