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README.md

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+# Rust Cache Action
+
+A GitHub Action that implements smart caching for rust/cargo projects
+
+## Inputs
+
+- `key` - An optional key for the `target` cache. This is useful in case you
+  have different jobs for test / check / clippy, etc
+
+## Example usage
+
+```yaml
+- uses: Swatinem/rust-cache@v1
+  with:
+    key: test
+```
+
+## Specifics
+
+This action tries to be better than just caching the following directories:
+
+```
+~/.cargo/registry
+~/.cargo/git
+target
+```
+
+It disables incremental compilation and only caches dependencies. The
+assumption is that we will likely recompile the own crate(s) anyway.
+
+It also separates the cache into 4 groups, each treated differently:
+
+- Index: `~/.cargo/registry/index/<registry>`:
+
+  This is always restored from its latest snapshot, and persisted based on the
+  most recent revision.
+
+- Registry / Cache: `~/.cargo/registry/cache/<registry>`:
+
+  Automatically keyed by the lockfile/toml hash, and is being pruned to only
+  persist the dependencies that are being used.
+
+- Registry / Git: `~/.cargo/registry/git/<registry>`:
+
+  Automatically keyed by the lockfile/toml hash. Pruning is still TODO.
+
+- target: `./target`
+
+  Automatically keyed by the lockfile/toml hash, and is being pruned to only
+  persist the dependencies that are being used. This is especially throwing
+  away any intermediate artifacts.