Change global enabling scheme for cc

The aim is to have an efficient test whether a phase or denot transformer should
be run.

Author: odersky

compiler/src/dotty/tools/dotc/Run.scala

@@ -168,10 +168,10 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
    */
   var pureFunsImportEncountered = false
 
-  /** Will be set to true if any of the compiled compilation units contains
-   *  a captureChecking language import.
+  /** Will be set to true if experimental.captureChecking is enabled
+   *  or any of the compiled compilation units contains a captureChecking language import.
    */
-  var ccImportEncountered = false
+  var ccEnabledSomewhere = Feature.enabledBySetting(Feature.captureChecking)(using ictx)
 
   private var myEnrichedErrorMessage = false
 

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

@@ -29,7 +29,7 @@ object CheckCaptures:
 
   class Pre extends PreRecheck, SymTransformer:
 
-    override def isEnabled(using Context) = true
+    override def isRunnable(using Context) = super.isRunnable && Feature.ccEnabledSomewhere
 
   	/**  - Reset `private` flags of parameter accessors so that we can refine them
      *     in Setup if they have non-empty capture sets.
@@ -190,7 +190,8 @@ class CheckCaptures extends Recheck, SymTransformer:
   import CheckCaptures.*
 
   def phaseName: String = "cc"
-  override def isEnabled(using Context) = true
+
+  override def isRunnable(using Context) = super.isRunnable && Feature.ccEnabledSomewhere
 
   def newRechecker()(using Context) = CaptureChecker(ctx)
 

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

@@ -103,8 +103,8 @@ object Feature:
 
   /** Is captureChecking enabled for any of the currently compiled compilation units? */
   def ccEnabledSomewhere(using Context) =
-    enabledBySetting(captureChecking)
-    || ctx.run != null && ctx.run.nn.ccImportEncountered
+    if ctx.run != null then ctx.run.nn.ccEnabledSomewhere
+    else enabledBySetting(captureChecking)
 
   def sourceVersionSetting(using Context): SourceVersion =
     SourceVersion.valueOf(ctx.settings.source.value)
@@ -174,7 +174,7 @@ object Feature:
       true
     else if fullFeatureName == captureChecking then
       ctx.compilationUnit.needsCaptureChecking = true
-      if ctx.run != null then ctx.run.nn.ccImportEncountered = true
+      if ctx.run != null then ctx.run.nn.ccEnabledSomewhere = true
       true
     else
       false

compiler/src/dotty/tools/dotc/core/Phases.scala

@@ -299,13 +299,24 @@ object Phases {
      */
     def phaseName: String
 
+    /** This property is queried when phases are first assembled.
+     *  If it is false, the phase will be dropped from the set of phases to traverse.
+     */
+    def isEnabled(using Context): Boolean = true
+
+    /** This property is queried before a phase is run.
+     *  If it is false, the phase is skipped.
+     */
     def isRunnable(using Context): Boolean =
       !ctx.reporter.hasErrors
         // TODO: This might test an unintended condition.
         // To find out whether any errors have been reported during this
         // run one calls `errorsReported`, not `hasErrors`.
         // But maybe changing this would prevent useful phases from running?
 
+    /** True for all phases except NoPhase */
+    def exists: Boolean = true
+
     /** If set, allow missing or superfluous arguments in applications
      *  and type applications.
      */
@@ -360,10 +371,6 @@ object Phases {
     /** Can this transform change the base types of a type? */
     def changesBaseTypes: Boolean = changesParents
 
-    def isEnabled(using Context): Boolean = true
-
-    def exists: Boolean = true
-
     def initContext(ctx: FreshContext): Unit = ()
 
     /** A hook that allows to transform the usual context passed to the function