gohcl: Decode missing hcl.Expression as a cty Null

Rather than writing a nil hcl.Expression, as a special case we'll deal
with this within the cty type system, which we assume is what the caller
wants if they are decoding into a hcl.Expression rather than a native Go
type.

gohcl/decode.go

@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"reflect"
 
+	"github.com/zclconf/go-cty/cty"
+
 	"github.com/hashicorp/hcl2/hcl"
 	"github.com/zclconf/go-cty/cty/convert"
 	"github.com/zclconf/go-cty/cty/gocty"
@@ -81,11 +83,25 @@ func decodeBodyToStruct(body hcl.Body, ctx *hcl.EvalContext, val reflect.Value)
 		}
 	}
 
-	for name, attr := range content.Attributes {
+	for name, fieldIdx := range tags.Attributes {
-		fieldIdx := tags.Attributes[name]
+		attr := content.Attributes[name]
 		field := val.Type().Field(fieldIdx)
 		fieldV := val.Field(fieldIdx)
 
+		if attr == nil {
+			if !exprType.AssignableTo(field.Type) {
+				continue
+			}
+
+			// As a special case, if the target is of type hcl.Expression then
+			// we'll assign an actual expression that evalues to a cty null,
+			// so the caller can deal with it within the cty realm rather
+			// than within the Go realm.
+			synthExpr := hcl.StaticExpr(cty.NullVal(cty.DynamicPseudoType), body.MissingItemRange())
+			fieldV.Set(reflect.ValueOf(synthExpr))
+			continue
+		}
+
 		switch {
 		case attrType.AssignableTo(field.Type):
 			fieldV.Set(reflect.ValueOf(attr))

gohcl/decode_test.go

@@ -19,6 +19,10 @@ func TestDecodeBody(t *testing.T) {
 		}
 	}
 
+	type withNameExpression struct {
+		Name hcl.Expression `hcl:"name"`
+	}
+
 	tests := []struct {
 		Body      map[string]interface{}
 		Target    interface{}
@@ -51,6 +55,60 @@ func TestDecodeBody(t *testing.T) {
 			}{}),
 			0,
 		},
+		{
+			map[string]interface{}{},
+			withNameExpression{},
+			func(v interface{}) bool {
+				if v == nil {
+					return false
+				}
+
+				wne, valid := v.(withNameExpression)
+				if !valid {
+					return false
+				}
+
+				if wne.Name == nil {
+					return false
+				}
+
+				nameVal, _ := wne.Name.Value(nil)
+				if !nameVal.IsNull() {
+					return false
+				}
+
+				return true
+			},
+			0,
+		},
+		{
+			map[string]interface{}{
+				"name": "Ermintrude",
+			},
+			withNameExpression{},
+			func(v interface{}) bool {
+				if v == nil {
+					return false
+				}
+
+				wne, valid := v.(withNameExpression)
+				if !valid {
+					return false
+				}
+
+				if wne.Name == nil {
+					return false
+				}
+
+				nameVal, _ := wne.Name.Value(nil)
+				if !nameVal.Equals(cty.StringVal("Ermintrude")).True() {
+					return false
+				}
+
+				return true
+			},
+			0,
+		},
 		{
 			map[string]interface{}{
 				"name": "Ermintrude",

gohcl/schema.go

@@ -43,9 +43,23 @@ func ImpliedBodySchema(val interface{}) (schema *hcl.BodySchema, partial bool) {
 	for _, n := range attrNames {
 		idx := tags.Attributes[n]
 		field := ty.Field(idx)
+		var required bool
+
+		switch {
+		case field.Type.AssignableTo(exprType):
+			// If we're decoding to hcl.Expression then absense can be
+			// 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:
+			required = true
+		default:
+			required = false
+		}
+
 		attrSchemas = append(attrSchemas, hcl.AttributeSchema{
 			Name:     n,
-			Required: field.Type.Kind() != reflect.Ptr,
+			Required: required,
 		})
 	}
 

gohcl/schema_test.go

@@ -179,6 +179,20 @@ func TestImpliedBodySchema(t *testing.T) {
 			},
 			true,
 		},
+		{
+			struct {
+				Expr hcl.Expression `hcl:"expr"`
+			}{},
+			&hcl.BodySchema{
+				Attributes: []hcl.AttributeSchema{
+					{
+						Name:     "expr",
+						Required: false,
+					},
+				},
+			},
+			false,
+		},
 	}
 
 	for _, test := range tests {