Relax From<array> for SmallVec to allow different length arrays.

Author: zachs18

src/lib.rs

@@ -2016,10 +2016,18 @@ impl<A: Array> From<Vec<A::Item>> for SmallVec<A> {
     }
 }
 
-impl<A: Array> From<A> for SmallVec<A> {
+impl<A1: Array, A2: Array<Item = A1::Item>> From<A2> for SmallVec<A1> {
     #[inline]
-    fn from(array: A) -> SmallVec<A> {
-        SmallVec::from_buf(array)
+    fn from(array: A2) -> SmallVec<A1> {
+        let m = A2::size();
+        let mut this = Self::with_capacity(m);
+        let array = mem::ManuallyDrop::new(array);
+        // SAFETY: m <= this.capacity()
+        unsafe {
+            ptr::copy_nonoverlapping(&*array as *const A2 as *const A2::Item, this.as_mut_ptr(), m);
+            this.set_len(m);
+        }
+        this
     }
 }
 

src/tests.rs

@@ -623,6 +623,16 @@ fn test_from() {
     let small_vec: SmallVec<[u8; 128]> = SmallVec::from(array);
     assert_eq!(&*small_vec, vec![99u8; 128].as_slice());
     drop(small_vec);
+
+    let array = [1; 128];
+    let small_vec: SmallVec<[u8; 1]> = SmallVec::from(array);
+    assert_eq!(&*small_vec, vec![1; 128].as_slice());
+    drop(small_vec);
+
+    let array = [99];
+    let small_vec: SmallVec<[u8; 128]> = SmallVec::from(array);
+    assert_eq!(&*small_vec, &[99u8]);
+    drop(small_vec);
 }
 
 #[test]