Merge pull request #8636 from dotty-staging/properly-optimize-locally

Properly inline DottyPredef.locally

Author: nicolasstucki

compiler/test/dotty/tools/backend/jvm/InlineBytecodeTests.scala

@@ -73,6 +73,37 @@ class InlineBytecodeTests extends DottyBytecodeTest {
       }
   }
 
+  @Test def inlineLocally = {
+    val source =
+         """
+         |class Foo {
+         |  def meth1: Unit = locally {
+         |    val a = 5
+         |    a
+         |  }
+         |
+         |  def meth2: Unit = {
+         |    val a = 5
+         |    a
+         |  }
+         |}
+         """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Foo.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+      val meth1      = getMethod(clsNode, "meth1")
+      val meth2      = getMethod(clsNode, "meth2")
+
+      val instructions1 = instructionsFromMethod(meth1)
+      val instructions2 = instructionsFromMethod(meth2)
+
+      assert(instructions1 == instructions2,
+        "`locally` was not properly inlined in `meth1`\n" +
+        diffInstructions(instructions1, instructions2))
+    }
+  }
+
   @Test def i4947 = {
     val source = """class Foo {
                    |  transparent inline def track[T](inline f: T): T = {

library/src/dotty/DottyPredef.scala

@@ -18,7 +18,36 @@ object DottyPredef {
 
   inline final def implicitly[T](implicit ev: T): T = ev
 
-  inline def locally[T](body: => T): T = body
+  /** Used to mark code blocks as being expressions, instead of being taken as part of anonymous classes and the like.
+   *  This is just a different name for [[identity]].
+   *
+   *  @example Separating code blocks from `new`:
+   *           {{{
+   *             val x = new AnyRef
+   *             {
+   *               val y = ...
+   *               println(y)
+   *             }
+   *             // the { ... } block is seen as the body of an anonymous class
+   *
+   *             val x = new AnyRef
+   *
+   *             {
+   *               val y = ...
+   *               println(y)
+   *             }
+   *             // an empty line is a brittle "fix"
+   *
+   *             val x = new AnyRef
+   *             locally {
+   *               val y = ...
+   *               println(y)
+   *             }
+   *             // locally guards the block and helps communicate intent
+   *           }}}
+   *  @group utilities
+   */
+  inline def locally[T](inline body: T): T = body
 
   /**
    * Retrieve the single value of a type with a unique inhabitant.