Respect cacheResult flag in safe initialization cache. (#16905)

The safe initialization checker only needs to cache entries on certain
`cachedEval` calls, specified by a `cacheResult` flag. This PR reduces
the size of the safe initialization cache during compilation with
`-Ysafe-init` by respecting the value of the flag, and only caching when
it is true.

Author: olhotak

compiler/src/dotty/tools/dotc/transform/init/Cache.scala

@@ -93,41 +93,46 @@ class Cache[Config, Res]:
    *
    *  The algorithmic skeleton is as follows:
    *
+   *      if don't cache result then
+   *        return eval(expr)
    *      if this.current.contains(config, expr) then
    *        return cached value
    *      else
    *        val assumed = this.last(config, expr) or bottom value if absent
    *        this.current(config, expr) = assumed
-   *        val actual = eval(exp)
+   *        val actual = eval(expr)
    *
    *        if assumed != actual then
    *          this.changed = true
    *          this.current(config, expr) = actual
    *
    */
   def cachedEval(config: Config, expr: Tree, cacheResult: Boolean, default: Res)(eval: Tree => Res): Res =
-    this.get(config, expr) match
-    case Some(value) => value
-    case None =>
-      val assumeValue: Res =
-        this.last.get(config, expr) match
-        case Some(value) => value
-        case None =>
-          this.last = this.last.updatedNested(config, expr, default)
-          default
-
-      this.current = this.current.updatedNested(config, expr, assumeValue)
-
-      val actual = eval(expr)
-      if actual != assumeValue then
-        // println("Changed! from = " + assumeValue + ", to = " + actual)
-        this.changed = true
-        // TODO: respect cacheResult to reduce cache size
-        this.current = this.current.updatedNested(config, expr, actual)
-        // this.current = this.current.removed(config, expr)
-      end if
-
-      actual
+    if !cacheResult then
+      eval(expr)
+    else
+      this.get(config, expr) match
+      case Some(value) => value
+      case None =>
+        val assumeValue: Res =
+          this.last.get(config, expr) match
+          case Some(value) => value
+          case None =>
+            this.last = this.last.updatedNested(config, expr, default)
+            default
+
+        this.current = this.current.updatedNested(config, expr, assumeValue)
+
+        val actual = eval(expr)
+        if actual != assumeValue then
+          // println("Changed! from = " + assumeValue + ", to = " + actual)
+          this.changed = true
+          this.current = this.current.updatedNested(config, expr, actual)
+          // this.current = this.current.removed(config, expr)
+        end if
+
+        actual
+    end if
   end cachedEval
 
   def hasChanged = changed

compiler/src/dotty/tools/dotc/transform/init/Semantic.scala

@@ -893,7 +893,7 @@ object Semantic:
 
               case Cold => Cold
 
-              case ref: Ref => eval(vdef.rhs, ref, enclosingClass)
+              case ref: Ref => eval(vdef.rhs, ref, enclosingClass, cacheResult = sym.is(Flags.Lazy))
 
               case _ =>
                  report.error("[Internal error] unexpected this value when accessing local variable, sym = " + sym.show + ", thisValue = " + thisValue2.show + Trace.show, Trace.position)
@@ -989,7 +989,7 @@ object Semantic:
             val errors = Reporter.stopEarly {
               val res = {
                 given Trace = Trace.empty
-                eval(body, thisV, klass)
+                eval(body, thisV, klass, cacheResult = true)
               }
               given Trace = Trace.empty.add(body)
               res.promote("The function return value is not hot. Found = " + res.show + ".")

tests/init/neg/apply2.scala

@@ -3,8 +3,8 @@ object O:
     println(n)
 
   class B:
-    val a = A(this)
+    val a = A(this)  // error
 
   val b = new B
-  val n = 10  // error
+  val n = 10
 end O

tests/init/pos/self-ref.scala

@@ -0,0 +1,9 @@
+class A {
+  def foo(a: Int) = {
+    lazy val x: Int = if (a == 0) x else 0
+    println(x)
+  }
+  foo(0)
+
+  val y = 5
+}