disallow once/passive in legacy mode, for now

src/compile/nodes/Element.ts

@@ -613,6 +613,15 @@ export default class Element extends Node {
 						});
 					}
 				}
+
+				if (component.options.legacy && (modifier === 'once' || modifier === 'passive')) {
+					// TODO this could be supported, but it would need a few changes to
+					// how event listeners work
+					component.error(handler, {
+						code: 'invalid-event-modifier',
+						message: `The '${modifier}' modifier cannot be used in legacy mode`
+					});
+				}
 			});
 		});
 	}

test/validator/index.js

@@ -32,6 +32,7 @@ describe("validate", () => {
 						warnings.push({ code, message, pos, start, end });
 					},
 					dev: config.dev,
+					legacy: config.legacy,
 					generate: false
 				});
 

test/validator/samples/event-modifiers-invalid/errors.json

@@ -1,15 +1,15 @@
 [{
-  "message": "Valid event modifiers are preventDefault, stopPropagation, capture, once or passive",
+	"message": "Valid event modifiers are preventDefault, stopPropagation, capture, once or passive",
-  "code": "invalid-event-modifier",
+	"code": "invalid-event-modifier",
-  "start": {
+	"start": {
-    "line": 1,
+		"line": 1,
-    "column": 8,
+		"column": 8,
-    "character": 8
+		"character": 8
-  },
+	},
-  "end": {
+	"end": {
-    "line": 1,
+		"line": 1,
-    "column": 36,
+		"column": 36,
-    "character": 36
+		"character": 36
-  },
+	},
-  "pos": 8
+	"pos": 8
 }]

test/validator/samples/event-modifiers-legacy/_config.js

@@ -0,0 +1,3 @@
+export default {
+	legacy: true
+};

test/validator/samples/event-modifiers-legacy/errors.json

@@ -0,0 +1,15 @@
+[{
+	"message": "The 'once' modifier cannot be used in legacy mode",
+	"code": "invalid-event-modifier",
+	"start": {
+		"line": 1,
+		"column": 8,
+		"character": 8
+	},
+	"end": {
+		"line": 1,
+		"column": 37,
+		"character": 37
+	},
+	"pos": 8
+}]

test/validator/samples/event-modifiers-legacy/input.html

@@ -0,0 +1 @@
+<button on:click|once="handleClick()"></button>