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> Increase the capacity of the vector (the total number of elements that the vector can hold without requiring reallocation) to a value that's greater or equal to new_cap.

I belive that the behaviour of reserve() is implementation defined.

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Because there's only a single function here, it has to either be Vec::reserve or Vec::reserve_exact

If you don't offer Vec::reserve_exact then people who needed that run out of RAM and will dub your stdlib garbage. If you don't offer Vec::reserve as we've seen C++ programmers will say "Skill issue" whenever a noob gets awful performance as a result. So, it's an easy choice.

That said MSVC,GCC and clang all implement it to allocate an exact value.

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