checker: add clearer errors for break/continue used within a $for loop (#16600)

vlib/v/checker/checker.v

@@ -1828,7 +1828,11 @@ fn (mut c Checker) branch_stmt(node ast.BranchStmt) {
 		c.error('`${node.kind.str()}` is not allowed in defer statements', node.pos)
 	}
 	if c.in_for_count == 0 {
-		c.error('${node.kind.str()} statement not within a loop', node.pos)
+		if c.inside_comptime_for_field {
+			c.error('${node.kind.str()} is not allowed within a compile-time loop', node.pos)
+		} else {
+			c.error('${node.kind.str()} statement not within a loop', node.pos)
+		}
 	}
 	if node.label.len > 0 {
 		if node.label != c.loop_label {

vlib/v/checker/tests/check_wrong_usage_of_break_and_continue.out

@@ -0,0 +1,28 @@
+vlib/v/checker/tests/check_wrong_usage_of_break_and_continue.vv:9:3: error: continue is not allowed within a compile-time loop
+    7 |     $for field in Test.fields {
+    8 |         println(field)
+    9 |         continue
+      |         ~~~~~~~~
+   10 |     }
+   11 |     $for field in Test.fields {
+vlib/v/checker/tests/check_wrong_usage_of_break_and_continue.vv:13:3: error: break is not allowed within a compile-time loop
+   11 |     $for field in Test.fields {
+   12 |         println(field)
+   13 |         break
+      |         ~~~~~
+   14 |     }
+   15 |     {
+vlib/v/checker/tests/check_wrong_usage_of_break_and_continue.vv:16:3: error: break statement not within a loop
+   14 |     }
+   15 |     {
+   16 |         break
+      |         ~~~~~
+   17 |         continue
+   18 |     }
+vlib/v/checker/tests/check_wrong_usage_of_break_and_continue.vv:17:3: error: continue statement not within a loop
+   15 |     {
+   16 |         break
+   17 |         continue
+      |         ~~~~~~~~
+   18 |     }
+   19 | }

vlib/v/checker/tests/check_wrong_usage_of_break_and_continue.vv

@@ -0,0 +1,19 @@
+struct Test {
+	a string
+	b string
+}
+
+fn main() {
+	$for field in Test.fields {
+		println(field)
+		continue
+	}
+	$for field in Test.fields {
+		println(field)
+		break
+	}
+	{
+		break
+		continue
+	}
+}

vlib/v/tests/comptime_for_break_test.v

@@ -0,0 +1,17 @@
+struct Test {
+	a string
+	b string
+}
+
+fn test_for() {
+	$for field in Test.fields {
+		for attr in field.attrs {
+			break
+		}
+	}
+	$for field in Test.fields {
+		for attr in field.attrs {
+			continue
+		}
+	}
+}