# Rust: When Cargo Code Coverage Works Locally But Fails in CI

A short post to help if you run into the same issue.

**TLDR:** If your code coverage reports show vastly different results between local development and CI, explicitly set `--engine llvm` in your cargo tarpaulin command.

I recently ran into a frustration you may be familiar with: your code works perfectly on your machine, but fails on CI. The most annoying part are the waiting times for CI to finish. You context-switch to other work, if you do not forget, return to debug. It breaks flow and wastes time. Stuff you do not want to be spending time on.

## The issue

My Rust code coverage reports were showing different results between local development (94%) and CI (22%). Same codebase, same test suite.

The original cargo command:

```bash
cargo tarpaulin --out xml
```

Relevant in this case: I test most of my functionality in the `tests` directory (integration/acceptance tests) and this is the coverage that seemed to be missing.

## The Solution

Add the `--engine llvm` flag to your cargo tarpaulin command:

```bash
cargo tarpaulin --engine llvm --out xml
```

This forces `tarpaulin` to use the coverage engine *that works* in all environments.

## Explanation

Cargo tarpaulin uses different default engines on different platforms. On Linux (CI), it defaults to the `ptrace` engine. On macOS (my local machine), it uses the `llvm` engine.

The `ptrace` engine traces system calls to measure coverage. The `ptrace` engine struggles with tests that spawn subprocesses (like acceptance tests running your compiled binary), whilst the `llvm` engine handles this correctly.