compiler: stabilize the interface code generation a little

vlib/compiler/cgen.v

@@ -470,23 +470,38 @@ fn (v &V) interface_table() string {
 		if t.cat != .interface_ {
 			continue
 		}
+		// interface_name is for example Speaker
 		interface_name := t.name
 		mut methods := ''
+		mut generated_casting_functions := ''
 		sb.writeln('// NR methods = $t.gen_types.len')
 		for i, gen_type in t.gen_types {
-			gen_concrete_type_name := gen_type.replace('*', '')
+			// ptr_ctype can be for example Cat OR Cat_ptr:
+			ptr_ctype := gen_type.replace('*', '_ptr')
+			// cctype is the Cleaned Concrete Type name, *without ptr*,
+			// i.e. cctype is always just Cat, not Cat_ptr:
+			cctype := gen_type.replace('*', '')
+
+			// Speaker_Cat_index = 0
+			interface_index_name := '_${interface_name}_${ptr_ctype}_index'
+
+			generated_casting_functions += '
+${interface_name} I_${cctype}_to_${interface_name}(${cctype} x) {
+  return (${interface_name}){
+           ._object = (void*) memdup(&x, sizeof(${cctype})),
+           ._interface_idx = ${interface_index_name} };
+}
+'
 			methods += '{\n'
 			for j, method in t.methods {
 				// Cat_speak
-				methods += ' (void*)    ${gen_concrete_type_name}_${method.name}'
+				methods += ' (void*)    ${cctype}_${method.name}'
 				if j < t.methods.len - 1 {
 					methods += ', \n'
 				}
 			}
 			methods += '\n},\n\n'
-			// Speaker_Cat_index = 0
-			concrete_type_name := gen_type.replace('*', '_ptr')
-			sb.writeln('int _${interface_name}_${concrete_type_name}_index = $i;')
+			sb.writeln('int ${interface_index_name} = $i;')
 		}
 		if t.gen_types.len > 0 {
 			// methods = '{TCCSKIP(0)}'
@@ -499,7 +514,10 @@ fn (v &V) interface_table() string {
 			// See https://github.com/zenith391/vgtk3/issues/7
 			sb.writeln('void* (* ${interface_name}_name_table[][1]) = ' + '{ {NULL} }; ')
 		}
-		continue
+		if generated_casting_functions.len > 0 {
+			sb.writeln('// Casting functions for interface "${interface_name}" :')
+			sb.writeln( generated_casting_functions )
+		}
 	}
 	return sb.str()
 }

vlib/compiler/fn.v

@@ -1115,12 +1115,18 @@ fn (p mut Parser) fn_call_args(f mut Fn, generic_param_types []string) {
 		if arg.typ.ends_with('er') || arg.typ[0] == `I` {
 			t := p.table.find_type(arg.typ)
 			if t.cat == .interface_ {
-				// perform((Speaker) { ._object = &dog,
-				// _interface_idx = _Speaker_Dog_index })
-				if !f.is_method {
+				// NB: here concrete_type_name can be 'Dog' OR 'Dog_ptr'
+				// cgen should have generated a _I_Dog_to_Speaker conversion function
+				// C: perform( _I_Dog_to_Speaker((Dog){...}) )
+				// In case of _ptr, there is no need for conversion, so the generated
+				// code will be just:
+				// C: perform( dog_ptr )
 				concrete_type_name := typ.replace('*', '_ptr')
-					p.cgen.set_placeholder(ph, '($arg.typ) { ._object = &')
-					p.gen(', ._interface_idx = _${arg.typ}_${concrete_type_name}_index} ')
+				// concrete_type_name here can be say Dog, or ui__Group_ptr (in vui)
+				//eprintln('arg.typ: $arg.typ | concrete_type_name: $concrete_type_name ')
+				if !concrete_type_name.ends_with('_ptr')  {
+					p.cgen.set_placeholder(ph, 'I_${concrete_type_name}_to_${arg.typ}(')
+					p.gen(')')
 				}
 				p.table.add_gen_type(arg.typ, typ)
 			}

vlib/compiler/tests/interfaces_map_test.v

@@ -0,0 +1,49 @@
+module main
+
+interface Speaker {
+	say() string
+}
+
+struct ChatRoom {
+mut:
+	talkers map[string]Speaker
+}
+
+fn new_room() &ChatRoom {
+	return &ChatRoom{
+		talkers: map[string]Speaker
+	}
+}
+
+fn (r mut ChatRoom) add(name string, s Speaker) {
+	r.talkers[name] = s
+}
+
+fn test_using_a_map_of_speaker_interfaces(){
+	mut room := new_room()
+	room.add('my cat', Cat{name: 'Tigga'} )
+	room.add('my dog', Dog{name: 'Pirin'} )
+	room.add('stray dog', Dog{name: 'Anoni'} )
+	room.add('me', Human{name: 'Bilbo'} )
+	room.add('she', Human{name: 'Maria'} )
+	mut text := ''
+	for name, subject in room.talkers {
+		line := '${name:12s}: ${subject.say()}'
+		println(line)
+		text += line
+	}
+	assert text.contains(' meows ')
+	assert text.contains(' barks ')
+	assert text.contains(' says ')
+}
+
+//
+
+struct Cat { name string }
+fn (c &Cat) say() string { return '${c.name} meows "MEOW!"' }
+
+struct Dog { name string }
+fn (d &Dog) say() string { return '${d.name} barks "Bau Bau!"' }
+
+struct Human { name string }
+fn (h &Human) say() string { return '${h.name} says "Hello"' }