Reuse beta-reduction logic from BetaReduce

Fixes a bug when beta-reducing inlined code. In some situations
the beta-reduction did not bind mutable variables.

compiler/src/dotty/tools/dotc/inlines/InlineReducer.scala

@@ -12,6 +12,8 @@ import NameKinds.{InlineAccessorName, InlineBinderName, InlineScrutineeName}
 import config.Printers.inlining
 import util.SimpleIdentityMap
 
+import dotty.tools.dotc.transform.BetaReduce
+
 import collection.mutable
 
 /** A utility class offering methods for rewriting inlined code */
@@ -163,26 +165,12 @@ class InlineReducer(inliner: Inliner)(using Context):
     */
   def betaReduce(tree: Tree)(using Context): Tree = tree match {
     case Apply(Select(cl, nme.apply), args) if defn.isFunctionType(cl.tpe) =>
-      val bindingsBuf = new DefBuffer
+      val bindingsBuf = new mutable.ListBuffer[ValDef]
       def recur(cl: Tree): Option[Tree] = cl match
         case Block((ddef : DefDef) :: Nil, closure: Closure) if ddef.symbol == closure.meth.symbol =>
           ddef.tpe.widen match
             case mt: MethodType if ddef.paramss.head.length == args.length =>
-              val argSyms = mt.paramNames.lazyZip(mt.paramInfos).lazyZip(args).map { (name, paramtp, arg) =>
-                arg.tpe.dealias match {
-                  case ref @ TermRef(NoPrefix, _) => ref.symbol
-                  case _ =>
-                    paramBindingDef(name, paramtp, arg, bindingsBuf)(
-                      using ctx.withSource(cl.source)
-                    ).symbol
-                }
-              }
-              val expander = new TreeTypeMap(
-                oldOwners = ddef.symbol :: Nil,
-                newOwners = ctx.owner :: Nil,
-                substFrom = ddef.paramss.head.map(_.symbol),
-                substTo = argSyms)
-              Some(expander.transform(ddef.rhs))
+              Some(BetaReduce.reduceApplication(ddef, args, bindingsBuf))
             case _ => None
         case Block(stats, expr) if stats.forall(isPureBinding) =>
           recur(expr).map(cpy.Block(cl)(stats, _))
@@ -193,7 +181,7 @@ class InlineReducer(inliner: Inliner)(using Context):
         case _ => None
       recur(cl) match
         case Some(reduced) =>
-          Block(bindingsBuf.toList, reduced).withSpan(tree.span)
+          seq(bindingsBuf.result(), reduced).withSpan(tree.span)
         case None =>
           tree
     case _ =>

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

@@ -9,6 +9,8 @@ import Symbols._, Contexts._, Types._, Decorators._
 import StdNames.nme
 import ast.TreeTypeMap
 
+import scala.collection.mutable.ListBuffer
+
 /** Rewrite an application
  *
  *    (((x1, ..., xn) => b): T)(y1, ..., yn)
@@ -70,9 +72,15 @@ object BetaReduce:
         original
   end apply
 
-  /** Beta-reduces a call to `ddef` with arguments `argSyms` */
+  /** Beta-reduces a call to `ddef` with arguments `args` */
   def apply(ddef: DefDef, args: List[Tree])(using Context) =
-    val bindings = List.newBuilder[ValDef]
+    val bindings = new ListBuffer[ValDef]()
+    val expansion1 = reduceApplication(ddef, args, bindings)
+    val bindings1 = bindings.result()
+    seq(bindings1, expansion1)
+
+  /** Beta-reduces a call to `ddef` with arguments `args` and registers new bindings */
+  def reduceApplication(ddef: DefDef, args: List[Tree], bindings: ListBuffer[ValDef])(using Context): Tree =
     val vparams = ddef.termParamss.iterator.flatten.toList
     assert(args.hasSameLengthAs(vparams))
     val argSyms =
@@ -84,7 +92,8 @@ object BetaReduce:
             val flags = Synthetic | (param.symbol.flags & Erased)
             val tpe = if arg.tpe.dealias.isInstanceOf[ConstantType] then arg.tpe.dealias else arg.tpe.widen
             val binding = ValDef(newSymbol(ctx.owner, param.name, flags, tpe, coord = arg.span), arg).withSpan(arg.span)
-            bindings += binding
+            if !(tpe.isInstanceOf[ConstantType] && isPureExpr(arg)) then
+              bindings += binding
             binding.symbol
 
     val expansion = TreeTypeMap(
@@ -99,8 +108,5 @@ object BetaReduce:
         case ConstantType(const) if isPureExpr(tree) => cpy.Literal(tree)(const)
         case _ => super.transform(tree)
     }.transform(expansion)
-    val bindings1 =
-      bindings.result().filterNot(vdef => vdef.tpt.tpe.isInstanceOf[ConstantType] && isPureExpr(vdef.rhs))
 
-    seq(bindings1, expansion1)
-  end apply
+    expansion1

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

@@ -600,15 +600,7 @@ class InlineBytecodeTests extends DottyBytecodeTest {
       val instructions = instructionsFromMethod(fun)
       val expected = // TODO room for constant folding
         List(
-          Op(ICONST_2),
-          VarOp(ISTORE, 1),
-          Op(ICONST_1),
-          VarOp(ISTORE, 2),
-          Op(ICONST_2),
-          VarOp(ILOAD, 2),
-          Op(IADD),
-          Op(ICONST_3),
-          Op(IADD),
+          IntOp(BIPUSH, 6),
           Op(IRETURN),
         )
       assert(instructions == expected,

tests/run/i16390.scala

@@ -0,0 +1,23 @@
+inline def cfor(inline body: Int => Unit): Unit =
+  var index = 0
+  while index < 3 do
+    body(index)
+    index = index + 1
+
+@main def Test =
+  assert(test1() == test2(), (test1(), test2()))
+
+def test1() =
+  val b = collection.mutable.ArrayBuffer.empty[() => Int]
+  cfor { x =>
+    b += (() => x)
+  }
+  b.map(_.apply()).toList
+
+def test2() =
+  val b = collection.mutable.ArrayBuffer.empty[() => Int]
+  var index = 0
+  while index < 3 do
+    ((x: Int) => b += (() => x)).apply(index)
+    index = index + 1
+  b.map(_.apply()).toList