crate_universe rule (#598)

This rule dynamically looks up crate dependencies as a repository rule.

See #2 for context.

Note that this rule is currently experimental, and offers absolutely no stability guarantees. Expect significant churn in near future.

.bazelignore

@@ -1,3 +1,4 @@
 docs
 examples
+examples/crate_universe
 examples/cargo_manifest_dir/external_crate

.gitignore

@@ -6,7 +6,7 @@
 # bazel
 /bazel-*
 /examples/bazel-*
-/examples/*/bazel-*
+/examples/crate_universe/*/bazel-*
 /docs/bazel-*
 
 # rustfmt

cargo/crate_universe_resolver/CONTRIBUTING.md

@@ -0,0 +1,40 @@
+# Contributing
+
+## Tour of the codebase
+
+We start at `workspace.bzl`, which invokes the resolver.
+
+The resolver:
+* `config.rs`: Deserializes the Config
+* `parser.rs`: Parses the Config, any `Cargo.toml` files, and any additional packages, into a single unified `Cargo.toml` file
+* `resolver.rs`: Resolves all of the crates.
+* `consolidator.rs`: Patches in any WORKSPACE-specified overrides, and deals with platform-specific concerns.
+* `renderer.rs`: Generates BUILD files for each crate, as well as functions that can be called from BUILD files.
+
+The code started off as a very hacky Week of Code project, and there are some clear remnants of that in the codebase - nothing is sacred, feel free to improve anything!
+
+Some areas have unit testing, there are a few (very brittle) integration tests, and some examples.
+
+## How to test local changes
+
+To use a local version, first build it:
+```console
+resolver $ cargo build
+```
+
+Then run a bazel build with this environment variable:
+```console
+RULES_RUST_RESOLVER_URL_OVERRIDE=file:///path/to/resolver/target/debug/resolver bazel build //whatever
+```
+
+To get verbose logging, edit `workspace.bzl` to set `RUST_LOG` to `debug` or `trace` instead of `info`. In particular, that will print out the generated Cargo.toml, and the path to the generated workspace file.
+
+Note that you may need to `bazel shutdown` between bazel commands. Hopefully not, but if you're seeing stale results, try it out. This only affects local development.
+
+## Testing
+
+To test with the `resolver` built from source:
+
+```bash
+bin/test
+```