checker: check labelled break/continue is inside a matching `for` loop (#6910)

vlib/v/checker/checker.v

@@ -65,6 +65,7 @@ mut:
 	vmod_file_content                string // needed for @VMOD_FILE, contents of the file, *NOT its path**
 	vweb_gen_types                   []table.Type // vweb route checks
 	prevent_sum_type_unwrapping_once bool // needed for assign new values to sum type, stopping unwrapping then
+	loop_label                       string // set when inside a labelled for loop
 }
 
 pub fn new_checker(table &table.Table, pref &pref.Preferences) Checker {
@@ -2424,6 +2425,19 @@ fn is_const_integer(cfield ast.ConstField) ?ast.IntegerLiteral {
 	return none
 }
 
+[inline]
+fn (mut c Checker) check_loop_label(label string, pos token.Position) {
+	if label.len == 0 {
+		// ignore
+		return
+	}
+	if c.loop_label.len != 0 {
+		c.error('nesting of labelled `for` loops is not supported', pos)
+		return
+	}
+	c.loop_label = label
+}
+
 fn (mut c Checker) stmt(node ast.Stmt) {
 	$if trace_checker ? {
 		stmt_pos := node.position()
@@ -2458,7 +2472,11 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			if c.in_for_count == 0 {
 				c.error('$node.kind.str() statement not within a loop', node.pos)
 			}
-			// TODO: check any node.label is in scope for goto
+			if node.label.len > 0 {
+				if node.label != c.loop_label {
+					c.error('invalid label name `$node.label`', node.pos)
+				}
+			}
 		}
 		ast.CompFor {
 			// node.typ = c.expr(node.expr)
@@ -2490,7 +2508,9 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			c.stmt(node.init)
 			c.expr(node.cond)
 			c.stmt(node.inc)
+			c.check_loop_label(node.label, node.pos)
 			c.stmts(node.stmts)
+			c.loop_label = ''
 			c.in_for_count--
 		}
 		ast.ForInStmt {
@@ -2545,7 +2565,9 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 				node.val_type = value_type
 				scope.update_var_type(node.val_var, value_type)
 			}
+			c.check_loop_label(node.label, node.pos)
 			c.stmts(node.stmts)
+			c.loop_label = ''
 			c.in_for_count--
 		}
 		ast.ForStmt {
@@ -2557,7 +2579,9 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			}
 			// TODO: update loop var type
 			// how does this work currenly?
+			c.check_loop_label(node.label, node.pos)
 			c.stmts(node.stmts)
+			c.loop_label = ''
 			c.in_for_count--
 		}
 		ast.GlobalDecl {

vlib/v/checker/tests/labelled_break_continue.out

@@ -0,0 +1,35 @@
+vlib/v/checker/tests/labelled_break_continue.vv:7:14: error: invalid label name `L2`
+    5 |         i++
+    6 |         for {
+    7 |             if i < 7 {continue L2}
+      |                       ~~~~~~~~
+    8 |             else {break L2}
+    9 |         }
+vlib/v/checker/tests/labelled_break_continue.vv:8:10: error: invalid label name `L2`
+    6 |         for {
+    7 |             if i < 7 {continue L2}
+    8 |             else {break L2}
+      |                   ~~~~~
+    9 |         }
+   10 |     }
+vlib/v/checker/tests/labelled_break_continue.vv:15:14: error: invalid label name `L1`
+   13 |         i = e
+   14 |         for {
+   15 |             if i < 3 {continue L1}
+      |                       ~~~~~~~~
+   16 |             else {break L1}
+   17 |         }
+vlib/v/checker/tests/labelled_break_continue.vv:16:10: error: invalid label name `L1`
+   14 |         for {
+   15 |             if i < 3 {continue L1}
+   16 |             else {break L1}
+      |                   ~~~~~
+   17 |         }
+   18 |     }
+vlib/v/checker/tests/labelled_break_continue.vv:21:11: error: nesting of labelled `for` loops is not supported
+   19 |     // check nested loops (not supported ATM)
+   20 |     L3: for ;; i++ {
+   21 |         L4: for {
+      |                 ^
+   22 |             if i < 17 {continue L3}
+   23 |             else {break L3}

vlib/v/checker/tests/labelled_break_continue.vv

@@ -0,0 +1,26 @@
+fn main() {
+	mut i := 4
+	// check branching to a later loop
+	L1: for {
+		i++
+		for {
+			if i < 7 {continue L2}
+			else {break L2}
+		}
+	}
+	// check branching to an earlier loop
+	L2: for e in [1,2,3,4] {
+		i = e
+		for {
+			if i < 3 {continue L1}
+			else {break L1}
+		}
+	}
+	// check nested loops (not supported ATM)
+	L3: for ;; i++ {
+		L4: for {
+			if i < 17 {continue L3}
+			else {break L3}
+		}
+	}
+}