Merge pull request #1574 from jsuereth/merge-2.10.0

Merge 2.10.0-RC2 into 2.10.x
SI-6616 Check that unsafe operations are only called on the presentation compiler thread.

The method that checks the actual constraint is @elidable, expecting it to be used
for nightly builds but stripped-off in release builds. This way we don't lose any
performance, but 'fail-fast' in IDE nightlies.

This assumes that release builds will have at least `-Xelide-below ASSERTION`, but
this pull request does not do that.
SI-6539 Annotation for methods unfit for post-typer ASTs

Motivated by the `.value` method in the SBT task-syntax branch,
which should only be called within the context of the argument
to a setting initialization macro.

The facility is akin to a fatal deprecation.
Refine @compileTimeOnly

 - Don't default the message, and show it exclusively.
 - Fix cut-and-pasto in the @since tag
 - Be tolerant if the annotaion class is missing, as seems to
   have been the case compiling the continuations plugin.
 - s/\t/  / in the test file to show the errors are positioned
   correctly.
 - Use defensive getOrElse
show developer guidelines on opening pull request

Includes "Improvements and typo fixes to CONTRIBUTING.md" by @heathercmiller
as well as feedback from @retronym.
Update tools/epfl-publish

Currently, during each Scala nightly build, the epfl-publish script rsyncs the resulting dists/archives to the repository machine (chara).

Now, as a result of a change introduced a while back, dists/archives/ now also contains a symbolic link, created at the end of the build when the targets "pack-archives.latest.*" are invoked (in src/build/pack.xml). That was introduced in:

scala/scala@506bcfe71c93160ebfa0ca9b8b170b4b54e844e9
scala/scala@cb99853c8655686dae1288cbcd44a42cf1ea6609

This link, created in dists/archives/, is copied over as-is at the end of the rsync to chara by epfl-publish. On chara, however, the link points to an invalid target (the path is absolute).

Separately, the repository directory on chara is rsync'd every 30 minutes over to the machine that serves www.scala-lang.org, via cron. Rsynch finds that the link target does not exist, and generates an email with the crontab log, which is then duly sent to a system mailbox. Every 30 minutes. Since March. Needless to say, the mailbox is pretty large by now.

The fix is trivial, but this needs to be included in all the branches that are tested during the nightly. Since RC2 has just been cut, and the code frozen, I'm not sure how this change can be included without disruption; therefore, I'll just send a pull request on 2.10.0-wip, and let Josh/Paul take the appropriate steps.
Fix type of the custom `ClassTag` in `PatternMatching.scala`

In fb315772 custom ClassTags were introduced to optimize array
creation in hot places. The idea was that if `Array[T]` is allocated
through `ClassTag[T]` (which is essentially a factory for Array[T])
we would introduce a custom class implementing `ClassTag[T]` interface
and would have specialized array allocation instruction instead of
having reflective call as it's being done with default (compiler
synthesized) implementation of `ClassTag[T]`.

In case of `PatternMatching.scala` I made a mistake of providing
`ClassTag[Array[Clause]]` instead of `ClassTag[Clause]` so that value
would never be used and compiler still would resolve implicit through
default mechanism. This has been discovered thanks to @paulp's work
on unused value detection.

Review by @paulp.
SI-6646 Fix regression in for desugaring.

The early check in the parser of pattern irrefutability,
added in c82ecab, failed to consider InitCaps and
`backquoted` identifiers.
SI-6644 Account for varargs in extmethod forwarder

Which sounded difficult, so instead I offshored the work
to the friendly republic of TreeGen.
-Yshow-trees-compact respects other options

The aforementioned tree printer now respects other tree printing
options such as -uniqid and -Xprint-types.

This helps debugging reify, which uses `nodePrinters`, not `showRaw`
to print its trees, because back then `showRaw` didn't exist yet.
Merge pull request #1602 from gkossakowski/wrong-implicit-classTag

Fix type of the custom `ClassTag` in `PatternMatching.scala`
Merge pull request #1609 from scalamacros/topic/showraw-nodeprinters

-Yshow-trees-compact respects other options
better error when typetagging local classes

Inspired by https://issues.scala-lang.org/browse/SI-6649
Merge pull request #1610 from scalamacros/topic/typetags-local-classes

better error when typetagging local classes
SI-6646 `ident` or Ident is always new binding.

The previous commit regressed in these cases:

    // no withFilter
    for (X <- List("A single ident is always a pattern")) println(X)
    for (`x` <- List("A single ident is always a pattern")) println(`x`)

At the top level of the LHS of a <-, such identifiers represent
new bindings, not stable identifier patterns.
Update comment.
Merge pull request #1606 from retronym/ticket/6646

SI-6646 Fix regression in for desugaring.
Merge pull request #1583 from dragos/issue/fix-6616

SI-6616 Check that unsafe operations are only called on the presentation...
Fix for SI-6662, macro failing too early.

Don't give up before you try tupling. Who knows what
someone might be doing with a Unit.
Merge pull request #1601 from cunei/patch-1

Update tools/epfl-publish