+lib/once: a little library to make sure stuff only gets called once

lib/once/Cargo.lock

@@ -0,0 +1,4 @@
+[root]
+name = "once"
+version = "0.1.0"
+

lib/once/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "once"
+version = "0.1.0"
+authors = ["Erin <erin@hashbang.sh>"]
+
+[dependencies]

lib/once/src/lib.rs

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+#![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();
+}