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# Features

Understand why and how features are enabled in a rust workspace. `Feature` is able to automatically fix missing feature propagation to dependencies. Eventually it will be ready for CI use to check an MR for consistent feature usage.

## Install

```bash
cargo install -f feature
```

## Example - Fixing feature propagation

Let's check that the `runtime-benchmarks` feature is properly passed down to all the dependencies of the `frame-support` crate in the workspace of [Substrate]:  

```bash
feature lint propagate-feature --manifest-path ../substrate/Cargo.toml --feature runtime-benchmarks --workspace -p frame-support
```

The output reveals that there are some dependencies that expose the feature but don't get it passed down:  

```pre
Analyzing workspace
crate "frame-support"
  feature "runtime-benchmarks"
    must propagate to:
      frame-system
      sp-runtime
      sp-staking
Generated 1 errors and 0 warnings and fixed 0 issues.
```

Without the `-p` it will detect many more problems. You can verify this for the [frame-support](https://github.com/paritytech/substrate/blob/ce2cee35f8f0fc5968ea6ffaffa6660dcd008804/frame/support/Cargo.toml#L71) which is indeed missing the feature for `sp-runtime` while that is clearly [sp-runtime](https://github.com/paritytech/substrate/blob/0b6aec52a90870c999856cd37f7d04789cdd8dfc/primitives/runtime/Cargo.toml#L43) it 🤔.

This can be fixed by applying the `--fix` flag like:  

```bash
feature lint propagate-feature --manifest-path ../substrate/Cargo.toml --feature runtime-benchmarks --workspace -p frame-support --fix
```

Which results in this diff:

```patch
-runtime-benchmarks = []
+runtime-benchmarks = [
+       "frame-system/runtime-benchmarks",
+       "sp-runtime/runtime-benchmarks",
+       "sp-staking/runtime-benchmarks"
+]
```

The auto-fix is currently a bit coarse, and does not check for optional dependencies. It will also not add the feature to crates that do not have it, but need it because of a dependency. This will be fixed soon.

## Example - Dependency tracing

Recently there was a build error in the [Substrate](https://github.com/paritytech/substrate) master CI which was caused by a downstream dependency [`snow`](https://github.com/mcginty/snow/issues/146). To investigate this, it is useful to see *how* Substrate depends on it.  

Let's find out how `node-cli` depends on `snow`:

```bash
feature trace --manifest-path substrate/Cargo.toml node-cli snow
```

output:

```
node-cli -> try-runtime-cli -> substrate-rpc-client -> sc-rpc-api ->sc-chain-spec -> sc-telemetry -> libp2p -> libp2p-webrtc -> libp2p-noise -> snow
```

So it comes from libp2p, okay. Good to know.

## Roadmap

- [ ] Add feature information to the enabled deps
- [ ] Allow manual skipping of dev dependencies (currently always skipped)
- [ ] Introduce filters for versions and features for argument `to`
- [ ] Optimize `shortest_path` function
- [ ] Create lint rules which can be used to validate that certain constraints in the work-space hold

<!-- LINKS -->
[Cumulus]: https://github.com/paritytech/cumulus
[Substrate]: https://github.com/paritytech/substrate