parser: disallow invalid expr in comptime $for (fix #23953) (#23959)

vlib/v/parser/comptime.v

@@ -361,6 +361,11 @@ fn (mut p Parser) comptime_for() ast.ComptimeFor {
 	mut typ_pos := p.tok.pos()
 	lang := p.parse_language()
 	mut typ := ast.void_type
+
+	if p.tok.lit.len == 0 {
+		p.error('invalid expr, use `${p.peek_tok.lit}` instead')
+		return ast.ComptimeFor{}
+	}
 	if p.tok.lit[0].is_capital() || p.tok.lit in p.imports {
 		typ = p.parse_any_type(lang, false, true, false)
 	} else {

vlib/v/parser/tests/comptime_for_invalid_expr_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/comptime_for_invalid_expr_err.vv:19:12: error: invalid expr, use `MyStruct` instead
+   17 |     }
+   18 | 
+   19 |     $for f in $MyStruct.fields {
+      |               ^
+   20 |         assert a.${f.field_name} == b.${f.field_name}
+   21 |     }

vlib/v/parser/tests/comptime_for_invalid_expr_err.vv

@@ -0,0 +1,22 @@
+module main
+
+struct MyStruct {
+	f_u8 u8
+	f_u16 u16
+}
+
+fn main() {
+	a := MyStruct {
+		f_u8 : u8(12)
+		f_u16 : u16(32)
+	}
+
+	b := MyStruct {
+		f_u8 : u8(12)
+		f_u16 : u16(32)
+	}
+
+	$for f in $MyStruct.fields {
+		assert a.${f.field_name} == b.${f.field_name}
+	}
+}