Plumb Allocator generic into std::vec::PeekMut

Author: cammeresi

library/alloc/src/vec/peek_mut.rs

@@ -1,6 +1,7 @@
 use core::ops::{Deref, DerefMut};
 
 use super::Vec;
+use crate::alloc::{Allocator, Global};
 use crate::fmt;
 
 /// Structure wrapping a mutable reference to the last item in a
@@ -11,8 +12,12 @@ use crate::fmt;
 ///
 /// [`peek_mut`]: Vec::peek_mut
 #[unstable(feature = "vec_peek_mut", issue = "122742")]
-pub struct PeekMut<'a, T> {
-    vec: &'a mut Vec<T>,
+pub struct PeekMut<
+    'a,
+    T,
+    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
+> {
+    vec: &'a mut Vec<T, A>,
 }
 
 #[unstable(feature = "vec_peek_mut", issue = "122742")]