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+# XPCOM components in Rust
+
+XPCOM components can be written in Rust.
+
+## A tiny example
+
+The following example shows a new type that implements `nsIObserver`.
+
+First, create a new empty crate (e.g. with `cargo init --lib`), and add the
+following dependencies in its `Cargo.toml` file.
+
+```toml
+[dependencies]
+libc = "0.2"
+nserror = { path = "../../../xpcom/rust/nserror" }
+nsstring = { path = "../../../xpcom/rust/nsstring" }
+xpcom = { path = "../../../xpcom/rust/xpcom" }
+```
+
+(The number of `../` occurrences will depend on the depth of the crate in the
+file hierarchy.)
+
+Next hook it into the build system according to the [build
+documentation](/build/buildsystem/rust.rst).
+
+The Rust code will need to import some basic types. `xpcom::interfaces`
+contains all the usual `nsI` interfaces.
+
+```rust
+use libc::c_char;
+use nserror::nsresult;
+use std::sync::atomic::{AtomicBool, Ordering};
+use xpcom::{interfaces::nsISupports, RefPtr};
+```
+
+The next part declares the implementation.
+
+```rust
+#[xpcom(implement(nsIObserver), atomic)]
+struct MyObserver {
+    ran: AtomicBool,
+}
+```
+
+This defines the implementation type, which will be refcounted in the specified
+way and implement the listed xpidl interfaces. It will also declare a second
+initializer struct `InitMyObserver` which can be used to allocate a new
+`MyObserver` using the `MyObserver::allocate` method.
+
+Next, all interface methods are declared in the `impl` block as `unsafe` methods.
+
+```rust
+impl MyObserver {
+    #[allow(non_snake_case)]
+    unsafe fn Observe(
+        &self,
+        _subject: *const nsISupports,
+        _topic: *const c_char,
+        _data: *const u16,
+    ) -> nsresult {
+        self.ran.store(true, Ordering::SeqCst);
+        nserror::NS_OK
+    }
+}
+```
+
+These methods always take `&self`, not `&mut self`, so we need to use interior
+mutability: `AtomicBool`, `RefCell`, `Cell`, etc. This is because all XPCOM
+objects are reference counted (like `Arc<T>`), so cannot provide exclusive access.
+
+XPCOM methods are unsafe by default, but the
+[xpcom_method!](https://searchfox.org/mozilla-central/source/xpcom/rust/xpcom/src/method.rs)
+macro can be used to clean this up. It also takes care of null-checking and
+hiding pointers behind references, lets you return a `Result` instead of an
+`nsresult,` and so on.
+
+To use this type within Rust code, do something like the following.
+
+```rust
+let observer = MyObserver::allocate(InitMyObserver {
+  ran: AtomicBool::new(false),
+});
+let rv = unsafe {
+  observer.Observe(x.coerce(),
+                   cstr!("some-topic").as_ptr(),
+                   ptr::null())
+};
+assert!(rv.succeeded());
+```
+
+The implementation has an (auto-generated) `allocate` method that takes in an
+initialization struct, and returns a `RefPtr` to the instance.
+
+`coerce` casts any XPCOM object to one of its base interfaces; in this case,
+the base interface is `nsISupports`. In C++, this would be handled
+automatically through inheritance, but Rust doesn’t have inheritance, so the
+conversion must be explicit.
+
+## Bigger examples
+
+The following XPCOM components are written in Rust.
+
+- [kvstore](https://searchfox.org/mozilla-central/source/toolkit/components/kvstore),
+  which exposes the LMDB key-value store (via the [Rkv
+  library](https://docs.rs/rkv)) The API is asynchronous, using `moz_task` to
+  schedule all I/O on a background thread, and supports getting, setting, and
+  iterating over keys.
+- [cert_storage](https://searchfox.org/mozilla-central/source/security/manager/ssl/cert_storage),
+  which stores lists of [revoked intermediate certificates](https://blog.mozilla.org/security/2015/03/03/revoking-intermediate-certificates-introducing-onecrl/).
+- [bookmark_sync](https://searchfox.org/mozilla-central/source/toolkit/components/places/bookmark_sync),
+  which [merges](https://mozilla.github.io/dogear) bookmarks from Firefox Sync
+  with bookmarks in the Places database.
+  [There's also some docs on how Rust interacts with Sync](/services/sync/rust-engines.rst)
+- [webext_storage_bridge](https://searchfox.org/mozilla-central/source/toolkit/components/extensions/storage/webext_storage_bridge),
+  which powers the WebExtension storage.sync API. It's a self-contained example
+  that pulls in a crate from application-services for the heavy lifting, wraps
+  that up in a Rust XPCOM component, and then wraps the component in a JS
+  interface. There's also some boilerplate there around adding a
+  `components.conf` file, and a dummy C++ header that declares the component
+  constructor. [It has some in-depth documentation on how it hangs together](../toolkit/components/extensions/webextensions/webext-storage.rst).