From 2fd9442afca446af8084c8b15feba2daa2fb4bb6 Mon Sep 17 00:00:00 2001
From: Petr Portnov <me@progrm-jarvis.ru>
Date: Mon, 20 Nov 2023 16:47:32 +0300
Subject: [PATCH] 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 | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/library/alloc/src/vec/spec_from_elem.rs b/library/alloc/src/vec/spec_from_elem.rs
index da43d17bf36..4e4150d9435 100644
--- a/library/alloc/src/vec/spec_from_elem.rs
+++ b/library/alloc/src/vec/spec_from_elem.rs
@@ -36,12 +36,12 @@ impl SpecFromElem for i8 {
         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 @@ impl SpecFromElem for u8 {
         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
     }
 }