Improve this qualifier warning

Author: som-snytt

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

@@ -3426,9 +3426,10 @@ object Parsers {
         }
         if mods1.is(Local) then
           val span = Span(startOffset, in.lastOffset)
+          val p = if mods1.is(Private) then "private" else "protected"
           report.errorOrMigrationWarning(
               em"""Ignoring [this] qualifier.
-                  |This syntax will be deprecated in the future; it should be dropped.
+                  |The syntax `$p[this]` will be deprecated in the future; just write `$p` instead.
                   |See: https://docs.scala-lang.org/scala3/reference/dropped-features/this-qualifier.html${rewriteNotice(`3.4-migration`)}""",
               in.sourcePos().withSpan(span),
               MigrationVersion.RemoveThisQualifier)

tests/warn/private-this-3.4.check

@@ -2,13 +2,13 @@
 4 |  private[this] def foo: Int = ??? // warn: migration warning
   |         ^^^^^^
   |         Ignoring [this] qualifier.
-  |         This syntax will be deprecated in the future; it should be dropped.
+  |         The syntax `private[this]` will be deprecated in the future; just write `private` instead.
   |         See: https://docs.scala-lang.org/scala3/reference/dropped-features/this-qualifier.html
   |         This construct can be rewritten automatically under -rewrite -source 3.4-migration.
 -- Warning: tests/warn/private-this-3.4.scala:5:11 ---------------------------------------------------------------------
 5 |  protected[this] def bar: Int = ??? // warn: migration warning
   |           ^^^^^^
   |           Ignoring [this] qualifier.
-  |           This syntax will be deprecated in the future; it should be dropped.
+  |           The syntax `protected[this]` will be deprecated in the future; just write `protected` instead.
   |           See: https://docs.scala-lang.org/scala3/reference/dropped-features/this-qualifier.html
   |           This construct can be rewritten automatically under -rewrite -source 3.4-migration.