add FieldsOf to inject fields of a struct directly (#138)

wire.go

@@ -30,7 +30,8 @@ type ProviderSet struct{}
 
 // NewSet creates a new provider set that includes the providers in its
 // arguments. Each argument is a function value, a struct (zero) value, a
-// provider set, a call to Bind, a call to Value, or a call to InterfaceValue.
+// provider set, a call to Bind, a call to Value, a call to InterfaceValue or a
+// call to FieldsOf.
 //
 // Passing a function value to NewSet declares that the function's first
 // return value type will be provided by calling the function. The arguments
@@ -135,3 +136,28 @@ func Value(interface{}) ProvidedValue {
 func InterfaceValue(typ interface{}, x interface{}) ProvidedValue {
 	return ProvidedValue{}
 }
+
+// StructFields is a collection of the fields from a struct.
+type StructFields struct{}
+
+// FieldsOf declares that the fields named of the given struct type will be used
+// to provide the types of those fields. The structType argument must be a
+// pointer to the struct or a pointer to a pointer to the struct it wishes to reference.
+//
+// The following example would provide *Foo and *Bar using S.MyFoo and S.MyBar respectively:
+//
+//  type S struct {
+//  	MyFoo *Foo
+//  	MyBar *Bar
+//  }
+//
+//  func NewStruct() S { /* ... */ }
+//  var Set = wire.NewSet(wire.FieldsOf(new(S), "MyFoo", "MyBar"))
+//
+//  or
+//
+//  func NewStruct() *S { /* ... */ }
+//  var Set = wire.NewSet(wire.FieldsOf(new(*S), "MyFoo", "MyBar"))
+func FieldsOf(structType interface{}, fieldNames ...string) StructFields {
+	return StructFields{}
+}