refactor(terminal): impl "[Process exited]" in Lua #38343

Problem:
"[Process exited]" is implemented in C with anonymous namespace
and users have no way to hide it.

Solution:
- Handle "TermClose" event in Lua.
- User can delete the "nvim.terminal" augroup to avoid "[Process exited]".

runtime/lua/vim/_core/defaults.lua

@@ -578,6 +578,54 @@ do
     end,
   })
 
+  local nvim_terminal_exitmsg_ns = vim.api.nvim_create_namespace('nvim.terminal.exitmsg')
+
+  --- @param buf integer
+  --- @param msg string
+  --- @param pos integer
+  local function set_terminal_exitmsg(buf, msg, pos)
+    vim.api.nvim_buf_set_extmark(buf, nvim_terminal_exitmsg_ns, pos, 0, {
+      virt_text = { { msg, nil } },
+      virt_text_pos = 'overlay',
+    })
+  end
+
+  vim.api.nvim_create_autocmd('TermClose', {
+    group = nvim_terminal_augroup,
+    nested = true,
+    desc = 'Displays the "[Process exited]" virtual text',
+    callback = function(ev)
+      if not vim.api.nvim_buf_is_valid(ev.buf) then
+        return
+      end
+
+      local buf = vim.bo[ev.buf]
+      local pos = ev.data.pos ---@type integer
+      local buf_has_exitmsg = #(
+          vim.api.nvim_buf_get_extmarks(ev.buf, nvim_terminal_exitmsg_ns, 0, -1, {})
+        ) > 0
+
+      -- `nvim_open_term` buffers do not have any attached chan
+      local msg = buf.channel == 0 and '[Terminal closed]'
+        or ('[Process exited %d]'):format(vim.v.event.status)
+
+      -- TermClose may be queued before TermOpen if the process
+      -- exits before `terminal_open` is called. Don't display
+      -- the msg now, let TermOpen display it.
+      if buf.buftype ~= 'terminal' or buf_has_exitmsg then
+        vim.api.nvim_create_autocmd('TermOpen', {
+          buffer = ev.buf,
+          once = true,
+          callback = function()
+            set_terminal_exitmsg(ev.buf, msg, pos)
+          end,
+        })
+        return
+      end
+      set_terminal_exitmsg(ev.buf, msg, pos)
+    end,
+  })
+
   vim.api.nvim_create_autocmd('TermRequest', {
     group = nvim_terminal_augroup,
     desc = 'Handles OSC foreground/background color requests',
@@ -694,6 +742,9 @@ do
       vim.keymap.set({ 'n', 'x', 'o' }, ']]', function()
         jump_to_prompt(nvim_terminal_prompt_ns, 0, ev.buf, vim.v.count1)
       end, { buffer = ev.buf, desc = 'Jump [count] shell prompts forward' })
+
+      -- If the terminal buffer is being reused, clear the previous exit msg
+      vim.api.nvim_buf_clear_namespace(ev.buf, nvim_terminal_exitmsg_ns, 0, -1)
     end,
   })