Support generating .hack files

fixes #111
refs #112

src/CodegenFile.hack

@@ -24,6 +24,7 @@ enum CodegenFileType: int {
   HACK_DECL = 1;
   HACK_PARTIAL = 2;
   HACK_STRICT = 3;
+  DOT_HACK = 4;
 }
 
 /**
@@ -223,6 +224,8 @@ final class CodegenFile {
         return '<?hh // partial';
       case CodegenFileType::HACK_STRICT:
         return '<?hh // strict';
+      case CodegenFileType::DOT_HACK:
+      return '';
     }
   }
 
@@ -298,7 +301,10 @@ final class CodegenFile {
       $builder->addLine($shebang);
     }
 
-    $builder->addLine($this->getFileTypeDeclaration());
+    $decl = $this->getFileTypeDeclaration();
+    if ($decl !== '') {
+      $builder->addLine($decl);
+    }
     $header = $this->config->getFileHeader();
     if ($header) {
       foreach ($header as $line) {

tests/CodegenFileTest.codegen

@@ -35,6 +35,20 @@ const string FOO = 'bar';
  */
 const string HERP = 'derp';
 
+!@#$%codegentest:testDotHackExecutable
+#!/usr/bin/env hhvm
+// Codegen Tests
+/**
+ * This file is generated. Do not modify it manually!
+ *
+ * @-generated SignedSource<<2e680a68425d6302b789a6588a8218e4>>
+ */
+
+<<__EntryPoint>>
+function main(): noreturn {
+  exit(0);
+}
+
 !@#$%codegentest:testExecutable
 #!/usr/bin/env hhvm
 <?hh // partial

tests/CodegenFileTest.hack

@@ -316,6 +316,22 @@ final class CodegenFileTest extends CodegenBaseTest {
 );
   }
 
+  public function testDotHackExecutable(): void {
+    $cgf = $this->getCodegenFactory();
+    $code = $cgf
+      ->codegenFile('no_file')
+      ->setFileType(CodegenFileType::DOT_HACK)
+      ->setShebangLine('#!/usr/bin/env hhvm')
+      ->addFunction(
+        $cgf->codegenFunction('main')
+          ->setReturnType('noreturn')
+          ->addEmptyUserAttribute('__EntryPoint')
+          ->setBody('exit(0);')
+      )
+      ->render();
+    expect_with_context(static::class, $code)->toBeUnchanged();
+  }
+
   public function testNoPseudoMainHeaderInStrict(): void {
     expect(() ==> {
       $cgf = $this->getCodegenFactory();