Make assert_impl_one! work with all combinations

Fixes the issue where subsequent traits were only checked against the
first trait and not to one another. This solution was made possible by
@SimonSapin demonstrating to me how to expand the input sequence into
combinations of inputs.

Closes #20.

src/assert_impl.rs

@@ -52,10 +52,16 @@
 macro_rules! assert_impl_one {
     ($type:ty: $t:path, $($ts:path),+ $(,)?) => {
         assert_impl_any!($type: $t, $($ts),+);
-        // FIXME: Only works against the first trait; needs to check all
-        // subsequent traits against one another.
+        assert_impl_one!(_priv $type: $t, $($ts),+);
+    };
+    // Expands into all combinations of trait pairs to ensure `$type` does not
+    // implement any pair of traits.
+    (_priv $type:ty: $t:path, $($ts:path),+) => {
         $(assert_not_impl_all!($type: $t, $ts);)+
+        assert_impl_one!(_priv $type: $($ts),+);
     };
+    // Finished passing along pairs, nothing to do.
+    (_priv $type:ty: $t:path) => {};
 }
 
 /// Asserts that the type implements _all_ of the given traits.

tests/trait_impl.rs

@@ -20,3 +20,20 @@ assert_impl_all!(str: Send, Sync, AsRef<[u8]>,);
 assert_impl_any!((): Send, Sync);
 assert_impl_any!((): Send, From<u8>);
 assert_impl_any!((): From<u8>, From<u16>, Send);
+
+#[allow(dead_code)]
+struct Foo;
+
+trait A {}
+trait B {}
+trait C {}
+
+impl B for Foo {}
+
+assert_impl_one!(Foo: A, B);
+assert_impl_one!(Foo: B, A);
+assert_impl_one!(Foo: B, C);
+assert_impl_one!(Foo: C, B);
+assert_impl_one!(Foo: A, B, C);
+assert_impl_one!(Foo: B, C, A);
+assert_impl_one!(Foo: C, A, B);