The current implementation of the `shrink_to_fit()` function of
`basic_string` swaps to the newly allocated buffer when the new buffer
has the same capacity as the existing one. While this is not incorrect,
it is truly unnecessary to swap to an equally-sized buffer. With equal
capacity, we should keep using the existing buffer and simply deallocate
the new one, avoiding the extra work of copying elements.
The desired behavior was documented in the following comment within the
function:
61ad08792a/libcxx/include/string (L3560-L3566)
However, the existing implementation did not exactly conform to this
guideline, which is a QoI matter.
This PR modifies the `shrink_to_fit()` function to ensure that the
buffer is only swapped when the new allocation is strictly smaller than
the existing one. When the capacities are equal, the new buffer will be
discarded without copying the elements. This is achieved by including
the `==` check in the above conditional logic.
The increasing_allocator<T> class, originally introduced to test shrink_to_fit()
for std::vector, std::vector<bool>, and std::basic_string, has duplicated
definitions across several test files. Given the potential utility of this
class for capacity-related tests in various sequence containers, this patch
refactors the definition of increasing_allocator<T> into a single, reusable
location.
