[ruby] Fixed Pseudo-Variables as Literals (#4840)

Although the parser rule for picking up pseudo-variables seems fine, it may sometimes choose to represent it as a local variable. This fixes this and adds the related tests.

joern-cli/frontends/rubysrc2cpg/src/main/scala/io/joern/rubysrc2cpg/parser/RubyNodeCreator.scala

@@ -640,9 +640,7 @@ class RubyNodeCreator extends RubyParserBaseVisitor[RubyNode] {
       case Defines.Proc | Defines.Lambda => ProcOrLambdaExpr(visit(ctx.block()).asInstanceOf[Block])(ctx.toTextSpan)
       case Defines.Loop =>
         DoWhileExpression(
-          SimpleIdentifier(Option(Defines.getBuiltInType(Defines.TrueClass)))(
-            ctx.methodIdentifier().toTextSpan.spanStart("true")
-          ),
+          StaticLiteral(Defines.getBuiltInType(Defines.TrueClass))(ctx.methodIdentifier().toTextSpan.spanStart("true")),
           ctx.block() match {
             case b: RubyParser.DoBlockBlockContext =>
               visit(b.doBlock().bodyStatement())
@@ -714,7 +712,13 @@ class RubyNodeCreator extends RubyParserBaseVisitor[RubyNode] {
   }
 
   override def visitLocalIdentifierVariable(ctx: RubyParser.LocalIdentifierVariableContext): RubyNode = {
-    SimpleIdentifier()(ctx.toTextSpan)
+    // Sometimes pseudo variables aren't given precedence in the parser, so we double-check here
+    ctx.getText match {
+      case "nil"   => StaticLiteral(getBuiltInType(Defines.NilClass))(ctx.toTextSpan)
+      case "true"  => StaticLiteral(getBuiltInType(Defines.TrueClass))(ctx.toTextSpan)
+      case "false" => StaticLiteral(getBuiltInType(Defines.FalseClass))(ctx.toTextSpan)
+      case _       => SimpleIdentifier()(ctx.toTextSpan)
+    }
   }
 
   override def visitClassName(ctx: RubyParser.ClassNameContext): RubyNode = {

joern-cli/frontends/rubysrc2cpg/src/test/scala/io/joern/rubysrc2cpg/querying/CallTests.scala

@@ -290,4 +290,13 @@ class CallTests extends RubyCode2CpgFixture(withPostProcessing = true) {
     augeasReceiv.argument(2).asInstanceOf[FieldIdentifier].canonicalName shouldBe "open"
   }
 
+  "`nil` keyword as a member access should be a literal" in {
+    val cpg    = code("nil.to_json")
+    val toJson = cpg.fieldAccess.codeExact("nil.to_json").head
+    val nilRec = toJson.argument(1).asInstanceOf[Literal]
+
+    nilRec.code shouldBe "nil"
+    nilRec.lineNumber shouldBe Option(1)
+  }
+
 }

joern-cli/frontends/rubysrc2cpg/src/test/scala/io/joern/rubysrc2cpg/querying/ControlStructureTests.scala

@@ -93,6 +93,9 @@ class ControlStructureTests extends RubyCode2CpgFixture {
     val List(breakNode) = cpg.break.l
     breakNode.code shouldBe "break"
     breakNode.lineNumber shouldBe Some(8)
+
+    // `loop` is lowered as a do-while loop with a true condition
+    cpg.controlStructure.condition("true").size shouldBe 1
   }
 
   "`if-end` statement is represented by an `IF` CONTROL_STRUCTURE node" in {