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-6632 SI-6633 Fixes issues and data corruption in ListBuffer

 - Disallow negative positions for ListBuffer#insert/insertAll/update
 - Fix data corruption issue in ListBuffer#insert
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.
SI-6640 Better reporting of deficient classpaths.

In a55788e, StubSymbols were introduced to fail-slow when
the classpath was deficient. This allowed compilation to
succeed in cases when one didn't actually use the part of
class A which referred to some missing class B.

But a few problems were introduced.

Firstly, when the deferred error eventually happened, it was
signalled with abort(msg), rather than through a thrown
MissingRequirementError. The latter is desirable, as it doesn't
lead to printing a stack trace.

Second, the actual error message changed, and no longer
included the name of the class file that refers to the missing
class.

Finally, it seems that we can end up with a stub term symbol
in a situation where a class symbol is desired. An assertion
in the constructor of ThisType throws trips when calling .isClass,
before the useful error message from StubSymbol can be emitted.

This commit addresses these points, and rewords the error
a little to be more accessible. The last point is the most fragile
in this arrangement, there might be some whack-a-mole
required to find other places that also need this.
I don't see a clean solution for this, but am open to suggestions.

We should really build a facility in partest to delete
specified classfiles between groups in separate compilation
tests, in order to have tests for this. I'll work on that as a followup.

For now, here's the result of my manual testing:

[info] Set current project to default-821d14 (in build file:/Users/jason/code/scratch1/)
> compile

[info] Compiling 1 Scala source to /Users/jason/code/scratch1/target/scala-2.10/classes...
[error]
[error]      while compiling: /Users/jason/code/scratch1/test.scala
[error]         during phase: typer
[error]      library version: version 2.10.0-RC2
[error]     compiler version: version 2.10.0-RC2
...
[error]   last tree to typer: Ident(SwingWorker)
[error]               symbol: <none> (flags: )
[error]    symbol definition: <none>
[error]        symbol owners:
[error]       context owners: object Test -> package <empty>
 ...
[error] uncaught exception during compilation: java.lang.AssertionError
[trace] Stack trace suppressed: run last compile:compile for the full output.
[error] (compile:compile) java.lang.AssertionError: assertion failed: value actors
[error] Total time: 2 s, completed Nov 10, 2012 3:18:34 PM
>
> set scalaHome := Some(file("/Users/jason/code/scala/build/pack"))
[info] Defining *:scala-home
[info] The new value will be used by no settings or tasks.
[info] Reapplying settings...
[info] Set current project to default-821d14 (in build file:/Users/jason/code/scratch1/)
^[compile
[info] Compiling 1 Scala source to /Users/jason/code/scratch1/target/scala-2.10/classes...
[error] /Users/jason/code/scratch1/test.scala:4: A signature in SwingWorker.class refers to term actors in package scala which is missing from the classpath.
[error] object Test extends SwingWorker
[error]                     ^
[error] one error found
[error] (compile:compile) Compilation failed
[error] Total time: 2 s, completed Nov 10, 2012 3:18:45 PM
Refine the message and triggering of MissingRequirementError.

 - To force a failure of the stub, call a new method `failIfStub`
   rather than `info`.
 - Offer a broader range of potential root causes in the
   error message.
-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-6440 Address regressions around MissingRequirementError

 Go back to using globalError to report when a stub's info is referenced,
 and only throw the MissingRequirementError when compilation really
 must abort due to having a StubTermSymbol in a place where a
 StubClassSymbol would have been a better choice.

 This situation arises when an entire package is missing from the
 classpath, as was the case in the reported bug.

 Adds `StoreReporterDirectTest`, which buffers messages issued
 during compilation for more structured interrogation. Use this
 in two test for manifests -- these tests were using a crude means
 of grepping compiler console output to focus on the relevant output,
 but this approach was insufficient with the new multi-line error
 message emitted as part of this change.

 Also used that base test class to add two new tests: one for
 the reported error (package missing), and another for a simpler
 error (class missing). The latter test shows how stub symbols
 allow code to compile if it doesn't the subset of signatures
 in some type that refer to a missing class.

 Gave the INFO/WARNING/ERROR members of Reporter sensible
 toString implementations; they inherit from Enumeration#Value
 in an unusual manner (why?) that means the built in toString of
 Enumeration printed `Severity@0`.
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.
sane printing of renamed imports

Having a select named "foo" with an underlying symbol named "bar"
and trying to make sense of all that by prettyprinting is very confusing