all: Add support for embedded struct types in object conversions (#1021)

* add embedded struct support via field indices

* add support for unexported embedded structs and better panic protection

* adjusting error messaging and adding tests

* add test for duplicates inside of embedded struct

* add exported struct to tests

* add ignore tests for entire embedded struct

* add a test for multiple levels of embedding

* comment

* comment

* move comment

* add documentation for embedded structs

* added changelog

* add tests for tfsdk tags on embedded structs

* refactor to use Tag.Lookup and add tests for empty tags

* added note changelog for existing structs

.changes/unreleased/ENHANCEMENTS-20240722-175116.yaml

@@ -0,0 +1,6 @@
+kind: ENHANCEMENTS
+body: 'all: Added embedded struct support for object to struct conversions with `tfsdk`
+  tags'
+time: 2024-07-22T17:51:16.833264-04:00
+custom:
+  Issue: "1021"

.changes/unreleased/NOTES-20240801-171654.yaml

@@ -0,0 +1,40 @@
+kind: NOTES
+body: |
+  Framework reflection logic (`Config.Get`, `Plan.Get`, etc.) for structs with
+  `tfsdk` field tags has been updated to support embedded structs that promote exported
+  fields. For existing structs that embed unexported structs with exported fields, a tfsdk
+  ignore tag (``tfsdk:"-"``) can be added to ignore all promoted fields.  
+
+  For example, the following struct will now return an error diagnostic:
+  ```go
+  type thingResourceModel struct {
+  	Attr1 types.String `tfsdk:"attr_1"`
+  	Attr2 types.Bool   `tfsdk:"attr_2"`
+  
+  	// Previously, this embedded struct was ignored, will now promote underlying fields
+  	embeddedModel
+  }
+  
+  type embeddedModel struct {
+  	// No `tfsdk` tag
+  	ExportedField string
+  }
+  ```
+  
+  To preserve the original behavior, a tfsdk ignore tag can be added to ignore the entire embedded struct:
+  ```go
+  type thingResourceModel struct {
+  	Attr1 types.String `tfsdk:"attr_1"`
+  	Attr2 types.Bool   `tfsdk:"attr_2"`
+  
+  	// This embedded struct will now be ignored
+  	embeddedModel      `tfsdk:"-"`
+  }
+  
+  type embeddedModel struct {
+  	ExportedField string
+  }
+  ```
+time: 2024-08-01T17:16:54.043432-04:00
+custom:
+  Issue: "1021"

internal/reflect/helpers.go

@@ -46,36 +46,87 @@ func commaSeparatedString(in []string) string {
 }
 
 // getStructTags returns a map of Terraform field names to their position in
-// the tags of the struct `in`. `in` must be a struct.
-func getStructTags(_ context.Context, in reflect.Value, path path.Path) (map[string]int, error) {
-	tags := map[string]int{}
-	typ := trueReflectValue(in).Type()
+// the fields of the struct `in`. `in` must be a struct.
+//
+// The position of the field in a struct is represented as an index sequence to support type embedding
+// in structs. This index sequence can be used to retrieve the field with the Go "reflect" package FieldByIndex methods:
+//   - https://pkg.go.dev/reflect#Type.FieldByIndex
+//   - https://pkg.go.dev/reflect#Value.FieldByIndex
+//   - https://pkg.go.dev/reflect#Value.FieldByIndexErr
+//
+// The following are not supported and will return an error if detected in a struct (including embedded structs):
+//   - Duplicate "tfsdk" tags
+//   - Exported fields without a "tfsdk" tag
+//   - Exported fields with an invalid "tfsdk" tag (must be a valid Terraform identifier)
+func getStructTags(ctx context.Context, typ reflect.Type, path path.Path) (map[string][]int, error) { //nolint:unparam // False positive, ctx is used below.
+	tags := make(map[string][]int, 0)
+
+	if typ.Kind() == reflect.Pointer {
+		typ = typ.Elem()
+	}
 	if typ.Kind() != reflect.Struct {
-		return nil, fmt.Errorf("%s: can't get struct tags of %s, is not a struct", path, in.Type())
+		return nil, fmt.Errorf("%s: can't get struct tags of %s, is not a struct", path, typ)
 	}
+
 	for i := 0; i < typ.NumField(); i++ {
 		field := typ.Field(i)
-		if field.PkgPath != "" {
-			// skip unexported fields
+		if !field.IsExported() && !field.Anonymous {
+			// Skip unexported fields. Unexported embedded structs (anonymous fields) are allowed because they may
+			// contain exported fields that are promoted; which means they can be read/set.
 			continue
 		}
-		tag := field.Tag.Get(`tfsdk`)
+
+		// This index sequence is the location of the field within the struct.
+		// For promoted fields from an embedded struct, the length of this sequence will be > 1
+		fieldIndexSequence := []int{i}
+		tag, tagExists := field.Tag.Lookup(`tfsdk`)
+
+		// "tfsdk" tags with "-" are being explicitly excluded
 		if tag == "-" {
-			// skip explicitly excluded fields
 			continue
 		}
-		if tag == "" {
+
+		// Handle embedded structs
+		if field.Anonymous {
+			if tagExists {
+				return nil, fmt.Errorf(`%s: embedded struct field %s cannot have tfsdk tag`, path.AtName(tag), field.Name)
+			}
+
+			embeddedTags, err := getStructTags(ctx, field.Type, path)
+			if err != nil {
+				return nil, fmt.Errorf(`error retrieving embedded struct %q field tags: %w`, field.Name, err)
+			}
+			for k, v := range embeddedTags {
+				if other, ok := tags[k]; ok {
+					otherField := typ.FieldByIndex(other)
+					return nil, fmt.Errorf("embedded struct %q promotes a field with a duplicate tfsdk tag %q, conflicts with %q tfsdk tag", field.Name, k, otherField.Name)
+				}
+
+				tags[k] = append(fieldIndexSequence, v...)
+			}
+			continue
+		}
+
+		// All non-embedded fields must have a tfsdk tag
+		if !tagExists {
 			return nil, fmt.Errorf(`%s: need a struct tag for "tfsdk" on %s`, path, field.Name)
 		}
+
+		// Ensure the tfsdk tag has a valid name
 		path := path.AtName(tag)
 		if !isValidFieldName(tag) {
-			return nil, fmt.Errorf("%s: invalid field name, must only use lowercase letters, underscores, and numbers, and must start with a letter", path)
+			return nil, fmt.Errorf("%s: invalid tfsdk tag, must only use lowercase letters, underscores, and numbers, and must start with a letter", path)
 		}
+
+		// Ensure there are no duplicate tfsdk tags
 		if other, ok := tags[tag]; ok {
-			return nil, fmt.Errorf("%s: can't use field name for both %s and %s", path, typ.Field(other).Name, field.Name)
+			otherField := typ.FieldByIndex(other)
+			return nil, fmt.Errorf("%s: can't use tfsdk tag %q for both %s and %s fields", path, tag, otherField.Name, field.Name)
 		}
-		tags[tag] = i
+
+		tags[tag] = fieldIndexSequence
 	}
+
 	return tags, nil
 }