deprecate 3.1-migration, err on import

Author: bishabosha

compiler/src/dotty/tools/dotc/config/SourceVersion.scala

@@ -17,5 +17,12 @@ enum SourceVersion:
 
 object SourceVersion extends Property.Key[SourceVersion]:
 
-  val allSourceVersionNames = values.toList.map(_.toString.toTermName)
+  /** language versions that may appear in a language import, are deprecated, but not removed from the standard library. */
+  val illegalSourceVersionNames = List("3.1-migration").map(_.toTermName)
+
+  /** language versions that the compiler recognises. */
+  val validSourceVersionNames = values.toList.map(_.toString.toTermName)
+
+  /** All source versions that can be recognised from a language import. e.g. `import language.3.1` */
+  val allSourceVersionNames = validSourceVersionNames ::: illegalSourceVersionNames
 end SourceVersion

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

@@ -3150,6 +3150,18 @@ object Parsers {
               syntaxError(i"source version import is only allowed at the toplevel", id.span)
             else if ctx.compilationUnit.sourceVersion.isDefined then
               syntaxError(i"duplicate source version import", id.span)
+            else if illegalSourceVersionNames.contains(imported) then
+              // find closest non-migration versions
+              val closest = closestNamed(
+                candidates = validSourceVersionNames,
+                missing = imported.show.replace("-migration", ""),
+                format = _.show.replace("-migration", "")
+              )
+              val baseMsg = i"`$imported` is not a valid source version"
+              val msg = closest match
+                case (_, member) :: _ => i"$baseMsg, did you mean language.`$member`?"
+                case _ => baseMsg
+              syntaxError(msg, id.span)
             else
               ctx.compilationUnit.sourceVersion = Some(SourceVersion.valueOf(imported.toString))
         case None =>

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -26,18 +26,43 @@ import ast.untpd
 import ast.tpd
 import transform.SymUtils._
 
-/**  Messages
-  *  ========
-  *  The role of messages is to provide the necessary details for a simple to
-  *  understand diagnostic event. Each message can be turned into a message
-  *  container (one of the above) by calling the appropriate method on them.
-  *  For instance:
-  *
-  *  ```scala
-  *  EmptyCatchBlock(tree).error(pos)   // res: Error
-  *  EmptyCatchBlock(tree).warning(pos) // res: Warning
-  *  ```
+/*  Messages
+ *  ========
+ *  The role of messages is to provide the necessary details for a simple to
+ *  understand diagnostic event. Each message can be turned into a message
+ *  container (one of the above) by calling the appropriate method on them.
+ *  For instance:
+ *
+ *  ```scala
+ *  EmptyCatchBlock(tree).error(pos)   // res: Error
+ *  EmptyCatchBlock(tree).warning(pos) // res: Warning
+ *  ```
+ */
+
+/** A list of possible candidate options with their Levenstein distances
+  * to the name of the missing member.
   */
+def closestNamed[T](candidates: List[T], missing: String, format: T => String, maxDist: Int = 3): List[(Int, T)] =
+
+  def levenshteinDistance(s1: String, s2: String): Int =
+    val dist = Array.ofDim[Int](s2.length + 1, s1.length + 1)
+    for
+      j <- 0 to s2.length
+      i <- 0 to s1.length
+    do
+      dist(j)(i) =
+        if j == 0 then i
+        else if i == 0 then j
+        else if s2(j - 1) == s1(i - 1) then dist(j - 1)(i - 1)
+        else (dist(j - 1)(i) min dist(j)(i - 1) min dist(j - 1)(i - 1)) + 1
+    dist(s2.length)(s1.length)
+  end levenshteinDistance
+
+  candidates
+    .map(candidate => (levenshteinDistance(format(candidate), missing), candidate))
+    .filter((d, candidate) => d <= maxDist && d < missing.length && d < format(candidate).length)
+    .sortBy((d, candidate) => (d, format(candidate)))  // sort by distance first, alphabetically second
+end closestNamed
 
   abstract class SyntaxMsg(errorId: ErrorMessageID) extends Message(errorId):
     def kind = MessageKind.Syntax
@@ -318,27 +343,10 @@ import transform.SymUtils._
             && !sym.flagsUNSAFE.isOneOf(Synthetic | Private))
         yield sym
 
-      // Calculate Levenshtein distance
-      def distance(s1: String, s2: String): Int =
-        val dist = Array.ofDim[Int](s2.length + 1, s1.length + 1)
-        for
-          j <- 0 to s2.length
-          i <- 0 to s1.length
-        do
-          dist(j)(i) =
-            if j == 0 then i
-            else if i == 0 then j
-            else if s2(j - 1) == s1(i - 1) then dist(j - 1)(i - 1)
-            else (dist(j - 1)(i) min dist(j)(i - 1) min dist(j - 1)(i - 1)) + 1
-        dist(s2.length)(s1.length)
-
       // A list of possible candidate symbols with their Levenstein distances
       // to the name of the missing member
-      def closest: List[(Int, Symbol)] = candidates
-        .toList
-        .map(sym => (distance(sym.name.show, missing), sym))
-        .filter((d, sym) => d <= maxDist && d < missing.length && d < sym.name.show.length)
-        .sortBy((d, sym) => (d, sym.name.show))  // sort by distance first, alphabetically second
+      def closest: List[(Int, Symbol)] =
+        closestNamed(candidates.toList, missing, format = _.name.show, maxDist)
 
       val enumClause =
         if ((name eq nme.values) || (name eq nme.valueOf)) && site.classSymbol.companionClass.isEnumClass then

library/src/scala/runtime/stdLibPatches/language.scala

@@ -149,10 +149,13 @@ object language:
   object `3.0`
 
   /** Set source version to 3.1-migration.
+    *
+    * This is a no-op, and should not be used. A syntax error will be reported upon import.
     *
     * @see [[https://docs.scala-lang.org/scala3/guides/migration/compatibility-intro.html]]
     */
   @compileTimeOnly("`3.1-migration` can only be used at compile time in import statements")
+  @deprecated("`3.1-migration` is not valid, use `3.1` instead", since = "3.2")
   object `3.1-migration`
 
   /** Set source version to 3.1

tests/neg/i12457.scala

@@ -1,3 +1,3 @@
-import language.`3.1-migration`
+import language.`3.1`
 
 trait X [ X <: Z , Z >: X [ R     ] ] // error

tests/neg/source-import-3-1-migration.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/source-import-3-1-migration.scala:1:16 -------------------------------------------------------------
+1 |import language.`3.1-migration` // error
+  |                ^^^^^^^^^^^^^^^
+  |                `3.1-migration` is not a valid source version, did you mean language.`3.1`?

tests/neg/source-import-3-1-migration.scala

@@ -0,0 +1 @@
+import language.`3.1-migration` // error