Document and extend Ivar

Documents `Ivar` and adds some common trait implementations for it

Author: Aatch

src/librustc/util/ivar.rs

@@ -8,8 +8,19 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::fmt;
 use std::cell::Cell;
 
+/// A write-once variable. When constructed, it is empty, and
+/// can only be set once.
+///
+/// Ivars ensure that data that can only be initialised once. A full
+/// implementation is used for concurrency and blocks on a read of an
+/// unfulfilled value. This implementation is more minimal and panics
+/// if you attempt to read the value before it has been set. It is also
+/// not `Sync`, but may be extended in the future to be usable as a true
+/// concurrency type.
+#[derive(PartialEq)]
 pub struct Ivar<T:Copy> {
     data: Cell<Option<T>>
 }
@@ -39,3 +50,21 @@ impl<T:Copy> Ivar<T> {
         self.get().unwrap()
     }
 }
+
+impl<T:fmt::Debug> fmt::Debug for Ivar<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self.get() {
+            Some(val) => write!(f, "Ivar {{ {:?} }}", val),
+            None => f.write_str("Ivar { <unfulfilled> }")
+        }
+    }
+}
+
+impl<T:Copy> Clone for Ivar<T> {
+    fn clone(&self) -> Ivar<T> {
+        match self.get() {
+            Some(val) => Ivar { data: Cell::new(Some(val)) },
+            None => Ivar::new()
+        }
+    }
+}