Add environment variable to override is_dev()

Useful for unit testing software that intentionally has different
behavior in prod and dev

src/Writer.hack

@@ -206,8 +206,13 @@ function root(): string {
   return $root;
 }
 
+<<__Memoize>>
 function is_dev(): bool {
-  return $is_dev;
+  \$override = \getenv('HH_FORCE_IS_DEV');
+  if (\$override === false) {
+    return $is_dev;
+  }
+  return (bool) \$override;
 }
 
 function map(): \Facebook\AutoloadMap\AutoloadMap {

tests/IsDevTest.hack

@@ -0,0 +1,75 @@
+/*
+ *  Copyright (c) 2015-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+
+namespace Facebook\AutoloadMap;
+
+use function Facebook\FBExpect\expect;
+use namespace HH\Lib\{Str, Vec};
+
+final class IsDevTest extends BaseTest {
+  public function testIsDevInUnitTest(): void {
+    expect(Generated\is_dev())->toBeTrue();
+  }
+
+  public function testInStandaloneExecutable(): void {
+    $root = __DIR__.'/fixtures/hh-only';
+    $builder = new RootImporter($root, IncludedRoots::DEV_AND_PROD);
+    $tempfile = \tempnam(\sys_get_temp_dir(), 'hh_autoload.tmp.').'.hack';
+
+    $builder = (new Writer())
+      ->setBuilder($builder)
+      ->setRoot($root)
+      ->setIsDev(true)
+      ->writeToFile($tempfile);
+    $is_dev = self::exec(
+      \PHP_BINARY,
+      '-d',
+      'auto_prepend_file='.$tempfile,
+      $root.'/is_dev.hack',
+    );
+    expect($is_dev)->toBeSame('bool(true)');
+
+    $builder->setIsDev(false)->writeToFile($tempfile);
+    $is_dev = self::exec(
+      \PHP_BINARY,
+      '-d',
+      'auto_prepend_file='.$tempfile,
+      $root.'/is_dev.hack',
+    );
+    expect($is_dev)->toBeSame('bool(false)');
+
+    $is_dev = self::exec(
+      '/bin/sh',
+      '-c',
+      vec[
+        \PHP_BINARY,
+        '-d',
+        'auto_prepend_file='.$tempfile,
+        $root.'/is_dev.hack',
+      ]
+        |> Vec\map($$, $a ==> \escapeshellarg($a))
+        |> Str\join($$, ' ')
+        |> \escapeshellcmd($$)
+        |> 'HH_FORCE_IS_DEV=1 '.$$
+    );
+    expect($is_dev)->toBeSame('bool(true)');
+
+  }
+
+  private static function exec(string ...$args): string {
+    $output = vec[];
+    $exit_code = -1;
+    Vec\map($args, $arg ==> \escapeshellarg($arg))
+      |> Str\join($$, ' ')
+      |> \exec($$, inout $output, inout $exit_code);
+    $output = Str\join($output, "\n");
+    invariant($exit_code === 0, 'exec failed: %s', $output);
+    return $output;
+  }
+}

tests/fixtures/hh-only/is_dev.hack

@@ -0,0 +1,14 @@
+/*
+ *  Copyright (c) 2015-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+namespace Facebook\AutoloadMap\Tests\IsDevEntrypoint;
+
+<<__EntryPoint>>
+function main(): void {
+    \var_dump(\Facebook\AutoloadMap\Generated\is_dev());
+}