Added trait for IterVariants, IterVariantNames.

This avoids the issues of the proposed PR by having both the inherent
method *and* the trait, and by using a different method name for the trait.
This is less than ideal, but solves the backwards compatibility issues well
enough.

Closes #32.

enum_derive/src/iter_variants.rs

@@ -20,6 +20,8 @@ macro_attr! {
 
 Derives a `$name::iter_variants() -> $itername` method.  The generated `$itername` type implements `Iterator<Item=$name>`, and yields each of the enumeration's variants.  This can only be used on an enum comprised on unitary variants.
 
+It also derives an implementation of the `IterVariants` trait.
+
 # Example
 
 ```rust
@@ -32,8 +34,11 @@ macro_attr! {
 }
 
 # fn main() {
+use enum_derive::iter_variants::IterVariants;
+
 let names: Vec<Cheese> = Cheese::iter_variants().collect();
 assert_eq!(names, vec![Cheese::RedLeicester, Cheese::Tilsit, Cheese::Stilton]);
+assert_eq!(<Cheese as IterVariants>::variants().next(), Some(Cheese::RedLeicester));
 # }
 ```
 */
@@ -70,6 +75,15 @@ macro_rules! IterVariants {
                 }
             }
         }
+
+        impl $crate::iter_variants::IterVariants for $name {
+            type Iter = $itername;
+
+            #[inline]
+            fn variants() -> Self::Iter {
+                $name::iter_variants()
+            }
+        }
     };
 
     (
@@ -89,6 +103,15 @@ macro_rules! IterVariants {
                 }
             }
         }
+
+        impl $crate::iter_variants::IterVariants for $name {
+            type Iter = $itername;
+
+            #[inline]
+            fn variants() -> Self::Iter {
+                $name::iter_variants()
+            }
+        }
     };
 
     (
@@ -187,6 +210,8 @@ macro_attr! {
 
 Derives a `$name::iter_variant_names() -> $itername` method.  The generated `$itername` type implements `Iterator<Item=&'static str>`, and yields the name of each of the enumeration's variants.  This can only be used on an enum comprised on unitary variants.
 
+It also derives an implementation of the `IterVariantNames` trait.
+
 # Example
 
 ```rust
@@ -198,8 +223,11 @@ macro_attr! {
 }
 
 # fn main() {
+use enum_derive::iter_variants::IterVariantNames;
+
 let names: Vec<&str> = Currency::iter_variant_names().collect();
 assert_eq!(names, vec!["Pounds", "FrenchFranks", "Lira", "DeutscheMark"]);
+assert_eq!(<Currency as IterVariantNames>::variant_names().next(), Some("Pounds"));
 # }
 ```
 */
@@ -236,6 +264,15 @@ macro_rules! IterVariantNames {
                 }
             }
         }
+
+        impl $crate::iter_variants::IterVariantNames for $name {
+            type Iter = $itername;
+
+            #[inline]
+            fn variant_names() -> Self::Iter {
+                $name::iter_variant_names()
+            }
+        }
     };
 
     (
@@ -255,6 +292,15 @@ macro_rules! IterVariantNames {
                 }
             }
         }
+
+        impl $crate::iter_variants::IterVariantNames for $name {
+            type Iter = $itername;
+
+            #[inline]
+            fn variant_names() -> Self::Iter {
+                $name::iter_variant_names()
+            }
+        }
     };
 
     (
@@ -342,3 +388,13 @@ macro_rules! IterVariantNames {
         }
     };
 }
+
+pub trait IterVariants: Sized {
+    type Iter: Iterator<Item=Self>;
+    fn variants() -> Self::Iter;
+}
+
+pub trait IterVariantNames {
+    type Iter: Iterator<Item=&'static str>;
+    fn variant_names() -> Self::Iter;
+}

enum_derive/src/lib.rs

@@ -48,7 +48,7 @@ IterVariants! { (Vars) enum ItAintRight { BabeNo, NoNo, BoyBoy } }
 #[macro_use] mod fmt;
 #[macro_use] mod from_str;
 #[macro_use] mod inner;
-#[macro_use] mod iter_variants;
+#[macro_use] pub mod iter_variants;
 #[macro_use] mod step_variants;
 #[macro_use] mod tag;