Add -Wperformance lints for *Ref boxing and nonlocal return

[som-snytt]

Warn when local variables are captured by lambdas or by-name args and moved to the heap.

The warnings are behind a new -Wperformance flag, as the user did nothing wrong but might be surprised by a difference in performance.

Similarly, -Xlint:nonlocal-return is also enabled by the new flag, though it is not removed from -Xlint.

Revives #8691 by @hrhino.

[som-snytt]

It would be nice to report whether boxings are eliminated by optimizer after closure inlining.

[SethTisue]

@som-snytt I don't doubt this will be merged in the end, but would you mind filling in the PR description, to aid reviewers? (But also as end-user documentation, since this is release-notable.)

[SethTisue]

Let's wait to merge until @lrytz is available.

[som-snytt]

This is under-specified, but 2.13.8 says

scala> StringContext.glob(Nil,"")
java.lang.NegativeArraySizeException: -1
  at scala.StringContext$.glob(StringContext.scala:233)
  ... 32 elided

and 2.13.9 will say

scala> StringContext.glob(Nil,"")
java.util.NoSuchElementException: head of empty list
  at scala.collection.immutable.Nil$.head(List.scala:662)
  at scala.collection.immutable.Nil$.head(List.scala:661)
  at scala.StringContext$.glob(StringContext.scala:229)
  ... 34 elided

[som-snytt]

Those other commits must have been prompted by the warning. But IIRC they are a balance of style rather than benchmark-driven.

Somewhere I must have seen someone's suggestion for the hand-rolled function object.

[SethTisue]

I see that back in February 2020 my take on Harrison's PR was:

agree this more properly would be part of a separate linter or Scalafix ruleset

and, well, I still wonder about that a bit. but I don't recall my whole thought process, there.

the implementation doesn't seem like a likely maintenance burden. and perhaps my previous thinking was merely pedantic.

[xuwei-k]

use class and AbstractFunction ? 🤔

• avoid some LazyVals, prefer AbstractFunction #9216

[som-snytt]

Thanks, I will also add a lint for that.