For the details about this PR, please see the blog post https://eed3si9n.com/sbt-remote-cache/.
* Add cache basics
* Refactor Attributed to use StringAttributeMap, which is Map[StringAttributeKey, String]
* Implement disk cache
* Rename Package to Pkg
* Virtualize packageBin
* Use HashedVirtualFileRef for packageBin
* Virtualize compile task
In https://github.com/sbt/sbt/pull/5981 I tried to work around the spruious post-macro "a pure expression does nothing" warning (https://github.com/scala/bug/issues/12112) by trying to remove some pure-looking expressions out of the tree.
This quickly backfired when it was reported that sbt 1.4.3 was not evaluating some code. This backs out the macro-level manipulation, and instead try to silence the warning at the reporter level. This feels safer, and it seems to work just as well.
Ref https://github.com/scala/bug/issues/12112
Current app transformation macro creates a tree that looks like:
```scala
FullInstance.app[[T0[x]](T0[Int], T0[Int]), Unit](scala.Tuple2(task2, task1), (($p$macro$3: (Int, Int)) => {
<synthetic> val $q$macro$2: Int = $p$macro$3._1;
<synthetic> val $q$macro$1: Int = $p$macro$3._2;
{
($q$macro$1: Int);
($q$macro$2: Int);
()
}
}))(AList.tuple2[Int, Int])
```
Starting Scala 2.12.12 the compiler's "pure expression does nothing" has become more enthusiastic/accurate in its reach, and it started warning about the naked reference to `$q$macro$1` that appears to do nothing, even though in reality it would trigger the tasks and do something in the context of sbt. It's just _that_ particular line ends up macroed away into a pure expression.
A somewhat bizarre workaround is to make a fake call to a method just to satisfy this warning. I've chosen `scala.Predef.identity` here so it can be composed together with the existing expression nesting when they exist.
Using the scala reflect library always introduces significant
classloading overhead. We can eliminate the classloading overhead by
generating StringTypeTags at compile time instead.
This sped up average project loading time by a few hundred milliseconds
on my computer. The ManagedLoggedReporter in zinc is still using the
type tag based apis but after the next sbt release, we can upgrade the
zinc apis. We also could consider breaking binary compatibility.
To demonstrate [-Yno-lub](http://eed3si9n.com/stricter-scala-with-ynolub), this shows the code changes that removes lubing (Not all subprojects are done).
After I made the changes, I switched the Scala back to normal 2.12.10.