feat: specialize SpecFromElem for ()

While a better approach would be to implement it for all ZSTs
which are `Copy` and have trivial `Clone`,
the last property cannot be detected for now.

Signed-off-by: Petr Portnov <me@progrm-jarvis.ru>

library/alloc/src/vec/spec_from_elem.rs

@@ -36,12 +36,12 @@ fn from_elem<A: Allocator>(elem: i8, n: usize, alloc: A) -> Vec<i8, A> {
         if elem == 0 {
             return Vec { buf: RawVec::with_capacity_zeroed_in(n, alloc), len: n };
         }
+        let mut v = Vec::with_capacity_in(n, alloc);
         unsafe {
-            let mut v = Vec::with_capacity_in(n, alloc);
             ptr::write_bytes(v.as_mut_ptr(), elem as u8, n);
             v.set_len(n);
-            v
         }
+        v
     }
 }
 
@@ -51,11 +51,26 @@ fn from_elem<A: Allocator>(elem: u8, n: usize, alloc: A) -> Vec<u8, A> {
         if elem == 0 {
             return Vec { buf: RawVec::with_capacity_zeroed_in(n, alloc), len: n };
         }
+        let mut v = Vec::with_capacity_in(n, alloc);
         unsafe {
-            let mut v = Vec::with_capacity_in(n, alloc);
             ptr::write_bytes(v.as_mut_ptr(), elem, n);
             v.set_len(n);
-            v
         }
+        v
+    }
+}
+
+// A better way would be to implement this for all ZSTs which are `Copy` and have trivial `Clone`
+// but this cannot be implemented currently
+impl SpecFromElem for () {
+    #[inline]
+    fn from_elem<A: Allocator>(elem: (), n: usize, alloc: A) -> Vec<(), A> {
+        let mut v = Vec::with_capacity_in(n, alloc);
+        // SAFETY: the capacity has just been set to `n` and `()`
+        // is a ZST with trivial `Clone` implementation
+        unsafe {
+            v.set_len(n);
+        }
+        v
     }
 }