checker: prohibit using non-lvalue as mut receiver (#10790)

vlib/v/builder/builder.v

@@ -70,9 +70,10 @@ pub fn (mut b Builder) front_stages(v_files []string) ? {
 	util.timing_start('PARSE')
 	b.parsed_files = parser.parse_files(v_files, b.table, b.pref, b.global_scope)
 	b.parse_imports()
-	util.get_timers().show('SCAN')
-	util.get_timers().show('PARSE')
-	util.get_timers().show_if_exists('PARSE stmt')
+	mut timers := util.get_timers()
+	timers.show('SCAN')
+	timers.show('PARSE')
+	timers.show_if_exists('PARSE stmt')
 	if b.pref.only_check_syntax {
 		return error('stop_after_parser')
 	}

vlib/v/builder/compile.v

@@ -43,7 +43,8 @@ pub fn compile(command string, pref &pref.Preferences) {
 		.js { b.compile_js() }
 		.native { b.compile_native() }
 	}
-	util.get_timers().show_remaining()
+	mut timers := util.get_timers()
+	timers.show_remaining()
 	if pref.is_stats {
 		compilation_time_micros := 1 + sw.elapsed().microseconds()
 		scompilation_time_ms := util.bold('${f64(compilation_time_micros) / 1000.0:6.3f}')

vlib/v/checker/checker.v

@@ -2106,6 +2106,9 @@ pub fn (mut c Checker) method_call(mut call_expr ast.CallExpr) ast.Type {
 		}
 		if method.params[0].is_mut {
 			to_lock, pos := c.fail_if_immutable(call_expr.left)
+			if !call_expr.left.is_lvalue() {
+				c.error('cannot pass expression as `mut`', call_expr.left.position())
+			}
 			// call_expr.is_mut = true
 			if to_lock != '' && rec_share != .shared_t {
 				c.error('$to_lock is `shared` and must be `lock`ed to be passed as `mut`',

vlib/v/checker/tests/mut_receiver_lit.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/mut_receiver_lit.vv:10:1: error: cannot pass expression as `mut`
+    8 | }
+    9 | 
+   10 | Box{}.set(0)
+      | ~~~~~

vlib/v/checker/tests/mut_receiver_lit.vv

@@ -0,0 +1,10 @@
+struct Box {
+mut:
+	value int
+}
+
+fn (mut box Box) set(value int) {
+	box.value = value
+}
+
+Box{}.set(0)

vlib/v/scanner/scanner.v

@@ -537,11 +537,12 @@ fn (mut s Scanner) end_of_file() token.Token {
 }
 
 pub fn (mut s Scanner) scan_all_tokens_in_buffer(mode CommentsMode) {
-	util.get_timers().measure_pause('PARSE')
+	mut timers := util.get_timers()
+	timers.measure_pause('PARSE')
 	util.timing_start('SCAN')
 	defer {
 		util.timing_measure_cumulative('SCAN')
-		util.get_timers().measure_resume('PARSE')
+		timers.measure_resume('PARSE')
 	}
 	oldmode := s.comments_mode
 	s.comments_mode = mode

vlib/v/tests/method_call_chain_test.v

@@ -1,30 +0,0 @@
-struct Test {
-mut:
-	val int
-}
-
-// this must return a reference, or else you'll get a C error
-// TODO: add a proper checker check for that case
-fn new(x int) &Test {
-	return &Test{x}
-}
-
-fn (mut t Test) inc() &Test {
-	t.val++
-	return unsafe { t }
-}
-
-fn (mut t Test) add(x int) &Test {
-	t.val += x
-	return unsafe { t }
-}
-
-fn (mut t Test) div(x int) &Test {
-	t.val /= x
-	return unsafe { t }
-}
-
-fn test_method_call_chains() {
-	mut x := new(4).inc().inc().inc().inc().add(4).div(2).inc()
-	assert x.val == 7
-}

vlib/v/util/timers.v

@@ -28,15 +28,18 @@ pub fn get_timers() &Timers {
 }
 
 pub fn timing_start(label string) {
-	get_timers().start(label)
+	mut t := get_timers()
+	t.start(label)
 }
 
 pub fn timing_measure(label string) {
-	get_timers().show(label)
+	mut t := get_timers()
+	t.show(label)
 }
 
 pub fn timing_measure_cumulative(label string) {
-	get_timers().measure_cumulative(label)
+	mut t := get_timers()
+	t.measure_cumulative(label)
 }
 
 pub fn timing_set_should_print(should_print bool) {