LazyList uses 2x memory for evaluated values compared to Stream

[lrytz]

In LazyList, an evaluated value is stored in the lazy val state: State[A], which contains an instance of final class Cons[A](val head: A, val tail: LazyList[A]) extends State[A].

So for every evaluated value there's an instance of LazyList plus an instance of Cons.

JOL output for a LazyList with 100 forced new AnyRefs:

scala.collection.immutable.LazyList@2d2e5f00d footprint:
     COUNT       AVG       SUM   DESCRIPTION
       100        16      1600   java.lang.Object
       101        24      2424   scala.collection.immutable.LazyList
       100        24      2400   scala.collection.immutable.LazyList$State$Cons
         1        16        16   scala.collection.immutable.LazyList$State$Empty$
       302                6440   (total)

Compare with Stream:

scala.collection.immutable.Stream$Cons@6ee12bacd footprint:
     COUNT       AVG       SUM   DESCRIPTION
       100        16      1600   java.lang.Object
       100        24      2400   scala.collection.immutable.Stream$Cons
         1        16        16   scala.collection.immutable.Stream$Empty$
       201                4016   (total)

24 bytes is the minimal size as we need two fields (head and tail).

Fortunately, LazyList internals are all private, so there's a chance to fix it. Serialization is already custom, so it's hopefully possible to remain unchanged.

[lrytz]

Draft PR for discussion: scala/scala#10937 @scala/collections