feat(diagnostic): pass diagnostics as data to DiagnosticChanged autocmd (#20173)

runtime/doc/diagnostic.txt

@@ -295,10 +295,18 @@ option in the "signs" table of |vim.diagnostic.config()| or 10 if unset).
 EVENTS                                                  *diagnostic-events*
 
                                                         *DiagnosticChanged*
-DiagnosticChanged       After diagnostics have changed.
+DiagnosticChanged       After diagnostics have changed. When used from Lua,
+                        the new diagnostics are passed to the autocmcd
+                        callback in the "data" table.
 
 Example: >
-    autocmd DiagnosticChanged * lua vim.diagnostic.setqflist({ open = false })
+
+    vim.api.nvim_create_autocmd('DiagnosticChanged', {
+      callback = function(args)
+        local diagnostics = args.data.diagnostics
+        vim.pretty_print(diagnostics)
+      end,
+    })
 <
 ==============================================================================
 Lua module: vim.diagnostic                                    *diagnostic-api*

runtime/lua/vim/diagnostic.lua

@@ -727,6 +727,7 @@ function M.set(namespace, bufnr, diagnostics, opts)
   vim.api.nvim_exec_autocmds('DiagnosticChanged', {
     modeline = false,
     buffer = bufnr,
+    data = { diagnostics = diagnostics },
   })
 end
 

test/functional/lua/diagnostic_spec.lua

@@ -1983,19 +1983,26 @@ end)
     end)
 
     it('triggers the autocommand when diagnostics are set', function()
-      eq(true, exec_lua [[
+      eq({true, true}, exec_lua [[
         -- Set a different buffer as current to test that <abuf> is being set properly in
         -- DiagnosticChanged callbacks
         local tmp = vim.api.nvim_create_buf(false, true)
         vim.api.nvim_set_current_buf(tmp)
 
-        vim.g.diagnostic_autocmd_triggered = 0
-        vim.cmd('autocmd DiagnosticChanged * let g:diagnostic_autocmd_triggered = +expand("<abuf>")')
+        local triggered = {}
+        vim.api.nvim_create_autocmd('DiagnosticChanged', {
+          callback = function(args)
+            triggered = {args.buf, #args.data.diagnostics}
+          end,
+        })
         vim.api.nvim_buf_set_name(diagnostic_bufnr, "test | test")
         vim.diagnostic.set(diagnostic_ns, diagnostic_bufnr, {
           make_error('Diagnostic', 0, 0, 0, 0)
         })
-        return vim.g.diagnostic_autocmd_triggered == diagnostic_bufnr
+        return {
+          triggered[1] == diagnostic_bufnr,
+          triggered[2] == 1,
+        }
       ]])
       end)