Fix span of private this warning

Author: som-snytt

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

@@ -3425,14 +3425,15 @@ object Parsers {
           else mods.withPrivateWithin(ident().toTypeName)
         }
         if mods1.is(Local) then
+          val span = Span(startOffset, in.lastOffset)
           report.errorOrMigrationWarning(
               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(),
+              in.sourcePos().withSpan(span),
               MigrationVersion.RemoveThisQualifier)
           if MigrationVersion.RemoveThisQualifier.needsPatch then
-            patch(source, Span(startOffset, in.lastOffset), "")
+            patch(source, span, "")
         mods1
       }
       else mods

tests/warn/private-this-3.4.check

@@ -1,14 +1,14 @@
--- Warning: tests/warn/private-this-3.4.scala:4:16 ---------------------------------------------------------------------
+-- Warning: tests/warn/private-this-3.4.scala:4:9 ----------------------------------------------------------------------
 4 |  private[this] def foo: Int = ??? // warn: migration 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.
--- Warning: tests/warn/private-this-3.4.scala:5:18 ---------------------------------------------------------------------
+  |         ^^^^^^
+  |         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/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.
-  |                  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.
+  |           ^^^^^^
+  |           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.