parser/checker: [ref_only] attribute

vlib/v/ast/ast.v

@@ -153,6 +153,7 @@ pub:
 	is_c        bool
 	is_js       bool
 	is_union    bool
+	attr string
 }
 
 pub struct InterfaceDecl {

vlib/v/checker/checker.v

@@ -232,6 +232,12 @@ pub fn (mut c Checker) struct_decl(decl ast.StructDecl) {
 		if sym.kind == .placeholder && !decl.is_c && !sym.name.starts_with('C.') {
 			c.error('unknown type `$sym.name`', field.pos)
 		}
+		if sym.kind == .struct_ {
+			info:=sym.info as table.Struct
+			if info.is_ref_only && !field.typ.is_ptr() {
+				c.error('`$sym.name` type can only be used as a reference: `&$sym.name`', field.pos)
+			}
+		}
 		if field.has_default_expr {
 			c.expected_type = field.typ
 			field_expr_type := c.expr(field.default_expr)

vlib/v/parser/struct.v

@@ -166,6 +166,7 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 			fields: fields
 			is_typedef: is_typedef
 			is_union: is_union
+			is_ref_only: p.attr == 'ref_only'
 		}
 		mod: p.mod
 		is_public: is_pub
@@ -193,6 +194,7 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 		is_c: is_c
 		is_js: is_js
 		is_union: is_union
+		attr: p.attr
 	}
 }
 

vlib/v/table/atypes.v

@@ -531,9 +531,10 @@ pub fn (kinds []Kind) str() string {
 
 pub struct Struct {
 pub mut:
-	fields     []Field
-	is_typedef bool // C. [typedef]
-	is_union   bool
+	fields      []Field
+	is_typedef  bool // C. [typedef]
+	is_union    bool
+	is_ref_only bool
 }
 
 pub struct Interface {
@@ -641,7 +642,7 @@ pub fn (table &Table) type_to_str(t Type) string {
 	return res
 }
 
-pub fn(t &Table) fn_to_str(func &Fn) string {
+pub fn (t &Table) fn_to_str(func &Fn) string {
 	mut sb := strings.new_builder(20)
 	sb.write('${func.name}(')
 	for i in 1 .. func.args.len {
@@ -655,7 +656,7 @@ pub fn(t &Table) fn_to_str(func &Fn) string {
 		}
 	}
 	sb.write(')')
-	if func.return_type != table.void_type {
+	if func.return_type != void_type {
 		sb.write(' ${t.type_to_str(func.return_type)}')
 	}
 	return sb.str()