cgen: properly support `typeof()`, sizeof(), ... in compile time `for` (#10896)

vlib/v/gen/c/cgen.v

@@ -36,8 +36,9 @@ fn string_array_to_map(a []string) map[string]bool {
 }
 
 struct Gen {
-	pref         &pref.Preferences
+	pref            &pref.Preferences
-	module_built string
+	module_built    string
+	field_data_type ast.Type // cache her to avoid map lookups
 mut:
 	table                  &ast.Table
 	out                    strings.Builder
@@ -225,6 +226,7 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 		module_built: module_built
 		timers: util.new_timers(timers_should_print)
 		inner_loop: &ast.EmptyStmt{}
+		field_data_type: ast.Type(table.find_type_idx('FieldData'))
 	}
 	g.timers.start('cgen init')
 	for mod in g.table.modules {
@@ -3422,7 +3424,8 @@ fn (mut g Gen) expr(node ast.Expr) {
 			g.select_expr(node)
 		}
 		ast.SizeOf {
-			node_typ := g.unwrap_generic(node.typ)
+			typ := if node.typ == g.field_data_type { g.comp_for_field_value.typ } else { node.typ }
+			node_typ := g.unwrap_generic(typ)
 			sym := g.table.get_type_symbol(node_typ)
 			if sym.language == .v && sym.kind in [.placeholder, .any] {
 				g.error('unknown type `$sym.name`', node.pos)
@@ -3431,7 +3434,8 @@ fn (mut g Gen) expr(node ast.Expr) {
 			g.write('/*SizeOf*/ sizeof(${util.no_dots(styp)})')
 		}
 		ast.IsRefType {
-			node_typ := g.unwrap_generic(node.typ)
+			typ := if node.typ == g.field_data_type { g.comp_for_field_value.typ } else { node.typ }
+			node_typ := g.unwrap_generic(typ)
 			sym := g.table.get_type_symbol(node_typ)
 			if sym.language == .v && sym.kind in [.placeholder, .any] {
 				g.error('unknown type `$sym.name`', node.pos)
@@ -3495,7 +3499,8 @@ fn (mut g Gen) expr(node ast.Expr) {
 }
 
 // T.name, typeof(expr).name
-fn (mut g Gen) type_name(typ ast.Type) {
+fn (mut g Gen) type_name(raw_type ast.Type) {
+	typ := if raw_type == g.field_data_type { g.comp_for_field_value.typ } else { raw_type }
 	sym := g.table.get_type_symbol(typ)
 	mut s := ''
 	if sym.kind == .function {
@@ -3511,7 +3516,12 @@ fn (mut g Gen) type_name(typ ast.Type) {
 }
 
 fn (mut g Gen) typeof_expr(node ast.TypeOf) {
-	sym := g.table.get_type_symbol(node.expr_type)
+	typ := if node.expr_type == g.field_data_type {
+		g.comp_for_field_value.typ
+	} else {
+		node.expr_type
+	}
+	sym := g.table.get_type_symbol(typ)
 	if sym.kind == .sum_type {
 		// When encountering a .sum_type, typeof() should be done at runtime,
 		// because the subtype of the expression may change:
@@ -3525,11 +3535,11 @@ fn (mut g Gen) typeof_expr(node ast.TypeOf) {
 	} else if sym.kind == .function {
 		info := sym.info as ast.FnType
 		g.write('_SLIT("${g.fn_decl_str(info)}")')
-	} else if node.expr_type.has_flag(.variadic) {
+	} else if typ.has_flag(.variadic) {
-		varg_elem_type_sym := g.table.get_type_symbol(g.table.value_type(node.expr_type))
+		varg_elem_type_sym := g.table.get_type_symbol(g.table.value_type(typ))
 		g.write('_SLIT("...${util.strip_main_name(varg_elem_type_sym.name)}")')
 	} else {
-		x := g.table.type_to_str(node.expr_type)
+		x := g.table.type_to_str(typ)
 		y := util.strip_main_name(x)
 		g.write('_SLIT("$y")')
 	}

vlib/v/tests/comptime_for_test.v

@@ -64,6 +64,7 @@ fn test_comptime_for_with_if() {
 
 fn test_comptime_for_fields() {
 	println(@FN)
+	mut fields_found := 0
 	$for field in App.fields {
 		println('  field: $field.name | ' + no_lines('$field'))
 		$if field.typ is string {
@@ -81,5 +82,19 @@ fn test_comptime_for_fields() {
 		if field.is_pub && field.is_mut {
 			assert field.name in ['g', 'h']
 		}
+		if field.name == 'f' {
+			assert sizeof(field) == 8
+			assert isreftype(field) == false
+			// assert typeof(field) == 'u64'
+			assert typeof(field).name == 'u64'
+			fields_found++
+		}
+		if field.name == 'g' {
+			// assert typeof(field) == 'string'
+			assert typeof(field).name == 'string'
+			assert isreftype(field) == true
+			fields_found++
+		}
 	}
+	assert fields_found == 2
 }