Warn (or throw?) when a value class gets boxed

[scabug]

Proposal: Add a compiler flag that makes it easier to know when an AnyVal allocation occurs.

AnyVals can radically improve performance, but allocations can occur in surprising places (removing any performance gains). SIP-15 documents the conditions under which allocation occurs, but that can be hard to remember when browsing a complicated program. Compiler support for detecting allocations would help a lot.

A couple of possible flags are:

• Warning - Generate a compiler warning when a use-site will require an AnyVal to be allocated.
• Throw on Allocate - Add a flag that causes an exception to be thrown when an AnyVal is allocated. The resulting stack trace should allow the programmer to track down the allocation easily.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9504?orig=1
Reporter: Justin Bailey (m4dc4p)

[SethTisue]

@som-snytt did you do a thing like this under -Wperformance?

[som-snytt]

I see the linked (duplicating) ticket links to the -Wperformance PR which warns for Ref heapification which is not all boxing.

[som-snytt]

I'll let @SethTisue choose which ticket to close. It's nice to honor earlier reports, like being the first to name a species, but the other ticket has slightly more attention due its high-profile reporter.

[SethTisue]

consolidating at #12271