Update the TastyFormat for matches with sub-cases (#23851)

Author: EugeneFlesselle

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

@@ -590,7 +590,7 @@ 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 { writeByte(SUBMATCH); pickleTree(selector) }
             else pickleTree(selector)
             tree.cases.foreach(pickleTree)
           }

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

@@ -1551,7 +1551,7 @@ class TreeUnpickler(reader: TastyReader,
                   readByte()
                   InlineMatch(readTree(), readCases(end))
                 }
-                else if nextByte == LAZY then // similarly to InlineMatch we use an arbitrary Cat.1 tag
+                else if nextByte == SUBMATCH then
                   readByte()
                   SubMatch(readTree(), readCases(end))
                 else Match(readTree(), readCases(end)))

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.

tasty/src/dotty/tools/tasty/TastyFormat.scala

@@ -104,7 +104,7 @@ Standard-Section: "ASTs" TopLevelStat*
                   INLINED        Length expr_Term call_Term? ValOrDefDef*          -- Inlined code from call, with given body `expr` and given bindings
                   LAMBDA         Length meth_Term target_Type?                     -- Closure over method `f` of type `target` (omitted id `target` is a function type)
                   IF             Length [INLINE] cond_Term then_Term else_Term     -- inline? if cond then thenPart else elsePart
-                  MATCH          Length (IMPLICIT | [INLINE] sel_Term) CaseDef*    -- (inline? sel | implicit) match caseDefs
+                  MATCH          Length (IMPLICIT | [INLINE | SUBMATCH] sel_Term) CaseDef* -- ((inline | if)? sel | implicit) match caseDefs
                   TRY            Length expr_Term CaseDef* finalizer_Term?         -- try expr catch {casdeDef} (finally finalizer)?
                   RETURN         Length meth_ASTRef expr_Term?                     -- return expr?,  `methASTRef` is method from which is returned
                   WHILE          Length cond_Term body_Term                        -- while cond do body
@@ -511,6 +511,7 @@ object TastyFormat {
   final val EMPTYCLAUSE = 45
   final val SPLITCLAUSE = 46
   final val TRACKED = 47
+  final val SUBMATCH = 48 // experimental.subCases
 
   // Tree Cat. 2:    tag Nat
   final val firstNatTreeTag = SHAREDterm