reflect: add example for StructTag

Fix a few minor vet quibbles while I'm here.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13235059

src/pkg/reflect/all_test.go

@@ -948,7 +948,7 @@ func TestMap(t *testing.T) {
 
 	newm := newmap.Interface().(map[string]int)
 	if len(newm) != len(m) {
-		t.Errorf("length after copy: newm=%d, m=%d", newm, m)
+		t.Errorf("length after copy: newm=%d, m=%d", len(newm), len(m))
 	}
 
 	for k, v := range newm {
@@ -3478,7 +3478,7 @@ func TestAllocsInterfaceBig(t *testing.T) {
 	}
 	v := ValueOf(S{})
 	if allocs := testing.AllocsPerRun(100, func() { v.Interface() }); allocs > 0 {
-		t.Errorf("allocs:", allocs)
+		t.Error("allocs:", allocs)
 	}
 }
 
@@ -3495,7 +3495,7 @@ func TestAllocsInterfaceSmall(t *testing.T) {
 	}
 	v := ValueOf(int64(0))
 	if allocs := testing.AllocsPerRun(100, func() { v.Interface() }); allocs > 0 {
-		t.Errorf("allocs:", allocs)
+		t.Error("allocs:", allocs)
 	}
 }
 

src/pkg/reflect/example_test.go

@@ -50,3 +50,17 @@ func ExampleMakeFunc() {
 	// 1 0
 	// 3.14 2.72
 }
+
+func ExampleStructTag() {
+	type S struct {
+		F string `species:"gopher" color:"blue"`
+	}
+
+	s := S{}
+	st := reflect.TypeOf(s)
+	field := st.Field(0)
+	fmt.Println(field.Tag.Get("color"), field.Tag.Get("species"))
+
+	// Output:
+	// blue gopher
+}