Make 'from a import b' throw ImportErrors

Before was throwing AttributeError, since we were just translating it to:
import a
b = a.b

Now there's a special importFrom runtime function that does essentially
the same thing, but throws the proper exception.

src/codegen/irgen/irgenerator.cpp

@@ -1453,7 +1453,11 @@ private:
             const std::string& name = alias->name;
             const std::string& asname = alias->asname.size() ? alias->asname : alias->name;
 
-            CompilerVariable* v = module->getattr(emitter, getEmptyOpInfo(exc_info), &name, false);
+            // TODO add patchpoints to this?
+            llvm::Value* r = emitter.createCall2(exc_info, g.funcs.importFrom, module->getValue(),
+                                                 embedConstantPtr(&name, g.llvm_str_type_ptr)).getInstruction();
+
+            CompilerVariable* v = new ConcreteCompilerVariable(UNKNOWN, r, true);
             _doSet(asname, v, exc_info);
             v->decvref(emitter);
         }

src/codegen/runtime_hooks.cpp

@@ -182,6 +182,7 @@ void initGlobalFuncs(GlobalState& g) {
     GET(getclsattr);
     GET(unaryop);
     GET(import);
+    GET(importFrom);
     GET(repr);
     GET(isinstance);
 

src/codegen/runtime_hooks.h

@@ -34,7 +34,7 @@ struct GlobalFuncs {
         *boxInstanceMethod, *boxBool, *unboxBool, *createTuple, *createDict, *createList, *createSlice,
         *createUserClass, *createClosure;
     llvm::Value* getattr, *setattr, *print, *nonzero, *binop, *compare, *augbinop, *unboxedLen, *getitem, *getclsattr,
-        *getGlobal, *setitem, *delitem, *unaryop, *import, *repr, *isinstance;
+        *getGlobal, *setitem, *delitem, *unaryop, *import, *importFrom, *repr, *isinstance;
 
     llvm::Value* checkUnpackingLength, *raiseAttributeError, *raiseAttributeErrorStr, *raiseNotIterableError,
         *assertNameDefined, *assertFail;

src/runtime/inline/link_forcer.cpp

@@ -74,6 +74,7 @@ void force() {
     FORCE(delitem);
     FORCE(unaryop);
     FORCE(import);
+    FORCE(importFrom);
     FORCE(repr);
     FORCE(isinstance);
 

src/runtime/objmodel.cpp

@@ -2959,4 +2959,16 @@ extern "C" Box* import(const std::string* name) {
 
     raiseExcHelper(ImportError, "No module named %s", name->c_str());
 }
+
+extern "C" Box* importFrom(Box* _m, const std::string* name) {
+    assert(_m->cls == module_cls);
+
+    BoxedModule* m = static_cast<BoxedModule*>(_m);
+
+    Box* r = m->getattr(*name, NULL, NULL);
+    if (r)
+        return r;
+
+    raiseExcHelper(ImportError, "cannot import name %s", name->c_str());
+}
 }

src/runtime/objmodel.h

@@ -41,6 +41,7 @@ extern "C" const std::string* getNameOfClass(BoxedClass* cls);
 
 // TODO sort this
 extern "C" void my_assert(bool b);
+extern "C" Box* importFrom(Box* obj, const std::string* attr);
 extern "C" Box* getattr(Box* obj, const char* attr);
 extern "C" void setattr(Box* obj, const char* attr, Box* attr_val);
 extern "C" bool nonzero(Box* obj);

test/tests/import_failure_test.py

-# expected: fail
-# - currently throws an AttributeError if we imported the module but the name didn't exist
-
 try:
     import non_existent_module
     assert 0, "shouldn't get here"