Suppressing repetitive warnings in the global initialization checker (#19898)

This PR suppresses repetitive warnings in the global initialization
checker to make the warnings more manageable to analyze.

Author: olhotak

Diff for: compiler/src/dotty/tools/dotc/transform/init/Checker.scala

@@ -50,7 +50,8 @@ class Checker extends Phase:
         Semantic.checkClasses(classes)(using checkCtx)
 
       if ctx.settings.YcheckInitGlobal.value then
-        Objects.checkClasses(classes)(using checkCtx)
+        val obj = new Objects
+        obj.checkClasses(classes)(using checkCtx)
     }
 
     units0

Diff for: compiler/src/dotty/tools/dotc/transform/init/Objects.scala

@@ -66,7 +66,7 @@ import dotty.tools.dotc.core.Flags.AbstractOrTrait
  *     whole-program analysis. However, the check is not modular in terms of project boundaries.
  *
  */
-object Objects:
+class Objects:
 
   // ----------------------------- abstract domain -----------------------------
 
@@ -1757,16 +1757,20 @@ object Objects:
       if cls.isAllOf(Flags.JavaInterface) then Bottom
       else evalType(tref.prefix, thisV, klass, elideObjectAccess = cls.isStatic)
 
+  val mutateErrorSet: mutable.Set[(ClassSymbol, ClassSymbol)] = mutable.Set.empty
   def errorMutateOtherStaticObject(currentObj: ClassSymbol, otherObj: ClassSymbol)(using Trace, Context) =
-    val msg =
-      s"Mutating ${otherObj.show} during initialization of ${currentObj.show}.\n" +
-      "Mutating other static objects during the initialization of one static object is forbidden. " + Trace.show
+    if mutateErrorSet.add((currentObj, otherObj)) then
+      val msg =
+        s"Mutating ${otherObj.show} during initialization of ${currentObj.show}.\n" +
+        "Mutating other static objects during the initialization of one static object is forbidden. " + Trace.show
 
-    report.warning(msg, Trace.position)
+      report.warning(msg, Trace.position)
 
+  val readErrorSet: mutable.Set[(ClassSymbol, ClassSymbol)] = mutable.Set.empty
   def errorReadOtherStaticObject(currentObj: ClassSymbol, otherObj: ClassSymbol)(using Trace, Context) =
-    val msg =
-      "Reading mutable state of " + otherObj.show + " during initialization of " + currentObj.show + ".\n" +
-      "Reading mutable state of other static objects is forbidden as it breaks initialization-time irrelevance. " + Trace.show
+    if readErrorSet.add((currentObj, otherObj)) then
+      val msg =
+        "Reading mutable state of " + otherObj.show + " during initialization of " + currentObj.show + ".\n" +
+        "Reading mutable state of other static objects is forbidden as it breaks initialization-time irrelevance. " + Trace.show
 
-    report.warning(msg, Trace.position)
+      report.warning(msg, Trace.position)