gohcl: allow optional attributes to be specified via struct tag

Previously we required optional attributes to be specified as pointers so that we could represent the empty vs. absent distinction.

For applications that don't need to make that distinction, representing "optional" as a struct tag is more convenient.

gohcl/decode_test.go

@@ -54,7 +54,17 @@ func TestDecodeBody(t *testing.T) {
 				Name *string `hcl:"name"`
 			}{}),
 			0,
-		},
+		}, // name nil
+		{
+			map[string]interface{}{},
+			struct {
+				Name string `hcl:"name,optional"`
+			}{},
+			deepEquals(struct {
+				Name string `hcl:"name,optional"`
+			}{}),
+			0,
+		}, // name optional
 		{
 			map[string]interface{}{},
 			withNameExpression{},

gohcl/schema.go

@@ -42,7 +42,9 @@ func ImpliedBodySchema(val interface{}) (schema *hcl.BodySchema, partial bool) {
 	sort.Strings(attrNames)
 	for _, n := range attrNames {
 		idx := tags.Attributes[n]
+		optional := tags.Optional[n]
 		field := ty.Field(idx)
+
 		var required bool
 
 		switch {
@@ -51,7 +53,7 @@ func ImpliedBodySchema(val interface{}) (schema *hcl.BodySchema, partial bool) {
 			// indicated via a null value, so we don't specify that
 			// the field is required during decoding.
 			required = false
-		case field.Type.Kind() != reflect.Ptr:
+		case field.Type.Kind() != reflect.Ptr && !optional:
 			required = true
 		default:
 			required = false
@@ -111,6 +113,7 @@ type fieldTags struct {
 	Blocks     map[string]int
 	Labels     []labelField
 	Remain     *int
+	Optional   map[string]bool
 }
 
 type labelField struct {
@@ -122,6 +125,7 @@ func getFieldTags(ty reflect.Type) *fieldTags {
 	ret := &fieldTags{
 		Attributes: map[string]int{},
 		Blocks:     map[string]int{},
+		Optional:   map[string]bool{},
 	}
 
 	ct := ty.NumField()
@@ -158,6 +162,9 @@ func getFieldTags(ty reflect.Type) *fieldTags {
 			}
 			idx := i // copy, because this loop will continue assigning to i
 			ret.Remain = &idx
+		case "optional":
+			ret.Attributes[name] = i
+			ret.Optional[name] = true
 		default:
 			panic(fmt.Sprintf("invalid hcl field tag kind %q on %s %q", kind, field.Type.String(), field.Name))
 		}

gohcl/schema_test.go

@@ -193,6 +193,20 @@ func TestImpliedBodySchema(t *testing.T) {
 			},
 			false,
 		},
+		{
+			struct {
+				Meh string `hcl:"meh,optional"`
+			}{},
+			&hcl.BodySchema{
+				Attributes: []hcl.AttributeSchema{
+					{
+						Name:     "meh",
+						Required: false,
+					},
+				},
+			},
+			false,
+		},
 	}
 
 	for _, test := range tests {