Add hook to simulate a release compiler

and some other cleanups

Author: odersky

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

@@ -216,9 +216,15 @@ object Config {
    */
   inline val reuseSymDenotations = true
 
-  /** Are experimental imports are allowed? By default same as
+  /** Turn on in order to test the CI assuming we have a stable release compiler
+   *  instead of a nightly or snapshot.
+   */
+  private val assumeNonExperimentalCompiler = false
+
+  /** Are experimental features are allowed? By default same as
    *  `Properties.experimental`. Can be re-assigned, e.g. to allow tests
    *  of experimental features.
    */
-  @sharable private[dotty] var allowExperimentalFeatures = Properties.experimental
+  @sharable private[dotty] var allowExperimentalFeatures =
+    Properties.experimental && !assumeNonExperimentalCompiler
 }

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

@@ -106,8 +106,8 @@ object Feature:
 
   /** Check that experimental compiler options are only set for snapshot or nightly compiler versions. */
   def checkExperimentalSettings(using Context): Unit =
-    ctx.settings.language.value
-      .filter(str => str.startsWith("experimental.") && str != "experimental.scala2macros")
-      .foreach(str => checkExperimentalFeature(s" $str"))
+    for setting <- ctx.settings.language.value
+        if setting.startsWith("experimental.") && setting != "experimental.scala2macros"
+    do checkExperimentalFeature(s" $setting")
 
 end Feature

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

@@ -32,7 +32,6 @@ import config.Feature
 import config.Feature.{sourceVersion, migrateTo3}
 import config.SourceVersion._
 import config.SourceVersion
-import config.Properties
 
 object Parsers {