[libc++][vector] Fixes shrink_to_fit. (#97895)

Summary:
This assures shrink_to_fit does not increase the allocated size.

Partly addresses #95161

---------

Co-authored-by: Mital Ashok <[email protected]>

Test Plan: 

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Differential Revision: https://phabricator.intern.facebook.com/D60251479

libcxx/include/vector

@@ -1443,7 +1443,11 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void vector<_Tp, _Allocator>::shrink_to_fit() _NOE
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
       allocator_type& __a = this->__alloc();
       __split_buffer<value_type, allocator_type&> __v(size(), size(), __a);
-      __swap_out_circular_buffer(__v);
+      // The Standard mandates shrink_to_fit() does not increase the capacity.
+      // With equal capacity keep the existing buffer. This avoids extra work
+      // due to swapping the elements.
+      if (__v.capacity() < capacity())
+        __swap_out_circular_buffer(__v);
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
     } catch (...) {
     }

libcxx/test/std/containers/sequences/vector/vector.capacity/shrink_to_fit.pass.cpp

@@ -71,11 +71,56 @@ TEST_CONSTEXPR_CXX20 bool tests() {
     return true;
 }
 
+#if TEST_STD_VER >= 23
+template <typename T>
+struct increasing_allocator {
+  using value_type         = T;
+  std::size_t min_elements = 1000;
+  increasing_allocator()   = default;
+
+  template <typename U>
+  constexpr increasing_allocator(const increasing_allocator<U>& other) noexcept : min_elements(other.min_elements) {}
+
+  constexpr std::allocation_result<T*> allocate_at_least(std::size_t n) {
+    if (n < min_elements)
+      n = min_elements;
+    min_elements += 1000;
+    return std::allocator<T>{}.allocate_at_least(n);
+  }
+  constexpr T* allocate(std::size_t n) { return allocate_at_least(n).ptr; }
+  constexpr void deallocate(T* p, std::size_t n) noexcept { std::allocator<T>{}.deallocate(p, n); }
+};
+
+template <typename T, typename U>
+bool operator==(increasing_allocator<T>, increasing_allocator<U>) {
+  return true;
+}
+
+// https://github.com/llvm/llvm-project/issues/95161
+constexpr bool test_increasing_allocator() {
+  std::vector<int, increasing_allocator<int>> v;
+  v.push_back(1);
+  assert(is_contiguous_container_asan_correct(v));
+  std::size_t capacity = v.capacity();
+  v.shrink_to_fit();
+  assert(v.capacity() <= capacity);
+  assert(v.size() == 1);
+  assert(is_contiguous_container_asan_correct(v));
+
+  return true;
+}
+#endif // TEST_STD_VER >= 23
+
 int main(int, char**)
 {
-    tests();
+  tests();
 #if TEST_STD_VER > 17
     static_assert(tests());
 #endif
+#if TEST_STD_VER >= 23
+    test_increasing_allocator();
+    static_assert(test_increasing_allocator());
+#endif
+
     return 0;
 }