add a11y-label-has-associated-control check (sveltejs#5074)

src/compiler/compile/nodes/Element.ts

@@ -61,6 +61,17 @@ const a11y_no_onchange = new Set([
 	'option'
 ]);
 
+const a11y_labelable = new Set([
+	"button",
+	"input",
+	"keygen",
+	"meter",
+	"output",
+	"progress",
+	"select",
+	"textarea"
+]);
+
 const invisible_elements = new Set(['meta', 'html', 'script', 'style']);
 
 const valid_modifiers = new Set([
@@ -507,6 +518,16 @@ export default class Element extends Node {
 			}
 		}
 
+		if (this.name === 'label') {
+			const has_input_child = this.children.some(i => (i instanceof Element && a11y_labelable.has(i.name) ));
+			if (!attribute_map.has('for') && !has_input_child) {
+				component.warn(this, {
+					code: `a11y-label-has-associated-control`,
+					message: `A11y: A form label must be associated with a control.`
+				});
+			}
+		}
+
 		if (a11y_no_onchange.has(this.name)) {
 			if (handlers_map.has('change') && !handlers_map.has('blur')) {
 				component.warn(this, {

test/validator/samples/a11y-label-has-associated-control/input.svelte

@@ -0,0 +1,6 @@
+<label>A</label>
+<label for="id">B</label>
+
+<label>C <input type="text" /></label>
+<label>D <button>D</button></label>
+<label>E <span></span></label>

test/validator/samples/a11y-label-has-associated-control/warnings.json

@@ -0,0 +1,32 @@
+[
+  {
+    "code": "a11y-label-has-associated-control",
+    "end": {
+      "character": 16,
+      "column": 16,
+      "line": 1
+    },
+    "message": "A11y: A form label must be associated with a control.",
+    "pos": 0,
+    "start": {
+      "character": 0,
+      "column": 0,
+      "line": 1
+    }
+  },
+  {
+    "code": "a11y-label-has-associated-control",
+    "end": {
+      "character": 149,
+      "column": 30,
+      "line": 6
+    },
+    "message": "A11y: A form label must be associated with a control.",
+    "pos": 119,
+    "start": {
+      "character": 119,
+      "column": 0,
+      "line": 6
+    }
+  }
+]