doc: state that only the latest stable toolchain is supported
This closes The content seemed to make more sense in the
installation section as opposed to the Troubleshooting section.
@@ -40,6 +40,20 @@ To add the sources manually, run the following command:
$ rustup component add rust-src
```
+=== Toolchain
+
+Rust Analyzer only officially supports using the latest stable toolchain.
+If you are using an older toolchain or have an override set, Rust Analyzer may fail to understand the Rust source.
+You will either need to update your toolchain or use an older version of Rust Analyzer that is compatible with your toolchain.
+
+If you are using an override in your project, you can still force Rust Analyzer to use the stable toolchain via the environment variable `RUSTUP_TOOLCHAIN`.
+For example, with VS Code or coc-rust-analyzer:
+
+[source,json]
+----
+{ "rust-analyzer.server.extraEnv": { "RUSTUP_TOOLCHAIN": "stable" } }
+----
+
=== VS Code
This is the best supported editor at the moment.
