Add #[postcard(bound = "...")] attribute for derive(Schema)

[ia0]

This doesn't fix #153 but provides a work-around (which is also present in serde).

[netlify]

✅ Deploy Preview for cute-starship-2d9c9b canceled.

Name	Link
🔨 Latest commit	c0c1ded
🔍 Latest deploy log	https://app.netlify.com/sites/cute-starship-2d9c9b/deploys/664370b45494810008cb7317

[netlify]

✅ Deploy Preview for cute-starship-2d9c9b canceled.

Name	Link
🔨 Latest commit	3b57a69
🔍 Latest deploy log	https://app.netlify.com/sites/cute-starship-2d9c9b/deploys/676983bd8b7c0c0008bc2be6

[max-heller]

Hi @ia0, I merged this with main since things have changed quite a bit since you opened the PR. Could you could add a section to the docs here describing the attribute?

@jamesmunns have you thought any more about the future of MaxSize? Currently, this only affects derive(Schema), but if we want to keep MaxSize as a derive, we should probably have derive(MaxSize) take this attribute into account or provide fine-grained attributes like #[postcard(bound(schema = "...", max_size = "..."))] (mirroring #[serde(bound(serialize = "...", deserialize = "..."))]).

[ia0]

Could you could add a section to the docs here describing the attribute?

Yes. However, note that I probably won't be able to spend too much time on this PR since I ended up writing and using my own serialization library wasefire-wire.

[max-heller]

However, note that I probably won't be able to spend too much time on this PR since I ended up writing and using my own serialization library wasefire-wire.

I think this is pretty much good to go, thanks for the contribution and the pointer to wasefire which looks quite interesting!

[jamesmunns]

@max-heller I haven't had a chance yet! I may have some time on postcard over the next two weeks. I'm inclined to favor the const fn version, but I know there are open design points on that as well right now. I wouldn't object to extending this change to MaxSize, but also fine if it stays as-is for the moment.

@ia0 I'm not sure if you've done a side-by-side with wasefire and postcard-rpc, I'd definitely be interested to help set one up to understand the differences in design you ended up with! I remember the use cases being pretty overlapping? Happy to coordinate in an issue or email if you think it would be useful.

[ia0]

I'm not sure if you've done a side-by-side with wasefire and postcard-rpc, I'd definitely be interested to help set one up to understand the differences in design you ended up with! I remember the use cases being pretty overlapping? Happy to coordinate in an issue or email if you think it would be useful.

No I didn't do anything precise because wasefire is much more opinionated (wasefire-wire is not really meant to be used outside wasefire even if the wire format is postcard compatible). The only question is whether postcard-rpc covers what wasefire-protocol needs. I don't know how much the schema changed since last time I checked, but that was the main issue. There was also things I didn't like about serde, but I don't remember how much of an issue it was. It's true that it would be good to check this at some point. I've created google/wasefire#708 but probably won't be able to look at it before late 2025.