Can I avoid memcpy? about arrayvec HOT 3 CLOSED

Ideally, I think it should be removed in optimization. Is this something you found in an optimized build?

If the ArrayVec is not a struct field, then you can use std::mem::forget(array) directly (if it is empty or the elements have no destructor).

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It looks like the memcpy / overwrite is not optimized out properly. We can automatically skip it, but only by using a nightly / unstable feature (std::intrinsics::needs_drop), but I've added a feature flag to crate nodrop called use_needs_drop that enables this.

To use it: Depend directly on crate nodrop, enable feature flag use_needs_drop for nodrop. To only do it temporarily you can use --features=nodrop/use_needs_drop.

Of course, the big memcpy is redundant (only a small part needs to be overwritten to set the flag that inhibits drop, but it's the cleanest way to implement it. Maybe we can improve it..

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Thank you for your advice.
As you mentioned, I was confused because memcpys appear in some functions and does not appear in some other functions, and now I understood as instability of optimization.

I will see std::mem::forget(array) at first.

Thank you!

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