Update private[this] deprecation warning and documentation (#19393)

Fixes #19339

Author: nicolasstucki

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

@@ -3151,7 +3151,8 @@ object Parsers {
         }
         if mods1.is(Local) then
           report.errorOrMigrationWarning(
-              em"""The [this] qualifier will be deprecated in the future; it should be dropped.
+              em"""Ignoring [this] qualifier.
+                  |This syntax will be deprecated in the future; it should be dropped.
                   |See: https://docs.scala-lang.org/scala3/reference/dropped-features/this-qualifier.html${rewriteNotice(`3.4-migration`)}""",
               in.sourcePos(),
               MigrationVersion.RemoveThisQualifier)

docs/_spec/APPLIEDreference/dropped-features/this-qualifier.md

@@ -4,7 +4,7 @@ title: "Dropped: private[this] and protected[this]"
 nightlyOf: https://docs.scala-lang.org/scala3/reference/dropped-features/this-qualifier.html
 ---
 
-The `private[this]` and `protected[this]` access modifiers will be deprecated and phased out.
+The `private[this]` and `protected[this]` access modifiers will be deprecated and phased out. Migration warning will be shown in Scala 3.4+.
 
 Previously, these modifiers were needed for
 

tests/neg-scalajs/js-native-members.check

@@ -1,31 +1,36 @@
 -- Warning: tests/neg-scalajs/js-native-members.scala:24:16 ------------------------------------------------------------
 24 |  private[this] def this(x: Int) = this() // ok
    |                ^
-   |                The [this] qualifier will be deprecated in the future; it should be dropped.
+   |                Ignoring [this] qualifier.
+   |                This syntax will be deprecated in the future; it should be dropped.
    |                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/neg-scalajs/js-native-members.scala:28:16 ------------------------------------------------------------
 28 |  private[this] val a: Int = js.native // error
    |                ^
-   |                The [this] qualifier will be deprecated in the future; it should be dropped.
+   |                Ignoring [this] qualifier.
+   |                This syntax will be deprecated in the future; it should be dropped.
    |                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/neg-scalajs/js-native-members.scala:32:16 ------------------------------------------------------------
 32 |  private[this] var d: Int = js.native // error
    |                ^
-   |                The [this] qualifier will be deprecated in the future; it should be dropped.
+   |                Ignoring [this] qualifier.
+   |                This syntax will be deprecated in the future; it should be dropped.
    |                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/neg-scalajs/js-native-members.scala:36:16 ------------------------------------------------------------
 36 |  private[this] def g(): Int = js.native // error
    |                ^
-   |                The [this] qualifier will be deprecated in the future; it should be dropped.
+   |                Ignoring [this] qualifier.
+   |                This syntax will be deprecated in the future; it should be dropped.
    |                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/neg-scalajs/js-native-members.scala:49:25 ------------------------------------------------------------
 49 |  class X3 private[this] () extends js.Object { // ok
    |                         ^
-   |                         The [this] qualifier will be deprecated in the future; it should be dropped.
+   |                         Ignoring [this] qualifier.
+   |                         This syntax will be deprecated in the future; it should be dropped.
    |                         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.
 -- [E003] Syntax Warning: tests/neg-scalajs/js-native-members.scala:58:44 ----------------------------------------------

tests/neg-scalajs/js-non-native-members-qualified-private.check

@@ -1,7 +1,8 @@
 -- Warning: tests/neg-scalajs/js-non-native-members-qualified-private.scala:52:28 --------------------------------------
 52 |      class B private[this] () extends js.Object // ok
    |                            ^
-   |                          The [this] qualifier will be deprecated in the future; it should be dropped.
+   |                          Ignoring [this] qualifier.
+   |                          This syntax will be deprecated in the future; it should be dropped.
    |                          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.
 -- Error: tests/neg-scalajs/js-non-native-members-qualified-private.scala:6:32 -----------------------------------------

tests/neg/private-this-3.4.check

@@ -1,12 +1,14 @@
 -- Error: tests/neg/private-this-3.4.scala:6:16 ------------------------------------------------------------------------
 6 |  private[this] def foo: Int = ??? // error: migration warning
   |                ^
-  |                The [this] qualifier will be deprecated in the future; it should be dropped.
+  |                Ignoring [this] qualifier.
+  |                This syntax will be deprecated in the future; it should be dropped.
   |                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.
 -- Error: tests/neg/private-this-3.4.scala:7:18 ------------------------------------------------------------------------
 7 |  protected[this] def bar: Int = ??? // error: migration warning
   |                  ^
-  |                  The [this] qualifier will be deprecated in the future; it should be dropped.
+  |                  Ignoring [this] qualifier.
+  |                  This syntax will be deprecated in the future; it should be dropped.
   |                  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.

tests/semanticdb/metac.expect

@@ -44,11 +44,13 @@ Occurrences:
 
 Diagnostics:
 [3:14..3:16): [warning] unused private member
-[4:16..4:16): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[4:16..4:16): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.
 [4:20..4:22): [warning] unused private member
-[7:18..7:18): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[7:18..7:18): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.
 
@@ -817,11 +819,13 @@ Occurrences:
 
 Diagnostics:
 [18:9..18:10): [warning] unused explicit parameter
-[20:23..20:23): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[20:23..20:23): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.
 [20:27..20:28): [warning] unused explicit parameter
-[22:23..22:23): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[22:23..22:23): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.
 [22:27..22:28): [warning] unused explicit parameter
@@ -4129,16 +4133,20 @@ Occurrences:
 
 Diagnostics:
 [2:20..2:21): [warning] unused explicit parameter
-[5:16..5:16): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[5:16..5:16): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.
-[7:16..7:16): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[7:16..7:16): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.
-[12:16..12:16): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[12:16..12:16): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.
-[13:16..13:16): [warning] The [this] qualifier will be deprecated in the future; it should be dropped.
+[13:16..13:16): [warning] Ignoring [this] qualifier.
+This syntax will be deprecated in the future; it should be dropped.
 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.