Avoid nulls in Array.items (#5736)

Put `Nothing` into an empty array rather than `null`. When running with `assert` on (unit tests), check the content of the array and `AssertError` quickly when a `null` is found.

engine/runtime/src/main/java/org/enso/interpreter/node/expression/builtin/runtime/GetStackTraceNode.java

@@ -21,7 +21,7 @@ public class GetStackTraceNode extends Node {
   public static Array stackTraceToArray(Throwable exception) {
     var elements = TruffleStackTrace.getStackTrace(exception);
     if (elements == null) return new Array();
-    var ret = new Array(elements.size());
+    var ret = Array.allocate(elements.size());
     for (int i = 0; i < elements.size(); i++) {
       var element = elements.get(i);
       ret.getItems()[i] = element.getGuestObject();

engine/runtime/src/main/java/org/enso/interpreter/runtime/callable/atom/Atom.java

@@ -133,7 +133,7 @@ public abstract class Atom implements TruffleObject {
   public Array getMembers(boolean includeInternal) {
     Map<String, Function> members = constructor.getDefinitionScope().getMethods().get(constructor.getType());
     if (members == null) {
-      return new Array(0);
+      return Array.allocate(0);
     }
     Object[] mems = members.keySet().toArray();
     return new Array(mems);

engine/runtime/src/main/java/org/enso/interpreter/runtime/data/Array.java

@@ -39,6 +39,7 @@ public final class Array implements TruffleObject {
       description = "Creates an array with given elements.",
       autoRegister = false)
   public Array(Object... items) {
+    assert noNulls(items);
     this.items = items;
   }
 
@@ -49,9 +50,25 @@ public final class Array implements TruffleObject {
    */
   @Builtin.Method(
       description = "Creates an uninitialized array of a given size.",
-      autoRegister = false)
-  public Array(long size) {
-    this.items = new Object[(int) size];
+      autoRegister = false,
+      name = "new")
+  public static Array allocate(long size) {
+    var arr = new Object[(int) size];
+    var ctx = EnsoContext.get(null);
+    var nothing = ctx.getBuiltins().nothing();
+    for (int i = 0; i < arr.length; i++) {
+      arr[i] = nothing;
+    }
+    return new Array(arr);
+  }
+
+  private static final boolean noNulls(Object[] arr) {
+    for (int i = 0; i < arr.length; i++) {
+      if (arr[i] == null) {
+        return false;
+      }
+    }
+    return true;
   }
 
   /** @return the elements of this array as a java array. */
@@ -107,7 +124,7 @@ public final class Array implements TruffleObject {
   /** @return an empty array */
   @Builtin.Method(description = "Creates an empty Array", autoRegister = false)
   public static Object empty() {
-    return new Array();
+    return allocate(0);
   }
 
   /** @return an identity array */

engine/runtime/src/main/java/org/enso/interpreter/runtime/data/Vector.java

@@ -74,7 +74,7 @@ public final class Vector implements TruffleObject {
     long slice_end = Math.min(this_length, end);
 
     if (slice_start >= slice_end) {
-      return new Vector(new Array(0));
+      return new Vector(Array.allocate(0));
     }
 
     if ((slice_start == 0) && (slice_end == this_length)) {

engine/runtime/src/test/java/org/enso/interpreter/test/ArrayTest.java

@@ -0,0 +1,52 @@
+package org.enso.interpreter.test;
+
+import java.io.ByteArrayOutputStream;
+import java.net.URI;
+import java.nio.file.Paths;
+import java.util.Map;
+import org.enso.polyglot.RuntimeOptions;
+import org.graalvm.polyglot.Context;
+import org.graalvm.polyglot.Language;
+import org.graalvm.polyglot.Source;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import org.junit.Before;
+import org.junit.Test;
+
+public class ArrayTest {
+  private Context ctx;
+
+  @Before
+  public void prepareCtx() {
+    this.ctx = Context.newBuilder()
+      .allowExperimentalOptions(true)
+      .allowIO(true)
+      .allowAllAccess(true)
+      .logHandler(new ByteArrayOutputStream())
+      .option(
+        RuntimeOptions.LANGUAGE_HOME_OVERRIDE,
+        Paths.get("../../distribution/component").toFile().getAbsolutePath()
+      ).build();
+    final Map<String, Language> langs = ctx.getEngine().getLanguages();
+    assertNotNull("Enso found: " + langs, langs.get("enso"));
+  }
+
+  @Test
+  public void dontExposeNullsOutOfArray() throws Exception {
+    final URI facUri = new URI("memory://choose.enso");
+    final Source facSrc = Source.newBuilder("enso", """
+    from Standard.Base import all
+
+    null =
+        a = Array.new 1
+        b = a.at 0
+        b
+    """, "nulls.enso")
+            .uri(facUri)
+            .buildLiteral();
+
+    var module = ctx.eval(facSrc);
+    var res = module.invokeMember("eval_expression", "null");
+    assertTrue("i null", res.isNull());
+  }
+}