Null out local variables that are no longer in use

[retronym]

scala/bug#9137

https://groups.google.com/forum/#!topic/scala-internals/zf36JWUoSAg

Here's a reference to a memory leak caused by inlining a Stream method: scala/scala#4284 (comment)

[lrytz]

Aleksey Shipilёv explains in https://shipilev.net/jvm-anatomy-park/8-local-var-reachability/ that the JIT may collect objects early if it is only referenced by local variables that are no longer in use. But we cannot rely on that, as we'd need it to work from the beginning, before the code is JITed.

[lrytz]

Done in scala/scala#7133

[lrytz]

Actually, not done in general, only done for local variables that were inlined into a method.