[adgjlsfhk1]

> The claim that using reserve_exact "throws away the amortized growth promise" is wrong

The key part is that multiple calls reserve_exact will cause repeated allocations. The classic example is if someone defines a `pushfive` function that uses reserve_exact to increase the size by 5, and then pushes 5 times. Calling this function in a loop will take quadratic time since each reserve_exact call increases the array size. With reserve, the runtime is linear as expected.

[lossolo]

Quadratic pushfive is just a cautionary tale about misusing reserve_exact. Basically use reserve_exact when you know the final size (reserve once), or you're doing a one off tight sizing where memory footprint matters.

Don't pre reserve inside pushfive, just push the 5 elements (Vec handles growth) or if you know how many times you'll call pushfive in a loop, reserve up front once vec.reserve(5 * times) or use reserve instead of reserve_exact for incremental growth.

[adgjlsfhk1]

that's exactly the footgun. reserve_exact usage needs to be analyzed globally, while `resere` has the extra fuzziness needed to ensure that you can't mess anything up too badly with it.

[lossolo]

Calling that a "footgun" feels really overstated.

Yes, reserve is the safer default because it gives you slack and preserves amortized growth even if your usage pattern is messy.

But "needs global analysis" isn't unique to reserve_exact. Lots of perf knobs do (chunk sizes, buffering, locking granularity etc). The fix to this is to use the tool where its preconditions actually hold, not to avoid the tool.

So what I'm basically saying is that reserve_exact isn't inherently dangerous, it just assumes you know the final size and won't exceed it etc. If you keep bumping it in tiny steps (pushfive style), that's just misuse so treating it as a flaw is unwarranted.

[dwattttt]

> reserve_exact isn't inherently dangerous

Dangerous is perhaps overstating it, but the only utility reserve(_exact) has is performance and predictability. If using the API can be worse than doing nothing, I think it warrants being referred to as a footgun.