diff --git a/lib/once/Cargo.lock b/lib/once/Cargo.lock
new file mode 100644
index 0000000..188f91d
--- /dev/null
+++ b/lib/once/Cargo.lock
@@ -0,0 +1,4 @@
+[root]
+name = "once"
+version = "0.1.0"
+
diff --git a/lib/once/Cargo.toml b/lib/once/Cargo.toml
new file mode 100644
index 0000000..931c5d9
--- /dev/null
+++ b/lib/once/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "once"
+version = "0.1.0"
+authors = ["Erin <erin@hashbang.sh>"]
+
+[dependencies]
diff --git a/lib/once/src/lib.rs b/lib/once/src/lib.rs
new file mode 100644
index 0000000..0b81bf0
--- /dev/null
+++ b/lib/once/src/lib.rs
@@ -0,0 +1,45 @@
+#![no_std]
+
+#[cfg(test)]
+extern crate std;
+
+#[macro_export]
+macro_rules! assert_first_call {
+    () => {
+        assert_first_call!("assertion failed: function called more than once");
+    };
+
+    ($($arg:tt)+) => {{
+        use ::core::sync::atomic::{AtomicBool, ATOMIC_BOOL_INIT, Ordering};
+        static CALLED: AtomicBool = ATOMIC_BOOL_INIT;
+        let called = CALLED.swap(true, Ordering::Relaxed);
+        assert!(called == false, $($arg)+);
+    }};
+}
+
+#[test]
+fn test_run_once() {
+    fn once() {
+        assert_first_call!();
+    }
+
+    once();
+}
+
+#[test]
+fn test_run_once_two_funcs() {
+    fn once1() {assert_first_call!();}
+    fn once2() {assert_first_call!();}
+
+    once1(); once2();
+}
+
+#[test]
+#[should_panic]
+fn test_run_twice() {
+    fn once() {
+        assert_first_call!();
+    }
+
+    once(); once();
+}