fix(lsp): filter code_action diagnostics to the cursor #38988

Problem:
Cursor-position `vim.lsp.buf.code_action()` requests include all diagnostics on the current line, so unrelated same-line diagnostics affect the returned actions.

Solution:
Filter same-line diagnostics to the cursor position for cursor-position requests.

2 files changed, 137 insertions, 9 deletions

diff --git a/runtime/lua/vim/lsp/buf.lua b/runtime/lua/vim/lsp/buf.lua
index ad98356ecf..926d82c986 100644
--- a/runtime/lua/vim/lsp/buf.lua
+++ b/runtime/lua/vim/lsp/buf.lua
@@ -1309,6 +1309,24 @@ local function on_code_action_results(results, opts)
   vim.ui.select(actions, select_opts, on_user_choice)
 end
 
+---@param diagnostic vim.Diagnostic
+---@param bufnr integer
+---@param lnum integer
+---@param col integer
+---@return boolean
+local function diagnostic_contains_cursor(diagnostic, bufnr, lnum, col)
+  local start = vim.pos(bufnr, diagnostic.lnum, diagnostic.col)
+  local finish =
+    vim.pos(bufnr, diagnostic.end_lnum or diagnostic.lnum, diagnostic.end_col or diagnostic.col)
+  local cursor = vim.pos(bufnr, lnum, col)
+
+  if start == finish then
+    return cursor == start
+  end
+
+  return start <= cursor and cursor < finish
+end
+
 --- Selects a code action (LSP: "textDocument/codeAction" request) available at cursor position.
 ---
 ---@param opts? vim.lsp.buf.code_action.Opts
@@ -1330,6 +1348,13 @@ function M.code_action(opts)
   local mode = api.nvim_get_mode().mode
   local bufnr = api.nvim_get_current_buf()
   local win = api.nvim_get_current_win()
+  local range = opts.range
+  if range == nil and (mode == 'v' or mode == 'V') then
+    range = range_from_selection(bufnr, mode)
+  end
+  local cursor = api.nvim_win_get_cursor(win)
+  local lnum = cursor[1] - 1
+  local col = cursor[2]
   local clients = lsp.get_clients({ bufnr = bufnr, method = 'textDocument/codeAction' })
   if not next(clients) then
     vim.notify(lsp._unsupported_method('textDocument/codeAction'), vim.log.levels.WARN)
@@ -1340,15 +1365,11 @@ function M.code_action(opts)
     ---@type lsp.CodeActionParams
     local params
 
-    if opts.range then
-      assert(type(opts.range) == 'table', 'code_action range must be a table')
-      local start = assert(opts.range.start, 'range must have a `start` property')
-      local end_ = assert(opts.range['end'], 'range must have a `end` property')
+    if range then
+      assert(type(range) == 'table', 'code_action range must be a table')
+      local start = assert(range.start, 'range must have a `start` property')
+      local end_ = assert(range['end'], 'range must have a `end` property')
       params = util.make_given_range_params(start, end_, bufnr, client.offset_encoding)
-    elseif mode == 'v' or mode == 'V' then
-      local range = range_from_selection(bufnr, mode)
-      params =
-        util.make_given_range_params(range.start, range['end'], bufnr, client.offset_encoding)
     else
       params = util.make_range_params(win, client.offset_encoding)
     end
@@ -1360,7 +1381,6 @@ function M.code_action(opts)
     else
       local ns_push = lsp.diagnostic.get_namespace(client.id)
       local diagnostics = {}
-      local lnum = api.nvim_win_get_cursor(0)[1] - 1
 
       client:_provider_foreach('textDocument/diagnostic', function(cap)
         local ns_pull = lsp.diagnostic.get_namespace(client.id, true, cap.identifier)
@@ -1371,6 +1391,11 @@ function M.code_action(opts)
       end)
 
       vim.list_extend(diagnostics, vim.diagnostic.get(bufnr, { namespace = ns_push, lnum = lnum }))
+      if range == nil then
+        diagnostics = vim.tbl_filter(function(diagnostic)
+          return diagnostic_contains_cursor(diagnostic, bufnr, lnum, col)
+        end, diagnostics)
+      end
       params.context = vim.tbl_extend('force', context, {
         ---@diagnostic disable-next-line: no-unknown
         diagnostics = vim.tbl_map(function(d)
diff --git a/test/functional/plugin/lsp/buf_spec.lua b/test/functional/plugin/lsp/buf_spec.lua
index a2afbe56f7..4f88108419 100644
--- a/test/functional/plugin/lsp/buf_spec.lua
+++ b/test/functional/plugin/lsp/buf_spec.lua
@@ -968,6 +968,109 @@ describe('vim.lsp.buf', function()
       }
     end)
 
+    it('uses diagnostics at cursor position', function()
+      exec_lua(create_server_definition)
+      local severity = exec_lua(function()
+        return vim.diagnostic.severity.ERROR
+      end)
+      local messages = exec_lua(function(severity_)
+        local server = _G._create_server({
+          capabilities = {
+            codeActionProvider = true,
+          },
+          handlers = {
+            ['textDocument/codeAction'] = function(_, _, callback)
+              callback(nil, {})
+            end,
+          },
+        })
+
+        local bufnr = vim.api.nvim_get_current_buf()
+        vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, { 'local first, second = 1, 2' })
+
+        local client_id = assert(vim.lsp.start({
+          name = 'dummy',
+          cmd = server.cmd,
+        }))
+
+        local expected_messages = 2 -- initialize, initialized
+        local function wait_for_messages()
+          assert(
+            vim.wait(200, function()
+              return #server.messages == expected_messages
+            end),
+            'Timed out waiting for expected number of messages. Current messages seen so far: '
+              .. vim.inspect(server.messages)
+          )
+        end
+
+        wait_for_messages()
+
+        vim.api.nvim_win_set_cursor(0, { 1, 15 })
+
+        local ns = vim.lsp.diagnostic.get_namespace(client_id)
+        vim.diagnostic.set(ns, bufnr, {
+          {
+            lnum = 0,
+            col = 6,
+            end_lnum = 0,
+            end_col = 11,
+            message = 'first',
+            severity = severity_,
+            user_data = {
+              lsp = {
+                range = {
+                  start = { line = 0, character = 6 },
+                  ['end'] = { line = 0, character = 11 },
+                },
+                message = 'first',
+                severity = severity_,
+              },
+            },
+          },
+          {
+            lnum = 0,
+            col = 13,
+            end_lnum = 0,
+            end_col = 19,
+            message = 'second',
+            severity = severity_,
+            user_data = {
+              lsp = {
+                range = {
+                  start = { line = 0, character = 13 },
+                  ['end'] = { line = 0, character = 19 },
+                },
+                message = 'second',
+                severity = severity_,
+              },
+            },
+          },
+        })
+
+        vim.lsp.buf.code_action()
+
+        expected_messages = expected_messages + 1
+        wait_for_messages()
+
+        vim.lsp.get_client_by_id(client_id):stop()
+
+        return server.messages
+      end, severity)
+
+      eq('textDocument/codeAction', messages[3].method)
+      eq({
+        {
+          range = {
+            start = { line = 0, character = 13 },
+            ['end'] = { line = 0, character = 19 },
+          },
+          message = 'second',
+          severity = severity,
+        },
+      }, messages[3].params.context.diagnostics)
+    end)
+
     it('fallback to command execution on resolve error', function()
       exec_lua(create_server_definition)
       local result = exec_lua(function()