https://medium.com/@sebastien.boulet/84826f949ae8

RemoteCacheExtensionsPlugin.scala

import org.apache.ivy.core.cache.ArtifactOrigin
import sbt.*
import sbt.Keys.*
import sbt.Project.inConfig
import sbt.internal.RemoteCache.*
import sbt.internal.inc.{HashUtil, JarUtils}
import sbt.internal.librarymanagement.IvyActions.mapArtifacts
import sbt.nio.Keys.*
import sbt.plugins.SemanticdbPlugin

import java.io.File

object RemoteCacheExtensionsPlugin extends AutoPlugin {
  // The version of the remote cache used to force the cache invalidation.
  // https://www.random.org/integers/?num=1&min=20000&max=536870911&col=1&base=16&format=plain
  private val remoteCacheVersion = "0bad107b"

  override def trigger: PluginTrigger = allRequirements

  override def requires = SemanticdbPlugin // don't let it override our own semanticdbTargetRoot

  object autoImport {
    val maybePushRemoteCache = taskKey[Unit](
      "Push remote cache artifact(s) to the cache server " +
        "if the artifact is not already present in the local repository."
    )
    // See https://github.com/sbt/sbt/issues/6286
    val maybePullRemoteCache = taskKey[Option[String]](
      "Retrieve the remote cache artifact unless the previous run already " +
        "attempted to retrieve the same remote cache id."
    )
    val cacheGenerators = settingKey[Seq[Task[Seq[File]]]](
      "List of tasks that generate cache files to include in remote cache artifacts."
    )
  }

  private val basePlaceholderRegex = "\\$\\{BASE\\}".r
  private val cachesDirectory      = "__caches"

  import autoImport.*

  override def globalSettings: Seq[Def.Setting[_]] = Seq(
    pushRemoteCacheTo := Some(MavenCache("local-cache", file("/tmp/remote-cache"))),
    allowMachinePath := false // Ensure ThisBuild / rootPaths contains all necessary paths to abstract machine-specific path
  )

  override def projectSettings: Seq[Def.Setting[_]] =
    remoteCacheSettings ++ Seq(Compile, Test).flatMap(remoteCacheConfigSettings)

  private val remoteCacheSettings: Seq[Def.Setting[_]] = Seq(
    maybePushRemoteCache := maybePushRemoteCache.?.all(anyConfigsInThisProject).value
  )

  def remoteCacheConfigSettings(config: Configuration): Seq[Setting[_]] = {
    inConfig(config)(
      configCacheSettings(compileArtifact(config, cachedClassifier(config))) ++
        Seq(
          // All tasks consuming classDirectory (where the remote cache is extracted)
          // should block until the pulling is over
          compileIncremental := compileIncremental.dependsOn(maybePullRemoteCache).value,
          products           := products.dependsOn(maybePullRemoteCache).value,
          copyResources      := copyResources.dependsOn(maybePullRemoteCache).value,
          maybePullRemoteCache := Def
            .taskDyn[Option[String]] {
              import CacheImplicits.*
              val log                      = streams.value.log
              val projectRoot              = baseDirectory.value
              val maybePulledRemoteCacheId = maybePullRemoteCache.previous.flatten
              // if we have uncommited changes, there is very little chance that we get a hit so don't bother trying
              if (hasLocalChanges(projectRoot)) {
                log.debug(s"local changes detected in $projectRoot, not attempting to pull")
                Def.task(maybePulledRemoteCacheId)
              } else {
                Def.taskDyn[Option[String]] { // lookup remoteCacheId within a nested task to avoid doing it unnecessarily
                  val currentPullRemoteCacheId = remoteCacheId.value
                  // don't try to pull something we already pulled or tried to pull before
                  if (maybePulledRemoteCacheId.contains(currentPullRemoteCacheId)) {
                    log.debug(
                      s"attempt to pull $currentPullRemoteCacheId detected, not attempting again"
                    )
                    Def.task(maybePulledRemoteCacheId)
                  } else pullRemoteCache.map(_ => Some(currentPullRemoteCacheId))
                }
              }
            }.value,
          maybePushRemoteCache := Def.taskDyn {
            val log        = streams.value.log
            val is         = (pushRemoteCache / ivySbt).value
            val projectId  = remoteCacheProjectId.value
            val config     = pushRemoteCacheConfiguration.value
            val module     = new is.Module((pushRemoteCache / moduleSettings).value)
            val artifacts  = Map(config.artifacts: _*)
            val classifier = artifacts.keySet.map(_.classifier).mkString
            val isArtifactInLocalCache = module.withModule(log) { case (ivy, md, _) =>
              // copied from https://github.com/sbt/librarymanagement/blob/99641bb/ivy/src/main/scala/sbt/internal/librarymanagement/IvyActions.scala#L162-L166
              def crossVersionMap(moduleSettings: ModuleSettings): Option[String => String] =
                moduleSettings match {
                  case i: InlineConfiguration => CrossVersion(i.module, i.scalaModuleInfo)
                  case _                      => None
                }

              val resolver     = ivy.getSettings.getDefaultRepositoryCacheManager
              val cross        = crossVersionMap(module.moduleSettings)
              val ivyArtifacts = mapArtifacts(md, cross, artifacts).map(_._1)
              ivyArtifacts
                .map(resolver.getSavedArtifactOrigin).forall(
                  !ArtifactOrigin.isUnknown(_)
                )
            }
            if (isArtifactInLocalCache) {
              log.debug(s"remote cache artifact already published for $projectId $classifier")
              Def.task(())
            } else {
              pushRemoteCache.result.map {
                case Inc(inc: Incomplete) =>
                  log.error(
                    s"Something went wrong when publishing the remote cache artifact for $projectId $classifier"
                  )
                  inc.directCause.foreach(log.trace(_))
                case _ =>
              }
            }
          }.value,
          packageCache / artifact := Artifact(moduleName.value, cachedClassifier(config)),
          // compute the remote cache id from the hash of the local cache id and the hash of the remote cache id of all dependencies
          remoteCacheId := Def.taskDyn {
            Def.value {
              val data        = settingsData.value
              val log         = streams.value.log
              val thisProject = thisProjectRef.value

              def remoteCacheIdTask(project: ProjectRef, configuration: String): Task[String] =
                (project / ConfigKey(configuration) / remoteCacheId).get(data) match {
                  case Some(value) => value
                  case _           => constant("")
                }

              val tasks: Seq[Task[String]] = internalDependencyConfigurations.value.flatMap {
                case (project, configurations) =>
                  configurations
                    // avoid circular call to remoteCacheId
                    .filterNot(_ == config.name && project == thisProject)
                    .map(c => remoteCacheIdTask(project, c))
              }
              // the parent value takes into account:
              //  - the sources (.scala)
              //  - the dependencies including transitive (.jar)
              //  - the extra options for the incremental compiler
              val parentValue = remoteCacheId.value
              log.debug(s"1 ${name.value} ${config.name} $parentValue")
              tasks.join.map(combineHash).map(hash => combineHash(Seq(hash, parentValue)))
            }
          }.value,
          remoteCacheId := {
            val log = streams.value.log
            // Take resources into account when generating the remote cache key
            //  - proto files have an impact on the generated code
            //  - other resources can have an impact on the test execution
            // But gitignored files should not have an impact on the remote cache key
            val excluded  = Set("parameters.conf", "parameters.conf.sample", "local.sbt")
            val resources = (unmanagedResources / inputFileStamps).value
            // Put the build definition in the hash as it can change the build output
            val buildFiles = (Global / checkBuildSources / inputFileStamps).value
            val inputs = (resources ++ buildFiles)
              .filterNot { case (path, _) =>
                excluded.contains(path.getFileName.toString)
              }.map { case (_, stamp0) =>
                stamp0.toString
              }
            val previous = remoteCacheId.value
            log.debug(s"2 ${name.value} ${config.name} $previous")
            combineHash(inputs :+ previous)
          },
          // also put the repositories used for the build as they are part of the incremental compile inputs
          remoteCacheId := {
            val log               = streams.value.log
            val resolversToString = fullResolvers.value.map(_.toString).mkString
            val previous          = remoteCacheId.value
            log.debug(s"3 ${name.value} ${config.name} $previous")
            combineHash(resolversToString, previous)
          },
          // combine with a version to allow for breaking changes in the remote cache format
          remoteCacheId := {
            val log      = streams.value.log
            val previous = remoteCacheId.value
            log.debug(s"4 ${name.value} ${config.name} $previous")
            combineHash(remoteCacheVersion, previous)
          },
          cacheGenerators := Seq.empty,
          packageCache := {
            // Call the parent task to create the remote cache jar containing the compiled classes, the unmanaged
            // resources and also the zinc incremental cache
            val cacheJar   = packageCache.value
            val log        = streams.value.log
            val base       = (ThisProject / baseDirectory).value
            val cacheFiles = Defaults.generate(cacheGenerators).value
            val baseRegex  = base.toString.r
            JarUtils.includeInJar(
              cacheJar,
              cacheFiles
                .map { file =>
                  log.debug(s"Including $file into package cache")
                  // Make cache file content machine-independent by transforming absolute paths to path relative to the
                  // project base directory. (awaiting https://github.com/sbt/sbt/issues/6298)
                  val transformedFile =
                    IO.withTemporaryFile(file.getName, "transformed", keepFile = true) {
                      temporaryFile =>
                        val content = IO.read(file)
                        val transformedContent =
                          baseRegex.replaceAllIn(content, basePlaceholderRegex.toString())
                        IO.write(temporaryFile, transformedContent)
                        temporaryFile
                    }
                  val jarEntry = s"$cachesDirectory/${IO.relativize(base, file).get}"
                  transformedFile -> jarEntry
                }
            )
            cacheJar
          },
          pullRemoteCache := {
            pullRemoteCache.value
            val log = streams.value.log
            // Cache files have been extracted into __caches by the parent pullRemoteCache,  we now move them to their
            // expected destination and turn them machine-*dependent* again by transforming relative paths
            // to absolute ones
            val base              = (ThisProject / baseDirectory).value
            val extractedCacheDir = classDirectory.value / cachesDirectory
            val cacheFiles        = extractedCacheDir.allPaths.filter(_.isFile).get
            cacheFiles.foreach { cacheFile =>
              val target = base / IO.relativize(extractedCacheDir, cacheFile).get
              log.debug(s"Restoring $target from package cache")
              val content = basePlaceholderRegex.replaceAllIn(IO.read(cacheFile), base.toString)
              IO.write(target, content)
            }
            IO.delete(extractedCacheDir)
          }
        )
    ) ++ semanticdbConfigSettings(config) ++ zincCacheWorkaroundsSettings(config)
  }

  private def semanticdbConfigSettings(config: Configuration): Seq[Def.Setting[_]] =
    inConfig(config)(
      Seq(
        // include semanticdb in the remote cache artifact as soon as it is enabled
        semanticdbIncludeInJar := semanticdbEnabled.value
      ) ++ SemanticdbPlugin.configurationSettings
    )

  // The scalac options are stored in the zinc cache, thus these options must be machine-independent.
  // https://github.com/sbt/sbt/issues/6027
  private def zincCacheWorkaroundsSettings(config: Configuration): Seq[Def.Setting[_]] = Seq(
    // relativize semantic DB target root which is part of the scalac options
    // https://github.com/sbt/sbt/issues/6027#issuecomment-717064450
    config / semanticdbTargetRoot := {
      val old = (config / semanticdbTargetRoot).value.toPath
      (LocalRootProject / baseDirectory).value.toPath.relativize(old).toFile
    }
  )

  private def cachedClassifier(conf: Configuration) = s"cached-${conf.name}"

  private val anyConfigsInThisProject = ScopeFilter(
    configurations = inAnyConfiguration
  )

  def hasLocalChanges(tree: File): Boolean =
    scala.sys.process.Process(s"git status --porcelain -- ${tree.getAbsolutePath}").!!.trim.nonEmpty

  private def combineHash(vs: Seq[String]): String = {
    val hashValue = HashUtil.farmHash(vs.sorted.mkString("").getBytes("UTF-8"))
    java.lang.Long.toHexString(hashValue)
  }

  def combineHash(str: String, strings: String*): String =
    combineHash(str +: strings)

}

TestQuickExtensionsPlugin.scala

import RemoteCacheExtensionsPlugin.autoImport.*
import sbt.*
import sbt.Keys.*
import sbt.Project.inConfig
import sbt.librarymanagement.Configuration

object TestQuickExtensionsPlugin extends AutoPlugin {

  override def trigger: PluginTrigger = allRequirements

  override def requires: Plugins = RemoteCacheExtensionsPlugin

  object autoImport {
    val runTest =
      taskKey[Unit]("Execute tests")
  }

  import autoImport.*

  override def projectSettings: Seq[Def.Setting[_]] =
    testQuickCacheConfigSettings(Test)

  def testQuickCacheConfigSettings(config: Configuration): Seq[Def.Setting[_]] =
    inConfig(config)(
      Seq(
        runTest := (if (Constants.useTestQuick) testQuick.toTask("").value else test.value),
        cacheGenerators += Def
          .task {
            val s             = (test / streams).value
            val succeededFile = Defaults.succeededFile(s.cacheDirectory)
            val cacheFiles    = if (succeededFile.isFile) Seq(succeededFile) else Seq.empty
            cacheFiles
          }.dependsOn(runTest).taskValue
      )
    )
}

Very helpful @gontard! Trying to figure out why my project is recompiling despite a perfect cache hit is proving very frustrating.

compileIncremental task logs can help: /target/streams/compile/compileIncremental/_global/streams/out