v: support `$for attr in Test.attributes` (#9836)

vlib/builtin/builtin.v

@@ -121,3 +121,18 @@ pub:
 	is_mut bool
 	typ    int
 }
+
+pub enum AttributeKind {
+	plain // [name]
+	string // ['name']
+	number // [123]
+	comptime_define // [if name]
+}
+
+pub struct StructAttribute {
+pub:
+	name    string
+	has_arg bool
+	arg     string
+	kind    AttributeKind
+}

vlib/v/ast/ast.v

@@ -786,6 +786,7 @@ pub:
 pub enum CompForKind {
 	methods
 	fields
+	attributes
 }
 
 pub struct CompFor {

vlib/v/ast/str.v

@@ -488,5 +488,6 @@ pub fn (e CompForKind) str() string {
 	match e {
 		.methods { return 'methods' }
 		.fields { return 'fields' }
+		.attributes { return 'attributes' }
 	}
 }

vlib/v/gen/c/comptime.v

@@ -468,6 +468,22 @@ fn (mut g Gen) comp_for(node ast.CompFor) {
 			}
 			g.comptime_var_type_map.delete(node.val_var)
 		}
+	} else if node.kind == .attributes {
+		if sym.info is ast.Struct {
+			if sym.info.attrs.len > 0 {
+				g.writeln('\tStructAttribute $node.val_var = {0};')
+			}
+			for attr in sym.info.attrs {
+				g.writeln('/* attribute $i */ {')
+
+				g.writeln('\t${node.val_var}.name = _SLIT("$attr.name");')
+				g.writeln('\t${node.val_var}.has_arg = $attr.has_arg;')
+				g.writeln('\t${node.val_var}.arg = _SLIT("$attr.arg");')
+				g.writeln('\t${node.val_var}.kind = AttributeKind_$attr.kind;')
+
+				g.writeln('}')
+			}
+		}
 	}
 	g.indent--
 	g.writeln('}// \$for')

vlib/v/parser/comptime.v

@@ -253,8 +253,15 @@ fn (mut p Parser) comp_for() ast.CompFor {
 			pos: var_pos
 		})
 		kind = .fields
+	} else if for_val == 'attributes' {
+		p.scope.register(ast.Var{
+			name: val_var
+			typ: p.table.find_type_idx('StructAttribute')
+			pos: var_pos
+		})
+		kind = .attributes
 	} else {
-		p.error_with_pos('unknown kind `$for_val`, available are: `methods` or `fields`',
+		p.error_with_pos('unknown kind `$for_val`, available are: `methods`, `fields` or `attributes`',
 			p.prev_tok.position())
 		return ast.CompFor{}
 	}

vlib/v/tests/comptime_attribute_selector_test.v

@@ -0,0 +1,14 @@
+[test: 'hello']
+[abc]
+struct Test {}
+
+fn test_attributes() {
+	$for attr in Test.attributes {
+		if attr.has_arg {
+			assert attr.name == 'test'
+			assert attr.arg == 'hello'
+		} else {
+			assert attr.name == 'abc'
+		}
+	}
+}