Refactor the way we mark experimental top-level definition

Move similar logic from PostTyper into `checkAndAdaptExperimentalImports`.
Also make the message of `@experimental` more precise for experimental
language settings.

Author: nicolasstucki

Diff for: compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -170,9 +170,8 @@ object Feature:
     ctx.settings.experimental.value ||
     experimentalAutoEnableFeatures.exists(enabled)
 
-  def isExperimentalEnabledBySetting(using Context): Boolean =
-    ctx.settings.experimental.value ||
-    experimentalAutoEnableFeatures.exists(enabledBySetting)
+  def experimentalEnabledByLanguageSetting(using Context): Option[TermName] =
+    experimentalAutoEnableFeatures.find(enabledBySetting)
 
   def isExperimentalEnabledByImport(using Context): Boolean =
     experimentalAutoEnableFeatures.exists(enabledByImport)

Diff for: compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -416,7 +416,7 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
             )
           }
         case tree: ValDef =>
-          annotateExperimental(tree.symbol)
+          annotateExperimentalCompanion(tree.symbol)
           registerIfHasMacroAnnotations(tree)
           checkErasedDef(tree)
           Checking.checkPolyFunctionType(tree.tpt)
@@ -425,7 +425,6 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
             checkStableSelection(tree.rhs)
           processValOrDefDef(super.transform(tree1))
         case tree: DefDef =>
-          annotateExperimental(tree.symbol)
           registerIfHasMacroAnnotations(tree)
           checkErasedDef(tree)
           Checking.checkPolyFunctionType(tree.tpt)
@@ -437,7 +436,7 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
           val sym = tree.symbol
           if (sym.isClass)
             VarianceChecker.check(tree)
-            annotateExperimental(sym)
+            annotateExperimentalCompanion(sym)
             checkMacroAnnotation(sym)
             if sym.isOneOf(GivenOrImplicit) then
               sym.keepAnnotationsCarrying(thisPhase, Set(defn.CompanionClassMetaAnnot), orNoneOf = defn.MetaAnnots)
@@ -583,16 +582,9 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
         else if tpe.derivesFrom(defn.NullClass) then
           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))
+    private def annotateExperimentalCompanion(sym: Symbol)(using Context): Unit =
       if sym.is(Module) then
         ExperimentalAnnotation.copy(sym.companionClass).foreach(sym.addAnnotation)
-      if !sym.hasAnnotation(defn.ExperimentalAnnot)
-        && Feature.isExperimentalEnabledBySetting && isTopLevelDefinitionInSource(sym)
-      then
-        sym.addAnnotation(ExperimentalAnnotation("Added by -experimental or -language:experimental.*", sym.span))
 
     // It needs to run at the phase of the postTyper --- otherwise, the test of the symbols will use
     // the transformed denotation with added `Serializable` and `AbstractFunction1`.

Diff for: compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -841,20 +841,23 @@ object Checking {
       }
 
     if ctx.owner.is(Package) || ctx.owner.name.startsWith(str.REPL_SESSION_LINE) then
+      def markTopLevelDefsAsExperimental(why: String): Unit =
+        for tree <- nonExperimentalStats(trees) do
+          tree match
+            case tree: MemberDef =>
+              val sym = tree.symbol
+              if !sym.isExperimental then
+                sym.addAnnotation(ExperimentalAnnotation(s"Added by $why", sym.span))
+            case tree =>
+              // There is no definition to attach the @experimental annotation
+              report.error(s"Implementation restriction: top-level `val _ = ...` is not supported with $why.", tree.srcPos)
       unitExperimentalLanguageImports match
-        case imp :: _ =>
-          // mark all top-level definitions as @experimental
-          for tree <- nonExperimentalStats(trees) do
-            tree match
-              case tree: MemberDef =>
-                // TODO move this out of checking (into posttyper?)
-                val sym = tree.symbol
-                if !sym.isExperimental then
-                  sym.addAnnotation(ExperimentalAnnotation(i"Added by top level $imp", sym.span))
-              case tree =>
-                // There is no definition to attach the @experimental annotation
-                report.error("Implementation restriction: top-level `val _ = ...` is not supported with experimental language imports.", tree.srcPos)
+        case imp :: _ => markTopLevelDefsAsExperimental(i"top level $imp")
         case _ =>
+          Feature.experimentalEnabledByLanguageSetting match
+            case Some(sel) => markTopLevelDefsAsExperimental(i"-language:experimental.$sel")
+            case _ if ctx.settings.experimental.value => markTopLevelDefsAsExperimental(i"-experimental")
+            case _ =>
     else
       for imp <- unitExperimentalLanguageImports do
         Feature.checkExperimentalFeature("feature local import", imp.srcPos)

Diff for: tests/neg-macros/i18677-a.check

@@ -7,10 +7,10 @@
   |The tree does not conform to the compiler's tree invariants.
   |
   |Macro was:
-  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-a/Test_2.scala") @scala.annotation.experimental("Added by -experimental or -language:experimental.*") @extendFoo class AFoo()
+  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-a/Test_2.scala") @scala.annotation.experimental("Added by -experimental") @extendFoo class AFoo()
   |
   |The macro returned:
-  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-a/Test_2.scala") @scala.annotation.experimental("Added by -experimental or -language:experimental.*") @extendFoo class AFoo() extends Foo
+  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-a/Test_2.scala") @scala.annotation.experimental("Added by -experimental") @extendFoo class AFoo() extends Foo
   |
   |Error:
   |assertion failed: Parents of class symbol differs from the parents in the tree for class AFoo

Diff for: tests/neg-macros/i18677-b.check

@@ -7,10 +7,10 @@
   |The tree does not conform to the compiler's tree invariants.
   |
   |Macro was:
-  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-b/Test_2.scala") @scala.annotation.experimental("Added by -experimental or -language:experimental.*") @extendFoo class AFoo()
+  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-b/Test_2.scala") @scala.annotation.experimental("Added by -experimental") @extendFoo class AFoo()
   |
   |The macro returned:
-  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-b/Test_2.scala") @scala.annotation.experimental("Added by -experimental or -language:experimental.*") @extendFoo class AFoo() extends Foo
+  |@scala.annotation.internal.SourceFile("tests/neg-macros/i18677-b/Test_2.scala") @scala.annotation.experimental("Added by -experimental") @extendFoo class AFoo() extends Foo
   |
   |Error:
   |assertion failed: Parents of class symbol differs from the parents in the tree for class AFoo

Diff for: tests/neg/experimental-import-with-top-level-val-underscore.check

@@ -1,6 +1,6 @@
 -- Error: tests/neg/experimental-import-with-top-level-val-underscore.scala:4:4 ----------------------------------------
 4 |val _ = // error
   |    ^
-  |    Implementation restriction: top-level `val _ = ...` is not supported with experimental language imports.
+  |Implementation restriction: top-level `val _ = ...` is not supported with top level import language.experimental.erasedDefinitions.
 5 |  println("Hello, world!")
 6 |  42

Diff for: tests/neg/experimental-message-experimental-flag.check

@@ -2,7 +2,7 @@
 -- Error: tests/neg/experimental-message-experimental-flag/Test_2.scala:3:10 -------------------------------------------
 3 |def g() = f() // error
   |          ^
-  |          method f is marked @experimental: Added by -experimental or -language:experimental.*
+  |          method f is marked @experimental: Added by -experimental
   |
   |          Experimental definition may only be used under experimental mode:
   |            1. in a definition marked as @experimental, or