Add (optional) support for typesize

[inklesspen]

typesize offers a trait to calculate the total memory size used by a type (including heap allocations). This can be important for caches; PaperCache uses typesize to determine how much memory a particular key or value uses; the Discord library serenity can optionally use it to track the size of its cached data, and so on.

While typesize includes optional implementations for some external libraries like arrayvec or url, it's currently impossible to fully implement TypeSize support for jiff from outside the jiff crate, because of how TimeZone pointers work.

I "implemented" support for most of jiff's core types in GnomedDev/typesize#10 (because for a wholly stack-based type the implementation can use default methods):

impl TypeSize for jiff::SignedDuration {}
impl TypeSize for jiff::Timestamp {}
// etc

However, the typesize dev would prefer to have the support be implemented in the jiff crate (which I can understand, since that will make it easier to keep up to date). And of course if we're going to have an implementation at all, it would be nice to support TimeZone.

If you're willing to accept a PR for this, I'd be happy to draft one.

[BurntSushi]

With respect to just getting a working implementation of TypeSize, I think Jiff can expose memory_usage() routines for types that allocate heap memory. Am I correct that that should let you implement TypeSize? If so, I'm very happy to have that.

But a dependency on typesize is a different matter I think. I'm extremely conservative with dependencies, even optional ones. And especially public dependencies. I think a crate basically needs to be an ecosystem standard and at least as stable as Jiff for me to pull it in as a dependency. If typesize becomes an ecosystem standard and demonstrates a commitment to API stability, then I'd probably be willing to add a dependency on it. But I don't think it's there yet.

While I can appreciate that maintaining more implementations in typesize is more work, I don't think it's a good enough reason for me to add a dependency on typesize in Jiff. A possible compromise is to wait until Jiff 1.0 is out (at some point this year). I plan to commit to its API indefinitely, so there shouldn't be too much maintenance overhead at that point.

[inklesspen]

TypeSize starts with calling core::mem::size_of for a type; it has extra_size() which is supposed to return the amount of memory not accounted for by core::mem::size_of, and then get_size() which has a default implementation that just does core::mem::size_of::<Self>() + self.extra_size().

So I think if you have a method alloc_usage() (or something like that) which gives the allocated size of types like TzIfOwned that will probably work. But I am asking the typesize dev to confirm.

[BurntSushi]

Yeah that's the same contract I use in regex-automata. Example.

[GnomedDev]

I would be happy with a method that implements extra_size but without the crate. That means I wouldn't have to maintain code that matches private internals.

[inklesspen]

Excellent. I'd be happy to make a PR to add memory_usage() routines, instead.