perf(lsp): include `previousResultId` in `DocumentDiagnosticParams` #32887

Problem:
Users of the Roslyn (C#) LSP have encountered significant delays when
retrieving pull diagnostics in large documents while using Neovim. For
instance, diagnostics in a 2000-line .cs file can take over 20 seconds
to display after edits in Neovim, whereas in VS Code, diagnostics for
the same file are displayed almost instantly.

As [mparq noted](https://github.com/seblj/roslyn.nvim/issues/93#issuecomment-2508940330)
in https://github.com/seblj/roslyn.nvim/issues/93, VS Code leverages
additional parameters specified in the [LSP documentation for
textDocument/diagnostic](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#documentDiagnosticParams),
specifically:

- previousResultId
- identifier

Solution:
When requesting diagnostics, Neovim should include the
`previousResultId` and `identifier` parameters as part of the request.
These parameters enable the server to utilize caching and return
incremental results.

Support for maintaining state is already present in the
[textDocument/semanticTokens implementation](https://github.com/neovim/neovim/blob/8f84167c30692555d3332565605e8a625aebc43c/runtime/lua/vim/lsp/semantic_tokens.lua#L289).
A similar mechanism can be implemented in `textDocument/diagnostic` handler.

1 files changed, 46 insertions, 14 deletions

diff --git a/runtime/lua/vim/lsp/diagnostic.lua b/runtime/lua/vim/lsp/diagnostic.lua
index f611a13c2f..700a3ea1c2 100644
--- a/runtime/lua/vim/lsp/diagnostic.lua
+++ b/runtime/lua/vim/lsp/diagnostic.lua
@@ -1,5 +1,6 @@
 local protocol = require('vim.lsp.protocol')
 local ms = protocol.Methods
+local util = vim.lsp.util
 
 local api = vim.api
 
@@ -7,6 +8,12 @@ local M = {}
 
 local augroup = api.nvim_create_augroup('nvim.lsp.diagnostic', {})
 
+---@class (private) vim.lsp.diagnostic.BufState
+---@field enabled boolean Whether diagnostics are enabled for this buffer
+---@field client_result_id table<integer, string?> Latest responded `resultId`
+---@type table<integer, vim.lsp.diagnostic.BufState?>
+local bufstates = {}
+
 local DEFAULT_CLIENT_ID = -1
 
 ---@param severity lsp.DiagnosticSeverity
@@ -256,7 +263,12 @@ function M.on_diagnostic(error, result, ctx)
     return
   end
 
-  handle_diagnostics(ctx.params.textDocument.uri, ctx.client_id, result.items, true)
+  local client_id = ctx.client_id
+  handle_diagnostics(ctx.params.textDocument.uri, client_id, result.items, true)
+
+  local bufnr = assert(ctx.bufnr)
+  local bufstate = assert(bufstates[bufnr])
+  bufstate.client_result_id[client_id] = result.resultId
 end
 
 --- Clear push diagnostics and diagnostic cache.
@@ -319,11 +331,6 @@ local function clear(bufnr)
   end
 end
 
----@class (private) lsp.diagnostic.bufstate
----@field enabled boolean Whether inlay hints are enabled for this buffer
----@type table<integer, lsp.diagnostic.bufstate>
-local bufstates = {}
-
 --- Disable pull diagnostics for a buffer
 --- @param bufnr integer
 --- @private
@@ -336,13 +343,38 @@ local function disable(bufnr)
 end
 
 --- Refresh diagnostics, only if we have attached clients that support it
----@param bufnr (integer) buffer number
----@param opts? table Additional options to pass to util._refresh
+---@param bufnr integer buffer number
+---@param client_id? integer Client ID to refresh (default: all clients)
+---@param only_visible? boolean Whether to only refresh for the visible regions of the buffer (default: false)
 ---@private
-local function _refresh(bufnr, opts)
-  opts = opts or {}
-  opts['bufnr'] = bufnr
-  vim.lsp.util._refresh(ms.textDocument_diagnostic, opts)
+local function _refresh(bufnr, client_id, only_visible)
+  if
+    only_visible
+    and vim.iter(api.nvim_list_wins()):all(function(window)
+      return api.nvim_win_get_buf(window) ~= bufnr
+    end)
+  then
+    return
+  end
+
+  local method = ms.textDocument_diagnostic
+  local clients = vim.lsp.get_clients({ bufnr = bufnr, method = method, id = client_id })
+  local bufstate = assert(bufstates[bufnr])
+
+  util._cancel_requests({
+    bufnr = bufnr,
+    clients = clients,
+    method = method,
+    type = 'pending',
+  })
+  for _, client in ipairs(clients) do
+    ---@type lsp.DocumentDiagnosticParams
+    local params = {
+      textDocument = util.make_text_document_params(bufnr),
+      previousResultId = bufstate.client_result_id[client.id],
+    }
+    client:request(method, params, nil, bufnr)
+  end
 end
 
 --- Enable pull diagnostics for a buffer
@@ -352,7 +384,7 @@ function M._enable(bufnr)
   bufnr = vim._resolve_bufnr(bufnr)
 
   if not bufstates[bufnr] then
-    bufstates[bufnr] = { enabled = true }
+    bufstates[bufnr] = { enabled = true, client_result_id = {} }
 
     api.nvim_create_autocmd('LspNotify', {
       buffer = bufnr,
@@ -365,7 +397,7 @@ function M._enable(bufnr)
         end
         if bufstates[bufnr] and bufstates[bufnr].enabled then
           local client_id = opts.data.client_id --- @type integer?
-          _refresh(bufnr, { only_visible = true, client_id = client_id })
+          _refresh(bufnr, client_id, true)
         end
       end,
       group = augroup,