diff --git a/ethereum-types/Cargo.toml b/ethereum-types/Cargo.toml
index f900f6e..1c52f96 100644
--- a/ethereum-types/Cargo.toml
+++ b/ethereum-types/Cargo.toml
@@ -3,6 +3,9 @@ name = "ethereum-types"
 version = "0.1.0"
 authors = ["debris <marek.kotewicz@gmail.com>"]
 
+[build-dependencies]
+rustc_version = "0.2"
+
 [dependencies]
 uint = { path = "../uint", version = "0.1" }
 fixed-hash = { path = "../fixed-hash", version = "0.1" }
diff --git a/ethereum-types/build.rs b/ethereum-types/build.rs
new file mode 100644
index 0000000..432acc3
--- /dev/null
+++ b/ethereum-types/build.rs
@@ -0,0 +1,17 @@
+// Copyright 2015-2017 Parity Technologies
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+extern crate rustc_version;
+
+use rustc_version::{version_meta, Channel};
+
+fn main() {
+	if let Channel::Nightly = version_meta().unwrap().channel {
+		println!("cargo:rustc-cfg=asm_available");
+	}
+}
diff --git a/ethereum-types/src/uint.rs b/ethereum-types/src/uint.rs
index 6b12c8a..f1595ad 100644
--- a/ethereum-types/src/uint.rs
+++ b/ethereum-types/src/uint.rs
@@ -7,6 +7,7 @@ impl U256 {
 	/// No overflow possible
 	#[cfg(all(asm_available, target_arch="x86_64"))]
 	pub fn full_mul(self, other: U256) -> U512 {
+		#![feature(asm)]
 		let self_t: &[u64; 4] = &self.0;
 		let other_t: &[u64; 4] = &other.0;
 		let mut result: [u64; 8] = unsafe { ::core::mem::uninitialized() };
diff --git a/uint/src/lib.rs b/uint/src/lib.rs
index 4a3dc8a..01d41a7 100644
--- a/uint/src/lib.rs
+++ b/uint/src/lib.rs
@@ -8,8 +8,6 @@
 
 //! Efficient large, fixed-size big integers and hashes.
 
-#![cfg_attr(asm_available, feature(asm))]
-
 #[doc(hidden)]
 pub extern crate byteorder;
 
diff --git a/uint/src/uint.rs b/uint/src/uint.rs
index 39e7aca..11da94e 100644
--- a/uint/src/uint.rs
+++ b/uint/src/uint.rs
@@ -77,6 +77,7 @@ macro_rules! uint_overflowing_add_reg {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! uint_overflowing_add {
+	#![feature(asm)]
 	(U256, $n_words:tt, $self_expr: expr, $other: expr) => ({
 		let mut result: [u64; $n_words] = unsafe { ::core::mem::uninitialized() };
 		let self_t: &[u64; $n_words] = &$self_expr.0;
@@ -223,6 +224,7 @@ macro_rules! uint_overflowing_sub_reg {
 #[macro_export]
 #[doc(hidden)]
 macro_rules! uint_overflowing_sub {
+	#![feature(asm)]
 	(U256, $n_words:tt, $self_expr: expr, $other: expr) => ({
 		let mut result: [u64; $n_words] = unsafe { ::core::mem::uninitialized() };
 		let self_t: &[u64; $n_words] = &$self_expr.0;
@@ -277,7 +279,7 @@ macro_rules! uint_overflowing_sub {
 			  "={al}"(overflow) /* $0 - $4 */
 
 			: "{rdi}"(&result[4] as *const u64) /* $5 */
-		 	 "{rsi}"(&self_t[4] as *const u64) /* $6 */
+			 "{rsi}"(&self_t[4] as *const u64) /* $6 */
 			  "0"(self_t[0]), "1"(self_t[1]), "2"(self_t[2]), "3"(self_t[3]),
 			  "m"(self_t[4]), "m"(self_t[5]), "m"(self_t[6]), "m"(self_t[7]),
 			  /* $7 - $14 */
@@ -298,6 +300,7 @@ macro_rules! uint_overflowing_sub {
 #[cfg(all(asm_available, target_arch="x86_64"))]
 #[macro_export]
 macro_rules! uint_overflowing_mul {
+	#![feature(asm)]
 	(U256, $n_words: expr, $self_expr: expr, $other: expr) => ({
 		let mut result: [u64; $n_words] = unsafe { ::core::mem::uninitialized() };
 		let self_t: &[u64; $n_words] = &$self_expr.0;