Undeprecate mimaFailOnNoPrevious to build-wide opt-out

Author: dwijnand

README.md

@@ -83,3 +83,42 @@ mimaBinaryIssueFilters ++= Seq(
   ProblemFilters.exclude[Problem]("com.example.mylibrary.internal.*"),
 )
 ```
+
+## Setting different mimaPreviousArtifacts
+
+From time to time you may need to set `mimaPreviousArtifacts` according to some conditions.  For
+instance, if you have already ported your project to Scala 2.13 and set it up for cross-building to Scala 2.13,
+but still haven't cut a release, you may want to define `mimaPreviousArtifacts` according to the Scala version,
+with something like:
+
+```scala
+mimaPreviousArtifacts := {
+  if (CrossVersion.partialVersion(scalaVersion.scala) == Some((2, 13))) Set.empty else {
+    Set("com.example" %% "my-library" % "1.2.3")
+  }
+}
+```
+
+or perhaps using some of sbt 1.2's new API:
+
+```scala
+import sbt.librarymanagement.{ SemanticSelector, VersionNumber }
+
+mimaPreviousArtifacts := {
+  if (VersionNumber(scalaVersion.value).matchesSemVer(SemanticSelector(">=2.13"))) Set.empty else {
+    Set("com.example" %% "my-library" % "1.2.3")
+  }
+}
+```
+
+## Make mimaReportBinaryIssues not fail
+
+The setting `mimaFailOnNoPrevious` (introduced in v0.4.0) defaults to `true` and will make
+`mimaReportBinaryIssues` fail if `mimaPreviousArtifacts` hasn't been set.
+
+To make `mimaReportBinaryIssues` not fail you may want to do one of the following:
+
+* set `mimaPreviousArtifacts` on all the projects that should be checking their binary compatibility
+* avoid calling `mimaPreviousArtifacts` when binary compatibility checking isn't needed
+* set `mimaFailOnNoPrevious := false` on specific projects that want to opt-out (alternatively `disablePlugins(MimaPlugin)`)
+* set `mimaFailOnNoPrevious in ThisBuild := false`, which disables it build-wide, effectively reverting back to the previous behaviour

sbtplugin/src/main/scala/com/typesafe/tools/mima/plugin/MimaKeys.scala

@@ -8,6 +8,7 @@ object MimaKeys extends MimaKeys
 class MimaKeys {
 
   final val mimaFailOnProblem      = settingKey[Boolean]("if true, fail the build on binary incompatibility detection.")
+  final val mimaFailOnNoPrevious   = settingKey[Boolean]("if true, fail the build if no previous artifacts are set.")
   final val mimaPreviousArtifacts  = settingKey[Set[ModuleID]]("Previous released artifacts used to test binary compatibility.")
   final val mimaPreviousClassfiles = taskKey[Map[ModuleID, File]]("Directories or jars containing the previous class files used to test compatibility with a given module.")
   final val mimaCurrentClassfiles  = taskKey[File]("Directory or jar containing the current class files used to test compatibility.")
@@ -22,7 +23,4 @@ class MimaKeys {
 
   final val mimaCheckDirection = settingKey[String]("Compatibility checking direction; default is \"backward\", but can also be \"forward\" or \"both\".")
 
-  @deprecated("No longer used", "0.5.0")
-  final val mimaFailOnNoPrevious = MimaPlugin.mimaFailOnNoPrevious
-
 }

sbtplugin/src/main/scala/com/typesafe/tools/mima/plugin/MimaPlugin.scala

@@ -79,12 +79,16 @@ object MimaPlugin extends AutoPlugin {
     val s = streams.value
     val log = new SbtLogger(s)
     val projectName = name.value
+    val failOnNoPrevious = mimaFailOnNoPrevious.value
     val currentClassfiles = mimaCurrentClassfiles.value
     val cp = (fullClasspath in mimaFindBinaryIssues).value
     val checkDirection = mimaCheckDirection.value
     mimaPreviousClassfiles.value match {
       case _: NoPreviousClassfiles.type =>
-        sys.error(s"$projectName: mimaPreviousArtifacts not set, not analyzing binary compatibility.")
+        val msg = s"$projectName: mimaPreviousArtifacts not set, not analyzing binary compatibility."
+        if (failOnNoPrevious) sys.error(msg)
+        else s.log.info(msg)
+        Iterator.empty
       case previousClassfiles if previousClassfiles.isEmpty =>
         s.log.info(s"$projectName: mimaPreviousArtifacts is empty, not analyzing binary compatibility.")
         Iterator.empty
@@ -126,8 +130,4 @@ object MimaPlugin extends AutoPlugin {
     override def apply(key: K) = throw new NoSuchElementException(s"key not found: $key")
   }
 
-  // internal un-deprecated version
-  private[mima] final val mimaFailOnNoPrevious =
-    settingKey[Boolean]("if true, fail the build if no previous artifacts are set.")
-
 }