In this case, the implementation is not free to ignore calls to reserve or reserve_exact -- the documentation makes explicit guarantees about this: "After calling reserve, capacity will be greater than or equal to self.len() + additional".

This matters for performance reasons, and also because unsafe code is allowed to use the extra capacity of the vector -- e.g. you can write to the unused space and then call set_len to insert elements in-place. If `reserve` did not guarantee the requested capacity was allocated, this would cause buffer overflows.

The documentation for these functions is very carefully worded to explain exactly what each method does and does not guarantee. Both functions have the exact same hard guarantees: after calling the function, the vector will have at least enough capacity for the additional elements.

The difference is in what the methods try to do without guaranteeing exact behavior: reserve follows the usual amortized growth pattern, while reserve_exact tries to avoid overallocating. These are described as best-effort performance optimizations rather than guarantees you can rely on because 1) the amortized growth strategy is subject to change between platforms and compiler versions, and 2) memory allocators typically don't support arbitrarily-sized blocks of memory, and instead round up to the nearest supported size.