Add parking_lot README

Author: sarsko

wrappers/parking_lot/README.md

@@ -0,0 +1,23 @@
+# Shuttle support for `parking_lot`
+
+This folder contains the implementation and wrapper that enables testing of [parking_lot](https://crates.io/crates/parking_lot) applications with Shuttle.
+
+## How to use
+
+To use it, add the following in your Cargo.toml:
+
+```
+[features]
+shuttle = [
+   "parking_lot/shuttle",
+]
+
+[dependencies]
+parking_lot = { package = "shuttle-parking_lot", version = "VERSION_NUMBER" }
+```
+
+The code will then behave as before when the `shuttle` feature flag is not provided, and will run with Shuttle-compatible primitives when the `shuttle` feature flag is provided.
+
+## Limitations
+
+Shuttle's parking_lot functionality is currently limited to a subset of the `Mutex` and `RwLock` primitives. If your project needs functionality which is not currently supported, please file an issue or, better yet, open a PR to contribute the functionality.

wrappers/parking_lot/parking_lot_impl/src/rwlock.rs

@@ -32,13 +32,11 @@ impl<T> RwLock<T> {
     /// Creates a new instance of an `RwLock<T>` which is unlocked
     /// and allows a maximum of `max_readers` concurrent readers.
     const fn with_max_readers(value: T, max_readers: usize) -> Self {
-        let sem = BatchSemaphore::const_new(max_readers, Fairness::StrictlyFair);
-        let rwlock = RwLock {
+        RwLock {
             max_readers,
-            sem,
+            sem: BatchSemaphore::const_new(max_readers, Fairness::StrictlyFair),
             inner: UnsafeCell::new(value),
-        };
-        rwlock
+        }
     }
 }