Add (gen > c) -> Added callconv attribute for function type aliases

vlib/v/gen/c/cgen.v

@@ -1343,7 +1343,21 @@ pub fn (mut g Gen) write_fn_typesymbol_declaration(sym ast.TypeSymbol) {
 	if !info.has_decl && (not_anon || is_fn_sig) && !func.return_type.has_flag(.generic)
 		&& !has_generic_arg {
 		fn_name := sym.cname
-		g.type_definitions.write_string('typedef ${g.typ(func.return_type)} (*$fn_name)(')
+
+		mut call_conv := ""
+
+		for attr in func.attrs {
+			match attr.name {
+				"callconv" {
+					if attr.has_arg {
+						call_conv = "__$attr.arg "
+					}
+				}
+				else {}
+			}
+		}
+
+		g.type_definitions.write_string('typedef ${g.typ(func.return_type)} ($call_conv*$fn_name)(')
 		for i, param in func.params {
 			g.type_definitions.write_string(g.typ(param.typ))
 			if i < func.params.len - 1 {

vlib/v/parser/parse_type.v

@@ -229,6 +229,20 @@ pub fn (mut p Parser) parse_multi_return_type() ast.Type {
 pub fn (mut p Parser) parse_fn_type(name string) ast.Type {
 	// p.warn('parse fn')
 	p.check(.key_fn)
+
+	for attr in p.attrs {
+		match attr.name {
+			"callconv" {
+				if attr.has_arg {
+					if attr.arg !in ['stdcall', 'fastcall', 'cdecl'] {
+						p.error_with_pos('unsupported calling convention, supported are stdcall, fastcall and cdecl', p.prev_tok.pos())
+					 }
+				}
+			}
+			else {}
+		}
+	}
+
 	mut has_generic := false
 	line_nr := p.tok.line_nr
 	args, _, is_variadic := p.fn_args()
@@ -255,6 +269,7 @@ pub fn (mut p Parser) parse_fn_type(name string) ast.Type {
 		return_type: return_type
 		return_type_pos: return_type_pos
 		is_method: false
+		attrs: p.attrs
 	}
 	// MapFooFn typedefs are manually added in cheaders.v
 	// because typedefs get generated after the map struct is generated