Keep inlined final vals in generated code

Though final vals with constants are guarantieed to be inlined in Dotty,
we need to keep the definition in case these are used from Scala 2.
Scala 2 misses some inlining oportunities.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/Memoize.scala

@@ -114,7 +114,9 @@ class Memoize extends MiniPhase with IdentityDenotTransformer { thisPhase =>
         EmptyTree
       }
 
-    if (sym.is(Accessor, butNot = NoFieldNeeded)) {
+    val constantFinalVal = sym.isAllOf(Accessor | Final, butNot = Mutable) && tree.rhs.isInstanceOf[Literal]
+
+    if (sym.is(Accessor, butNot = NoFieldNeeded) && !constantFinalVal) {
       val field = sym.field.orElse(newField).asTerm
 
       def adaptToField(tree: Tree): Tree =

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -869,11 +869,9 @@ trait Checking {
   /** Check that `tree` can be right hand-side or argument to `inline` value or parameter. */
   def checkInlineConformant(tpt: Tree, tree: Tree, sym: Symbol)(using Context): Unit = {
     if sym.is(Inline, butNot = DeferredOrTermParamOrAccessor) && !ctx.erasedTypes && !Inliner.inInlineMethod then
-      // final vals can be marked inline even if they're not pure, see Typer#patchFinalVals
-      val purityLevel = if (sym.is(Final)) Idempotent else Pure
       tpt.tpe.widenTermRefExpr.dealias.normalized match
         case tp: ConstantType =>
-          if !(exprPurity(tree) >= purityLevel) then
+          if !(exprPurity(tree) >= Pure) then
             ctx.error(em"inline value must be pure", tree.sourcePos)
         case _ =>
           val pos = if tpt.span.isZeroExtent then tree.sourcePos else tpt.sourcePos

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1464,6 +1464,7 @@ class Namer { typer: Typer =>
 
       // println(s"final inherited for $sym: ${inherited.toString}") !!!
       // println(s"owner = ${sym.owner}, decls = ${sym.owner.info.decls.show}")
+      // TODO Scala 3.1: only check for inline vals (no final ones)
       def isInlineVal = sym.isOneOf(FinalOrInline, butNot = Method | Mutable)
 
       // Widen rhs type and eliminate `|' but keep ConstantTypes if

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1856,30 +1856,9 @@ class Typer extends Namer
     val vdef1 = assignType(cpy.ValDef(vdef)(name, tpt1, rhs1), sym)
     checkSignatureRepeatedParam(sym)
     checkInlineConformant(tpt1, rhs1, sym)
-    patchFinalVals(vdef1)
     vdef1.setDefTree
   }
 
-  /** Adds inline to final vals with idempotent rhs
-   *
-   *  duplicating scalac behavior: for final vals that have rhs as constant, we do not create a field
-   *  and instead return the value. This seemingly minor optimization has huge effect on initialization
-   *  order and the values that can be observed during superconstructor call
-   *
-   *  see remark about idempotency in TreeInfo#constToLiteral
-   */
-  private def patchFinalVals(vdef: ValDef)(using Context): Unit = {
-    def isFinalInlinableVal(sym: Symbol): Boolean =
-      sym.is(Final, butNot = Mutable) &&
-      isIdempotentExpr(vdef.rhs) /* &&
-      ctx.scala2Mode (stay compatible with Scala2 for now) */
-    val sym = vdef.symbol
-    sym.info match {
-      case info: ConstantType if isFinalInlinableVal(sym) && !ctx.settings.YnoInline.value => sym.setFlag(Inline)
-      case _ =>
-    }
-  }
-
   def typedDefDef(ddef: untpd.DefDef, sym: Symbol)(using Context): Tree = {
     if (!sym.info.exists) { // it's a discarded synthetic case class method, drop it
       assert(sym.is(Synthetic) && desugar.isRetractableCaseClassMethodName(sym.name))

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

@@ -345,7 +345,7 @@ class InlineBytecodeTests extends DottyBytecodeTest {
   }
 
     // Testing that a is not boxed
-    @Test def i4522 = {
+  @Test def i4522 = {
     val source = """class Foo {
                    |  def test: Int = {
                    |    var a = 10
@@ -418,4 +418,27 @@ class InlineBytecodeTests extends DottyBytecodeTest {
 
     }
   }
+
+  @Test def finalVals = {
+    val source = """class Test:
+                   |  final val a = 1 // should be inlined but not erased
+                   |  inline val b = 2 // should be inlined and erased
+                   |  def test: Int = a + b
+                 """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Test.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+
+      val fun = getMethod(clsNode, "test")
+      val instructions = instructionsFromMethod(fun)
+      val expected = List(Op(ICONST_3), Op(IRETURN))
+      assert(instructions == expected,
+        "`a and b were not properly inlined in `test`\n" + diffInstructions(instructions, expected))
+
+      val methods = clsNode.methods.asScala.toList.map(_.name)
+      assert(methods == List("<init>", "a", "test"), clsNode.methods.asScala.toList.map(_.name))
+    }
+  }
+
 }

tests/run/erased-inline-vals.scala

@@ -41,7 +41,7 @@ class D:
   assert(classOf[B].getDeclaredFields.isEmpty)
 
   assert(classOf[C].getDeclaredMethods.size == 2)
-  assert(classOf[C].getDeclaredFields.size == 1)
+  assert(classOf[C].getDeclaredFields.isEmpty)
 
   assert(classOf[D].getDeclaredMethods.isEmpty)
   assert(classOf[D].getFields.isEmpty)