internal/wire: use set to determine which argument to use in zero-call injectors

internal/wire/testdata/ReturnArgumentAsInterface/foo/foo.go

@@ -0,0 +1,25 @@
+// Copyright 2019 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"
+
+func main() {
+	fmt.Println(injectStringer("Hello, World!"))
+}
+
+type MyString string
+
+func (s MyString) String() string { return string(s) }

internal/wire/testdata/ReturnArgumentAsInterface/foo/wire.go

@@ -0,0 +1,28 @@
+// Copyright 2019 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 (
+	"fmt"
+
+	"github.com/google/wire"
+)
+
+func injectStringer(s MyString) fmt.Stringer {
+	wire.Build(wire.Bind(new(fmt.Stringer), new(MyString)))
+	return nil
+}

internal/wire/testdata/ReturnArgumentAsInterface/pkg

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

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

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

internal/wire/wire.go

@@ -367,12 +367,12 @@ func (g *gen) inject(pos token.Pos, name string, sig *types.Signature, set *Prov
 	}
 
 	// Perform one pass to collect all imports, followed by the real pass.
-	injectPass(name, sig, calls, &injectorGen{
+	injectPass(name, sig, calls, set, &injectorGen{
 		g:       g,
 		errVar:  disambiguate("err", g.nameInFileScope),
 		discard: true,
 	})
-	injectPass(name, sig, calls, &injectorGen{
+	injectPass(name, sig, calls, set, &injectorGen{
 		g:       g,
 		errVar:  disambiguate("err", g.nameInFileScope),
 		discard: false,
@@ -586,7 +586,7 @@ type injectorGen struct {
 
 // injectPass generates an injector given the output from analysis.
 // The sig passed in should be verified.
-func injectPass(name string, sig *types.Signature, calls []call, ig *injectorGen) {
+func injectPass(name string, sig *types.Signature, calls []call, set *ProviderSet, ig *injectorGen) {
 	params := sig.Params()
 	injectSig, err := funcOutput(sig)
 	if err != nil {
@@ -643,12 +643,7 @@ func injectPass(name string, sig *types.Signature, calls []call, ig *injectorGen
 		}
 	}
 	if len(calls) == 0 {
-		for i := 0; i < params.Len(); i++ {
-			if types.Identical(injectSig.out, params.At(i).Type()) {
-				ig.p("\treturn %s", ig.paramNames[i])
-				break
-			}
-		}
+		ig.p("\treturn %s", ig.paramNames[set.For(injectSig.out).Arg().Index])
 	} else {
 		ig.p("\treturn %s", ig.localNames[len(calls)-1])
 	}