feat: introduce no_std support for ff, curve, and crypto

[tdelabro]

No description provided.

[xJonathanLEI]

Found this clippy lint that might be useful:

rust-lang/rust-clippy#7724

Clippy rule link:

https://rust-lang.github.io/rust-clippy/master/#std_instead_of_core

[xJonathanLEI]

Note that we should also add CI steps for building without the std feature to make sure we're not accidentally breaking no_std support.

Edit: found this: https://github.com/KodrAus/rust-no-std

[xJonathanLEI]

Hmmm I just tried running this:

console~ ~$ cargo build --package starknet-crypto --no-default-features~ ~

to make sure it really works for no_std, but I'm getting errors. Any ideas?

Edit: never mind it looks like I have to also enable alloc feature when using no_std.

[xJonathanLEI]

Is it possible that we make the crate work with everything turned off (not even alloc), but with reduced functionality? Anything that requires alloc gets gated by any(feature = "std", feature = "alloc"). Then at least the crate would work with a bare default-features = false.

Or maybe we should just unconditionally enable alloc when std is off - the crate doesn't build without it anyways.

[tdelabro]

Is it possible that we make the crate work with everything turned off (not even alloc), but with reduced functionality?

Yeah totally. alloc is only used by starknet-ff here:

impl Serialize for FieldElement {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: serde::Serializer,
    {
        serializer.serialize_str(&ToString::to_string(&self))
    }
}

impl<'de> Deserialize<'de> for FieldElement {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        let value = String::deserialize(deserializer)?;
        Self::from_str(&value).map_err(serde::de::Error::custom)
    }
}

So what we can do is:

[dependencies]
serde = { version = "1.0.152", default-features = false, optional = true }

[features]
serde = ["alloc", "dep:serde"]
alloc = [""serde?/alloc"]

[tdelabro]

@xJonathanLEI I pushed a new version. Where the tree crates are full #![no_std] and don't relly at all on std.
They occasionally need alloc to perform some specific actions that are behind feature flags, such as serde or display-field-element.

I also removed thiserror this error entirely and implemented it manually instead.

I think you will like this version better

[xJonathanLEI]

Thanks for updating this! Looks good to me apart from a few minor issues

[xJonathanLEI]

LGTM. Thanks a lot for working on this!