subxt `PairSigner` trait bounds not satisfied

Question

When I try to create a PairSigner, then the Pair traits are not satisfied.

GitHub repo to check the code

With this code:

let pair = sp_core::sr25519::Pair::from_phrase(mnemonic_phrase.as_str(), None).unwrap();

let signer = PairSigner::<PolkadotConfig, sp_core::sr25519::Pair>::new(pair);

I get this error:

error[E0599]: the function or associated item `new` exists for struct `PairSigner<WithExtrinsicParams<SubstrateConfig, BaseExtrinsicParams<SubstrateConfig, PlainTip>>, Pair>`, but its trait bounds were not satisfied
  --> src/main.rs:23:72
   |
23 |     let signer = PairSigner::<PolkadotConfig, sp_core::sr25519::Pair>::new(pair);
   |                                                                        ^^^ function or associated item cannot be called due to unsatisfied trait bounds
   |
  ::: /Users/martin.jensen/.cargo/registry/src/github.com-1ecc6299db9ec823/sp-core-18.0.0/src/sr25519.rs:83:1
   |
83 | pub struct Pair(Keypair);
   | ---------------
   | |
   | doesn't satisfy `_: CryptoType`
   | doesn't satisfy `sp_core::sr25519::Pair: sp_core::crypto::Pair`
   |
   = note: the following trait bounds were not satisfied:
           `sp_core::sr25519::Pair: sp_core::crypto::Pair`
           `sp_core::sr25519::Pair: sp_core::crypto::CryptoType`
           which is required by `sp_core::sr25519::Pair: sp_core::crypto::Pair`

For more information about this error, try `rustc --explain E0599`.

This is my toml dependencies which I followed from the subxt example toml:

[dependencies]
structopt = "0.3.26"
tokio = { version = "1.26", features = ["rt-multi-thread", "macros", "time"] }
sp-keyring = "20.0.0"
sp-core = { version = "18.0.0", default-features = false }
sp-runtime = "20.0.0"
futures = "0.3.27"
codec = { package = "parity-scale-codec", version = "3.0.0", default-features = false, features = ["derive", "full", "bit-vec"] }
hex = "0.4.3"
tracing-subscriber = "0.3.11"
subxt = "0.27.1"

Answer 1

This is a simple case of the subxt and sp_* dependencies being out of sync; I think you used Subxt master branch to take the dependencies you used, but you're pointing at an older version of Subxt in your Cargo.toml. In other words, your dependencies are:

sp-keyring = "20.0.0"
sp-core = { version = "18.0.0", default-features = false }
sp-runtime = "20.0.0"
subxt = "0.26.0"

But If you look at the dependencies for Subxt 0.26.0 (https://crates.io/crates/subxt/0.26.0/dependencies) you see it depends on:

sp-keyring = "12.0.0"
sp-core = "11.0.0"
sp-runtime = "12.0.0"

So, some options are:

• downgrade the versions of the sp_* libraries that you're using to match what Subxt 0.26 is using.
• upgrade your version of Subxt to be in line with the sp_* libraries you're using (I think you'll still need to downgrade the sp_* ones a bit because we need to do a new release with latest).
• use subxt::ext::sp_core:: and subxt::ext::sp_runtime:: instead of specifying those deps yourself to get at the same versions that Subxt uses (may only be available in a more recent version of Subxt than the one you're using; can't recall!)

Answer 2

Add this import:

use sp_core::{Pair};

And then the method from_phrase() returns you a tuple: (Pair, Seed) get only the Pair in your method new:

let pair = sp_core::sr25519::Pair::from_phrase(mnemonic_phrase.as_str(), None).unwrap();

let signer = PairSigner::<PolkadotConfig, sp_core::sr25519::Pair>::new(pair.0);

Or you can use the method from_string():

let pair = sp_core::sr25519::Pair::from_string(mnemonic_phrase.as_str(), None).unwrap();

let signer = PairSigner::<PolkadotConfig, sp_core::sr25519::Pair>::new(pair);

Answer 3

Just wanted to add the solution that worked for me for any future visitors. (Tested with subxt v0.28)

To avoid manually checking and specifying sp_* versions, which did not solve my issues, I used the re-exported structs from subxt itself. I did the following:

use subxt::ext::sp_core::Pair;
use subxt::tx::PairSigner;

...

pub async fn something() -> Result<(), Box<dyn Error>> {

...

    let pair = subxt::ext::sp_core::sr25519::Pair::from_string_with_seed(<'0x'-PREFIXED PRIVATE SEED>, None).unwrap();
    let signer: PairSigner<SubstrateConfig, subxt::ext::sp_core::sr25519::Pair> = PairSigner::<SubstrateConfig, subxt::ext::sp_core::sr25519::Pair>::new(pair.0);

...
}

This will presumably avoid having to check and update the version of each sp_* dependency if the subxt version is changed.

If any subxt maintainer reads this, it could be great with some in-depth explanations/tutorials/guides to set up the tool with a custom Substrate chain. The provided examples and the rust documentation pages did not quite satisfy my needs. That said, the idea of having a tool for interacting with a live chain, which can be validated on compile-time through generated runtime APIs is super cool. I appreciate your hard work.