Deprecation warnings for old syntax: var x = _

* In `3.4` we emit the deprecation warning
* In `future` we emit we make this syntax an error
* Add patch. Not ideal because we need to use the full path of `uninitialized`

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

@@ -3655,11 +3655,14 @@ object Parsers {
           subExpr() match
             case rhs0 @ Ident(name) if placeholderParams.nonEmpty && name == placeholderParams.head.name
                 && !tpt.isEmpty && mods.is(Mutable) && lhs.forall(_.isInstanceOf[Ident]) =>
-              if sourceVersion.isAtLeast(future) then
-                deprecationWarning(
-                  em"""`= _` has been deprecated; use `= uninitialized` instead.
-                      |`uninitialized` can be imported with `scala.compiletime.uninitialized`.""",
-                  rhsOffset)
+              report.gradualErrorOrMigrationWarning(
+                em"""`= _` has been deprecated; use `= uninitialized` instead.
+                        |`uninitialized` can be imported with `scala.compiletime.uninitialized`.${rewriteNotice(`3.4-migration`)}""",
+                in.sourcePos(rhsOffset),
+                warnFrom = `3.4`,
+                errorFrom = future)
+              if sourceVersion.isMigrating && sourceVersion.isAtLeast(`3.4-migration`) then
+                patch(source, Span(rhsOffset, rhsOffset + 1), "scala.compiletime.uninitialized")
               placeholderParams = placeholderParams.tail
               atSpan(rhs0.span) { Ident(nme.WILDCARD) }
             case rhs0 => rhs0

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -60,6 +60,7 @@ class CompilationTests {
       compileFile("tests/rewrites/rewrites.scala", defaultOptions.and("-source", "3.0-migration").and("-rewrite", "-indent")),
       compileFile("tests/rewrites/rewrites3x.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/rewrites3x-fatal-warnings.scala", defaultOptions.and("-rewrite", "-source", "future-migration", "-Xfatal-warnings")),
+      compileFile("tests/rewrites/uninitialized-var.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/with-type-operator.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/filtering-fors.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
       compileFile("tests/rewrites/refutable-pattern-bindings.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),

language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala

@@ -15,6 +15,7 @@ import scala.collection._
 import scala.jdk.CollectionConverters._
 import scala.util.control.NonFatal
 import scala.io.Codec
+import scala.compiletime.uninitialized
 
 import dotc._
 import ast.{Trees, tpd, untpd}
@@ -54,14 +55,14 @@ class DottyLanguageServer extends LanguageServer
   import lsp4j.jsonrpc.messages.{Either => JEither}
   import lsp4j._
 
-  private[this] var rootUri: String = _
+  private[this] var rootUri: String = uninitialized
 
-  private[this] var myClient: DottyClient = _
+  private[this] var myClient: DottyClient = uninitialized
   def client: DottyClient = myClient
 
-  private[this] var myDrivers: mutable.Map[ProjectConfig, InteractiveDriver] = _
+  private[this] var myDrivers: mutable.Map[ProjectConfig, InteractiveDriver] = uninitialized
 
-  private[this] var myDependentProjects: mutable.Map[ProjectConfig, mutable.Set[ProjectConfig]] = _
+  private[this] var myDependentProjects: mutable.Map[ProjectConfig, mutable.Set[ProjectConfig]] = uninitialized
 
   def drivers: Map[ProjectConfig, InteractiveDriver] = thisServer.synchronized {
     if myDrivers == null then

library/src/scala/util/control/NonLocalReturns.scala

@@ -1,5 +1,7 @@
 package scala.util.control
 
+import scala.compiletime.uninitialized
+
 /** Library implementation of nonlocal return.
  *
  *  Usage:
@@ -21,7 +23,7 @@ package scala.util.control
 object NonLocalReturns {
   @deprecated("Use scala.util.boundary.Break instead", "3.3")
   class ReturnThrowable[T] extends ControlThrowable {
-    private var myResult: T = _
+    private var myResult: T = uninitialized
     def throwReturn(result: T): Nothing = {
       myResult = result
       throw this

presentation-compiler/src/main/dotty/tools/pc/MetalsDriver.scala

@@ -7,6 +7,8 @@ import dotty.tools.dotc.interactive.InteractiveDriver
 import dotty.tools.dotc.reporting.Diagnostic
 import dotty.tools.dotc.util.SourceFile
 
+import scala.compiletime.uninitialized
+
 /**
  * MetalsDriver is a wrapper class that provides a compilation cache for InteractiveDriver.
  * MetalsDriver skips running compilation if
@@ -29,7 +31,7 @@ class MetalsDriver(
     override val settings: List[String]
 ) extends InteractiveDriver(settings):
 
-  @volatile private var lastCompiledURI: URI = _
+  @volatile private var lastCompiledURI: URI = uninitialized
 
   private def alreadyCompiled(uri: URI, content: Array[Char]): Boolean =
     compilationUnits.get(uri) match

project/Build.scala

@@ -578,7 +578,8 @@ object Build {
   lazy val commonDottyCompilerSettings = Seq(
        // Note: bench/profiles/projects.yml should be updated accordingly.
        Compile / scalacOptions ++= Seq("-Yexplicit-nulls", "-Ysafe-init"),
-
+      // Use source 3.3 to avoid fatal migration warnings on scalajs-ir
+      scalacOptions ++= Seq("-source", "3.3"),
       // Generate compiler.properties, used by sbt
       (Compile / resourceGenerators) += Def.task {
         import java.util._

sbt-test/compilerReporter/i14576/Test.scala

@@ -10,8 +10,5 @@ object Test:
   def f(x: Text) = println(x.str)
   f("abc")
 
-  // private[this] and = _ are deprecated under -source:future
-  private[this] var x: AnyRef = _
-
   // under -source:future, `_` is deprecated for wildcard arguments of types: use `?` instead
   val xs: List[_] = Nil

sbt-test/compilerReporter/i14576/build.sbt

@@ -24,7 +24,7 @@ lazy val root = (project in file("."))
     },
     assertDeprecationSummary := {
       assert {
-        FakePrintWriter.messages.exists(_.contains("there were 3 deprecation warnings; re-run with -deprecation for details"))
+        FakePrintWriter.messages.exists(_.contains("there was 1 deprecation warning; re-run with -deprecation for details"))
       }
     },
     assertNoDeprecationSummary := {

tests/neg/uninitialized-future-migration.check

@@ -0,0 +1,6 @@
+-- Error: tests/neg/uninitialized-future-migration.scala:7:15 ----------------------------------------------------------
+7 |  var a: Int = _ // error
+  |               ^
+  |               `= _` has been deprecated; use `= uninitialized` instead.
+  |               `uninitialized` can be imported with `scala.compiletime.uninitialized`.
+  |               This construct can be rewritten automatically under -rewrite -source future-migration.

tests/neg/uninitialized-future-migration.scala

@@ -0,0 +1,8 @@
+//> using options -Werror
+
+import scala.language.`future-migration`
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // error
+  var b: Int = uninitialized

tests/neg/uninitialized-future.scala

@@ -0,0 +1,6 @@
+import scala.language.future
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // error
+  var b: Int = uninitialized

tests/pos/uninitialized-future-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`future-migration`
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // warn
+  var b: Int = uninitialized

tests/rewrites/uninitialized-var.check

@@ -0,0 +1,2 @@
+class Foo:
+  var a: Int = scala.compiletime.uninitialized

tests/rewrites/uninitialized-var.scala

@@ -0,0 +1,2 @@
+class Foo:
+  var a: Int = _