wire: report an error if a func with wire.Build in it is an invalid injector (google/go-cloud#487)

internal/wire/testdata/InvalidInjector/foo/foo.go

@@ -0,0 +1,36 @@
+// Copyright 2018 The Go Cloud 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() {
+	foo := injectFoo()
+	bar := injectBar()
+	fmt.Println(foo)
+	fmt.Println(bar)
+}
+
+type Foo int
+type Bar int
+
+func provideFoo() Foo {
+	return Foo(42)
+}
+func provideBar() Bar {
+	return Bar(99)
+}

internal/wire/testdata/InvalidInjector/foo/wire.go

@@ -0,0 +1,33 @@
+// Copyright 2018 The Go Cloud 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/go-cloud/wire"
+)
+
+func injectFoo() Foo {
+	// This non-call statement makes this an invalid injector.
+	 _ = 42
+	panic(wire.Build(provideFoo))
+}
+
+func injectBar() Bar {
+	// Two call statements are also invalid.
+	panic(wire.Build(provideBar))
+	panic(wire.Build(provideBar))
+}

internal/wire/testdata/InvalidInjector/pkg

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

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

@@ -0,0 +1,4 @@
+a call to wire.Build indicates that this function is an injector, but injectors
+must consist of only the wire.Build call and an optional return
+a call to wire.Build indicates that this function is an injector, but injectors
+must consist of only the wire.Build call and an optional return