Scala 2.13 compiler flags

build.sbt

val scalacOptions ++= Seq(
  "-encoding",
  "utf-8",                         // Specify character encoding used by source files.
  "-deprecation",                  // Emit warning and location for usages of deprecated APIs.
  "-explaintypes",                 // Explain type errors in more detail.
  "-feature",                      // Emit warning and location for usages of features that should be imported explicitly.
  "-language:existentials",        // Existential types (besides wildcard types) can be written and inferred
  "-language:experimental.macros", // Allow macro definition (besides implementation and application)
  "-language:higherKinds",         // Allow higher-kinded types
  "-language:implicitConversions", // Allow definition of implicit functions called views
  "-language:postfixOps",          // Allow post fix ops
  "-unchecked",                    // Enable additional warnings where generated code depends on assumptions.
  "-Xcheckinit",                   // Wrap field accessors to throw an exception on uninitialized access.
  "-Xfatal-warnings",              // Fail the compilation if there are any warnings.
  "-Xlint:-serial",                // Enable all except for serial. see scalac -Xlint:help for more
  "-Ybackend-parallelism", java.lang.Runtime.getRuntime.availableProcessors().toString, // Scala 2.12.5 compiler flag to run some compilation tasks in parallel
  "-Ywarn-dead-code",              // Warn when dead code is identified.
  "-Ywarn-extra-implicit",         // Warn when more than one implicit parameter section is defined.
  "-Ywarn-numeric-widen",          // Warn when numerics are widened.
  "-Ywarn-value-discard"           // Warn when non-Unit expression results are unused.

  // Stats and scalac-profiling - see https://www.scala-lang.org/blog/2018/06/04/scalac-profiling.html
  // uncomment the bloop plugin in project/plugins.sbt
//  ,"-Ystatistics:typer", "-Ybackend-parallelism", "1" // stats
//  ,"-Ycache-plugin-class-loader:last-modified"
//  ,"-Ycache-plugin-class-loader:last-modified"
//  ,"-Xplugin:/Users/USER/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/ch/epfl/scala/scalac-profiling_2.12/1.0.0/scalac-profiling_2.12-1.0.0.jar"
//  ,"-P:scalac-profiling:no-profiledb"
//  ,"-P:scalac-profiling:show-profiles"
//  ,"-P:scalac-profiling:sourceroot:PATH_TO_PROJECT_ROOT
)

I had both -Xlint:unused and all the -Ywarn-unused but you don't need both. See https://stackoverflow.com/a/45677366/434405

java.lang.Runtime.getRuntime.availableProcessors().toString

Nice, comparing to many other online article that suggests using a hardcoded number like 4 or 8.