[nomerge] Use foldLeft instead of /:

Ref scala/bug#11411
Ref scala/scala#6646

This backports the /: deprecation to aid the transition from 2.12.x to 2.13.x.

Author: eed3si9n

library/src/scala/collection/GenTraversableOnce.scala

@@ -199,6 +199,7 @@ trait GenTraversableOnce[+A] extends Any {
    *           }}}
    *           where `x,,1,,, ..., x,,n,,` are the elements of this $coll.
    */
+  @deprecated("Use foldLeft instead of /:", "2.12.10")
   def /:[B](z: B)(op: (B, A) => B): B
 
   /** Applies a binary operator to all elements of this $coll and a start value,
@@ -234,6 +235,7 @@ trait GenTraversableOnce[+A] extends Any {
    *           }}}
    *           where `x,,1,,, ..., x,,n,,` are the elements of this $coll.
    */
+  @deprecated("Use foldRight instead of :\\", "2.12.10")
   def :\[B](z: B)(op: (A, B) => B): B
 
   /** Applies a binary operator to a start value and all elements of this $coll,

library/src/scala/collection/TraversableOnce.scala

@@ -151,8 +151,10 @@ trait TraversableOnce[+A] extends Any with GenTraversableOnce[A] {
     None
   }
 
+  @deprecated("Use foldLeft instead of /:", "2.12.10")
   def /:[B](z: B)(op: (B, A) => B): B = foldLeft(z)(op)
 
+  @deprecated("Use foldRight instead of :\\", "2.12.10")
   def :\[B](z: B)(op: (A, B) => B): B = foldRight(z)(op)
 
   def foldLeft[B](z: B)(op: (B, A) => B): B = {

library/src/scala/collection/parallel/ParIterableLike.scala

@@ -1472,8 +1472,10 @@ self: ParIterableLike[T, Repr, Sequential] =>
 
   /* alias methods */
 
+  @deprecated("Use foldLeft instead of /:", "2.12.10")
   def /:[S](z: S)(op: (S, T) => S): S = foldLeft(z)(op)
 
+  @deprecated("Use foldRight instead of :\\", "2.12.10")
   def :\[S](z: S)(op: (T, S) => S): S = foldRight(z)(op)
 
   /* debug information */