Rollup merge of #148534 - WaffleLapkin:push_within_capacity_now_with_50_percent_more_mut, r=Amanieu

Merge `Vec::push{,_mut}_within_capacity`

Implements rust-lang/libs-team#689.

cc #135974, #100486

r? libs-api

Author: matthiaskrgr

library/alloc/src/vec/mod.rs

@@ -2564,8 +2564,8 @@ impl<T, A: Allocator> Vec<T, A> {
         let _ = self.push_mut(value);
     }
 
-    /// Appends an element if there is sufficient spare capacity, otherwise an error is returned
-    /// with the element.
+    /// Appends an element and returns a reference to it if there is sufficient spare capacity,
+    /// otherwise an error is returned with the element.
     ///
     /// Unlike [`push`] this method will not reallocate when there's insufficient capacity.
     /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
@@ -2601,8 +2601,20 @@ impl<T, A: Allocator> Vec<T, A> {
     /// Takes *O*(1) time.
     #[inline]
     #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
-    pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
-        self.push_mut_within_capacity(value).map(|_| ())
+    // #[unstable(feature = "push_mut", issue = "135974")]
+    pub fn push_within_capacity(&mut self, value: T) -> Result<&mut T, T> {
+        if self.len == self.buf.capacity() {
+            return Err(value);
+        }
+
+        unsafe {
+            let end = self.as_mut_ptr().add(self.len);
+            ptr::write(end, value);
+            self.len += 1;
+
+            // SAFETY: We just wrote a value to the pointer that will live the lifetime of the reference.
+            Ok(&mut *end)
+        }
     }
 
     /// Appends an element to the back of a collection, returning a reference to it.
@@ -2654,36 +2666,6 @@ impl<T, A: Allocator> Vec<T, A> {
         }
     }
 
-    /// Appends an element and returns a reference to it if there is sufficient spare capacity,
-    /// otherwise an error is returned with the element.
-    ///
-    /// Unlike [`push_mut`] this method will not reallocate when there's insufficient capacity.
-    /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
-    ///
-    /// [`push_mut`]: Vec::push_mut
-    /// [`reserve`]: Vec::reserve
-    /// [`try_reserve`]: Vec::try_reserve
-    ///
-    /// # Time complexity
-    ///
-    /// Takes *O*(1) time.
-    #[unstable(feature = "push_mut", issue = "135974")]
-    // #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
-    #[inline]
-    #[must_use = "if you don't need a reference to the value, use `Vec::push_within_capacity` instead"]
-    pub fn push_mut_within_capacity(&mut self, value: T) -> Result<&mut T, T> {
-        if self.len == self.buf.capacity() {
-            return Err(value);
-        }
-        unsafe {
-            let end = self.as_mut_ptr().add(self.len);
-            ptr::write(end, value);
-            self.len += 1;
-            // SAFETY: We just wrote a value to the pointer that will live the lifetime of the reference.
-            Ok(&mut *end)
-        }
-    }
-
     /// Removes the last element from a vector and returns it, or [`None`] if it
     /// is empty.
     ///