goose: require pointer for first argument to goose.Bind (google/go-cloud#31)

Fixes google/go-cloud#15

README.md

@@ -247,13 +247,13 @@ func ProvideBar() *Bar {
 
 var BarFooer = goose.NewSet(
 	ProvideBar,
-	goose.Bind(Fooer(nil), (*Bar)(nil)))
+	goose.Bind(new(Fooer), new(Bar)))
 ```
 
-The first argument to `goose.Bind` is a nil value for the interface type and the
-second argument is a zero value of the concrete type. An interface binding does
-not necessarily need to have a provider in the same set that provides the
-concrete type.
+The first argument to `goose.Bind` is a pointer to a value of the desired
+interface type and the second argument is a zero value of the concrete type. An
+interface binding does not necessarily need to have a provider in the same set
+that provides the concrete type.
 
 [type identity]: https://golang.org/ref/spec#Type_identity
 [return concrete types]: https://github.com/golang/go/wiki/CodeReviewComments#interfaces

goose.go

@@ -43,11 +43,12 @@ func Build(...interface{}) string {
 type Binding struct{}
 
 // Bind declares that a concrete type should be used to satisfy a
-// dependency on iface.
+// dependency on the type of iface, which must be a pointer to an
+// interface type.
 //
 // Example:
 //
-//	var MySet = goose.NewSet(goose.Bind(MyInterface(nil), new(MyStruct)))
+//	var MySet = goose.NewSet(goose.Bind(new(MyInterface), new(MyStruct)))
 func Bind(iface, to interface{}) Binding {
 	return Binding{}
 }

internal/goose/parse.go

@@ -496,21 +496,25 @@ func processBind(fset *token.FileSet, info *types.Info, call *ast.CallExpr) (*If
 		return nil, fmt.Errorf("%v: call to Bind takes exactly two arguments", fset.Position(call.Pos()))
 	}
 	// TODO(light): Verify that arguments are simple expressions.
-	iface := info.TypeOf(call.Args[0])
-	methodSet, ok := iface.Underlying().(*types.Interface)
+	ifaceArgType := info.TypeOf(call.Args[0])
+	ifacePtr, ok := ifaceArgType.(*types.Pointer)
 	if !ok {
-		return nil, fmt.Errorf("%v: first argument to bind must be of interface type; found %s", fset.Position(call.Pos()), types.TypeString(iface, nil))
+		return nil, fmt.Errorf("%v: first argument to bind must be a pointer to an interface type; found %s", fset.Position(call.Pos()), types.TypeString(ifaceArgType, nil))
+	}
+	methodSet, ok := ifacePtr.Elem().Underlying().(*types.Interface)
+	if !ok {
+		return nil, fmt.Errorf("%v: first argument to bind must be a pointer to an interface type; found %s", fset.Position(call.Pos()), types.TypeString(ifaceArgType, nil))
 	}
 	provided := info.TypeOf(call.Args[1])
-	if types.Identical(iface, provided) {
+	if types.Identical(ifacePtr.Elem(), provided) {
 		return nil, fmt.Errorf("%v: cannot bind interface to itself", fset.Position(call.Pos()))
 	}
 	if !types.Implements(provided, methodSet) {
-		return nil, fmt.Errorf("%v: %s does not implement %s", fset.Position(call.Pos()), types.TypeString(provided, nil), types.TypeString(iface, nil))
+		return nil, fmt.Errorf("%v: %s does not implement %s", fset.Position(call.Pos()), types.TypeString(provided, nil), types.TypeString(ifaceArgType, nil))
 	}
 	return &IfaceBinding{
 		Pos:      call.Pos(),
-		Iface:    iface,
+		Iface:    ifacePtr.Elem(),
 		Provided: provided,
 	}, nil
 }

internal/goose/testdata/ImportedInterfaceBinding/bar/bar.go

@@ -17,8 +17,8 @@ package main
 import (
 	"fmt"
 
-	"github.com/google/go-cloud/goose"
 	"foo"
+	"github.com/google/go-cloud/goose"
 )
 
 func main() {
@@ -39,4 +39,4 @@ func provideBar() *Bar {
 
 var Set = goose.NewSet(
 	provideBar,
-	goose.Bind(foo.Fooer(nil), (*Bar)(nil)))
+	goose.Bind((*foo.Fooer)(nil), (*Bar)(nil)))

internal/goose/testdata/InterfaceBinding/foo/foo.go

@@ -42,4 +42,4 @@ func provideBar() *Bar {
 
 var Set = goose.NewSet(
 	provideBar,
-	goose.Bind(Fooer(nil), (*Bar)(nil)))
+	goose.Bind((*Fooer)(nil), (*Bar)(nil)))

internal/goose/testdata/InterfaceBindingReuse/foo/foo_goose.go

@@ -24,5 +24,5 @@ func injectFooBar() FooBar {
 	panic(goose.Build(
 		provideBar,
 		provideFooBar,
-		goose.Bind(Fooer(nil), (*Bar)(nil))))
+		goose.Bind((*Fooer)(nil), (*Bar)(nil))))
 }