Add runtime errors when constructing any element with children or categories

- children declarations are meant to be (and appear to be) parse errors
  with the new syntax
- categories can be fully be replaced with interfaces; let's ban them
  here in expectation of a hack/parser ban in the future
- remove the consistency validation trait: it can't be used even before
  this diff, but doubly so now that it throws
- raising exceptions from the constructor gives clearer traces than from
  render

Author: fredemmott

src/ChildValidation/ConsistencyValidation.hack

@@ -23,15 +23,4 @@ trait Validation {
   ): mixed {
     return static::getChildrenDeclaration()->legacySerialize();
   }
-
-  final public function validateChildren(): void {
-    invariant(
-      $this->__xhpChildrenDeclaration() ===
-        x\element::__NO_LEGACY_CHILDREN_DECLARATION,
-      "The XHPChildValidation trait can not be used with a 'children' ".
-      "declaration; override 'getChildrenDeclaration()'' instead",
-    );
-
-    parent::validateChildren();
-  }
 }

src/ChildValidation/Validation.hack

@@ -52,6 +52,15 @@ abstract xhp class node implements \XHPChild {
     Traversable<\XHPChild> $children,
     dynamic ...$debug_info
   ) {
+    invariant(
+      $this->__xhpChildrenDeclaration() === self::__NO_LEGACY_CHILDREN_DECLARATION,
+      'The `children` keyword is no longer supported',
+    );
+    invariant(
+      $this->__xhpCategoryDeclaration() === self::__NO_LEGACY_CATEGORY_DECLARATION,
+      'The `category` keyword is no longer supported',
+    );
+
     foreach ($children as $child) {
       $this->appendChild($child);
     }
@@ -573,10 +582,11 @@ abstract xhp class node implements \XHPChild {
    * categories an element belongs to. Each category is a key with value 1.
    */
   protected function __xhpCategoryDeclaration(): darray<string, int> {
-    return darray[];
+    return self::__NO_LEGACY_CATEGORY_DECLARATION;
   }
 
   const int __NO_LEGACY_CHILDREN_DECLARATION = -31337;
+  const darray<string, int> __NO_LEGACY_CATEGORY_DECLARATION = darray["\0INVALID\0" => 0];
 
   /**
    * Defined in elements by the `children` keyword. This returns a pattern of

src/core/Node.hack

@@ -429,11 +429,15 @@ class ChildRuleTest extends Facebook\HackTest\HackTest {
   }
 
   public async function testCommaCategory(): Awaitable<void> {
-    $x =
-      <test:needs_comma_category>
-        <test:has_comma_category />
-      </test:needs_comma_category>;
-    expect(await $x->toStringAsync())->toEqual('<div></div>');
+    expect(
+      () ==>
+        <test:needs_comma_category>
+          <test:has_comma_category />
+        </test:needs_comma_category>,
+    )->toThrow(
+      InvariantException::class,
+      'no longer supported',
+    );
   }
 
   public async function testFrags(): Awaitable<void> {