no_std support

[quodlibetor]

mentioned in 195, we need a good story around no_std/alloc support.

The parts of this that jump right to mind:

• Breaking up the core mathy parts of chrono into their own crates/features
• Refactoring various stringy bits to be in terms of Write
• parts of chrono around parsing allocate, too, I believe.

[quodlibetor]

Honestly I'm not sure how far Chrono is from supporting no_std when the default features are turned off. I think some people might already be using it semi-successfully? Code that people would like to work, but does not, would be welcome.

For example, this seems to work:

#![no_std]

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        use chrono::TimeZone;
        let _ = chrono::Utc.ymd(1999, 1,1).and_hms(1, 2, 3);
    }
}

[alex]

My understanding is that testing #![no_std] with cargo test doesn't necessarily prove what you expect, because libtest links std.

[quodlibetor]

that makes sense, this also works, though:

# Cargo.toml
[package]
name = "nostd"
version = "0.1.0"
authors = ["Brandon W Maister <quodlibetor@gmail.com>"]
edition = "2018"

[dependencies]
chrono = { version = "0.4", default-features = false }

// src/lib.rs
#![no_std]

use chrono::TimeZone;

pub fn dt() -> chrono::DateTime<chrono::Utc> {
    chrono::Utc.ymd(1999, 1,1).and_hms(1, 2, 3)
}

$ cargo build
   Compiling nostd v0.1.0 (/Users/bwm/github/chrono/examples/nostd)
    Finished dev [unoptimized + debuginfo] target(s) in 0.14s

[quodlibetor]

I'm pretty sure that chrono doesn't work well in no-std environments, but I don't know what people would expect to work, or how folks would like to be able to interact with it. Examples of code that doesn't work that anyone would expect to would be great.

[alex]

I haven't explored too deeply on what does or does not work, but the code I hope would be possible to make work is: https://github.com/alex/rust-asn1/blob/0fe97f6cdddc7ba718e12b3f0880dc2f924915b5/src/parser.rs#L281-L324

[quodlibetor]

Do you have an example of the exact error that you're getting? Most of the code in there seems like it should either already work or be trivial to make work.

edit: oh no I said should be trivial.

[alex]

I don't, it's entirely possible this works fine! I generally assume crates aren't no_std friendly unless they explicitly document it :-)

[quodlibetor]

haha lmk how it goes and I'll document it 😛

[CryZe]

Considering my crate makes heavy use of chrono, I'll take a shot at this. Atm I've polyfilled chrono, but that shouldn't be necessary.

[quodlibetor]

thanks!

[CryZe]

Alright, I have a working branch now. The only remaining problem is that I needed to get rid of all the dev-dependencies