Unnecessary full compilations are performed when synchronizing BSP projects

[atty303]

When the BSP's buildTargetScalacOptions is requested, the following code will be executed.

mill/bsp/src/mill/bsp/MillBuildServer.scala

Lines 448 to 450 in 5266b79

	val classpath =
	evaluateInformativeTask(evaluator, m.compileClasspath, Agg.empty[PathRef])
	.map(_.path.toNIO.toUri.toString)

The compileClasspath target depends on the localClasspath target.

mill/scalalib/src/JavaModule.scala

Lines 269 to 275 in 5266b79

	/**
	* The output classfiles/resources from this module, excluding upstream
	* modules and third-party dependencies
	*/
	def localClasspath: T[Seq[PathRef]] = T {
	resources() ++ Agg(compile().classes)
	}

Since the localClasspath target depends on the compile target, the compilation of all modules will be executed.
For BSP synchronization, I think it is enough to get the dest of the compile target, and there is no need to actually compile it.

I came up with the following potential fixes.

• Get only the dest of the compile target from the localClasspath target. (Seems impossible).
• Evaluate each element of the compileClasspath from the BSP separately, and do not evaluate the compile. However, this cannot handle the case where compileClasspath is customized.
• Skip compilation when evaluating targets from BSP (by communicate with ZincWorker)

Does anyone have any ideas?

[lefou]

I've come with a solution for the compilation problem in pull request: #1536