Only evaluate transparent inline unapply once

Fixes #19369

Author: nicolasstucki

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

@@ -172,10 +172,10 @@ object Inlines:
   /** Try to inline a pattern with an inline unapply method. Fail with error if the maximal
    *  inline depth is exceeded.
    *
-   *  @param unapp   The tree of the pattern to inline
+   *  @param fun The function of an Unapply node
    *  @return   An `Unapply` with a `fun` containing the inlined call to the unapply
    */
-  def inlinedUnapply(unapp: tpd.UnApply)(using Context): Tree =
+  def inlinedUnapplyFun(fun: tpd.Tree)(using Context): Tree =
     // We cannot inline the unapply directly, since the pattern matcher relies on unapply applications
     // as signposts what to do. On the other hand, we can do the inlining only in typer, not afterwards.
     // So the trick is to create a "wrapper" unapply in an anonymous class that has the inlined unapply
@@ -189,7 +189,6 @@ object Inlines:
     // transforms the patterns into terms, the `inlinePatterns` phase removes this anonymous class by β-reducing
     // the call to the `unapply`.
 
-    val fun = unapp.fun
     val sym = fun.symbol
 
     val newUnapply = AnonClass(ctx.owner, List(defn.ObjectType), sym.coord) { cls =>
@@ -199,7 +198,7 @@ object Inlines:
       val unapplyInfo = fun.tpe.widen
       val unapplySym = newSymbol(cls, sym.name.toTermName, Synthetic | Method, unapplyInfo, coord = sym.coord).entered
 
-      val unapply = DefDef(unapplySym.asTerm, argss => fun.appliedToArgss(argss).withSpan(unapp.span))
+      val unapply = DefDef(unapplySym.asTerm, argss => fun.appliedToArgss(argss).withSpan(fun.span))
 
       if sym.is(Transparent) then
         // Inline the body and refine the type of the unapply method
@@ -214,9 +213,8 @@ object Inlines:
         List(unapply)
     }
 
-    val newFun = newUnapply.select(sym.name).withSpan(unapp.span)
-    cpy.UnApply(unapp)(fun = newFun)
-  end inlinedUnapply
+    newUnapply.select(sym.name).withSpan(fun.span)
+  end inlinedUnapplyFun
 
   /** For a retained inline method, another method that keeps track of
    *  the body that is kept at runtime. For instance, an inline method

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

@@ -34,6 +34,7 @@ import Denotations.SingleDenotation
 import annotation.threadUnsafe
 
 import scala.util.control.NonFatal
+import dotty.tools.dotc.inlines.Inlines
 
 object Applications {
   import tpd.*
@@ -1446,7 +1447,23 @@ trait Applications extends Compatibility {
             unapplyArgType
 
         val dummyArg = dummyTreeOfType(ownType)
-        val unapplyApp = typedExpr(untpd.TypedSplice(Apply(unapplyFn, dummyArg :: Nil)))
+        val (newUnapplyFn, unapplyApp) =
+          val unapplyAppCall = withMode(Mode.NoInline) {
+            typedExpr(untpd.TypedSplice(Apply(unapplyFn, dummyArg :: Nil)))
+          }
+          def inlinedUnapplyFnAndApp(unapp: Tree): (Tree, Tree) = unapp match {
+            case DynamicUnapply(_) =>
+              report.error(em"Structural unapply is not supported", unapplyFn.srcPos)
+              (unapplyFn, unapplyAppCall)
+            case Apply(fn, `dummyArg` :: Nil) =>
+              val inlinedUnapplyFn = Inlines.inlinedUnapplyFun(fn)
+              (inlinedUnapplyFn, inlinedUnapplyFn.appliedToArgs(`dummyArg` :: Nil))
+            case Apply(fn, args) =>
+              val (fn1, app) = inlinedUnapplyFnAndApp(fn)
+              (fn1, tpd.cpy.Apply(unapp)(app, args))
+          }
+          if unapplyAppCall.symbol.isAllOf(Transparent | Inline) then inlinedUnapplyFnAndApp(unapplyAppCall)
+          else (unapplyFn, unapplyAppCall)
         def unapplyImplicits(unapp: Tree): List[Tree] = {
           val res = List.newBuilder[Tree]
           def loop(unapp: Tree): Unit = unapp match {
@@ -1475,7 +1492,7 @@ trait Applications extends Compatibility {
             List.fill(argTypes.length - args.length)(WildcardType)
         }
         val unapplyPatterns = bunchedArgs.lazyZip(argTypes) map (typed(_, _))
-        val result = assignType(cpy.UnApply(tree)(unapplyFn, unapplyImplicits(unapplyApp), unapplyPatterns), ownType)
+        val result = assignType(cpy.UnApply(tree)(newUnapplyFn, unapplyImplicits(unapplyApp), unapplyPatterns), ownType)
         unapp.println(s"unapply patterns = $unapplyPatterns")
         if (ownType.stripped eq selType.stripped) || ownType.isError then result
         else tryWithTypeTest(Typed(result, TypeTree(ownType)), selType)

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

@@ -1938,7 +1938,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             if (bounds != null) sym.info = bounds
           }
           b
-        case t: UnApply if t.symbol.is(Inline) => Inlines.inlinedUnapply(t)
+        case t: UnApply if t.symbol.is(Inline) =>
+          assert(!t.symbol.is(Transparent))
+          cpy.UnApply(t)(fun = Inlines.inlinedUnapplyFun(t.fun)) // TODO inline these in the inlining phase (see #19382)
         case t => t
       }
   }

tests/pos-macros/i19369/Macro_1.scala

@@ -0,0 +1,11 @@
+import scala.quoted._
+
+object Unapplier:
+  transparent inline def unapply(arg: Any): Option[Int] = ${unapplyImpl('arg)}
+
+  def unapplyImpl(using Quotes)(argExpr: Expr[Any]): Expr[Option[Int]] =
+    executionCount += 1
+    assert(executionCount == 1, "macro should only expand once")
+    '{Some(1)}
+
+  private var executionCount = 0

tests/pos-macros/i19369/Test_2.scala

@@ -0,0 +1,2 @@
+@main def main() =
+  val Unapplier(result) = Some(5)

tests/run/i8530.check

@@ -3,3 +3,5 @@ MyBoooleanUnapply
 3
 (4,5)
 5
+6
+7

tests/run/i8530.scala

@@ -13,6 +13,12 @@ object MySeqUnapply:
 object MyWhiteboxUnapply:
   transparent inline def unapply(x: Int): Option[Any] = Some(x)
 
+object MyWhiteboxUnapply1:
+  transparent inline def unapply(using DummyImplicit)(x: Int)(using DummyImplicit): Option[Any] = Some(x)
+
+object MyWhiteboxUnapply2:
+  transparent inline def unapply(using DummyImplicit)(using DummyImplicit)(x: Int)(using DummyImplicit)(using DummyImplicit): Option[Any] = Some(x)
+
 
 @main def Test =
   1 match
@@ -30,4 +36,10 @@ object MyWhiteboxUnapply:
   5 match
     case MyWhiteboxUnapply(x) => println(x: Int)
 
+  6 match
+    case MyWhiteboxUnapply1(x) => println(x: Int)
+
+  7 match
+    case MyWhiteboxUnapply2(x) => println(x: Int)
+
 end Test