Problems With `app_crypto!` Macro

Question

I have the following error while building:

error[E0599]: no function or associated item named `generate_with_phrase` found for struct `crain_pow::app::Pair` in the current scope
   --> node/src/service.rs:211:49
    |
211 |         let (pair, phrase, _) = crain_pow::app::Pair::generate_with_phrase(None);
    |                                                       ^^^^^^^^^^^^^^^^^^^^ function or associated item not found in `crain_pow::app::Pair`
    |
    = help: items from traits can only be used if the trait is in scope

A Pair type is declared using app_crypto! macro in another file:

pub mod app {
    use sp_application_crypto::{app_crypto, sr25519};
    use sp_core::crypto::KeyTypeId;

    pub const ID: KeyTypeId = KeyTypeId(*b"crn1");
    app_crypto!(sr25519, ID);
}

Which enables another macro which implements the generate_with_phrase function (code from substrate's github repo):

/// Implements functions for the `Pair` trait when `feature = "std"` is enabled.
#[doc(hidden)]
#[cfg(feature = "std")]
#[macro_export]
macro_rules! app_crypto_pair_functions_if_std {
    ($pair:ty) => {
        fn generate_with_phrase(password: Option<&str>) -> (Self, String, Self::Seed) {
            let r = <$pair>::generate_with_phrase(password);
            (Self(r.0), r.1, r.2)
        }

        fn from_phrase(
            phrase: &str,
            password: Option<&str>,
        ) -> Result<(Self, Self::Seed), $crate::SecretStringError> {
            <$pair>::from_phrase(phrase, password).map(|r| (Self(r.0), r.1))
        }
    };
}

I see that is requires sp-application-crypto's std feature to be enabled (#[cfg(feature = "std")]). So I enable it in my package's Cargo.toml:

[dependencies]
sp-application-crypto = { version = "6.0.0", default-features = false, git = "https://github.com/paritytech/substrate.git", branch = "polkadot-v0.9.19" }
***snip***
[features]
default = ["std"]
std = [
    "sp-application-crypto/std", # enables sp-application-crypto/full_crypto as well
]

So the std (and full_crypto as well) feature requirement is satisfied. But I still get the error saying that generate_with_phrase function is not implemented.

What might be the problem?

(P.S. This is a MR adding that function long time ago into primitives)

Answer 1

The issue here seems to be with how you are importing this crate.

Rust's feature system is quite strange in behavior, and if there is any situation where you try to use the app_crypto! macro outside of std, you will hit this kind of error.

I suggest that you simply import this crate like so:

[dependencies]
sp-application-crypto = { version = "6.0.0", git = "https://github.com/paritytech/substrate.git", branch = "polkadot-v0.9.19" }

Which involves no features at all, and due to the configuration of the sp-application-crypto, this will imply std by default.

Answer 2

Ok. I've found the solution.

Rust has quite a weird behavior of traits. Let's say we have:
libA with traitA (which hash show() function),
libB with struct structB and impl traitA for structB
libC with code: structB::show()

The last line won't compile. Because, as it turns out, you have to first import traitA into libC with use libA::traitA.

In my case:
libA was sp_core,
libB was pow crate,
libC was node crate,

The solution:
(in the beginning of node/src/service.rs)
use sp_core::Pair;
That's it! It had nothing to do with Cargo.toml's at all!