Optimize enclosingInlineds

inlineContext already normalized most of InlinedCalls, normalize all of it
instead. The new code isn't much more complex, the only tricky bit is assuming
we don't need to normalize the pre-normalized list.

In fact, this can be simplified further!

Author: Blaisorblade

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -1082,21 +1082,24 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   /** A key to be used in a context property that tracks enclosing inlined calls */
   private val InlinedCalls = new Property.Key[List[Tree]]
 
-  override def inlineContext(call: Tree)(implicit ctx: Context): Context =
-    ctx.fresh.setProperty(InlinedCalls, call :: enclosingInlineds)
-
-  /** All enclosing calls that are currently inlined, from innermost to outermost.
-   *  EmptyTree calls cancel the next-enclosing non-empty call in the list
-   */
-  def enclosingInlineds(implicit ctx: Context): List[Tree] = {
-    def normalize(ts: List[Tree]): List[Tree] = ts match {
-      case t :: (ts1 @ (t1 :: ts2)) if t.isEmpty => normalize(if (t1.isEmpty) ts1 else ts2)
-      case t :: ts1 => t :: normalize(ts1)
-      case Nil => Nil
+  /** Record an enclosing inlined call.
+    * EmptyTree calls (for parameters) cancel the next-enclosing non-empty call in the list
+    */
+  override def inlineContext(call: Tree)(implicit ctx: Context): Context = {
+    // We assume enclosingInlineds is already normalized, and only process the new call with the head.
+    val oldIC = enclosingInlineds
+    val newIC = (call, oldIC) match {
+      case (t, ts1 @ (t1 :: ts2)) if t.isEmpty => if (t1.isEmpty) ts1 else ts2
+      case _ => call :: oldIC
     }
-    normalize(ctx.property(InlinedCalls).getOrElse(Nil))
+    ctx.fresh.setProperty(InlinedCalls, newIC)
   }
 
+  /** All enclosing calls that are currently inlined, from innermost to outermost.
+    */
+  def enclosingInlineds(implicit ctx: Context): List[Tree] =
+    ctx.property(InlinedCalls).getOrElse(Nil)
+
   /** The source file where the symbol of the `inline` method referred to by `call`
    *  is defined
    */