The Issue with Printing Standard Output in Anchor Tests using Rust
As a developer working on an Anchor project, you’re likely no stranger to the frustrations of debugging tests. One common issue that can arise is when testing the anchor contract using Rust’s Test module doesn’t output any standard error messages. In this article, we’ll explore the reasons behind this behavior and provide solutions to resolve it.
The Problem: Printing Standard Output in Texts
When running a test with cargo test, Anchor’s Test module runs your tests as separate processes. By default, these processes do not output any standard error messages, even if an error occurs during execution. This is because the process is designed to be lightweight and efficient, allowing it to handle errors without consuming too much memory or disk space.
However, this can be problematic when trying to debug issues, especially in tests where you rely on specific behavior from the contract. For instance, let’s say you’re testing a transaction signing functionality using println! statements:
use anchor_lang::prelude::*;
#[anchorcontract]
pub mod my_contract {
use super::{get_token_info, sign_transaction};
#[actix_test]
async fn test_sign_transaction() {
// Get the token info from the local storage
let mute token_info = get_token_info();
// Sign a transaction
let _ = sign_transaction(token_info);
// Check that no error occurred during signing
assert!(false);
}
}
When you run this test, the output will be:
ok
As expected. However, when running an anchor test command and checking the standard output of the process using a tool like wsl-actix or cargo test --verbose, you won’t see any messages.
The Solution: Using the --no-std Flag
To resolve this issue, you can use the --no-std flag when running your anchor tests. This flag tells Anchor to run in non-standard mode, where it will output standard error and warning messages, including those from Rust’s Test module.
Here’s an updated command:
cargo test --no-std -p my_contract
By using --no-std, you’ll see both standard error messages (e.g., “Error: …”) and warning messages, which will help you identify the issue with your code. This solution should work for most Anchor projects.
Alternative Solution: Using a Custom Logging Mechanism
If you prefer not to use the --no-std flag or want more control over logging, you can create a custom logging mechanism using the log crate. Here’s an example:
Create a new file named logger.rs in your project directory and add the following code:
use std::io;
use log::{info, warn};
fn main() {
info!("Starting test...");
}
#[cfg(text)]
mod tests {
use super::*;
#[text]
fn my_test() {
// Test code here...
}
}
In your Cargo.toml file:
[dependencies]
log = "0.4.14"
Now, when you run your test with cargo test, the standard output will include logs from your custom logging mechanism.
Remember to update your tests to use the new logging module, like this:
“`rust
use anchor_lang::prelude::*;
#[anchorcontract]
pub mod my_contract {
use super::{get_token_info, sign_transaction};
#[actix_test]
async fn test_sign_transaction() {
// Get the token info from the local storage
let mute token_info = get_token_info();
// Sign a transaction using custom logging
log::info!(“Signing transaction…”);
let _ = sign_transaction(token_info);
// Check that no error occurred during signing
assert!