improve --files CLI UX

To avoid getting false positive errors (missing SemanticDB), scalafix
executions with `--files` should not aggregate as they are by nature
specific to a given project / Configuration.

- prevent usage of scalafixAll --files
- validate --files values against unmanagedSourceDirectories
  - allow precise completions
  - avoid aggregated executions: since requested files are not valid
    outside the project on which the task input was crafted, sbt simply
    ignores the run

src/main/scala/scalafix/internal/sbt/ScalafixCompletions.scala

@@ -10,8 +10,11 @@ import sbt.complete.DefaultParsers._
 
 class ScalafixCompletions(
     workingDirectory: Path,
+    // Unfortunately, local rules will not show up as completions in the parser, as that parser can only
+    // depend on settings, while local rules classpath must be looked up via tasks
     loadedRules: () => Seq[ScalafixRule],
-    terminalWidth: Option[Int]
+    terminalWidth: Option[Int],
+    allowedTargetFilesPrefixes: Seq[Path] // Nil means all values are valid
 ) {
 
   private type P = Parser[String]
@@ -28,14 +31,18 @@ class ScalafixCompletions(
       valueParser: ArgP
   ): KVArgP = {
     val keyParser = Parser.oneOf((key +: keyAliases).map(literal)).examples(key)
-    val sepValueParser = (sep ~> valueParser).!!!("missing or invalid value")
+    val sepValueParser =
+      if (valueParser.toString.contains("!!!")) sep ~> valueParser
+      else (sep ~> valueParser).!!!("missing or invalid value")
     keyParser.^^^(sepValueParser)
   }
 
   private def uri(protocol: String): Parser[ShellArgs.Rule] =
     token(protocol + ":") ~> NotQuoted.map(x => ShellArgs.Rule(s"$protocol:$x"))
 
-  private val filepathParser: Parser[Path] = {
+  private def filepathParser(
+      filterPrefixes: Option[Seq[Path]] = None
+  ): Parser[Path] = {
     def toAbsolutePath(path: Path, cwd: Path): Path = {
       if (path.isAbsolute) path
       else cwd.resolve(path)
@@ -65,12 +72,25 @@ class ScalafixCompletions(
       }
     }
 
+    def isAllowedPrefix(path: Path): Boolean = filterPrefixes match {
+      case Some(prefixes) =>
+        val absolutePath = path.toAbsolutePath.toString
+        prefixes.map(_.toAbsolutePath.toString).exists { prefix =>
+          absolutePath.startsWith(prefix) || prefix.startsWith(absolutePath)
+        }
+      case None => true
+    }
+
     string
-      .examples(new AbsolutePathExamples(workingDirectory))
+      .examples(
+        exampleSource = new AbsolutePathExamples(workingDirectory),
+        maxNumberOfExamples = 25,
+        removeInvalidExamples = true
+      )
       .map { f =>
         toAbsolutePath(Paths.get(f), workingDirectory)
       }
-      .filter(f => Files.exists(f), x => x)
+      .filter(f => Files.exists(f) && isAllowedPrefix(f), x => x)
   }
 
   private val namedRule: P = {
@@ -130,7 +150,7 @@ class ScalafixCompletions(
   def hide(p: P): P = p.examples()
 
   def parser: Parser[ShellArgs] = {
-    val pathParser: Parser[Path] = token(filepathParser)
+    val pathParser: Parser[Path] = token(filepathParser())
     val fileRule: Parser[ShellArgs.Rule] =
       (token("file:") ~> pathParser)
         .map(path => ShellArgs.Rule(path.toUri.toString))
@@ -166,7 +186,15 @@ class ScalafixCompletions(
         gitDiffParser.map(v => ShellArgs.Extra("--diff-base", Some(v)))
       }
       val files: KVArgP = valueAfterKey("--files", "-f") {
-        pathParser.map(v => ShellArgs.File(v.toString))
+        val errorMessage =
+          if (allowedTargetFilesPrefixes.nonEmpty)
+            "--files value(s) must reference existing files or directories in unmanagedSourceDirectories; " +
+              "are you running scalafix on the right project / Configuration?"
+          else
+            "--files can only be used on project-level invocations (i.e. myproject / scalafix --files=f.scala)"
+        token(filepathParser(Some(allowedTargetFilesPrefixes)))
+          .map(v => ShellArgs.File(v.toString))
+          .!!!(errorMessage)
       }
       val rules: KVArgP = valueAfterKey("--rules", "-r")(rule)
 

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

@@ -127,13 +127,6 @@ object ScalafixPlugin extends AutoPlugin {
       Invisible
     )
 
-  private val scalafixCompletions: SettingKey[ScalafixCompletions] =
-    SettingKey(
-      "scalafixCompletions",
-      "Implementation detail - do not use",
-      Invisible
-    )
-
   // Memoize ScalafixInterface instances initialized with a custom tool classpath across projects & configurations
   // during task execution, to amortize the classloading cost when invoking scalafix concurrently on many targets
   private val scalafixInterfaceCache
@@ -223,13 +216,6 @@ object ScalafixPlugin extends AutoPlugin {
       scalafixDependencies = (ThisBuild / scalafixDependencies).value,
       scalafixCustomResolvers = (ThisBuild / scalafixResolvers).value
     ),
-    scalafixCompletions := new ScalafixCompletions(
-      workingDirectory = (ThisBuild / baseDirectory).value.toPath,
-      // Unfortunately, local rules will not show up as completions in the parser, as that parser can only
-      // depend on settings, while local rules classpath must be looked up via tasks
-      loadedRules = () => scalafixInterfaceProvider.value().availableRules(),
-      terminalWidth = Some(JLineAccess.terminalWidth)
-    ),
     scalafixInterfaceCache := new BlockingCache[
       ToolClasspath,
       ScalafixInterface
@@ -299,14 +285,22 @@ object ScalafixPlugin extends AutoPlugin {
 
   }
 
-  private def scalafixAllInputTask(): Def.Initialize[InputTask[Unit]] =
+  private def scalafixAllInputTask(): Def.Initialize[InputTask[Unit]] = {
+    val parser = Def.setting(
+      new ScalafixCompletions(
+        workingDirectory = (ThisBuild / baseDirectory).value.toPath,
+        loadedRules = () => scalafixInterfaceProvider.value().availableRules(),
+        terminalWidth = Some(JLineAccess.terminalWidth),
+        allowedTargetFilesPrefixes = Nil
+      )
+    )(_.parser)
     // workaround https://github.com/sbt/sbt/issues/3572 by invoking directly what Def.inputTaskDyn would via macro
-    InputTask
-      .createDyn(InputTask.initParserAsInput(scalafixCompletions(_.parser)))(
-        Def.task(shellArgs =>
-          scalafixAllTask(shellArgs, thisProject.value, resolvedScoped.value)
-        )
+    InputTask.createDyn(InputTask.initParserAsInput(parser))(
+      Def.task(shellArgs =>
+        scalafixAllTask(shellArgs, thisProject.value, resolvedScoped.value)
       )
+    )
+  }
 
   private def scalafixAllTask(
       shellArgs: ShellArgs,
@@ -343,12 +337,23 @@ object ScalafixPlugin extends AutoPlugin {
 
   private def scalafixInputTask(
       config: Configuration
-  ): Def.Initialize[InputTask[Unit]] =
+  ): Def.Initialize[InputTask[Unit]] = {
+    val parser = Def.setting(
+      new ScalafixCompletions(
+        workingDirectory = (ThisBuild / baseDirectory).value.toPath,
+        loadedRules = () => scalafixInterfaceProvider.value().availableRules(),
+        terminalWidth = Some(JLineAccess.terminalWidth),
+        allowedTargetFilesPrefixes =
+          (scalafix / unmanagedSourceDirectories).value.map(_.toPath)
+      )
+    )(_.parser)
+
     // workaround https://github.com/sbt/sbt/issues/3572 by invoking directly what Def.inputTaskDyn would via macro
     InputTask
-      .createDyn(InputTask.initParserAsInput(scalafixCompletions(_.parser)))(
+      .createDyn(InputTask.initParserAsInput(parser))(
         Def.task(shellArgs => scalafixTask(shellArgs, config))
       )
+  }
 
   private def scalafixTask(
       shellArgs: ShellArgs,

src/sbt-test/sbt-scalafix/explicit-files/.scalafix.conf

@@ -1,2 +1,2 @@
-rules = [DisableSyntax]
+rules = [DisableSyntax, RemoveUnused]
 DisableSyntax.keywords = [null]

src/sbt-test/sbt-scalafix/explicit-files/build.sbt

@@ -1,5 +1,13 @@
 import _root_.scalafix.sbt.{BuildInfo => Versions}
 
-val explicit = project.settings(
-  scalaVersion := Versions.scala212
+inThisBuild(
+  List(
+    scalaVersion := Versions.scala212,
+    scalacOptions += "-Ywarn-unused",
+    semanticdbEnabled := true,
+    semanticdbVersion := scalafixSemanticdb.revision
+  )
 )
+
+val ok_and_ko = project
+val ok_only = project

src/sbt-test/sbt-scalafix/explicit-files/ok_only/src/main/scala/OK.scala

@@ -0,0 +1 @@
+class OK {}

src/sbt-test/sbt-scalafix/explicit-files/src/main/scala/OK.scala

@@ -0,0 +1 @@
+class OK {}

src/sbt-test/sbt-scalafix/explicit-files/test

@@ -1,14 +1,38 @@
--> scalafix
+> ok_only / scalafix
+-> ok_and_ko / scalafix
 
--> scalafix -f=explicit/src/main/scala/KO.scala
--> scalafix --files=explicit/src/main/scala/KO.scala
 
--> scalafix -f=explicit/src/main/scala/OK.scala --files=explicit/src/main/scala/KO.scala
--> scalafix --files=explicit/src/main/scala/OK.scala -f=explicit/src/main/scala/KO.scala
--> scalafix -f=explicit/src/main/scala/KO.scala -f=explicit/src/main/scala/OK.scala
+# CORRECT USAGES
+# --------------
 
-> scalafix -f=explicit/src/main/scala/OK.scala
-> scalafix --files=explicit/src/main/scala/OK.scala
+# Targeting valid files within a project containing invalid files succeeds, with or without a key/value separator
+> ok_and_ko / scalafix -f=ok_and_ko/src/main/scala/OK.scala
+> ok_and_ko / scalafix --files=ok_and_ko/src/main/scala/OK.scala
+> ok_and_ko / scalafix -f ok_and_ko/src/main/scala/OK.scala
+> ok_and_ko / scalafix --files ok_and_ko/src/main/scala/OK.scala
 
-> scalafix -f explicit/src/main/scala/OK.scala
-> scalafix --files explicit/src/main/scala/OK.scala
+# Targeting invalid files fails
+-> ok_and_ko / scalafix -f=ok_and_ko/src/main/scala/KO.scala
+
+# Targeting valid and invalid files fails
+-> ok_and_ko / scalafix -f=ok_and_ko/src/main/scala/OK.scala -f=ok_and_ko/src/main/scala/KO.scala
+-> ok_and_ko / scalafix -f=ok_and_ko/src/main/scala/KO.scala -f=ok_and_ko/src/main/scala/OK.scala
+
+# Targeting valid files from the root project succeeds
+> scalafix -f=src/main/scala/OK.scala
+
+
+# INCORRECT USAGES
+# ----------------
+
+# Targeting non-existing files fails
+-> ok_only / scalafix -f=ok_only/src/main/scala/NotHere.scala
+
+# Targeting with scalafixAll valid files from the correct project fails
+-> ok_only / scalafixAll -f=ok_only/src/main/scala/OK.scala
+
+# Targeting valid files from a different project fails
+-> ok_only / scalafix -f=ok_and_ko/src/main/scala/OK.scala
+
+# Targeting valid files from a different configuration fails
+-> ok_only / Test / scalafix -f=ok_only/src/main/scala/OK.scala