Nathan Kleyn

Scala by day — Rust, Ruby and Haskell by night. I hail from London.

Hi! I currently work for Intent HQ as Head of Engineering, writing functional Scala by day and Rust by night. I write for SitePoint about Ruby, and I’m on Twitter and on GitHub. You can get in touch with me at mail/at/

Recommended Scalac Flags For 2.13

13 May 2019

Following in the footsteps of @tpolecat’s wonderful “Recommended Scalac Flags for 2.12”, and previous version for 2.11, I’d like to share with you an updated list for Scala 2.13.

Scala 2.13 has removed a significant number of old lints and compiler flags, so you will find the previous list no longer works as-is with Scala >= 2.13.0.

This list is current as of Lightbend Scala 2.13.0-RC1.

scalacOptions ++= Seq(
  "-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
  "-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:adapted-args", // Warn if an argument list is modified to match the receiver.
  "-Xlint:constant", // Evaluation of a constant arithmetic expression results in an error.
  "-Xlint:delayedinit-select", // Selecting member of DelayedInit.
  "-Xlint:doc-detached", // A Scaladoc comment appears to be detached from its element.
  "-Xlint:inaccessible", // Warn about inaccessible types in method signatures.
  "-Xlint:infer-any", // Warn when a type argument is inferred to be `Any`.
  "-Xlint:missing-interpolator", // A string literal appears to be missing an interpolator id.
  "-Xlint:nullary-override", // Warn when non-nullary `def f()' overrides nullary `def f'.
  "-Xlint:nullary-unit", // Warn when nullary methods return Unit.
  "-Xlint:option-implicit", // Option.apply used implicit view.
  "-Xlint:package-object-classes", // Class or object defined in package object.
  "-Xlint:poly-implicit-overload", // Parameterized overloaded implicit methods are not visible as view bounds.
  "-Xlint:private-shadow", // A private field (or class parameter) shadows a superclass field.
  "-Xlint:stars-align", // Pattern sequence wildcard must align with sequence component.
  "-Xlint:type-parameter-shadow", // A local type parameter shadows a type already in scope.
  "-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-unused:implicits", // Warn if an implicit parameter is unused.
  "-Ywarn-unused:imports", // Warn if an import selector is not referenced.
  "-Ywarn-unused:locals", // Warn if a local definition is unused.
  "-Ywarn-unused:params", // Warn if a value parameter is unused.
  "-Ywarn-unused:patvars", // Warn if a variable bound in a pattern is unused.
  "-Ywarn-unused:privates", // Warn if a private member is unused.
  "-Ywarn-value-discard", // Warn when non-Unit expression results are unused.
  "-Ybackend-parallelism", "8", // Enable paralellisation — change to desired number!
  "-Ycache-plugin-class-loader:last-modified", // Enables caching of classloaders for compiler plugins
  "-Ycache-macro-class-loader:last-modified", // and macro definitions. This can lead to performance improvements.