Use iter::repeat_n to implement Vec::extend_with

This replaces the manual `Vec::extend_with` implementation with
`iter::repeat_n` and `Vec::extend_trusted`. This simplifies the
code and gets LLVM to remove the special case for the last element
when `T` is trivial to clone.

Author: paolobarbolini

library/alloc/src/vec/mod.rs

@@ -3258,31 +3258,7 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
     #[track_caller]
     /// Extend the vector by `n` clones of value.
     fn extend_with(&mut self, n: usize, value: T) {
-        self.reserve(n);
-
-        unsafe {
-            let mut ptr = self.as_mut_ptr().add(self.len());
-            // Use SetLenOnDrop to work around bug where compiler
-            // might not realize the store through `ptr` through self.set_len()
-            // don't alias.
-            let mut local_len = SetLenOnDrop::new(&mut self.len);
-
-            // Write all elements except the last one
-            for _ in 1..n {
-                ptr::write(ptr, value.clone());
-                ptr = ptr.add(1);
-                // Increment the length in every step in case clone() panics
-                local_len.increment_len(1);
-            }
-
-            if n > 0 {
-                // We can write the last element directly without cloning needlessly
-                ptr::write(ptr, value);
-                local_len.increment_len(1);
-            }
-
-            // len set by scope guard
-        }
+        self.extend_trusted(iter::repeat_n(value, n));
     }
 }