Replace pickling of sub matches with an annotation

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1061,6 +1061,7 @@ class Definitions {
   @tu lazy val ScalaStaticAnnot: ClassSymbol = requiredClass("scala.annotation.static")
   @tu lazy val SerialVersionUIDAnnot: ClassSymbol = requiredClass("scala.SerialVersionUID")
   @tu lazy val SilentIntoAnnot: ClassSymbol = requiredClass("scala.annotation.internal.$into")
+  @tu lazy val SilentSubMatchAnnot: ClassSymbol = requiredClass("scala.annotation.internal.$subMatch")
   @tu lazy val TailrecAnnot: ClassSymbol = requiredClass("scala.annotation.tailrec")
   @tu lazy val ThreadUnsafeAnnot: ClassSymbol = requiredClass("scala.annotation.threadUnsafe")
   @tu lazy val ConstructorOnlyAnnot: ClassSymbol = requiredClass("scala.annotation.constructorOnly")

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -590,7 +590,10 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
             if (tree.isInline)
               if (selector.isEmpty) writeByte(IMPLICIT)
               else { writeByte(INLINE); pickleTree(selector) }
-            else if tree.isSubMatch then { writeByte(LAZY); pickleTree(selector) }
+            else if tree.isSubMatch then
+              // Temporary measure until we can change TastyFormat
+              val annot = New(defn.SilentSubMatchAnnot.typeRef, Nil)
+              pickleTree(selector.annotated(annot))
             else pickleTree(selector)
             tree.cases.foreach(pickleTree)
           }

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1542,7 +1542,7 @@ class TreeUnpickler(reader: TastyReader,
               val tpt = ifBefore(end)(readTpt(), EmptyTree)
               Closure(Nil, meth, tpt)
             case MATCH =>
-              simplifyLub(
+              simplifyLub:
                 if (nextByte == IMPLICIT) {
                   readByte()
                   InlineMatch(EmptyTree, readCases(end))
@@ -1551,10 +1551,9 @@ class TreeUnpickler(reader: TastyReader,
                   readByte()
                   InlineMatch(readTree(), readCases(end))
                 }
-                else if nextByte == LAZY then // similarly to InlineMatch we use an arbitrary Cat.1 tag
-                  readByte()
-                  SubMatch(readTree(), readCases(end))
-                else Match(readTree(), readCases(end)))
+                else readTree() match
+                  case Typed(sel, tpt) if tpt.tpe.hasAnnotation(defn.SilentSubMatchAnnot) => SubMatch(sel, readCases(end))
+                  case sel => Match(sel, readCases(end))
             case RETURN =>
               val from = readSymRef()
               val expr = ifBefore(end)(readTree(), EmptyTree)

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

@@ -3171,6 +3171,7 @@ object Parsers {
           val t = inSepRegion(InCase)(postfixExpr(Location.InGuard))
           t.asSubMatch
         case other =>
+          // the guard is reinterpreted as a sub-match when there is no leading IF or ARROW token
           val t = grd1.asSubMatch
           grd1 = EmptyTree
           t

docs/_docs/reference/experimental/sub-cases.md

@@ -36,11 +36,6 @@ e.g., `case Some(x) => x.version match ...`.
 If none of the sub-cases succeed, then control flow returns to the outer match expression and proceeds as though the current case had not matched.
 For example, `Some(Document("...", Version.Stable(2, 1)))` matches the first pattern, but none of its sub-cases, and we therefore obtain the result `"unsupported"`. 
 
-More generally, sub-matches also allow:
-- Arbitrary nesting, e.g. sub-sub-matches are supported.
-- Interleaved boolean guards, e.g. `case Some(x: Int) if x != 0 if x match ...`.
-- Interleaving pattern extractors and computations for the scrutinees of sub-matches.
-
 
 ## Motivation
 
@@ -62,11 +57,21 @@ def version(d: Option[Document]) = d match
   case _ => "unsupported"
 ```
 
+## Details
 
+Sub-cases allow:
+- Arbitrary nesting, e.g. sub-sub-matches are supported.
+- Interleaving boolean guards, e.g. `case Some(x: Int) if x != 0 if x match ...`.
+- Interleaving pattern extractors and computations for the scrutinees of sub-matches.
 
+Sub-cases are supported for:
+- match clauses
+- catch clauses
+- partial functions
 
+Similarly to catch clauses, match expressions with a single case can now be written on single line (without braces),
+e.g., `Some(1) match case Some(x) => x`.
 
+Exhaustivity and reachability checking conservatively assume the sub-cases to be partial, similarly boolean guards.
 
-
-
-
+A sub-match is inlined iff the outer match is inlined, with the same semantics as the usual match expressions.

library/src/scala/annotation/internal/$subMatch.scala

@@ -0,0 +1,7 @@
+package scala.annotation.internal
+
+import scala.annotation.experimental
+
+/** An internal annotation on (part of) that serves as a marker for sub matches. */
+@experimental
+class $subMatch extends annotation.StaticAnnotation

project/Build.scala

@@ -1201,6 +1201,7 @@ object Build {
         file(s"${baseDirectory.value}/src/scala/annotation/internal/SourceFile.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/reachCapability.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/$$into.scala"),
+        file(s"${baseDirectory.value}/src/scala/annotation/internal/$$subMatch.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/TASTYLongSignature.java"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/readOnlyCapability.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/unshared.scala"),
@@ -1339,6 +1340,7 @@ object Build {
         file(s"${baseDirectory.value}/src/scala/annotation/internal/SourceFile.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/reachCapability.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/$$into.scala"),
+        file(s"${baseDirectory.value}/src/scala/annotation/internal/$$subMatch.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/TASTYLongSignature.java"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/readOnlyCapability.scala"),
         file(s"${baseDirectory.value}/src/scala/annotation/internal/unshared.scala"),