Make separation checking controlled by language import

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -259,7 +259,10 @@ class CheckCaptures extends Recheck, SymTransformer:
     /** The set of symbols that were rechecked via a completer */
     private val completed = new mutable.HashSet[Symbol]
 
-    var needAnotherRun = false
+    /** Set on recheckClassDef since there we see all language imports */
+    private var sepChecksEnabled = false
+
+    private var needAnotherRun = false
 
     def resetIteration()(using Context): Unit =
       needAnotherRun = false
@@ -511,7 +514,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             // The path-use.scala neg test contains an example.
             val underlying = CaptureSet.ofTypeDeeply(c1.widen)
             capt.println(i"Widen reach $c to $underlying in ${env.owner}")
-            if ccConfig.useSepChecks then
+            if sepChecksEnabled then
               recur(underlying.filter(!_.isTerminalCapability), env, null)
                 // we don't want to disallow underlying Fresh instances, since these are typically locally created
                 // fresh capabilities. We don't need to also follow the hidden set since separation
@@ -1144,6 +1147,7 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  is already done in the TypeApply.
      */
     override def recheckClassDef(tree: TypeDef, impl: Template, cls: ClassSymbol)(using Context): Type =
+      if Feature.enabled(Feature.separationChecking) then sepChecksEnabled = true
       val localSet = capturedVars(cls)
       for parent <- impl.parents do // (1)
         checkSubset(capturedVars(parent.tpe.classSymbol), localSet, parent.srcPos,
@@ -2014,7 +2018,7 @@ class CheckCaptures extends Recheck, SymTransformer:
       end checker
 
       checker.traverse(unit)(using ctx.withOwner(defn.RootClass))
-      if ccConfig.useSepChecks then SepCheck(this).traverse(unit)
+      if sepChecksEnabled then SepCheck(this).traverse(unit)
       if !ctx.reporter.errorsReported then
         // We dont report errors here if previous errors were reported, because other
         // errors often result in bad applied types, but flagging these bad types gives

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -32,6 +32,7 @@ object Feature:
   val saferExceptions = experimental("saferExceptions")
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
+  val separationChecking = experimental("separationChecking")
   val into = experimental("into")
   val modularity = experimental("modularity")
   val quotedPatternsWithPolymorphicFunctions = experimental("quotedPatternsWithPolymorphicFunctions")
@@ -40,7 +41,7 @@ object Feature:
   def experimentalAutoEnableFeatures(using Context): List[TermName] =
     defn.languageExperimentalFeatures
       .map(sym => experimental(sym.name))
-      .filterNot(_ == captureChecking) // TODO is this correct?
+      .filterNot(sym => sym == captureChecking || sym == separationChecking) // TODO is this correct?
 
   val values = List(
     (nme.help, "Display all available features"),
@@ -60,6 +61,7 @@ object Feature:
     (saferExceptions, "Enable safer exceptions"),
     (pureFunctions, "Enable pure functions for capture checking"),
     (captureChecking, "Enable experimental capture checking"),
+    (separationChecking, "Enable experimental separation checking (requires captureChecking)"),
     (into, "Allow into modifier on parameter types"),
     (modularity, "Enable experimental modularity features"),
     (packageObjectValues, "Enable experimental package objects as values"),

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -926,6 +926,7 @@ object Checking {
         val name = Feature.experimental(sel.name)
         name == Feature.scala2macros
         || name == Feature.captureChecking
+        || name == Feature.separationChecking
       trees.filter {
         case Import(qual, selectors) =>
           languageImport(qual) match

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -37,7 +37,7 @@ class CompilationTests {
       compileFilesInDir("tests/pos-special/sourcepath/outer", defaultOptions.and("-sourcepath", "tests/pos-special/sourcepath")),
       compileFile("tests/pos-special/sourcepath/outer/nested/Test4.scala", defaultOptions.and("-sourcepath", "tests/pos-special/sourcepath")),
       compileFilesInDir("tests/pos-scala2", defaultOptions.and("-source", "3.0-migration")),
-      compileFilesInDir("tests/pos-custom-args/captures", defaultOptions.and("-language:experimental.captureChecking", "-source", "3.8")),
+      compileFilesInDir("tests/pos-custom-args/captures", defaultOptions.and("-language:experimental.captureChecking", "-language:experimental.separationChecking")),
       compileFile("tests/pos-special/utf8encoded.scala", defaultOptions.and("-encoding", "UTF8")),
       compileFile("tests/pos-special/utf16encoded.scala", defaultOptions.and("-encoding", "UTF16")),
       compileDir("tests/pos-special/i18589", defaultOptions.and("-Wsafe-init").without("-Ycheck:all")),
@@ -149,7 +149,7 @@ class CompilationTests {
     aggregateTests(
       compileFilesInDir("tests/neg", defaultOptions, FileFilter.exclude(TestSources.negScala2LibraryTastyExcludelisted)),
       compileFilesInDir("tests/neg-deep-subtype", allowDeepSubtypes),
-      compileFilesInDir("tests/neg-custom-args/captures", defaultOptions.and("-language:experimental.captureChecking", "-source", "3.8")),
+      compileFilesInDir("tests/neg-custom-args/captures", defaultOptions.and("-language:experimental.captureChecking", "-language:experimental.separationChecking")),
       compileFile("tests/neg-custom-args/sourcepath/outer/nested/Test1.scala", defaultOptions.and("-sourcepath", "tests/neg-custom-args/sourcepath")),
       compileDir("tests/neg-custom-args/sourcepath2/hi", defaultOptions.and("-sourcepath", "tests/neg-custom-args/sourcepath2", "-Xfatal-warnings")),
       compileList("duplicate source", List(
@@ -172,7 +172,7 @@ class CompilationTests {
     aggregateTests(
       compileFilesInDir("tests/run", defaultOptions.and("-Wsafe-init")),
       compileFilesInDir("tests/run-deep-subtype", allowDeepSubtypes),
-      compileFilesInDir("tests/run-custom-args/captures", allowDeepSubtypes.and("-language:experimental.captureChecking", "-source", "3.8")),
+      compileFilesInDir("tests/run-custom-args/captures", allowDeepSubtypes.and("-language:experimental.captureChecking", "-language:experimental.separationChecking")),
       // Run tests for legacy lazy vals.
       compileFilesInDir("tests/run", defaultOptions.and("-Wsafe-init", "-Ylegacy-lazy-vals", "-Ycheck-constraint-deps"), FileFilter.include(TestSources.runLazyValsAllowlist)),
     ).checkRuns()

library/src/scala/runtime/stdLibPatches/language.scala

@@ -84,6 +84,13 @@ object language:
     @compileTimeOnly("`captureChecking` can only be used at compile time in import statements")
     object captureChecking
 
+    /** Experimental support for separation checking; requires captureChecking also to be enabled.
+     *
+     *  @see [[https://dotty.epfl.ch/docs/reference/experimental/cc]]
+     */
+    @compileTimeOnly("`separationChecking` can only be used at compile time in import statements")
+    object separationChecking
+
     /** Experimental support for automatic conversions of arguments, without requiring
      *  a language import `import scala.language.implicitConversions`.
      *

project/Build.scala

@@ -1512,7 +1512,7 @@ object Build {
     settings(scala2LibraryBootstrappedSettings).
     settings(
       moduleName := "scala2-library-cc",
-      scalacOptions ++= Seq("-source", "3.8"), // for separation checking
+      scalacOptions += "-language:experimental.separationChecking" // for separation checking
     )
 
   lazy val scala2LibraryBootstrappedSettings = Seq(

project/MiMaFilters.scala

@@ -16,6 +16,8 @@ object MiMaFilters {
 
         ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language.2.13"),
         ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$2$u002E13$"),
+        ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language#experimental.separationChecking"),
+        ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$experimental$separationChecking$"),
 
         ProblemFilters.exclude[DirectMissingMethodProblem]("scala.Conversion.underlying"),
         ProblemFilters.exclude[MissingClassProblem]("scala.Conversion$"),

tests/neg-custom-args/captures/capture-vars-subtyping.scala → tests/neg/capture-vars-subtyping.scala

@@ -1,5 +1,5 @@
 import language.experimental.captureChecking
-import language.`3.7` // no separation checking, TODO enable
+// no separation checking, TODO enable and move to neg-customargs
 import caps.*
 
 def test[C^] =

tests/neg-custom-args/captures/cc-fresh-levels.scala → tests/neg/cc-fresh-levels.scala

@@ -1,5 +1,5 @@
-//> using options -source 3.7
 import language.experimental.captureChecking
+// no separation checking
 import caps.*
 class IO
 class Ref[X](init: X):