nvim-lsp: Add settings for rust-analyzer

Because the settings are quite complex they are auto-generated. This
commit adds the script creating it & the generated options.

plugins/nvim-lsp/basic-servers.nix

@@ -193,6 +193,9 @@ let
       name = "rust-analyzer";
       description = "Enable rust-analyzer, for Rust.";
       serverName = "rust_analyzer";
+
+      extraOptions = import ./rust-analyzer-config.nix lib;
+      settings = cfg: { rust-analyzer = cfg; };
     }
     {
       name = "tailwindcss";

plugins/nvim-lsp/update_ra.md

@@ -0,0 +1,13 @@
+# Updating rust-analyzer options
+
+Because there a large number of rust-analyzer options it's difficult to handle them by hand.
+
+The options can be fetched from the [rust-analyzer package.json](https://github.com/rust-lang/rust-analyzer/blob/master/editors/code/package.json).
+
+There is a derivation on the top-level flake that allows to build it easily, you just have to run:
+
+```bash
+nix build .#rustAnalyzerOptions
+```
+
+You can then copy the `result/share/rust-analyzer-config.nix` to the correct location.