Rust and defmt-test

You can use OnMCU as the Cargo runner for defmt-test. Add --ignore-trailing-args so Cargo and rust-analyzer can append their test arguments.

Configure the Cargo runner

Add the target and runner to the project's Cargo configuration:

[build]
target = "thumbv7em-none-eabihf"

[target.thumbv7em-none-eabihf]
runner = [
  "onmcu",
  "run",
  "--board",
  "NUCLEO-H743ZI",
  "--ignore-trailing-args",
  "--file",
]
rustflags = [
  "-C", "link-arg=--nmagic",
  "-C", "link-arg=-Tlink.x",
  "-C", "link-arg=-Tdefmt.x",
]

[env]
DEFMT_LOG = "info"

The argument order matters. Cargo appends the linked test ELF, then its filters and harness flags. The command above becomes:

onmcu run \
  --board NUCLEO-H743ZI \
  --ignore-trailing-args \
  --file target/thumbv7em-none-eabihf/debug/deps/integration-… \
  integration::tests::passes --exact --nocapture

--file consumes the ELF path. --ignore-trailing-args discards the remaining development-tool arguments.

Add defmt-test

For an integration-test target:

[dev-dependencies]
defmt = "1"
defmt-rtt = "1"
defmt-test = "0.5"
panic-probe = { version = "1", features = ["print-defmt"] }

[[test]]
name = "integration"
harness = false

Choose the logger, panic handler, HAL, and chip features for your target. A minimal test file looks like this:

#![no_std]
#![no_main]

use defmt_rtt as _;
use panic_probe as _;

#[defmt_test::tests]
mod tests {
    #[test]
    fn arithmetic_works() {
        defmt::assert_eq!(2 + 2, 4);
    }
}

Run it from a terminal:

cargo test --test integration

defmt-test uses semihosting to halt the device after the suite. OnMCU reads that exit, stops the run, and returns the result without waiting for a timeout.

Run from rust-analyzer in VS Code

1. Open the crate as the VS Code workspace so rust-analyzer loads its .cargo/config.toml.
2. Confirm onmcu is on the environment PATH visible to VS Code.
3. Authenticate with onmcu login.
4. Open the test file and select Run Test above the #[test] function or #[defmt_test::tests] module.

rust-analyzer calls Cargo, which builds the test ELF and starts OnMCU. The output appears in VS Code's Rust test terminal.

If VS Code was already running when you installed the CLI, restart it so the extension receives the updated PATH.

CI authentication

Do not use the keyring in CI. Extend the runner with --api-key-from-env, then provide ONMCU_API_KEY through the job's secret environment:

[target.thumbv7em-none-eabihf]
runner = [
  "onmcu",
  "run",
  "--board",
  "NUCLEO-H743ZI",
  "--api-key-from-env",
  "--ignore-trailing-args",
  "--file",
]

- name: Run embedded tests
  env:
    ONMCU_API_KEY: ${{ secrets.ONMCU_API_KEY }}
  run: cargo test --test integration

If you build the ELF in a separate GitHub Actions step, use the OnMCU Action. Keep the Cargo runner when you want to use the same cargo test command locally and in CI.

Why the flag is opt-in

Without --ignore-trailing-args, unexpected values produce a usage error with exit code 2. This catches misspelled CLI options. Enable the flag only in runner configurations where Cargo or an editor is expected to append test arguments.