Rust: add rust-analyzer update instructions

Author: redsun82

rust/README.md

@@ -62,3 +62,51 @@ Sometimes, especially if resolving conflicts on generated files, you might need
 bazel run @codeql//rust/codegen -- --force
 ```
 for code generation to succeed.
+
+### Updating `rust-analyzer`
+
+Here's a rundown of the typical actions to perform to do a rust-analyzer (and other dependencies) update. A one-time setup consists in
+installing [`cargo-edit`](https://crates.io/crates/cargo-edit) with `cargo install cargo-edit`. On Ubuntu that also requires
+`sudo apt install libssl-dev pkg-config`.
+
+1. from the root of the `codeql` repo checkout, run an upgrade, and commit the changes (skipping `pre-commit` hooks if you have them enabled):
+   ```
+   cargo upgrade --incompatible --pinned
+   ```
+2. Look at a diff of the `Cargo.toml` files: if all `ra_ap_` prefixed dependencies have been updated to the same number, go on to the next step.
+   Otherwise, it means the latest `rust-analyzer` update has not been fully rolled out to all its crates in `crates.io`.
+   _All `ra_ap_` versions must agree!_
+   Downgrade by hand to the minimum one you see, and run a `cargo update` after that to fix the `Cargo.lock` file.
+3. Commit the changes, skipping `pre-commit` hooks if you have them enabled:
+   ```
+   git commit -am 'Cargo: upgrade dependencies' --no-verify
+   ```
+4. Regenerate vendored bazel files, commit the changes:
+   ```
+   misc/bazel/3rdparty/update_tree_sitter_extractors_deps.sh
+   git add .
+   git commit -am 'Bazel: regenerate vendored cargo dependencies' --no-verify
+   ```
+5. Run codegen
+   ```
+   bazel run //rust/codegen
+   ```
+   Take note whether `rust/schema/ast.py` was changed. That might need tweaks, new tests and/or downgrade/upgrade scripts down the line
+6. Try compiling
+   ```
+   bazel run //rust:install
+   ```
+   * if it succeeds: good! You can move on to the next step.
+   * if it fails while compiling rust-analyzer dependencies, you need to update the rust toolchain. Sometimes the error will tell you
+     so explcitly, but it may happen that the error is more obscure. To update the rust toolchain:
+      * you will need to open a PR on the internal repo updating `RUST_VERSION` in `MODULE.bazel`. In general you can have this merged
+        independently of the changes in `codeql`.
+      * in `codeql`, update both `RUST_VERSION` in `MODULE.bazel` _and_ `rust-toolchain.toml` files. You may want to also update the
+        nightly toolchain in `rust/extractor/src/nightly-toolchain/rust-toolchain.toml` to a more recent date while you're at it.
+   * if it fails while compiling rust extractor code, you will need to adapt it to the new library version.
+
+   If you had to do any changes, commit them. If you updated the rust toolchain, running `rust/lint.py` might reformat or apply new
+   lints to the code.
+7. If in step 5 the schema was updated, add upgrade/downgrade scripts and a change note
+8. Check with CI if everything is in order.
+9. Run DCA. Iterate on the code if needed.