Support setting and using namespaces

src/CodegenFile.php

@@ -39,6 +39,8 @@ final class CodegenFile {
   private ?Map<string, Vector<string>> $rekey = null;
   private bool $createOnly = false;
   private ?ICodegenFormatter $formatter;
+  private ?string $fileNamespace;
+  private Map<string, ?string> $useNamespaces = Map {};
 
   public function __construct(
     private IHackCodegenConfig $config,
@@ -270,6 +272,15 @@ public function setDoClobber(bool $do_force): this {
 
   private function getContent(): string {
     $builder = hack_builder();
+    $builder->addLineIf(
+      $this->fileNamespace !== null,
+      'namespace %s;',
+      $this->fileNamespace,
+    );
+    foreach ($this->useNamespaces as $ns => $as) {
+      $builder->addLine($as === null ? "use $ns;" : "use $ns as $as;");
+    }
+
     foreach ($this->beforeTypes as $type) {
       $builder->ensureNewLine()->newLine();
       $builder->add($type->render());
@@ -330,6 +341,33 @@ public function setGeneratedFrom(
     return $this;
   }
 
+  public function setNamespace(string $file_namespace): this {
+    invariant($this->fileNamespace === null, 'namespace has already been set');
+    $this->fileNamespace = $file_namespace;
+    return $this;
+  }
+
+  public function useNamespace(string $ns, ?string $as = null): this {
+    invariant(
+      !$this->useNamespaces->contains($ns),
+      $ns.' is already being used',
+    );
+    $this->useNamespaces[$ns] = $as;
+    return $this;
+  }
+
+  public function useClass(string $ns, ?string $as = null): this {
+    return $this->useNamespace($ns, $as);
+  }
+
+  public function useFunction(string $ns, ?string $as = null): this {
+    return $this->useNamespace('function '.$ns, $as);
+  }
+
+  public function useConst(string $ns, ?string $as = null): this {
+    return $this->useNamespace('const '.$ns, $as);
+  }
+
   /**
    * If called, save() will only write the file if it doesn't exist
    */

test/CodegenFileTestCase.codegen

@@ -30,6 +30,23 @@ function fun(): int {
   return 0;
 }
 
+!@#$%codegentest:testNamespace
+<?hh
+// Codegen Tests
+/**
+ * This file is generated. Do not modify it manually!
+ *
+ * @-generated SignedSource<<1b0ad98452e2fc568c0eda46ed43952e>>
+ */
+namespace MyNamespace;
+use Another\Space;
+use My\Space\Bar as bar;
+use function My\Space\my_function as f;
+use const My\Space\MAX_RETRIES;
+
+class Foo {
+}
+
 !@#$%codegentest:testNoSignature
 <?hh
 // Codegen Tests

test/CodegenFileTestCase.php

@@ -230,4 +230,19 @@ public function testNoSignature() {
     self::assertUnchanged($code);
   }
 
+  public function testNamespace() {
+    $code = test_codegen_file('no_file')
+      ->setNamespace('MyNamespace')
+      ->useNamespace('Another\Space')
+      ->useClass('My\Space\Bar', 'bar')
+      ->useFunction('My\Space\my_function', 'f')
+      ->useConst('My\Space\MAX_RETRIES')
+      ->addClass(
+        codegen_class('Foo')
+      )
+      ->render();
+
+    self::assertUnchanged($code);
+  }
+
 }