Rollup merge of rust-lang#139773 - thaliaarchi:vec-into-iter-last, r=workingjubilee

Implement `Iterator::last` for `vec::IntoIter`

Avoid iterating everything when we have random access to the last element.

Author: Zalathar

alloc/src/vec/into_iter.rs

@@ -258,6 +258,11 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         self.len()
     }
 
+    #[inline]
+    fn last(mut self) -> Option<T> {
+        self.next_back()
+    }
+
     #[inline]
     fn next_chunk<const N: usize>(&mut self) -> Result<[T; N], core::array::IntoIter<T, N>> {
         let mut raw_ary = [const { MaybeUninit::uninit() }; N];

std/src/sys/args/common.rs

@@ -49,8 +49,8 @@ impl Iterator for Args {
     }
 
     #[inline]
-    fn last(mut self) -> Option<OsString> {
-        self.iter.next_back()
+    fn last(self) -> Option<OsString> {
+        self.iter.last()
     }
 
     #[inline]