Bug 1766045 - Document generating JS bindings with UniFFI r=teshaq,markh

Differential Revision: https://phabricator.services.mozilla.com/D144470

docs/writing-rust-code/uniffi.md

@@ -0,0 +1,54 @@
+# Generating Javascript bindings with UniFFI
+
+Firefox supports auto-generating JS bindings for Rust components using [UniFFI](https://mozilla.github.io/uniffi-rs/).
+
+## How it works
+
+The Rust crate contains a
+[UniFFI Definition Language (UDL) file](https://mozilla.github.io/uniffi-rs/udl_file_spec.html), which describes the
+interface to generate bindings for.
+
+The UniFFI core generates the scaffolding: Rust code which acts as the FFI layer from the UDL file.  The functions of
+this layer all use the C calling convention and all structs use a C layout, this is the de facto standard for FFI
+interoperability.
+
+The [`uniffi-bindgen-gecko-js`](https://searchfox.org/mozilla-central/source/toolkit/components/uniffi-bindgen-gecko-js)
+tool, which lives in the Firefox source tree, generates 2 things:
+  - A JS interface for the scaffolding code, which uses [WebIDL](/dom/bindings/webidl/index.rst)
+  - A JSM module that uses the scaffolding to provide the bindings API.
+
+Currently, this generated code gets checked in to source control.  We are working on a system to avoid this and
+auto-generate it at build time instead (see [bugzilla 1756214](https://bugzilla.mozilla.org/show_bug.cgi?id=1756214)).
+
+## Before creating new bindings with UniFFI
+
+Keep a few things in mind before you create a new set of bindings:
+
+ - **UniFFI was not written to maximize performance.**  It's code is efficient enough to handle many use cases, but at this
+   point should probably be avoided for performance critical components.
+ - **uniffi-bindgen-gecko-js bindings run with chrome privileges.**  Make sure this is acceptable for your project
+ - **Only a subset of Rust types can be exposed via the FFI.**  Check the [UniFFI Book](https://mozilla.github.io/uniffi-rs/) to see what
+   types are compatible with UniFFI.
+
+If any of these are blockers for your work, consider discussing it further with the UniFFI devs to see if we can support
+your project:
+
+  - Chat with us on `#uniffi` on Matrix/Element
+  - File an issue on [mozilla/uniffi](https://github.com/mozilla/uniffi-rs/)
+
+## Creating new bindings with UniFFI
+
+Here's how you can create a new set of bindings using UniFFI:
+  - UniFFI your crate (if it isn't already):
+    - [Create a UDL file to describe your interface](https://mozilla.github.io/uniffi-rs/udl_file_spec.html)
+    - [Set up your Rust crate to generate scaffolding](https://mozilla.github.io/uniffi-rs/tutorial/Rust_scaffolding.html)
+  - Add your crate as a Firefox dependency (if it isn't already)
+    - If the code will exist in the mozilla-central repo:
+      - Create a new directory for the Rust crate
+      - Edit `toolkit/library/rust/shared/Cargo.toml` and add a dependency to your library path
+    - If the code exists in an external repo:
+      - Edit `toolkit/library/rust/shared/Cargo.toml` and add a dependency to your library URL
+      - Run `mach vendor rust` to vendor in your Rust code
+  - Generate bindings code for your crate
+    - TODO: create a system for this and document it here.  I think we can do something like we do for the test
+      fixtures, where devs don't need to touch that many moz.build files.