Support shape subtyping

fixes #98

Author: fredemmott

src/CodegenShape.php

@@ -39,6 +39,16 @@ public function setManualAttrsID(?string $id = null): this {
     return $this;
   }
 
+  private bool $allowSubtyping = false;
+  public function allowsSubtyping(): bool {
+    return $this->allowSubtyping;
+  }
+
+  public function setAllowsSubtyping(bool $value): this {
+    $this->allowSubtyping = $value;
+    return $this;
+  }
+
   public function appendToBuilder(HackBuilder $builder): HackBuilder {
     $builder->addLine('shape(')->indent();
 
@@ -61,6 +71,10 @@ public function appendToBuilder(HackBuilder $builder): HackBuilder {
         ->endManualSection();
     }
 
+    if ($this->allowsSubtyping()) {
+      $builder->ensureNewLine()->addLine('...');
+    }
+
     return $builder->unindent()->add(')');
   }
 }

tests/CodegenShapeTest.codegen

@@ -1,4 +1,11 @@
 @generated
+!@#$%codegentest:testImplicitSubtyping
+shape(
+  ?'x' => int,
+  ?'y' => int,
+  'url' => string,
+  ...
+)
 !@#$%codegentest:testMultipleNestedShapes
 shape(
   'test' => shape(

tests/CodegenShapeTest.php

@@ -36,6 +36,20 @@ public function testShapeOptionalFields(): void {
     expect_with_context(static::class, $shape->render())->toBeUnchanged();
   }
 
+  public function testImplicitSubtyping(): void {
+    $shape = $this
+      ->getCodegenFactory()
+      ->codegenShape(
+        (new CodegenShapeMember('x', 'int'))->setIsOptional(),
+        (new CodegenShapeMember('y', 'int'))->setIsOptional(),
+        new CodegenShapeMember('url', 'string'),
+      )
+      ->setAllowsSubtyping(true);
+
+		expect_with_context(static::class, $shape->render())->toBeUnchanged();
+
+  }
+
   public function testNestedShape(): void {
     $nested = $this
       ->getCodegenFactory()