Allow experimental language imports in experimental scopes

Also allow top-level experimental language imports if all top-level definitions are experimental

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

@@ -97,9 +97,9 @@ object Feature:
     else
       false
 
-  def checkExperimentalFeature(which: String, srcPos: SrcPos)(using Context) =
+  def checkExperimentalFeature(which: String, srcPos: SrcPos, note: => String = "")(using Context) =
     if !isExperimentalEnabled then
-      report.error(i"Experimental $which may only be used with a nightly or snapshot version of the compiler", srcPos)
+      report.error(i"Experimental $which may only be used with a nightly or snapshot version of the compiler$note", srcPos)
 
   def checkExperimentalDef(sym: Symbol, srcPos: SrcPos)(using Context) =
     if !isExperimentalEnabled then

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

@@ -3114,10 +3114,6 @@ object Parsers {
       languageImport(tree) match
         case Some(prefix) =>
           in.languageImportContext = in.languageImportContext.importContext(imp, NoSymbol)
-          if prefix == nme.experimental
-             && selectors.exists(sel => Feature.experimental(sel.name) != Feature.scala2macros && Feature.experimental(sel.name) != Feature.erasedDefinitions)
-          then
-            Feature.checkExperimentalFeature("features", imp.srcPos)
           for
             case ImportSelector(id @ Ident(imported), EmptyTree, _) <- selectors
             if allSourceVersionNames.contains(imported)

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -448,6 +448,10 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           throw ex
       }
 
+    override def transformStats(trees: List[Tree], exprOwner: Symbol)(using Context): List[Tree] =
+      try super.transformStats(trees, exprOwner)
+      finally Checking.checkExperimentalImports(trees)
+
     /** Transforms the rhs tree into a its default tree if it is in an `erased` val/def.
      *  Performed to shrink the tree that is known to be erased later.
      */

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

@@ -719,6 +719,47 @@ object Checking {
         checkValue(tree)
       case _ =>
     tree
+
+  /** Check that experimental language imports in `trees`
+   *  are done only in experimental scopes, or in a top-level
+   *  scope with only @experimental definitions.
+   */
+  def checkExperimentalImports(trees: List[Tree])(using Context): Unit =
+
+    def nonExperimentalStat(trees: List[Tree]): Tree = trees match
+      case (_: Import | EmptyTree) :: rest =>
+        nonExperimentalStat(rest)
+      case (tree @ TypeDef(_, impl: Template)) :: rest if tree.symbol.isPackageObject =>
+        nonExperimentalStat(impl.body).orElse(nonExperimentalStat(rest))
+      case (tree: PackageDef) :: rest =>
+        nonExperimentalStat(tree.stats).orElse(nonExperimentalStat(rest))
+      case (tree: MemberDef) :: rest =>
+        if tree.symbol.isExperimental || tree.symbol.is(Synthetic) then
+          nonExperimentalStat(rest)
+        else
+          tree
+      case tree :: rest =>
+        tree
+      case Nil =>
+        EmptyTree
+
+    for case imp @ Import(qual, selectors) <- trees do
+      languageImport(qual) match
+        case Some(nme.experimental)
+        if !ctx.owner.isInExperimentalScope
+            && selectors.exists(sel => experimental(sel.name) != scala2macros && experimental(sel.name) != erasedDefinitions) =>
+          def check(stable: => String) =
+            checkExperimentalFeature("features", imp.srcPos,
+              s"\n\nNote: the scope enclosing the import is not considered experimental because it contains the\nnon-experimental $stable")
+          if ctx.owner.is(Package) then
+            // allow top-level experimental imports if all definitions are @experimental
+            nonExperimentalStat(trees) match
+              case EmptyTree =>
+              case tree: MemberDef => check(i"${tree.symbol}")
+              case tree => check(i"expression ${tree}")
+          else checkExperimentalFeature("features", imp.srcPos)
+        case _ =>
+  end checkExperimentalImports
 }
 
 trait Checking {

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

@@ -42,6 +42,7 @@ class CompilationTests {
       compileFilesInDir("tests/pos-custom-args/erased", defaultOptions.and("-language:experimental.erasedDefinitions")),
       compileFilesInDir("tests/pos", defaultOptions.and("-Ysafe-init")),
       compileFilesInDir("tests/pos-deep-subtype", allowDeepSubtypes),
+      compileFilesInDir("tests/pos-custom-args/no-experimental", defaultOptions.and("-Yno-experimental")),
       compileDir("tests/pos-special/java-param-names", defaultOptions.withJavacOnlyOptions("-parameters")),
       compileFile(
         // succeeds despite -Xfatal-warnings because of -nowarn
@@ -178,7 +179,7 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/matchable.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/i7314.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/feature-shadowing.scala", defaultOptions.and("-Xfatal-warnings", "-feature")),
-      compileDir("tests/neg-custom-args/hidden-type-errors",  defaultOptions.and("-explain")),
+      compileDir("tests/neg-custom-args/hidden-type-errors", defaultOptions.and("-explain")),
     ).checkExpectedErrors()
   }
 

docs/docs/reference/other-new-features/experimental-defs.md

@@ -10,7 +10,11 @@ This annotation can be placed on term or type definitions.
 
 ### References to experimental definitions
 
-Experimental definitions can only be referenced in an experimental scope. Experimental scopes are defined as follows.
+- Experimental definitions can only be referenced in an experimental scope.
+- Experimental language features can be imported in an experimental scope.
+- Experimental language features can be imported at the top-level if all top-level definitions are @experimental.
+
+Experimental scopes are defined as follows.
 
 (1) The RHS of an experimental `def`, `val`, `var`, `given` or `type` is an experimental scope.
 

tests/neg-custom-args/no-experimental/experimental-2.scala

@@ -0,0 +1,5 @@
+class Test7 {
+  import scala.language.experimental
+  import experimental.genericNumberLiterals // error: no aliases can be used to refer to a language import
+  val x: BigInt = 13232202002020202020202   // error
+}

tests/neg-custom-args/no-experimental/experimental-imports.scala

@@ -0,0 +1,46 @@
+import annotation.experimental
+
+@experimental
+object Object1:
+  import language.experimental.fewerBraces
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  erased def f = 1
+
+object Object2:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions
+  erased def f = 1
+
+@experimental
+object Class1:
+  import language.experimental.fewerBraces
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  erased def f = 1
+
+object Class2:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions
+  erased def f = 1
+
+@experimental
+def fun1 =
+  import language.experimental.fewerBraces
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  erased def f = 1
+
+def fun2 =
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions
+  erased def f = 1

tests/neg-custom-args/no-experimental/experimental-nested-imports-2.scala

@@ -0,0 +1,33 @@
+import annotation.experimental
+
+class Class1:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1
+  def g = 1
+
+object Object1:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1
+  def g = 1
+
+def fun1 =
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1
+  def g = 1
+
+val value1 =
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1
+  def g = 1

tests/neg-custom-args/no-experimental/experimental-nested-imports-3.scala

@@ -0,0 +1,25 @@
+import annotation.experimental
+
+class Class1:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+
+object Object1:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+
+def fun1 =
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+
+val value1 =
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition

tests/neg-custom-args/no-experimental/experimental-nested-imports.scala

@@ -0,0 +1,29 @@
+import annotation.experimental
+
+class Class1:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1
+
+object Object1:
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1
+
+def fun1 =
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1
+
+val value1 =
+  import language.experimental.fewerBraces // error
+  import language.experimental.namedTypeArguments // error
+  import language.experimental.genericNumberLiterals // error
+  import language.experimental.erasedDefinitions // ok: only check at erased definition
+  @experimental def f = 1

tests/neg-custom-args/no-experimental/experimental.scala

@@ -26,9 +26,3 @@ class Test2 {
 class Test6 {
   import scala.language.experimental  // ok
 }
-
-class Test7 {
-  import scala.language.experimental
-  import experimental.genericNumberLiterals // error: no aliases can be used to refer to a language import
-  val x: BigInt = 13232202002020202020202   // error
-}

tests/pos-custom-args/no-experimental/experimental-imports-empty.scala

@@ -0,0 +1,5 @@
+import annotation.experimental
+import language.experimental.fewerBraces
+import language.experimental.namedTypeArguments
+import language.experimental.genericNumberLiterals
+import language.experimental.erasedDefinitions

tests/pos-custom-args/no-experimental/experimental-imports-top.scala

@@ -0,0 +1,5 @@
+import language.experimental.erasedDefinitions
+import annotation.experimental
+
+@experimental
+erased def f = 1