Avoiding redundant tuple formations.

In a case like

     val (x, y) = if (...) (a, b) else (c, d)

we can pass the tuples directly. Previously, this would have been translated to

    val (x, y) = (if (...) (a, b) else (c, d): @unchecked) match {
      case (s, t) => (s, t)
    }

2 files changed, 17 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/ast/Desugar.scala b/src/dotty/tools/dotc/ast/Desugar.scala
index b74b455eb..54d668193 100644
--- a/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/src/dotty/tools/dotc/ast/Desugar.scala
@@ -355,9 +355,15 @@ object desugar {
     case _ =>
       val rhsUnchecked = makeAnnotated(defn.UncheckedAnnot, rhs)
       val vars = getVariables(pat)
+      val isMatchingTuple: Tree => Boolean = {
+        case Tuple(es) => es.length == vars.length
+        case _ => false
+      }
       val ids = for ((named, _) <- vars) yield Ident(named.name)
       val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids))
-      val matchExpr = Match(rhsUnchecked, caseDef :: Nil)
+      val matchExpr =
+        if (forallResults(rhs, isMatchingTuple)) rhs
+        else Match(rhsUnchecked, caseDef :: Nil)
       vars match {
         case Nil =>
           matchExpr
diff --git a/src/dotty/tools/dotc/ast/TreeInfo.scala b/src/dotty/tools/dotc/ast/TreeInfo.scala
index 29ff492d4..e1715ae26 100644
--- a/src/dotty/tools/dotc/ast/TreeInfo.scala
+++ b/src/dotty/tools/dotc/ast/TreeInfo.scala
@@ -260,6 +260,16 @@ trait TreeInfo[T >: Untyped] { self: Trees.Instance[T] =>
     case Bind(_, y) => unbind(y)
     case y          => y
   }
+
+  /** Checks whether predicate `p` is true for all result parts of this epression,
+   *  where we zoom into Ifs, Matches, and Blocks.
+   */
+  def forallResults(tree: Tree, p: Tree => Boolean): Boolean = tree match {
+    case If(_, thenp, elsep) => forallResults(thenp, p) && forallResults(elsep, p)
+    case Match(_, cases) => cases forall (c => forallResults(c.body, p))
+    case Block(_, expr) => forallResults(expr, p)
+    case _ => p(tree)
+  }
 }
 
 trait TypedTreeInfo extends TreeInfo[Type] {self: Trees.Instance[Type] =>