Remove a function call from Vec::peek_mut construction

This style aligns with that of `BinaryHeap::peek_mut`.

Author: cammeresi

library/alloc/src/vec/mod.rs

@@ -784,7 +784,7 @@ impl<T> Vec<T> {
     #[inline]
     #[unstable(feature = "vec_peek_mut", issue = "122742")]
     pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
-        PeekMut::new(self)
+        if self.is_empty() { None } else { Some(PeekMut { vec: self }) }
     }
 
     /// Decomposes a `Vec<T>` into its raw components: `(pointer, length, capacity)`.

library/alloc/src/vec/peek_mut.rs

@@ -17,7 +17,7 @@ pub struct PeekMut<
     T,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
 > {
-    vec: &'a mut Vec<T, A>,
+    pub(super) vec: &'a mut Vec<T, A>,
 }
 
 #[unstable(feature = "vec_peek_mut", issue = "122742")]
@@ -28,10 +28,6 @@ impl<T: fmt::Debug> fmt::Debug for PeekMut<'_, T> {
 }
 
 impl<'a, T> PeekMut<'a, T> {
-    pub(crate) fn new(vec: &'a mut Vec<T>) -> Option<Self> {
-        if vec.is_empty() { None } else { Some(Self { vec }) }
-    }
-
     /// Removes the peeked value from the vector and returns it.
     #[unstable(feature = "vec_peek_mut", issue = "122742")]
     pub fn pop(this: Self) -> T {
@@ -45,8 +41,9 @@ impl<'a, T> Deref for PeekMut<'a, T> {
     type Target = T;
 
     fn deref(&self) -> &Self::Target {
+        let idx = self.vec.len() - 1;
         // SAFETY: PeekMut is only constructed if the vec is non-empty
-        unsafe { self.vec.get_unchecked(self.vec.len() - 1) }
+        unsafe { self.vec.get_unchecked(idx) }
     }
 }