no-std not compiling

[claravanstaden]

I am trying to use this crate with the Substrate framework, which requires the no-std feature. It looks like this crate supports running in no-std environments, if I look at Cargo.toml. However, when I try to compile the crate I get the following compiler errors:

error[E0433]: failed to resolve: could not find `prelude` in `alloc`
  --> src/lib.rs:15:20
   |
15 |     pub use alloc::prelude::v1::*;
   |                    ^^^^^^^ could not find `prelude` in `alloc`

error[E0433]: failed to resolve: use of undeclared type `Vec`
  --> src/keys.rs:58:27
   |
58 |             let mut prk = Vec::<u8>::with_capacity(1 + ikm.len());
   |                           ^^^ not found in this scope
   |
help: consider importing this struct
   |
5  | use alloc::vec::Vec;
   |

I will try to fix these in a forked version, but just wanted to get your input in case I might be missing something.

Is the crate only supported for nightly builds?

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> src/lib.rs:4:5
  |
4 |     feature(alloc),
  |     ^^^^^^^^^^^^^^

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> src/lib.rs:5:5
  |
5 |     feature(alloc_prelude),
  |     ^^^^^^^^^^^^^^^^^^^^^^

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> src/lib.rs:6:5
  |
6 |     feature(prelude_import)
  |     ^^^^^^^^^^^^^^^^^^^^^^^

[kirk-baird]

Hey sorry for the slow reply, does PR #53 resolve you issue such that it will now compile without std?

[tbraun96]

No, that PR did not actually solve the issue. The command #![cfg_attr(not(std), no_std)] is not equivalent to #![no_std] (see: rust-lang/rust#42190 (comment)).

Related comments:

milagro_bls/Cargo.toml

Line 13 in d3fc0a4

amcl = { path = "./incubator-milagro-crypto-rust", default-features = false, features = ["bls381"]}

There, it points to https://github.com/sigp/incubator-milagro-crypto-rust/tree/057d238936c0cbbe3a59dfae6f2405db1090f474
In that repo, we see the package amcl ’s lib.rs file: https://github.com/sigp/incubator-milagro-crypto-rust/blob/057d238936c0cbbe3a59dfae6f2405db1090f474/src/lib.rs
So, what I did to try forcing a no_std environment was clone the amcl crate above, and add #![no_std] to the lib.rs and see if it would compile. It did not compile.

[claravanstaden]

@tbraun96 good point!

@kirk-baird I think supporting no-std is actually a little more involved than PR #53 accomplished. Essentially a clean way to do it is so like this:

Declare imports: https://github.com/serde-rs/serde/blob/master/serde/src/lib.rs#L151
Usage: https://github.com/serde-rs/serde/blob/master/serde/src/ser/mod.rs#L110

So you toggle between std and core/alloc dependent on if feature std is enabled or not, preferably in a central place in the crate and giving it your own mod name, like lib in serde's case, or std. Then you import it (use lib::*) where any std dependency is used (which is usually everywhere 😋 ).

I attempted this for the ssz_rs crate, PR here: ralexstokes/ssz-rs#25