Drop FreeSeqFactory from stdlib-cc (#19849)

FreeSeqFactory was a construction to demonstrate type safety for certain
iterableFactory.from calls where we rely in the fact that for all Seqs
iterableFactory has an eager implementation of from.

While that shows that we _can_ make it typesafe, it does not work at
runtime as a drop-in replacement for stdlib since of course stdlib does
not have a FreeSeqFactory.

This commit drops FreeSeqFactory and adds three unsafeAssumePure calls
instead, with explanations.

Fixes #19845

Author: Linyxus

Diff for: scala2-library-cc/src/scala/collection/Factory.scala

@@ -296,15 +296,10 @@ object IterableFactory {
   }
 }
 
-// !!! Needed to add this separate trait
-trait FreeSeqFactory[+CC[A]] extends IterableFactory[CC]:
-  def from[A](source: IterableOnce[A]^): CC[A]
-  override def apply[A](elems: A*): CC[A] = from(elems)
-
 /**
   * @tparam CC Collection type constructor (e.g. `List`)
   */
-trait SeqFactory[+CC[A] <: SeqOps[A, Seq, Seq[A]]] extends FreeSeqFactory[CC] {
+trait SeqFactory[+CC[A] <: SeqOps[A, Seq, Seq[A]]] extends IterableFactory[CC] {
   import SeqFactory.UnapplySeqWrapper
   final def unapplySeq[A](x: CC[A] @uncheckedVariance): UnapplySeqWrapper[A] = new UnapplySeqWrapper(x) // TODO is uncheckedVariance sound here?
 }

Diff for: scala2-library-cc/src/scala/collection/Seq.scala

@@ -81,8 +81,6 @@ trait SeqOps[+A, +CC[_], +C] extends Any with SeqViewOps[A, CC, C] { self =>
 
   override def view: SeqView[A] = new SeqView.Id[A](this)
 
-  def iterableFactory: FreeSeqFactory[CC]
-
   /** Get the element at the specified index. This operation is provided for convenience in `Seq`. It should
     * not be assumed to be efficient unless you have an `IndexedSeq`. */
   @throws[IndexOutOfBoundsException]
@@ -167,7 +165,7 @@ trait SeqOps[+A, +CC[_], +C] extends Any with SeqViewOps[A, CC, C] { self =>
   def prependedAll[B >: A](prefix: IterableOnce[B]^): CC[B] = iterableFactory.from(prefix match {
     case prefix: Iterable[B] => new View.Concat(prefix, this)
     case _ => prefix.iterator ++ iterator
-  })
+  }).unsafeAssumePure // assume pure OK since iterableFactory.from is eager for Seq
 
   /** Alias for `prependedAll` */
   @`inline` override final def ++: [B >: A](prefix: IterableOnce[B]^): CC[B] = prependedAll(prefix)
@@ -532,6 +530,7 @@ trait SeqOps[+A, +CC[_], +C] extends Any with SeqViewOps[A, CC, C] { self =>
 
   @deprecated("Use .reverseIterator.map(f).to(...) instead of .reverseMap(f)", "2.13.0")
   def reverseMap[B](f: A => B): CC[B] = iterableFactory.from(new View.Map(View.fromIteratorProvider(() => reverseIterator), f))
+    .unsafeAssumePure // assume pure OK since iterableFactory.from is eager for Seq
 
   /** Iterates over distinct permutations of elements.
    *
@@ -947,7 +946,8 @@ trait SeqOps[+A, +CC[_], +C] extends Any with SeqViewOps[A, CC, C] { self =>
     */
   def patch[B >: A](from: Int, other: IterableOnce[B]^, replaced: Int): CC[B] =
     iterableFactory.from(new View.Patched(this, from, other, replaced))
-
+      .unsafeAssumePure // assume pure OK since iterableFactory.from is eager for Seq
+      
   /** A copy of this $coll with one single replaced element.
     *  @param  index  the position of the replacement
     *  @param  elem   the replacing element

Diff for: scala2-library-cc/src/scala/collection/generic/IsSeq.scala

@@ -84,7 +84,7 @@ object IsSeq {
           def toIterable: Iterable[Char] = new immutable.WrappedString(s)
           protected[this] def coll: String = s
           protected[this] def fromSpecific(coll: IterableOnce[Char]^): String = coll.iterator.mkString
-          def iterableFactory: FreeSeqFactory[immutable.ArraySeq] = immutable.ArraySeq.untagged
+          def iterableFactory: IterableFactory[immutable.ArraySeq] = immutable.ArraySeq.untagged
           override def empty: String = ""
           protected[this] def newSpecificBuilder: mutable.Builder[Char, String] = new StringBuilder
           def iterator: Iterator[Char] = s.iterator
@@ -102,7 +102,7 @@ object IsSeq {
           def toIterable: Iterable[A] = mutable.ArraySeq.make[A](a)
           protected def coll: Array[A] = a
           protected def fromSpecific(coll: IterableOnce[A]^): Array[A] = Array.from(coll)
-          def iterableFactory: FreeSeqFactory[mutable.ArraySeq] = mutable.ArraySeq.untagged
+          def iterableFactory: IterableFactory[mutable.ArraySeq] = mutable.ArraySeq.untagged
           override def empty: Array[A] = Array.empty[A]
           protected def newSpecificBuilder: mutable.Builder[A, Array[A]] = Array.newBuilder
           def iterator: Iterator[A] = a.iterator