Turn capture checking on when language import is present

Author: odersky

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

@@ -9,10 +9,12 @@ import util.{FreshNameCreator, SourceFile, NoSource}
 import util.Spans.Span
 import ast.{tpd, untpd}
 import tpd.{Tree, TreeTraverser}
+import ast.Trees.Import
 import typer.Nullables
 import transform.SymUtils._
 import core.Decorators._
 import config.SourceVersion
+import StdNames.nme
 import scala.annotation.internal.sharable
 
 class CompilationUnit protected (val source: SourceFile) {
@@ -51,6 +53,9 @@ class CompilationUnit protected (val source: SourceFile) {
    */
   var needsStaging: Boolean = false
 
+  /** Will be set to true if the unit contains a captureChecking language import */
+  var needsCaptureChecking: Boolean = false
+
   var suspended: Boolean = false
   var suspendedAtInliningPhase: Boolean = false
 
@@ -111,6 +116,7 @@ object CompilationUnit {
       force.traverse(unit1.tpdTree)
       unit1.needsStaging = force.containsQuote
       unit1.needsInlining = force.containsInline
+      unit1.needsCaptureChecking = force.containsCaptureChecking
     }
     unit1
   }
@@ -138,11 +144,17 @@ object CompilationUnit {
   private class Force extends TreeTraverser {
     var containsQuote = false
     var containsInline = false
+    var containsCaptureChecking = false
     def traverse(tree: Tree)(using Context): Unit = {
       if (tree.symbol.isQuote)
         containsQuote = true
       if tree.symbol.is(Flags.Inline) then
         containsInline = true
+      tree match
+        case Import(qual, selectors)
+        if tpd.languageImport(qual).isDefined && selectors.contains(nme.captureChecking) =>
+          containsCaptureChecking = true
+        case _ =>
       traverseChildren(tree)
     }
   }

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

@@ -26,7 +26,7 @@ object CheckCaptures:
 
   class Pre extends PreRecheck, SymTransformer:
 
-    override def isEnabled(using Context) = Feature.ccEnabled
+    override def isEnabled(using Context) = true
 
   	/** Reset `private` flags of parameter accessors so that we can refine them
      *  in Setup if they have non-empty capture sets. Special handling of some
@@ -133,13 +133,14 @@ class CheckCaptures extends Recheck, SymTransformer:
   import CheckCaptures.*
 
   def phaseName: String = "cc"
-  override def isEnabled(using Context) = Feature.ccEnabled
+  override def isEnabled(using Context) = true
 
   def newRechecker()(using Context) = CaptureChecker(ctx)
 
   override def run(using Context): Unit =
-    checkOverrides.traverse(ctx.compilationUnit.tpdTree)
-    super.run
+    if Feature.ccEnabled then
+      checkOverrides.traverse(ctx.compilationUnit.tpdTree)
+      super.run
 
   override def transformSym(sym: SymDenotation)(using Context): SymDenotation =
     if Synthetics.needsTransform(sym) then Synthetics.transformFromCC(sym)

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

@@ -82,7 +82,8 @@ object Feature:
   def pureFunsEnabled(using Context) =
     enabled(pureFunctions) || ccEnabled
 
-  def ccEnabled(using Context) = enabled(captureChecking)
+  def ccEnabled(using Context) =
+    enabledBySetting(captureChecking) || ctx.compilationUnit.needsCaptureChecking
 
   def sourceVersionSetting(using Context): SourceVersion =
     SourceVersion.valueOf(ctx.settings.source.value)

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

@@ -426,6 +426,7 @@ object StdNames {
     val canEqual_ : N           = "canEqual"
     val canEqualAny : N         = "canEqualAny"
     val caps: N                 = "caps"
+    val captureChecking: N      = "captureChecking"
     val checkInitialized: N     = "checkInitialized"
     val classOf: N              = "classOf"
     val classType: N            = "classType"

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3311,8 +3311,11 @@ object Parsers {
           for
             case ImportSelector(id @ Ident(imported), EmptyTree, _) <- selectors
           do
-            if globalOnlyImports.contains(QualifiedName(prefix, imported.asTermName)) && !outermost then
+            val fullFeatureName = QualifiedName(prefix, imported.asTermName)
+            if globalOnlyImports.contains(fullFeatureName) && !outermost then
               syntaxError(i"this language import is only allowed at the toplevel", id.span)
+            if fullFeatureName == Feature.captureChecking then
+              ctx.compilationUnit.needsCaptureChecking = true
             if allSourceVersionNames.contains(imported) && prefix.isEmpty then
               if !outermost then
                 syntaxError(i"source version import is only allowed at the toplevel", id.span)

tests/pos/byname-purefuns-adapt/A_1.scala

@@ -0,0 +1,3 @@
+object A:
+  def f(x: => Int) = ()
+

tests/pos/byname-purefuns-adapt/B_2.scala

@@ -0,0 +1,5 @@
+import language.experimental.captureChecking
+object B:
+  def test(x: => Int) = A.f(x)
+
+