sp-core `full_crypto` is no_std incompatible

[Slesarew]

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• This is not a support question.

Description of bug

When building for no_std targets, sp-core dependency fails due to issue with libsecp256k1

$cargo build --target=thumbv7m-none-eabi

<truncated>

error[E0463]: can't find crate for `std`
  --> /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/subtle-2.4.1/src/lib.rs:88:1
   |
88 | extern crate std;
   | ^^^^^^^^^^^^^^^^^ can't find crate
   |
   = note: the `thumbv7m-none-eabi` target may not support the standard library
   = help: consider building the standard library from source with `cargo build -Zbuild-std`

See more details and proposed fix paritytech/libsecp256k1#146

Steps to reproduce

1. Install appropriate targets
2. $cargo generate --git https://github.com/rust-embedded/cortex-m-quickstart
3. give test project a name, cd
4. add to Cargo.toml sp-core = {version = "7.0.0", default-features = false, features = ["full_crypto"]} dependency
5. $cargo build --target=thumbv7m-none-eabi

[bkchr]

There was recently an issue around no_std incompatibility. Which was solved? #12225

Maybe the problem is the target architecture.

[Slesarew]

Looks similar, but this time it's ECDSA, different crypto and different crate. Issue you refer to is about ed25519-zebra crate, while this is libsecp256k1 crate, and it still persists even in 7.0.0

With a tweak proposed above it builds nicely, should be a simple fix since the lib is also owned by Parity.

[bkchr]

Ahh the problem is that libsecp256k1 is pulled in build-dependencies.

The solution to your problem is to add:

[workspace]
resolver = "2"

in your workspace Cargo.toml.

[Slesarew]

It does work indeed, did not notice that generate script still uses 2018 rust version. Setting rust edition to 2021 seems even better here.