Rollup merge of #112263 - GrishaVar:remove-extend-element, r=scottmcm

Remove ExtendElement, ExtendWith, extend_with

Related to  #104624, broken up into two commits. The first removes wrapper trait ExtendWith and its only implementer struct ExtendElement. The second may have perf issues so may be reverted/removed if no alternate fix is found; it should be profiled.

r? `@scottmcm`

library/alloc/src/vec/mod.rs

@@ -2355,7 +2355,7 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
         let len = self.len();
 
         if new_len > len {
-            self.extend_with(new_len - len, ExtendElement(value))
+            self.extend_with(new_len - len, value)
         } else {
             self.truncate(new_len);
         }
@@ -2469,26 +2469,10 @@ impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
     }
 }
 
-// This code generalizes `extend_with_{element,default}`.
-trait ExtendWith<T> {
-    fn next(&mut self) -> T;
-    fn last(self) -> T;
-}
-
-struct ExtendElement<T>(T);
-impl<T: Clone> ExtendWith<T> for ExtendElement<T> {
-    fn next(&mut self) -> T {
-        self.0.clone()
-    }
-    fn last(self) -> T {
-        self.0
-    }
-}
-
-impl<T, A: Allocator> Vec<T, A> {
+impl<T: Clone, A: Allocator> Vec<T, A> {
     #[cfg(not(no_global_oom_handling))]
-    /// Extend the vector by `n` values, using the given generator.
-    fn extend_with<E: ExtendWith<T>>(&mut self, n: usize, mut value: E) {
+    /// Extend the vector by `n` clones of value.
+    fn extend_with(&mut self, n: usize, value: T) {
         self.reserve(n);
 
         unsafe {
@@ -2500,15 +2484,15 @@ impl<T, A: Allocator> Vec<T, A> {
 
             // Write all elements except the last one
             for _ in 1..n {
-                ptr::write(ptr, value.next());
+                ptr::write(ptr, value.clone());
                 ptr = ptr.add(1);
-                // Increment the length in every step in case next() panics
+                // 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.last());
+                ptr::write(ptr, value);
                 local_len.increment_len(1);
             }
 

library/alloc/src/vec/spec_from_elem.rs

@@ -3,7 +3,7 @@ use core::ptr;
 use crate::alloc::Allocator;
 use crate::raw_vec::RawVec;
 
-use super::{ExtendElement, IsZero, Vec};
+use super::{IsZero, Vec};
 
 // Specialization trait used for Vec::from_elem
 pub(super) trait SpecFromElem: Sized {
@@ -13,7 +13,7 @@ pub(super) trait SpecFromElem: Sized {
 impl<T: Clone> SpecFromElem for T {
     default fn from_elem<A: Allocator>(elem: Self, n: usize, alloc: A) -> Vec<Self, A> {
         let mut v = Vec::with_capacity_in(n, alloc);
-        v.extend_with(n, ExtendElement(elem));
+        v.extend_with(n, elem);
         v
     }
 }
@@ -25,7 +25,7 @@ impl<T: Clone + IsZero> SpecFromElem for T {
             return Vec { buf: RawVec::with_capacity_zeroed_in(n, alloc), len: n };
         }
         let mut v = Vec::with_capacity_in(n, alloc);
-        v.extend_with(n, ExtendElement(elem));
+        v.extend_with(n, elem);
         v
     }
 }