REPL: make truncation behaviour configurable

via `-Xrepl-max-print-elements`.

The default behaviour is unchanged:
```
scala> 1.to(300).toList
val res0: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 22 ... large output truncated, print value to show all
```

The user can now configure a higher threshold before truncating kicks in:
```
scala -Xrepl-max-print-elements:2000

scala> 1.to(300).toList
// prints the entire list
```

Author: mpollmeier

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -221,6 +221,7 @@ private sealed trait XSettings:
   val XprintSuspension: Setting[Boolean] = BooleanSetting("-Xprint-suspension", "Show when code is suspended until macros are compiled.")
   val Xprompt: Setting[Boolean] = BooleanSetting("-Xprompt", "Display a prompt after each error (debugging option).")
   val XreplDisableDisplay: Setting[Boolean] = BooleanSetting("-Xrepl-disable-display", "Do not display definitions in REPL.")
+  val XreplMaxPrintElements: Setting[Int] = IntSetting("-Xrepl-max-print-elements", "Number of elements to be printed before output is truncated.", 1000)
   val XverifySignatures: Setting[Boolean] = BooleanSetting("-Xverify-signatures", "Verify generic signatures in generated bytecode.")
   val XignoreScala2Macros: Setting[Boolean] = BooleanSetting("-Xignore-scala2-macros", "Ignore errors when compiling code that calls Scala2 macros, these will fail at runtime.")
   val XimportSuggestionTimeout: Setting[Int] = IntSetting("-Ximport-suggestion-timeout", "Timeout (in ms) for searching for import suggestions when errors are reported.", 8000)

compiler/src/dotty/tools/repl/Rendering.scala

@@ -23,17 +23,15 @@ import dotc.reporting.Diagnostic
  *       `ReplDriver#resetToInitial` is called, the accompanying instance of
  *       `Rendering` is no longer valid.
  */
-private[repl] class Rendering(parentClassLoader: Option[ClassLoader] = None) {
+private[repl] class Rendering(parentClassLoader: Option[ClassLoader] = None,
+                              maxPrintElements: Int = 1000) {
 
   import Rendering._
 
-  private val MaxStringElements: Int = 1000  // no need to mkString billions of elements
-
   private var myClassLoader: AbstractFileClassLoader = _
 
   private var myReplStringOf: Object => String = _
 
-
   /** Class loader used to load compiled code */
   private[repl] def classLoader()(using Context) =
     if (myClassLoader != null && myClassLoader.root == ctx.settings.outputDir.value) myClassLoader
@@ -63,12 +61,12 @@ private[repl] class Rendering(parentClassLoader: Option[ClassLoader] = None) {
           val meth = scalaRuntime.getMethod(renderer, classOf[Object], classOf[Int], classOf[Boolean])
           val truly = java.lang.Boolean.TRUE
 
-          (value: Object) => meth.invoke(null, value, Integer.valueOf(MaxStringElements), truly).asInstanceOf[String]
+          (value: Object) => meth.invoke(null, value, Integer.valueOf(maxPrintElements), truly).asInstanceOf[String]
         } catch {
           case _: NoSuchMethodException =>
             val meth = scalaRuntime.getMethod(renderer, classOf[Object], classOf[Int])
 
-            (value: Object) => meth.invoke(null, value, Integer.valueOf(MaxStringElements)).asInstanceOf[String]
+            (value: Object) => meth.invoke(null, value, Integer.valueOf(maxPrintElements)).asInstanceOf[String]
         }
       }
       myClassLoader
@@ -83,8 +81,8 @@ private[repl] class Rendering(parentClassLoader: Option[ClassLoader] = None) {
   private[repl] def truncate(str: String): String = {
     val showTruncated = " ... large output truncated, print value to show all"
     val ncp = str.codePointCount(0, str.length) // to not cut inside code point
-    if ncp <= MaxStringElements then str
-    else str.substring(0, str.offsetByCodePoints(0, MaxStringElements - 1)) + showTruncated
+    if ncp <= maxPrintElements then str
+    else str.substring(0, str.offsetByCodePoints(0, maxPrintElements - 1)) + showTruncated
   }
 
   /** Return a String representation of a value we got from `classLoader()`. */

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -111,8 +111,12 @@ class ReplDriver(settings: Array[String],
     if (rootCtx.settings.outputDir.isDefault(using rootCtx))
       rootCtx = rootCtx.fresh
         .setSetting(rootCtx.settings.outputDir, new VirtualDirectory("<REPL compilation output>"))
+      
     compiler = new ReplCompiler
-    rendering = new Rendering(classLoader)
+    rendering = new Rendering(
+      classLoader, 
+      maxPrintElements = rootCtx.settings.XreplMaxPrintElements.valueIn(rootCtx.settingsState)
+    )
   }
 
   private var rootCtx: Context = _