cgen: fix appending struct to interface array (#14388)

vlib/v/gen/c/struct.v

@@ -183,6 +183,9 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 					}
 				}
 				if !cloned {
+					inside_cast_in_heap := g.inside_cast_in_heap
+					g.inside_cast_in_heap = 0 // prevent use of pointers in child structs
+
 					if field_type_sym.kind == .array_fixed && sfield.expr is ast.Ident {
 						fixed_array_info := field_type_sym.info as ast.ArrayFixed
 						g.write('{')
@@ -202,6 +205,7 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 						}
 						g.expr_with_cast(sfield.expr, sfield.typ, sfield.expected_type)
 					}
+					g.inside_cast_in_heap = inside_cast_in_heap // restore value for further struct inits
 				}
 				if is_multiline {
 					g.writeln(',')

vlib/v/tests/append_struct_to_interface_array_test.v

@@ -0,0 +1,43 @@
+struct Vec {
+mut:
+	x f64
+	y f64
+	z f64
+}
+
+struct Plane {
+	position Vec
+	normal   Vec
+}
+
+struct Sphere {
+	position Vec
+	radius   f64
+}
+
+interface Object {
+	position Vec
+}
+
+fn test_append_struct_to_interface_array() {
+	mut scene := []Object{}
+
+	scene << Plane{
+		position: Vec{0, -10, 0}
+		normal: Vec{0, -1, 0}
+	}
+	scene << Sphere{
+		position: Vec{0, 0, -20}
+		radius: 7
+	}
+
+	println(scene)
+
+	assert scene.len == 2
+	assert scene[0].position.x == 0
+	assert scene[0].position.y == -10
+	assert scene[0].position.z == 0
+	assert scene[1].position.x == 0
+	assert scene[1].position.y == 0
+	assert scene[1].position.z == -20
+}