reflect: add method StructTag.Lookup

The Lookup method provides a way to extract a tag value, while
determining whether the tag key exists in the struct field's tag.

Fixes #14883

Change-Id: I7460cb68f0ca1aaa025935050b9e182efcb64db3
Reviewed-on: https://go-review.googlesource.com/20864Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/example_test.go

@@ -67,6 +67,34 @@ func ExampleStructTag() {
 	// blue gopher
 }
 
+func ExampleStructTag_Lookup() {
+	type S struct {
+		F0 string `alias:"field_0"`
+		F1 string `alias:""`
+		F2 string
+	}
+
+	s := S{}
+	st := reflect.TypeOf(s)
+	for i := 0; i < st.NumField(); i++ {
+		field := st.Field(i)
+		if alias, ok := field.Tag.Lookup("alias"); ok {
+			if alias == "" {
+				fmt.Println("(blank)")
+			} else {
+				fmt.Println(alias)
+			}
+		} else {
+			fmt.Println("(not specified)")
+		}
+	}
+
+	// Output:
+	// field_0
+	// (blank)
+	// (not specified)
+}
+
 func ExampleTypeOf() {
 	// As interface types are only used for static typing, a
 	// common idiom to find the reflection Type for an interface

src/reflect/type.go

@@ -976,8 +976,20 @@ type StructTag string
 // Get returns the value associated with key in the tag string.
 // If there is no such key in the tag, Get returns the empty string.
 // If the tag does not have the conventional format, the value
-// returned by Get is unspecified.
+// returned by Get is unspecified. To determine whether a tag is
+// explicitly set to the empty string, use Lookup.
 func (tag StructTag) Get(key string) string {
+	v, _ := tag.Lookup(key)
+	return v
+}
+
+// Lookup returns the value associated with key in the tag string.
+// If the key is present in the tag the value (which may be empty)
+// is returned. Otherwise the returned value will be the empty string.
+// The ok return value reports whether the value was explicitly set in
+// the tag string. If the tag does not have the conventional format,
+// the value returned by Lookup is unspecified.
+func (tag StructTag) Lookup(key string) (value string, ok bool) {
 	// When modifying this code, also update the validateStructTag code
 	// in golang.org/x/tools/cmd/vet/structtag.go.
 
@@ -1025,10 +1037,10 @@ func (tag StructTag) Get(key string) string {
 			if err != nil {
 				break
 			}
-			return value
+			return value, true
 		}
 	}
-	return ""
+	return "", false
 }
 
 // Field returns the i'th struct field.