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This PR optimizes the input iterator overload of `assign(_InputIterator, _InputIterator)` in `std::vector<_Tp, _Allocator>` by directly assigning to already initialized memory, rather than first destroying existing elements and then constructing new ones. By eliminating unnecessary destruction and construction, the proposed algorithm enhances the performance by up to 2x for trivial element types (e.g., `std::vector<int>`), up to 2.6x for non-trivial element types like `std::vector<std::string>`, and up to 3.4x for more complex non-trivial types (e.g., `std::vector<std::vector<int>>`). ### Google Benchmarks Benchmark tests (`libcxx/test/benchmarks/vector_operations.bench.cpp`) were conducted for the `assign()` implementations before and after this patch. The tests focused on trivial element types like `std::vector<int>`, and non-trivial element types such as `std::vector<std::string>` and `std::vector<std::vector<int>>`. #### Before ``` ------------------------------------------------------------------------------------------------- Benchmark Time CPU Iterations ------------------------------------------------------------------------------------------------- BM_AssignInputIterIter/vector_int/1024/1024 1157 ns 1169 ns 608188 BM_AssignInputIterIter<32>/vector_string/1024/1024 14559 ns 14710 ns 47277 BM_AssignInputIterIter<32>/vector_vector_int/1024/1024 26846 ns 27129 ns 25925 ``` #### After ``` ------------------------------------------------------------------------------------------------- Benchmark Time CPU Iterations ------------------------------------------------------------------------------------------------- BM_AssignInputIterIter/vector_int/1024/1024 561 ns 566 ns 1242251 BM_AssignInputIterIter<32>/vector_string/1024/1024 5604 ns 5664 ns 128365 BM_AssignInputIterIter<32>/vector_vector_int/1024/1024 7927 ns 8012 ns 88579 ```
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