cgen: fix `for in` with mutable structs (#12368)

vlib/v/gen/c/cgen.v

@@ -2165,7 +2165,11 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 		g.writeln('\tif (${t_var}.state != 0) break;')
 		val := if node.val_var in ['', '_'] { g.new_tmp_var() } else { node.val_var }
 		val_styp := g.typ(node.val_type)
+		if node.val_is_mut {
+			g.writeln('\t$val_styp* $val = ($val_styp*)${t_var}.data;')
+		} else {
 			g.writeln('\t$val_styp $val = *($val_styp*)${t_var}.data;')
+		}
 	} else {
 		typ_str := g.table.type_to_str(node.cond_type)
 		g.error('for in: unhandled symbol `$node.cond` of type `$typ_str`', node.pos)

vlib/v/tests/for_in_mut_struct_val_test.v

@@ -0,0 +1,45 @@
+struct Struct {
+mut:
+	array [][]int
+}
+
+fn (s Struct) rows() StructsRowIterator {
+	return StructsRowIterator{
+		array: s.array
+		position: 0
+	}
+}
+
+struct StructsRowIterator {
+	Struct
+mut:
+	position int
+}
+
+fn (mut s StructsRowIterator) next() ?[]int {
+	if s.position >= s.array.len {
+		return error('out of range')
+	}
+	defer {
+		s.position++
+	}
+	return s.array[s.position]
+}
+
+fn test_for_in_mut_struct_val() {
+	mut s := Struct{
+		array: [[1, 2, 3], [4, 5, 6]]
+	}
+	println(s)
+	mut si := s.rows()
+	println(si)
+
+	mut rets := []string{}
+	for mut row in si {
+		println(row)
+		rets << '$row'
+	}
+	assert rets.len == 2
+	assert rets[0] == '[1, 2, 3]'
+	assert rets[1] == '[4, 5, 6]'
+}