bench and optimize from_slice

This adds "bench_from_slice(_vec)(_small)" benches, renaming the previous
versions to "bench_from_iter*" (because they were using `From<&[T]>`, which
calls `FromIterator` internally) and optimizes the `from_slice` constructor
considerably.

Before:
```
test bench_from_slice                  ... bench:          42 ns/iter (+/- 0)
test bench_from_slice_small            ... bench:          12 ns/iter (+/- 0)
```
After:
```
test bench_from_slice                  ... bench:          27 ns/iter (+/- 0)
test bench_from_slice_small            ... bench:           8 ns/iter (+/- 0)
```

benches/bench.rs

@@ -17,6 +17,7 @@ trait Vector<T>: for<'a> From<&'a [T]> + Extend<T> + ExtendFromSlice<T> {
     fn remove(&mut self, p: usize) -> T;
     fn insert(&mut self, n: usize, val: T);
     fn from_elem(val: T, n: usize) -> Self;
+    fn from_elems(val: &[T]) -> Self;
 }
 
 impl<T: Copy> Vector<T> for Vec<T> {
@@ -43,6 +44,10 @@ impl<T: Copy> Vector<T> for Vec<T> {
     fn from_elem(val: T, n: usize) -> Self {
         vec![val; n]
     }
+
+    fn from_elems(val: &[T]) -> Self {
+        val.to_owned()
+    }
 }
 
 impl<T: Copy> Vector<T> for SmallVec<[T; VEC_SIZE]> {
@@ -69,6 +74,10 @@ impl<T: Copy> Vector<T> for SmallVec<[T; VEC_SIZE]> {
     fn from_elem(val: T, n: usize) -> Self {
         smallvec![val; n]
     }
+
+    fn from_elems(val: &[T]) -> Self {
+        SmallVec::from_slice(val)
+    }
 }
 
 macro_rules! make_benches {
@@ -92,6 +101,8 @@ make_benches! {
         bench_remove_small => gen_remove(VEC_SIZE as _),
         bench_extend => gen_extend(SPILLED_SIZE as _),
         bench_extend_small => gen_extend(VEC_SIZE as _),
+        bench_from_iter => gen_from_iter(SPILLED_SIZE as _),
+        bench_from_iter_small => gen_from_iter(VEC_SIZE as _),
         bench_from_slice => gen_from_slice(SPILLED_SIZE as _),
         bench_from_slice_small => gen_from_slice(VEC_SIZE as _),
         bench_extend_from_slice => gen_extend_from_slice(SPILLED_SIZE as _),
@@ -112,6 +123,8 @@ make_benches! {
         bench_remove_vec_small => gen_remove(VEC_SIZE as _),
         bench_extend_vec => gen_extend(SPILLED_SIZE as _),
         bench_extend_vec_small => gen_extend(VEC_SIZE as _),
+        bench_from_iter_vec => gen_from_iter(SPILLED_SIZE as _),
+        bench_from_iter_vec_small => gen_from_iter(VEC_SIZE as _),
         bench_from_slice_vec => gen_from_slice(SPILLED_SIZE as _),
         bench_from_slice_vec_small => gen_from_slice(VEC_SIZE as _),
         bench_extend_from_slice_vec => gen_extend_from_slice(SPILLED_SIZE as _),
@@ -179,14 +192,22 @@ fn gen_extend<V: Vector<u64>>(n: u64, b: &mut Bencher) {
     });
 }
 
-fn gen_from_slice<V: Vector<u64>>(n: u64, b: &mut Bencher) {
+fn gen_from_iter<V: Vector<u64>>(n: u64, b: &mut Bencher) {
     let v: Vec<u64> = (0..n).collect();
     b.iter(|| {
         let vec = V::from(&v);
         vec
     });
 }
 
+fn gen_from_slice<V: Vector<u64>>(n: u64, b: &mut Bencher) {
+    let v: Vec<u64> = (0..n).collect();
+    b.iter(|| {
+        let vec = V::from_elems(&v);
+        vec
+    });
+}
+
 fn gen_extend_from_slice<V: Vector<u64>>(n: u64, b: &mut Bencher) {
     let v: Vec<u64> = (0..n).collect();
     b.iter(|| {

lib.rs

@@ -946,9 +946,25 @@ impl<A: Array> SmallVec<A> where A::Item: Copy {
     ///
     /// For slices of `Copy` types, this is more efficient than `SmallVec::from(slice)`.
     pub fn from_slice(slice: &[A::Item]) -> Self {
-        let mut vec = Self::new();
-        vec.extend_from_slice(slice);
-        vec
+        let len = slice.len();
+        if len <= A::size() {
+            SmallVec {
+                capacity: len,
+                data: SmallVecData::from_inline(unsafe {
+                    let mut data: A = mem::uninitialized();
+                    ptr::copy_nonoverlapping(slice.as_ptr(), data.ptr_mut(), len);
+                    data
+                })
+            }
+        } else {
+            let mut b = slice.to_vec();
+            let ptr = b.as_mut_ptr();
+            mem::forget(b);
+            SmallVec {
+                capacity: len,
+                data: SmallVecData::from_heap(ptr, len),
+            }
+        }
     }
 
     /// Copy elements from a slice into the vector at position `index`, shifting any following