Can’t get logging to work in a custom pallet when running unit tests

Question

I can’t get logging to work in a custom pallet when running unit tests:

RUST_LOG=runtime=debug cargo test --features rococo-local -- --nocapture

log::debug!("") doesn’t work and println!() doesn’t compile because of no-std. (edited)

The custom pallet is importing the log facade:

use frame_support::{
    dispatch::{DispatchInfo, DispatchResult, PostDispatchInfo},
    ensure, log,
    pallet_prelude::*,
    traits::IsSubType,
};

Cargo.toml does not have a log crate but I tried adding log = "0.4.17" to dependencies and also dev-dependencies but that had no effect. I did not try adding it to both.

I tried RUST_LOG=debug, RUST_LOG=runtime=debug and RUST_LOG=test=debug to no avail.

IMPORTANT NOTE: The log is not in the unit tests. It’s in the custom pallet lib.rs which is being called by the unit test.

Answer 1

This is a Rust question.

log is just some sort of "code".

You need a trigger to activate it.

e.g.

https://github.com/paritytech/substrate/blob/fbddfbd76c60c6fda0024e8a44e82ad776033e4b/frame/merkle-mountain-range/src/tests.rs#L473

Answer 2

Using logging didn't work but this did:

sp_std::if_std! {
  println!("debug log");
}