throw better error messages for invalid LHS in replacement

com.oracle.truffle.r.nodes/src/com/oracle/truffle/r/nodes/RASTBuilder.java

@@ -58,6 +58,7 @@ import com.oracle.truffle.r.runtime.RInternalError;
 import com.oracle.truffle.r.runtime.RRuntime;
 import com.oracle.truffle.r.runtime.builtins.FastPathFactory;
 import com.oracle.truffle.r.runtime.data.REmpty;
+import com.oracle.truffle.r.runtime.data.RNull;
 import com.oracle.truffle.r.runtime.env.frame.FrameSlotChangeMonitor;
 import com.oracle.truffle.r.runtime.nodes.EvaluatedArgumentsVisitor;
 import com.oracle.truffle.r.runtime.nodes.RCodeBuilder;
@@ -287,15 +288,18 @@ public final class RASTBuilder implements RCodeBuilder<RSyntaxNode> {
         RSyntaxElement current = lhs;
         while (!(current instanceof RSyntaxLookup)) {
             if (!(current instanceof RSyntaxCall)) {
-                throw RError.error(RError.NO_CALLER, RError.Message.NON_LANG_ASSIGNMENT_TARGET);
+                if (current instanceof RSyntaxConstant && ((RSyntaxConstant) current).getValue() == RNull.instance) {
+                    throw RError.error(RError.NO_CALLER, RError.Message.INVALID_LHS, "NULL");
+                } else {
+                    throw RError.error(RError.NO_CALLER, RError.Message.NON_LANG_ASSIGNMENT_TARGET);
+                }
             }
             RSyntaxCall call = (RSyntaxCall) current;
             calls.add(call);
 
             RSyntaxElement syntaxLHS = call.getSyntaxLHS();
             if (call.getSyntaxArguments().length == 0 || !(syntaxLHS instanceof RSyntaxLookup || isNamespaceLookupCall(syntaxLHS))) {
-                // TODO: this should only be signaled when run, not when parsed
-                throw RInternalError.unimplemented("proper error message for RError.INVALID_LHS");
+                return new ReplacementNode.LHSError(source, operator, new RSyntaxElement[]{lhs, rhs});
             }
             current = call.getSyntaxArguments()[0];
         }

com.oracle.truffle.r.nodes/src/com/oracle/truffle/r/nodes/control/ReplacementNode.java

@@ -24,6 +24,7 @@ package com.oracle.truffle.r.nodes.control;
 
 import java.util.List;
 
+import com.oracle.truffle.api.CompilerDirectives;
 import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.nodes.ExplodeLoop;
 import com.oracle.truffle.api.source.SourceSection;
@@ -31,6 +32,8 @@ import com.oracle.truffle.r.nodes.access.RemoveAndAnswerNode;
 import com.oracle.truffle.r.nodes.access.WriteVariableNode;
 import com.oracle.truffle.r.nodes.function.visibility.SetVisibilityNode;
 import com.oracle.truffle.r.runtime.ArgumentsSignature;
+import com.oracle.truffle.r.runtime.RError;
+import com.oracle.truffle.r.runtime.RError.Message;
 import com.oracle.truffle.r.runtime.nodes.RNode;
 import com.oracle.truffle.r.runtime.nodes.RSourceSectionNode;
 import com.oracle.truffle.r.runtime.nodes.RSyntaxCall;
@@ -119,4 +122,41 @@ public final class ReplacementNode extends RSourceSectionNode implements RSyntax
     public ArgumentsSignature getSyntaxSignature() {
         return ArgumentsSignature.empty(2);
     }
+
+    /**
+     * Used by the parser for assignments that miss a left hand side. This node will raise an error
+     * once executed.
+     */
+    public static final class LHSError extends RSourceSectionNode implements RSyntaxNode, RSyntaxCall {
+
+        private final String operator;
+        private final RSyntaxElement[] arguments;
+
+        public LHSError(SourceSection sourceSection, String operator, RSyntaxElement[] arguments) {
+            super(sourceSection);
+            this.operator = operator;
+            this.arguments = arguments;
+        }
+
+        @Override
+        public Object execute(VirtualFrame frame) {
+            CompilerDirectives.transferToInterpreter();
+            throw RError.error(this, Message.INVALID_LHS, "NULL");
+        }
+
+        @Override
+        public RSyntaxElement getSyntaxLHS() {
+            return RSyntaxLookup.createDummyLookup(null, operator, true);
+        }
+
+        @Override
+        public RSyntaxElement[] getSyntaxArguments() {
+            return arguments;
+        }
+
+        @Override
+        public ArgumentsSignature getSyntaxSignature() {
+            return ArgumentsSignature.empty(2);
+        }
+    }
 }