rust-analyzer: add tests with neovim

pkgs/development/tools/rust/rust-analyzer/default.nix

@@ -1,5 +1,6 @@
 { lib
 , stdenv
+, callPackage
 , fetchFromGitHub
 , rustPlatform
 , CoreServices
@@ -54,7 +55,11 @@ rustPlatform.buildRustPackage rec {
     runHook postInstallCheck
   '';
 
-  passthru.updateScript = ./update.sh;
+  passthru = {
+    updateScript = ./update.sh;
+    # FIXME: Pass overrided `rust-analyzer` once `buildRustPackage` also implements #119942
+    tests.neovim-lsp = callPackage ./test-neovim-lsp.nix { };
+  };
 
   meta = with lib; {
     description = "A modular compiler frontend for the Rust language";

pkgs/development/tools/rust/rust-analyzer/test-neovim-lsp.nix

@@ -0,0 +1,49 @@
+{ runCommandNoCC, cargo, neovim, rust-analyzer, rustc }:
+runCommandNoCC "test-neovim-rust-analyzer" {
+  nativeBuildInputs = [ cargo neovim rust-analyzer rustc ];
+
+  testRustSrc = /* rust */ ''
+    fn main() {
+      let mut var = vec![None];
+      var.push(Some("hello".to_owned()));
+    }
+  '';
+
+  nvimConfig = /* lua */ ''
+    vim.lsp.buf_attach_client(vim.api.nvim_get_current_buf(), vim.lsp.start_client({
+      cmd = { "rust-analyzer" },
+      handlers = {
+        ["$/progress"] = function(_, msg, ctx)
+          if msg.token == "rustAnalyzer/Indexing" and msg.value.kind == "end" then
+            vim.cmd("goto 23") -- let mut |var =...
+            vim.lsp.buf.hover()
+          end
+        end,
+        ["textDocument/hover"] = function(_, msg, ctx)
+          -- Keep newlines.
+          io.write(msg.contents.value)
+          vim.cmd("q")
+        end,
+      },
+      on_error = function(code)
+        print("error: " .. code)
+        vim.cmd("q")
+      end
+    }))
+  '';
+
+} ''
+  # neovim requires a writable HOME.
+  export HOME="$(pwd)"
+
+  cargo new --bin test-rust-analyzer
+  cd test-rust-analyzer
+  cat <<<"$testRustSrc" >src/main.rs
+  cat <<<"$nvimConfig" >script.lua
+
+  # `-u` doesn't work
+  result="$(nvim --headless +'lua dofile("script.lua")' src/main.rs)"
+  echo "$result"
+  [[ "$result" == *"var: Vec<Option<String>>"* ]]
+  touch $out
+''