Extend caching in the space engine beyond local context

Previously isSubspace calls would be cached only as part of individual
Space object, so when recursing through their ADT contents, we were not
able to reuse the previously computed information. Now the cache is
shared across the whole space engine run.

Author: jchyb

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -48,6 +48,9 @@ import SpaceEngine.*
  *
  */
 
+/** A key to be used in a context property that caches the unpickled trees */
+private val IsSubspaceCacheKey = new Property.Key[mutable.HashMap[(Space, Space), Boolean]]
+
 /** space definition */
 sealed trait Space extends Showable:
 
@@ -60,7 +63,8 @@ sealed trait Space extends Showable:
     if (a ne a2) || (b ne b2) then a2.isSubspace(b2)
     else if a == Empty then true
     else if b == Empty then false
-    else isSubspaceCache.getOrElseUpdate(b, computeIsSubspace(a, b))
+    else
+      ctx.property(IsSubspaceCacheKey).get.getOrElseUpdate((a, b), computeIsSubspace(a, b))
 
   @sharable private var mySimplified: Space | Null = null
 
@@ -968,6 +972,8 @@ object SpaceEngine {
   end checkReachability
 
   def checkMatch(m: Match)(using Context): Unit =
-    if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
-    if reachabilityCheckable(m.selector) then checkReachability(m)
+    inContext(ctx.withProperty(IsSubspaceCacheKey, Some(mutable.HashMap.empty))) {
+      if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
+      if reachabilityCheckable(m.selector) then checkReachability(m)
+    }
 }

tests/pos/i23317.scala

@@ -0,0 +1,26 @@
+import scala.quoted.*
+
+def goImpl(using Quotes): Expr[Int] =
+  List.empty[Type[?]] match
+    case Nil =>
+      Expr(0)
+    case '[t1] :: Nil =>
+      Expr(1)
+    case '[t1] :: '[t2] :: Nil =>
+      Expr(2)
+    case '[t1] :: '[t2] :: '[t3] :: Nil =>
+      Expr(3)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: Nil =>
+      Expr(4)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: Nil =>
+      Expr(5)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: Nil =>
+      Expr(6)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: '[t7] :: Nil =>
+      Expr(7)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: '[t7] :: '[t8] :: Nil =>
+      Expr(8)
+    case '[t1] :: '[t2] :: '[t3] :: '[t4] :: '[t5] :: '[t6] :: '[t7] :: '[t8] :: '[t9] :: Nil =>
+      Expr(9)
+    case _ =>
+      Expr(999)