Just be careful, because moving heavy things to be dropped to another thread can change the semantics of the program. For instance, consider what happens if within that heavy thing you had a BufWriter: unless its buffer is empty, dropping it writes the buffer, so now your file is being written and closed in a random moment in the future, instead of being guaranteed to have been sent to the kernel and closed when the function returns.
And it can even be worse if it's holding a limited resource, like a file descriptor or a database connection. That is, I wouldn't recommend using this trick unless you're sure that the only thing the "heavy thing" is holding is memory (and even then, keep in mind that memory can also be a limited resource).
I only know a very little rust, but since it's generally a good practice to never defer writing (or other side effects) to an ambiguous future point in time - with memory allocations as the only plausible exception - is there any way in rust to make sure one doesn't accidentally move complex objects with drop side-effects into other threads?
Granted the way the type system work you usually know the type of a variable quite well, but could this happen with opaque types?
I'm very much out of my depth, but it felt like one of those things that could really bite you if you are unaware, as happened with finalizers in Java decades ago.
> I only know a very little rust, but since it's generally a good practice to never defer writing (or other side effects) to an ambiguous future point in time - with memory allocations as the only plausible exception - is there any way in rust to make sure one doesn't accidentally move complex objects with drop side-effects into other threads?
If you're the one creating the structure, you could opt it out of Send, that'd make it… not sendable. So it wouldn't be able to cross thread-boundaries. For instance Rc is !Send, you simply can not send it across a thread-boundary (because it's a non-threadsafe reference-counting handle).
If you don't control the type, then you'd have to wrap it (newtype pattern) or remember to manually mem::drop it. The latter would obviously have no safety whatsoever, the former you might be able to lint for I guess, though even that is limited or complicated (because of type inference the problematic type might never get explicitly mentioned).
Considering that writing files can also block the process you probably don't want to have that in your latency-sensitive parts either, so you'll have to optimize that one way or another anyway.
For the more general problem you have can also dedicate more threads to the task or apply backpressure.
A while ago I stumbled over a proposal to move a shared pointer (this was C++ code) to a thread in order to trigger the freeing of a legacy data structure there (the multi-thousand delete calls caused the watchdog of the main thread to fail).
However, keeping the shared pointer reference in the main thread for too long resulted in the possibility that the "clean-up" thread ran while the main thread still had a hold on the shared pointer... Resulting in a low chance of the "clean-up" thread doing nothing and the main thread still locking up. People here got taught to use shared pointers to prevent memory management errors, but it can really cause a lot of unexpected non-determinism when used blindly.
Even ignoring the shared_ptr abuse, at least for allocating and freeing many small objects Java would most probably be (a lot?) faster than non-optimized allocations and frees in native binaries. But in my case it was legacy code running on an embedded device.
It seems like the caller should ensure that the buffer is written before giving away ownership. Also, what happens if there is an error writing during finalization/destruction/etc? Seems like you'd want to find out about such errors earlier if at all possible.
And it can even be worse if it's holding a limited resource, like a file descriptor or a database connection. That is, I wouldn't recommend using this trick unless you're sure that the only thing the "heavy thing" is holding is memory (and even then, keep in mind that memory can also be a limited resource).