checker, cgen: allow implementing an interface with an embedded struct (#9042)

vlib/v/checker/checker.v

@@ -2060,7 +2060,7 @@ fn (mut c Checker) type_implements(typ table.Type, inter_typ table.Type, pos tok
 	}
 	if mut inter_sym.info is table.Interface {
 		for ifield in inter_sym.info.fields {
-			if field := typ_sym.find_field(ifield.name) {
+			if field := c.table.find_field_with_embeds(typ_sym, ifield.name) {
 				if ifield.typ != field.typ {
 					exp := c.table.type_to_str(ifield.typ)
 					got := c.table.type_to_str(field.typ)

vlib/v/gen/c/cgen.v

@@ -6164,13 +6164,14 @@ fn (mut g Gen) interface_table() string {
 		iinidx_minimum_base := 1000 // NB: NOT 0, to avoid map entries set to 0 later, so `if already_generated_mwrappers[name] > 0 {` works.
 		mut current_iinidx := iinidx_minimum_base
 		for st in inter_info.types {
+			st_sym := g.table.get_type_symbol(st)
 			// cctype is the Cleaned Concrete Type name, *without ptr*,
 			// i.e. cctype is always just Cat, not Cat_ptr:
 			cctype := g.cc_type(st, true)
 			$if debug_interface_table ? {
 				eprintln(
 					'>> interface name: $ityp.name | concrete type: $st.debug() | st symname: ' +
-					g.table.get_type_symbol(st).name)
+					st_sym.name)
 			}
 			// Speaker_Cat_index = 0
 			interface_index_name := '_${interface_name}_${cctype}_index'
@@ -6189,7 +6190,20 @@ fn (mut g Gen) interface_table() string {
 			for field in inter_info.fields {
 				cname := c_name(field.name)
 				field_styp := g.typ(field.typ)
-				cast_struct.writeln('\t\t.$cname = ($field_styp*)((char*)x + __offsetof($cctype, $cname)),')
+				if _ := st_sym.find_field(field.name) {
+					cast_struct.writeln('\t\t.$cname = ($field_styp*)((char*)x + __offsetof($cctype, $cname)),')
+				} else {
+					// the field is embedded in another struct
+					cast_struct.write_string('\t\t.$cname = ($field_styp*)((char*)x')
+					for embed_type in st_sym.struct_info().embeds {
+						embed_sym := g.table.get_type_symbol(embed_type)
+						if _ := embed_sym.find_field(field.name) {
+							cast_struct.write_string(' + __offsetof($cctype, $embed_sym.embed_name()) + __offsetof($embed_sym.cname, $cname)')
+							break
+						}
+					}
+					cast_struct.writeln('),')
+				}
 			}
 			cast_struct.write_string('\t}')
 			cast_struct_str := cast_struct.str()
@@ -6208,7 +6222,6 @@ $staticprefix $interface_name* I_${cctype}_to_Interface_${interface_name}_ptr($c
 			if g.pref.build_mode != .build_module {
 				methods_struct.writeln('\t{')
 			}
-			st_sym := g.table.get_type_symbol(st)
 			mut method := table.Fn{}
 			for _, m in ityp.methods {
 				for mm in st_sym.methods {

vlib/v/table/table.v

@@ -296,6 +296,32 @@ pub fn (t &Table) find_field(s &TypeSymbol, name string) ?Field {
 	return none
 }
 
+// search for a given field, looking through embedded fields
+pub fn (t &Table) find_field_with_embeds(sym &TypeSymbol, field_name string) ?Field {
+	if f := t.find_field(sym, field_name) {
+		return f
+	} else {
+		// look for embedded field
+		if sym.info is Struct {
+			mut found_fields := []Field{}
+			mut embed_of_found_fields := []Type{}
+			for embed in sym.info.embeds {
+				embed_sym := t.get_type_symbol(embed)
+				if f := t.find_field(embed_sym, field_name) {
+					found_fields << f
+					embed_of_found_fields << embed
+				}
+			}
+			if found_fields.len == 1 {
+				return found_fields[0]
+			} else if found_fields.len > 1 {
+				return error('ambiguous field `$field_name`')
+			}
+		}
+		return err
+	}
+}
+
 [inline]
 pub fn (t &Table) find_type_idx(name string) int {
 	return t.type_idxs[name]

vlib/v/tests/interface_test.v

@@ -305,3 +305,28 @@ fn animal_match(a Animal) {
 	}
 }
 */
+
+interface II {
+mut:
+    my_field int
+}
+
+struct AA {
+    BB
+}
+
+struct BB {
+	pad [10]byte
+mut:
+    my_field int
+}
+
+fn main() {
+	mut aa := AA{}
+	mut ii := II(aa)
+	assert ii.my_field == 0
+	aa.my_field = 123
+	assert ii.my_field == 123
+	ii.my_field = 1234
+	assert aa.my_field == 1234
+}