Implement PackedOption to address #19.

The PackedOption<T> struct uses the same amount of memory as T, but can
represent None via a reserved value.

lib/cretonne/src/lib.rs

@@ -24,3 +24,4 @@ mod constant_hash;
 mod predicates;
 mod legalizer;
 mod ref_slice;
+mod packed_option;

lib/cretonne/src/packed_option.rs

@@ -0,0 +1,121 @@
+//! Compact representation of `Option<T>` for types with a reserved value.
+//!
+//! Small Cretonne types like the 32-bit entity references are often used in tables and linked
+//! lists where an `Option<T>` is needed. Unfortunately, that would double the size of the tables
+//! because `Option<T>` is twice as big as `T`.
+//!
+//! This module provides a `PackedOption<T>` for types that have a reserved value that can be used
+//! to represent `None`.
+
+use std::fmt;
+
+/// Types that have a reserved value which can't be created any other way.
+pub trait ReservedValue: Eq {
+    /// Create an instance of the reserved value.
+    fn reserved_value() -> Self;
+}
+
+/// Packed representation of `Option<T>`.
+#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Hash)]
+pub struct PackedOption<T: ReservedValue>(T);
+
+impl<T: ReservedValue> PackedOption<T> {
+    /// Returns `true` if the packed option is a `None` value.
+    pub fn is_none(&self) -> bool {
+        self.0 == T::reserved_value()
+    }
+
+    /// Expand the packed option into a normal `Option`.
+    pub fn expand(self) -> Option<T> {
+        if self.is_none() { None } else { Some(self.0) }
+    }
+}
+
+impl<T: ReservedValue> Default for PackedOption<T> {
+    /// Create a default packed option representing `None`.
+    fn default() -> PackedOption<T> {
+        PackedOption(T::reserved_value())
+    }
+}
+
+impl<T: ReservedValue> From<T> for PackedOption<T> {
+    /// Convert `t` into a packed `Some(x)`.
+    fn from(t: T) -> PackedOption<T> {
+        debug_assert!(t != T::reserved_value(),
+                      "Can't make a PackedOption from the reserved value.");
+        PackedOption(t)
+    }
+}
+
+impl<T: ReservedValue> From<Option<T>> for PackedOption<T> {
+    /// Convert an option into its packed equivalent.
+    fn from(opt: Option<T>) -> PackedOption<T> {
+        match opt {
+            None => Self::default(),
+            Some(t) => t.into(),
+        }
+    }
+}
+
+impl<T: ReservedValue> Into<Option<T>> for PackedOption<T> {
+    fn into(self) -> Option<T> {
+        self.expand()
+    }
+}
+
+impl<T> fmt::Debug for PackedOption<T>
+    where T: ReservedValue + fmt::Debug
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if self.is_none() {
+            write!(f, "None")
+        } else {
+            write!(f, "Some({:?})", self.0)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // Dummy entity class, with no Copy or Clone.
+    #[derive(Debug, PartialEq, Eq)]
+    struct NoC(u32);
+
+    impl ReservedValue for NoC {
+        fn reserved_value() -> Self {
+            NoC(13)
+        }
+    }
+
+    #[test]
+    fn moves() {
+        let x = NoC(3);
+        let somex: PackedOption<NoC> = x.into();
+        assert!(!somex.is_none());
+        assert_eq!(somex.expand(), Some(NoC(3)));
+
+        let none: PackedOption<NoC> = None.into();
+        assert!(none.is_none());
+        assert_eq!(none.expand(), None);
+    }
+
+    // Dummy entity class, with Copy.
+    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
+    struct Ent(u32);
+
+    impl ReservedValue for Ent {
+        fn reserved_value() -> Self {
+            Ent(13)
+        }
+    }
+
+    #[test]
+    fn copies() {
+        let x = Ent(2);
+        let some: PackedOption<Ent> = x.into();
+        assert_eq!(some.expand(), x.into());
+        assert_eq!(some, x.into());
+    }
+}