Fix #18609: Add language.3.4 and language.3.4-migration.

Author: sjrd

Diff for: compiler/src/dotty/tools/dotc/config/SourceVersion.scala

@@ -10,6 +10,7 @@ enum SourceVersion:
   case `3.2-migration`, `3.2`
   case `3.3-migration`, `3.3`
   case `3.4-migration`, `3.4`
+  // !!! Keep in sync with scala.runtime.stdlibPatches.language !!!
   case `future-migration`, `future`
 
   val isMigrating: Boolean = toString.endsWith("-migration")

Diff for: library/src/scala/runtime/stdLibPatches/language.scala

@@ -231,4 +231,21 @@ object language:
   @compileTimeOnly("`3.3` can only be used at compile time in import statements")
   object `3.3`
 
+  /** Set source version to 3.4-migration.
+    *
+    * @see [[https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html]]
+    */
+  @compileTimeOnly("`3.4-migration` can only be used at compile time in import statements")
+  object `3.4-migration`
+
+  /** Set source version to 3.4
+    *
+    * @see [[https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html]]
+    */
+  @compileTimeOnly("`3.4` can only be used at compile time in import statements")
+  object `3.4`
+
+  // !!! Keep in sync with dotty.tools.dotc.config.SourceVersion !!!
+  // Also add tests in `tests/pos/source-import-3-x.scala` and `tests/pos/source-import-3-x-migration.scala`
+
 end language

Diff for: tests/pos/source-import-3-3-migration.scala

@@ -0,0 +1 @@
+import language.`3.3-migration`

Diff for: tests/pos/source-import-3-3.scala

@@ -0,0 +1 @@
+import language.`3.3`

Diff for: tests/pos/source-import-3-4-migration.scala

@@ -0,0 +1 @@
+import language.`3.4-migration`

Diff for: tests/pos/source-import-3-4.scala

@@ -0,0 +1 @@
+import language.`3.4`