feat(api): add err field to nvim_echo() opts

Problem:  We want to deprecate `nvim_err_write(ln)()` but there is no
          obvious replacement (from Lua). Meanwhile we already have
          `nvim_echo()` with an `opts` argument.
Solution: Add `err` argument to `nvim_echo()` that directly maps to
          `:echoerr`.

2 files changed, 39 insertions, 0 deletions

diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index 578fa361e8..e0ab31f702 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -3680,6 +3680,30 @@ describe('API', function()
       async_meths.nvim_echo({ { 'msg\nmsg' }, { 'msg' } }, false, {})
       eq('', exec_capture('messages'))
     end)
+
+    it('can print error message', function()
+      async_meths.nvim_echo({ { 'Error\nMessage' } }, false, { err = true })
+      screen:expect([[
+                                                |
+        {1:~                                       }|*3
+        {3:                                        }|
+        {9:Error}                                   |
+        {9:Message}                                 |
+        {6:Press ENTER or type command to continue}^ |
+      ]])
+      feed(':messages<CR>')
+      screen:expect([[
+        ^                                        |
+        {1:~                                       }|*6
+                                                |
+      ]])
+      async_meths.nvim_echo({ { 'Error' }, { 'Message', 'Special' } }, false, { err = true })
+      screen:expect([[
+        ^                                        |
+        {1:~                                       }|*6
+        {9:Error}{16:Message}                            |
+      ]])
+    end)
   end)
 
   describe('nvim_open_term', function()
diff --git a/test/functional/ui/messages_spec.lua b/test/functional/ui/messages_spec.lua
index 77ffc475b0..06048c665c 100644
--- a/test/functional/ui/messages_spec.lua
+++ b/test/functional/ui/messages_spec.lua
@@ -315,6 +315,21 @@ describe('ui/ext_messages', function()
     })
     feed('<Esc>')
     command('set showmode')
+
+    -- kind=echoerr for nvim_echo() err
+    feed(':call nvim_echo([["Error"], ["Message", "Special"]], 1, #{ err:1 })<CR>')
+    screen:expect({
+      cmdline = { {
+        abort = false,
+      } },
+      messages = {
+        {
+          content = { { 'Error', 9, 6 }, { 'Message', 16, 99 } },
+          history = true,
+          kind = 'echoerr',
+        },
+      },
+    })
   end)
 
   it(':echoerr', function()