Issue an error if the user has nested foreach statements.

Partially addresses issue #280.  (We should support them properly,
but at least now we don't silently generate incorrect code.)

ctx.cpp

@@ -584,6 +584,18 @@ FunctionEmitContext::EndLoop() {
 
 void
 FunctionEmitContext::StartForeach() {
+    // Issue an error if we're in a nested foreach...
+    for (int i = 0; i < (int)controlFlowInfo.size(); ++i) {
+        if (controlFlowInfo[i]->type == CFInfo::Foreach) {
+            Error(currentPos, "Nested \"foreach\" statements are currently illegal.");
+            break;
+            // Don't return here, however, and in turn allow the caller to
+            // do the rest of its codegen and then call EndForeach()
+            // normally--the idea being that this gives a chance to find
+            // any other errors inside the body of the foreach loop...
+        }
+    }
+
     // Store the current values of various loop-related state so that we
     // can restore it when we exit this loop.
     llvm::Value *oldMask = GetInternalMask();

tests_errors/nested-foreach.ispc

@@ -0,0 +1,11 @@
+// Nested "foreach" statements are currently illegal
+
+void func();
+
+void foo(uniform int a, uniform int b) {
+    foreach (i = 0 ... a) {
+        foreach (j = 0 ... b) {
+            func();
+        }
+    }
+}