Auto merge of #123 - llogiq:specialization, r=nox

First specialization

I added a new `specialization` feature that will specialize `From<&[_: Copy]>` to use `from_slice`, which offers a nice performance boost.

Alas, I could not get any measurable perf improvement on `insert_many` or `extend` yet, so I'll leave them out for now.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-smallvec/123)
<!-- Reviewable:end -->

Author: bors-servo

Cargo.toml

@@ -14,6 +14,7 @@ documentation = "http://doc.servo.org/smallvec/"
 std = []
 union = []
 default = ["std"]
+specialization = []
 
 [lib]
 name = "smallvec"

lib.rs

@@ -31,6 +31,7 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(not(feature = "std"), feature(alloc))]
 #![cfg_attr(feature = "union", feature(untagged_unions))]
+#![cfg_attr(feature = "specialization", feature(specialization))]
 #![deny(missing_docs)]
 
 
@@ -1156,11 +1157,40 @@ where A::Item: Deserialize<'de>,
     }
 }
 
+
+#[cfg(feature = "specialization")]
+trait SpecFrom<A: Array, S> {
+    fn spec_from(slice: S) -> SmallVec<A>;
+}
+
+#[cfg(feature = "specialization")]
+impl<'a, A: Array> SpecFrom<A, &'a [A::Item]> for SmallVec<A> where A::Item: Clone {
+    #[inline]
+    default fn spec_from(slice: &'a [A::Item]) -> SmallVec<A> {
+        slice.into_iter().cloned().collect()
+    }
+}
+
+#[cfg(feature = "specialization")]
+impl<'a, A: Array> SpecFrom<A, &'a [A::Item]> for SmallVec<A> where A::Item: Copy {
+    #[inline]
+    fn spec_from(slice: &'a [A::Item]) -> SmallVec<A> {
+        SmallVec::from_slice(slice)
+    }
+}
+
 impl<'a, A: Array> From<&'a [A::Item]> for SmallVec<A> where A::Item: Clone {
+    #[cfg(not(feature = "specialization"))]
     #[inline]
     fn from(slice: &'a [A::Item]) -> SmallVec<A> {
         slice.into_iter().cloned().collect()
     }
+
+    #[cfg(feature = "specialization")]
+    #[inline]
+    fn from(slice: &'a [A::Item]) -> SmallVec<A> {
+        SmallVec::spec_from(slice)
+    }
 }
 
 impl<A: Array> From<Vec<A::Item>> for SmallVec<A> {