Add a stub Rust crate (#12595)

docs/deprecation_policy.md

@@ -18,6 +18,12 @@ documented at [https://endoflife.date/python](https://endoflife.date/python) and
 [https://endoflife.date/postgresql](https://endoflife.date/postgresql).
 
 
+A Rust compiler is required to build Synapse from source. For any given release
+the minimum required version may be bumped up to a recent Rust version, and so
+people building from source should ensure they can fetch recent versions of Rust
+(e.g. by using [rustup](https://rustup.rs/)).
+
+
 Context
 -------
 
@@ -31,3 +37,10 @@ long process.
 By following the upstream support life cycles Synapse can ensure that its
 dependencies continue to get security patches, while not requiring system admins
 to constantly update their platform dependencies to the latest versions.
+
+For Rust, the situation is a bit different given that a) the Rust foundation
+does not generally support older Rust versions, and b) the library ecosystem
+generally bump their minimum support Rust versions frequently. In general, the
+Synapse team will try to avoid updating the dependency on Rust to the absolute
+latest version, but introducing a formal policy is hard given the constraints of
+the ecosystem.

docs/development/contributing_guide.md

@@ -28,6 +28,9 @@ The source code of Synapse is hosted on GitHub. You will also need [a recent ver
 
 For some tests, you will need [a recent version of Docker](https://docs.docker.com/get-docker/).
 
+A recent version of the Rust compiler is needed to build the native modules. The
+easiest way of installing the latest version is to use [rustup](https://rustup.rs/).
+
 
 # 3. Get the source.
 
@@ -114,6 +117,11 @@ Some documentation also exists in [Synapse's GitHub
 Wiki](https://github.com/matrix-org/synapse/wiki), although this is primarily
 contributed to by community authors.
 
+When changes are made to any Rust code then you must call either `poetry install`
+or `maturin develop` (if installed) to rebuild the Rust code. Using [`maturin`](https://github.com/PyO3/maturin)
+is quicker than `poetry install`, so is recommended when making frequent
+changes to the Rust code.
+
 
 # 8. Test, test, test!
 <a name="test-test-test"></a>
@@ -195,7 +203,7 @@ The database file can then be inspected with:
 sqlite3 _trial_temp/test.db
 ```
 
-Note that the database file is cleared at the beginning of each test run. Thus it 
+Note that the database file is cleared at the beginning of each test run. Thus it
 will always only contain the data generated by the *last run test*. Though generally
 when debugging, one is only running a single test anyway.
 

docs/setup/installation.md

@@ -196,6 +196,10 @@ System requirements:
 - Python 3.7 or later, up to Python 3.10.
 - At least 1GB of free RAM if you want to join large public rooms like #matrix:matrix.org
 
+If building on an uncommon architecture for which pre-built wheels are
+unavailable, you will need to have a recent Rust compiler installed. The easiest
+way of installing the latest version is to use [rustup](https://rustup.rs/).
+
 To install the Synapse homeserver run:
 
 ```sh