Enable experimental mode when experimental feature is imported

The `@experimental` flag is added to definitions in the scope where
the the experimental language feature is imported.

Author: nicolasstucki

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

@@ -32,6 +32,22 @@ object Feature:
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
   val into = experimental("into")
+  val relaxedExtensionImports = experimental("relaxedExtensionImports")
+
+  // TODO compute this list
+  // TODO remove features that do not enable experimental mode
+  val experimentalAutoEnableFeatures: List[TermName] = List(
+    namedTypeArguments,
+    genericNumberLiterals,
+    erasedDefinitions,
+    fewerBraces,
+    saferExceptions,
+    clauseInterleaving,
+    pureFunctions,
+    captureChecking,
+    into,
+    relaxedExtensionImports,
+  )
 
   val globalOnlyImports: Set[TermName] = Set(pureFunctions, captureChecking)
 
@@ -132,19 +148,20 @@ object Feature:
       false
 
   def checkExperimentalFeature(which: String, srcPos: SrcPos, note: => String = "")(using Context) =
-    if !isExperimentalEnabled then
+    if !isExperimentalGloballyEnabled && !isExperimentalByImportEnabled && !isExperimentalUnstableEnabled then
       report.error(
         em"""Experimental $which may only be used under experimental mode:
             |  1. In a definition marked as @experimental
-            |  2. Compiling with the -experimental compiler flag
-            |  3. With a nightly or snapshot version of the compiler$note
+            |  2. An experimental language import is in scope
+            |  3. Compiling with the -experimental compiler flag
+            |  4. With a nightly or snapshot version of the compiler$note
           """, srcPos)
 
   private def ccException(sym: Symbol)(using Context): Boolean =
     ccEnabled && defn.ccExperimental.contains(sym)
 
   def checkExperimentalDef(sym: Symbol, srcPos: SrcPos)(using Context) =
-    if !isExperimentalEnabled then
+    if !isExperimentalGloballyEnabled && !isExperimentalByImportEnabled && !isExperimentalUnstableEnabled then
       val experimentalSym =
         if sym.hasAnnotation(defn.ExperimentalAnnot) then sym
         else if sym.owner.hasAnnotation(defn.ExperimentalAnnot) then sym.owner
@@ -162,8 +179,22 @@ object Feature:
         if setting.startsWith("experimental.") && setting != "experimental.macros"
     do checkExperimentalFeature(s"feature $setting", NoSourcePosition)
 
-  def isExperimentalEnabled(using Context): Boolean =
-    (Properties.experimental && !ctx.settings.YnoExperimental.value) || ctx.settings.experimental.value
+
+  /** Experimental mode enabled by default in nightly or snapshot version of the compiler without `-Yno-experimental` */
+  def isExperimentalUnstableEnabled(using Context): Boolean =
+    Properties.experimental && !ctx.settings.YnoExperimental.value
+
+  /** Experimental mode enabled in this compilation unit
+   *  - Compiled with `-experimental`
+   *  - Compiled with `-language:experimental.xyz`
+   */
+  def isExperimentalGloballyEnabled(using Context): Boolean =
+    ctx.settings.experimental.value ||
+    experimentalAutoEnableFeatures.exists(enabledBySetting)
+
+  /** Experimental mode enabled by a `language.experimental` import in scope */
+  def isExperimentalByImportEnabled(using Context): Boolean =
+    experimentalAutoEnableFeatures.exists(enabledByImport)
 
   /** Handle language import `import language.<prefix>.<imported>` if it is one
    *  of the global imports `pureFunctions` or `captureChecking`. In this case

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

@@ -1254,6 +1254,19 @@ object SymDenotations {
     final def isTopLevelClass(using Context): Boolean =
       !this.exists || this.isEffectiveRoot || this.is(PackageClass) || this.owner.is(PackageClass)
 
+    /** The top-level definition containing this denotation.
+     *  Returns top-level class or members of top-level package objects.
+     *  If this is not in a definition, returns NoSymbol.
+     */
+    @tailrec final def topLevelDefinition(using Context): Symbol =
+      if !this.exists || this.is(Package) then NoSymbol
+      else if this.isTopLevelDefinition then this.symbol
+      else this.owner.topLevelDefinition
+
+    final def isTopLevelDefinition(using Context): Boolean =
+      !this.is(Package) && !this.name.isPackageObjectName &&
+      (this.owner.is(Package) || (this.owner.isPackageObject && !this.isConstructor))
+
     /** The package class containing this denotation */
     final def enclosingPackageClass(using Context): Symbol =
       if (this.is(PackageClass)) symbol else owner.enclosingPackageClass

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

@@ -545,15 +545,22 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
           report.error("`erased` definition cannot be implemented with en expression of type Null", tree.srcPos)
 
     private def annotateExperimental(sym: Symbol)(using Context): Unit =
-      def isTopLevelDefinitionInSource(sym: Symbol) =
-        !sym.is(Package) && !sym.name.isPackageObjectName &&
-        (sym.owner.is(Package) || (sym.owner.isPackageObject && !sym.isConstructor))
-      if !sym.hasAnnotation(defn.ExperimentalAnnot)
-        && (ctx.settings.experimental.value && isTopLevelDefinitionInSource(sym))
-        || (sym.is(Module) && sym.companionClass.hasAnnotation(defn.ExperimentalAnnot))
-      then
+      def companionOfExperimental =
+        sym.is(Module) && sym.companionClass.hasAnnotation(defn.ExperimentalAnnot)
+      def topLevelDefinitionWithExperimentalEnabledInUnit =
+        sym.isTopLevelDefinition && (Feature.isExperimentalGloballyEnabled || Feature.isExperimentalByImportEnabled)
+      if !sym.hasAnnotation(defn.ExperimentalAnnot) && (companionOfExperimental || topLevelDefinitionWithExperimentalEnabledInUnit) then
         sym.addAnnotation(Annotation(defn.ExperimentalAnnot, sym.span))
 
+      // TODO do this on import statement
+      val topLevelDefinition = sym.topLevelDefinition
+      if topLevelDefinition.exists &&
+        !topLevelDefinition.hasAnnotation(defn.ExperimentalAnnot)
+        && Feature.isExperimentalByImportEnabled
+      then
+        topLevelDefinition.addAnnotation(Annotation(defn.ExperimentalAnnot, sym.span))
+        if topLevelDefinition.is(Module) then topLevelDefinition.companionModule.addAnnotation(Annotation(defn.ExperimentalAnnot, sym.span))
+
     private def scala2LibPatch(tree: TypeDef)(using Context) =
       val sym = tree.symbol
       if compilingScala2StdLib && sym.is(ModuleClass) then

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

@@ -265,7 +265,9 @@ Experimental definitions can only be referenced in an experimental scope. Experi
 
    </details>
 
-6. Any code compiled using a [_Nightly_](https://search.maven.org/artifact/org.scala-lang/scala3-compiler_3) or _Snapshot_ version of the compiler is considered to be in an experimental scope.
+6. An experimental language feature is imported in the current scope
+
+7. Any code compiled using a [_Nightly_](https://search.maven.org/artifact/org.scala-lang/scala3-compiler_3) or _Snapshot_ version of the compiler is considered to be in an experimental scope.
 Can use the `-Yno-experimental` compiler flag to disable it and run as a proper release.
 
 In any other situation, a reference to an experimental definition will cause a compilation error.

docs/_spec/TODOreference/other-new-features/experimental-defs.md

@@ -216,7 +216,7 @@ Experimental definitions can only be referenced in an experimental scope. Experi
 
    <details>
    <summary>Example 1</summary>
-   
+
    ```scala
    import scala.annotation.experimental
 
@@ -242,7 +242,7 @@ Experimental definitions can only be referenced in an experimental scope. Experi
      }
    }
    ```
-   
+
    </details>
 
 5. Annotations of an experimental definition are in experimental scopes. Examples:
@@ -265,7 +265,9 @@ Experimental definitions can only be referenced in an experimental scope. Experi
 
    </details>
 
-6. Any code compiled using a [_Nightly_](https://search.maven.org/artifact/org.scala-lang/scala3-compiler_3) or _Snapshot_ version of the compiler is considered to be in an experimental scope.
+6. An experimental language feature is imported in the current scope
+
+7. Any code compiled using a [_Nightly_](https://search.maven.org/artifact/org.scala-lang/scala3-compiler_3) or _Snapshot_ version of the compiler is considered to be in an experimental scope.
 Can use the `-Yno-experimental` compiler flag to disable it and run as a proper release.
 
 In any other situation, a reference to an experimental definition will cause a compilation error.

tests/neg/experimental-erased.scala

@@ -2,4 +2,4 @@
 
 import scala.language.experimental.erasedDefinitions
 
-erased def erasedFun(erased x: Int): Int = x // error // error
+erased def erasedFun(erased x: Int): Int = x

tests/neg/experimental-nested-imports-2.scala

@@ -1,6 +1,6 @@
 //> using options -Yno-experimental
 
-import scala.language.experimental.namedTypeArguments // error
+import scala.language.experimental.namedTypeArguments
 
 def namedTypeArgumentsFun[T, U]: Int =
   namedTypeArgumentsFun[T = Int, U = Int]

tests/neg/experimental-nested-imports.scala

@@ -1,7 +1,7 @@
 //> using options -Yno-experimental
 
 class Test0 {
-  import language.experimental.namedTypeArguments // error
+  import language.experimental.namedTypeArguments
   object Foo {
     inline def f[S, T](x: S): T = ???
     def g(x: Int) = f[T = Any](x)

tests/neg/experimentalErased.scala

@@ -11,9 +11,9 @@ object Object1:
   erased def f = 1
 
 object Object2:
-  import language.experimental.fewerBraces // error
-  import language.experimental.namedTypeArguments // error
-  import language.experimental.genericNumberLiterals // error
+  import language.experimental.fewerBraces
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
   import language.experimental.erasedDefinitions
   erased def f = 1
 
@@ -26,9 +26,9 @@ object Class1:
   erased def f = 1
 
 object Class2:
-  import language.experimental.fewerBraces // error
-  import language.experimental.namedTypeArguments // error
-  import language.experimental.genericNumberLiterals // error
+  import language.experimental.fewerBraces
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
   import language.experimental.erasedDefinitions
   erased def f = 1
 
@@ -41,8 +41,8 @@ def fun1 =
   erased def f = 1
 
 def fun2 =
-  import language.experimental.fewerBraces // error
-  import language.experimental.namedTypeArguments // error
-  import language.experimental.genericNumberLiterals // error
+  import language.experimental.fewerBraces
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
   import language.experimental.erasedDefinitions
   erased def f = 1

tests/neg/expeimental-flag-with-lang-feature-1.scala renamed to tests/pos/expeimental-flag-with-lang-feature-1.scala

@@ -0,0 +1,31 @@
+//> using options -Yno-experimental
+
+import annotation.experimental
+
+class Class1:
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1
+  def g = 1
+
+object Object1:
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1
+  def g = 1
+
+def fun1 =
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1
+  def g = 1
+
+val value1 =
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1
+  def g = 1

tests/neg/expeimental-flag-with-lang-feature-2.scala renamed to tests/pos/expeimental-flag-with-lang-feature-2.scala

@@ -0,0 +1,27 @@
+//> using options -Yno-experimental
+
+import annotation.experimental
+
+class Class1:
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1
+
+object Object1:
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1
+
+def fun1 =
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1
+
+val value1 =
+  import language.experimental.namedTypeArguments
+  import language.experimental.genericNumberLiterals
+  import language.experimental.erasedDefinitions
+  @experimental def f = 1