Deprecate private[this] and protected[this] in 3.1

Add docs to state that they will be dropped.

Author: odersky

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

@@ -2642,7 +2642,11 @@ object Parsers {
         if (mods.is(Local) || mods.hasPrivateWithin)
           syntaxError(DuplicatePrivateProtectedQualifier())
         inBrackets {
-          if (in.token == THIS) { in.nextToken(); mods | Local }
+          if in.token == THIS then
+            if ctx.settings.strict.value then
+              deprecationWarning("The [this] qualifier is deprecated in Scala 3.1; it should be dropped.")
+            in.nextToken()
+            mods | Local
           else mods.withPrivateWithin(ident().toTypeName)
         }
       }

docs/docs/internals/syntax.md

@@ -331,7 +331,7 @@ LocalModifier     ::=  ‘abstract’
                     |  ‘lazy’
                     |  ‘inline’
 AccessModifier    ::=  (‘private’ | ‘protected’) [AccessQualifier]
-AccessQualifier   ::=  ‘[’ (id | ‘this’) ‘]’
+AccessQualifier   ::=  ‘[’ id ‘]’
 
 Annotation        ::=  ‘@’ SimpleType {ParArgumentExprs}                        Apply(tpe, args)
 

docs/docs/reference/dropped-features/this-qualifier.md

@@ -0,0 +1,14 @@
+---
+layout: doc-page
+title: Dropped: private[this] and protected[this]
+---
+
+The `private[this]` and `protected[this]` access modifiers are deprecated and will be phased out.
+
+Previously, these modifier were needed
+
+ - for avoiding the generation of getters and setters
+ - for excluding code under a `private[this]` from variance checks. (Scala 2 also excludes `protected[this]` but this was found to be unsound and was therefore removed).
+
+The compiler now infers for `private` members the fact that they are only accessed via `this`. Such members are treated as if they had been declared `private[this]`. `protected[this]` is dropped without a replacement.
+

docs/sidebar.yml

@@ -171,6 +171,8 @@ sidebar:
               url: docs/reference/dropped-features/weak-conformance.html
             - title: Nonlocal Returns
               url: docs/reference/dropped-features/nonlocal-returns.html
+            - title: `[this]` Qualifier
+              url: docs/reference/dropped-features/this-qualifier.html
     - title: Contributing
       subsection:
         - title: Contribute Knowledge