add --triggered support to scalafixOnCompile

src/main/scala/scalafix/sbt/ScalafixPlugin.scala

@@ -50,6 +50,12 @@ object ScalafixPlugin extends AutoPlugin {
           "but can be disabled with `scalafixCaching := false`."
       )
 
+    val scalafixArgsOnCompile: TaskKey[Seq[String]] =
+      taskKey[Seq[String]](
+        "Pass extra arguments to scalafix only when scalafix is triggred by compilation.  " +
+          "Only --triggered argument is passed by default."
+      )
+
     val scalafixCaching: SettingKey[Boolean] =
       settingKey[Boolean](
         "Cache scalafix invocations (off by default, on if scalafixOnCompile := true)."
@@ -107,7 +113,7 @@ object ScalafixPlugin extends AutoPlugin {
               compile.value // evaluated first, before the potential scalafix evaluation
             if (scalafixOnCompile.value)
               scalafix
-                .toTask("")
+                .toTask(scalafixArgsOnCompile.value.mkString(" ", " ", ""))
                 .map(_ => oldCompile)
             else Def.task(oldCompile)
           }.value,
@@ -184,6 +190,7 @@ object ScalafixPlugin extends AutoPlugin {
   override lazy val globalSettings: Seq[Def.Setting[_]] = Seq(
     scalafixConfig := None, // let scalafix-cli try to infer $CWD/.scalafix.conf
     scalafixOnCompile := false,
+    scalafixArgsOnCompile := Seq("--triggered"),
     scalafixResolvers :=
       // Repository.defaults() defaults to Repository.ivy2Local() and Repository.central(). These can be overridden per
       // env variable, e.g., export COURSIER_REPOSITORIES="ivy2Local|central|sonatype:releases|jitpack|https://corporate.com/repo".

src/sbt-test/sbt-scalafix/scalafixOnCompile/.scalafix.conf

@@ -1,2 +1,9 @@
 rules = [DisableSyntax, RemoveUnused]
-DisableSyntax.noNulls = true
+DisableSyntax.noNulls = true
+
+DisableSyntax.noVars = true
+RemoveUnused.privates = false
+triggered = {
+    DisableSyntax.noVars = false
+    RemoveUnused.privates = true
+}

src/sbt-test/sbt-scalafix/scalafixOnCompile/args/src/test/scala/Variables.scala

@@ -0,0 +1,4 @@
+object Variables {
+  private var a = 1
+  def b: Int = return 2
+}

src/sbt-test/sbt-scalafix/scalafixOnCompile/build.sbt

@@ -5,7 +5,7 @@ inThisBuild(
     scalaVersion := Versions.scala212,
     scalacOptions ++= List(
       "-Yrangepos",
-      "-Ywarn-unused-import"
+      "-Ywarn-unused"
     )
   )
 )
@@ -21,3 +21,8 @@ lazy val rewrite = project
     addCompilerPlugin(scalafixSemanticdb)
   )
   .settings(scalafixConfigSettings(IntegrationTest): _*)
+
+lazy val args = project
+  .settings(
+    addCompilerPlugin(scalafixSemanticdb)
+  )

src/sbt-test/sbt-scalafix/scalafixOnCompile/test

@@ -23,4 +23,15 @@
 
 # check configuration granularity for scalafixOnCompile
 > set scalafixOnCompile.in(lint, Test) := false
-> lint/test:compile
+> lint/test:compile
+
+# check that triggered rules are respected on compilation, and ignored on explicit invocation.
+#   `private var` is detected by `DisableSyntax.noVars = true` on explicit invocation,
+#   and removed by `triggered.RemoveUnused.privates = true` on compilation.
+-> args/test:scalafix
+> args/test:compile
+> args/test:scalafix
+
+# check extra scalafix arguments
+> set scalafixArgsOnCompile.in(args, Test) += "--settings DisableSyntax.noReturns = true"
+-> args/test:compile