Teach len() howto rewrite itself

-15% for fasta.py

src/runtime/builtin_modules/builtins.cpp

@@ -1048,8 +1048,12 @@ void setupBuiltins() {
 
     repr_obj = new BoxedBuiltinFunctionOrMethod(boxRTFunction((void*)repr, UNKNOWN, 1));
     builtins_module->giveAttr("repr", repr_obj);
-    len_obj = new BoxedBuiltinFunctionOrMethod(boxRTFunction((void*)len, UNKNOWN, 1));
+
+    auto len_func = boxRTFunction((void*)len, UNKNOWN, 1);
+    len_func->internal_callable = lenCallInternal;
+    len_obj = new BoxedBuiltinFunctionOrMethod(len_func);
     builtins_module->giveAttr("len", len_obj);
+
     hash_obj = new BoxedBuiltinFunctionOrMethod(boxRTFunction((void*)hash, UNKNOWN, 1));
     builtins_module->giveAttr("hash", hash_obj);
     abs_obj = new BoxedBuiltinFunctionOrMethod(boxRTFunction((void*)abs_, UNKNOWN, 1));

src/runtime/objmodel.cpp

@@ -1900,6 +1900,25 @@ extern "C" BoxedInt* lenInternal(Box* obj, LenRewriteArgs* rewrite_args) {
     return static_cast<BoxedInt*>(rtn);
 }
 
+Box* lenCallInternal(BoxedFunctionBase* func, CallRewriteArgs* rewrite_args, ArgPassSpec argspec, Box* arg1, Box* arg2,
+                     Box* arg3, Box** args, const std::vector<const std::string*>* keyword_names) {
+    if (argspec != ArgPassSpec(1))
+        return callFunc(func, rewrite_args, argspec, arg1, arg2, arg3, args, keyword_names);
+
+    if (rewrite_args) {
+        LenRewriteArgs lrewrite_args(rewrite_args->rewriter, rewrite_args->arg1, rewrite_args->destination);
+        Box* rtn = lenInternal(arg1, &lrewrite_args);
+        if (!lrewrite_args.out_success) {
+            rewrite_args = 0;
+        } else {
+            rewrite_args->out_rtn = lrewrite_args.out_rtn;
+            rewrite_args->out_success = true;
+        }
+        return rtn;
+    }
+    return lenInternal(arg1, NULL);
+}
+
 extern "C" BoxedInt* len(Box* obj) {
     static StatCounter slowpath_len("slowpath_len");
     slowpath_len.log();

src/runtime/objmodel.h

@@ -100,6 +100,8 @@ struct BinopRewriteArgs;
 extern "C" Box* binopInternal(Box* lhs, Box* rhs, int op_type, bool inplace, BinopRewriteArgs* rewrite_args);
 
 struct CallRewriteArgs;
+Box* lenCallInternal(BoxedFunctionBase* f, CallRewriteArgs* rewrite_args, ArgPassSpec argspec, Box* arg1, Box* arg2,
+                     Box* arg3, Box** args, const std::vector<const std::string*>* keyword_names);
 Box* typeCallInternal(BoxedFunctionBase* f, CallRewriteArgs* rewrite_args, ArgPassSpec argspec, Box* arg1, Box* arg2,
                       Box* arg3, Box** args, const std::vector<const std::string*>* keyword_names);