feat(vim.ui): vim.ui.open() DWIM mode

Problem:
The `gx` mapping has too much logic in it, so the logic is not in user
configs or plugins, and it lacks test coverage.

Solution:
Move the logic into `vim.ui.open()`.

runtime/lua/vim/_defaults.lua

@@ -97,37 +97,13 @@ do
 
   --- Map |gx| to call |vim.ui.open| on the <cfile> at cursor.
   do
-    local function do_open(uri)
-      local cmd, err = vim.ui.open(uri)
-      local rv = cmd and cmd:wait(1000) or nil
-      if cmd and rv and rv.code ~= 0 then
-        err = ('vim.ui.open: command %s (%d): %s'):format(
-          (rv.code == 124 and 'timeout' or 'failed'),
-          rv.code,
-          vim.inspect(cmd.cmd)
-        )
-      end
-      return err
-    end
-
     local gx_desc =
       'Opens filepath or URI under cursor with the system handler (file explorer, web browser, …)'
     vim.keymap.set({ 'n' }, 'gx', function()
-      for _, url in ipairs(require('vim.ui')._get_urls()) do
-        local err = do_open(url)
-        if err then
-          vim.notify(err, vim.log.levels.ERROR)
-        end
-      end
+      vim.ui.open()
     end, { desc = gx_desc })
     vim.keymap.set({ 'x' }, 'gx', function()
-      local lines =
-        vim.fn.getregion(vim.fn.getpos('.'), vim.fn.getpos('v'), { type = vim.fn.mode() })
-      -- Trim whitespace on each line and concatenate.
-      local err = do_open(table.concat(vim.iter(lines):map(vim.trim):totable()))
-      if err then
-        vim.notify(err, vim.log.levels.ERROR)
-      end
+      vim.ui.open()
     end, { desc = gx_desc })
   end
 

runtime/lua/vim/ui.lua

@@ -124,7 +124,7 @@ end
 --- end
 --- ```
 ---
----@param path string Path or URL to open
+---@param path? string Path or URL to open, or `nil` to get path or URL at cursor.
 ---
 ---@return vim.SystemObj|nil # Command object, or nil if not found.
 ---@return nil|string # Error message on failure, or nil on success.
@@ -132,32 +132,29 @@ end
 ---@see |vim.system()|
 function M.open(path)
   vim.validate({
-    path = { path, 'string' },
+    path = { path, 'string', true },
   })
-  local is_uri = path:match('%w+:')
-  if not is_uri then
-    path = vim.fs.normalize(path)
-  end
 
+  local function do_open(uri)
     local cmd --- @type string[]
     local opts --- @type vim.SystemOpts
 
     opts = { text = true, detach = true }
 
     if vim.fn.has('mac') == 1 then
-    cmd = { 'open', path }
+      cmd = { 'open', uri }
     elseif vim.fn.has('win32') == 1 then
       if vim.fn.executable('rundll32') == 1 then
-      cmd = { 'rundll32', 'url.dll,FileProtocolHandler', path }
+        cmd = { 'rundll32', 'url.dll,FileProtocolHandler', uri }
       else
         return nil, 'vim.ui.open: rundll32 not found'
       end
     elseif vim.fn.executable('wslview') == 1 then
-    cmd = { 'wslview', path }
+      cmd = { 'wslview', uri }
     elseif vim.fn.executable('explorer.exe') == 1 then
-    cmd = { 'explorer.exe', path }
+      cmd = { 'explorer.exe', uri }
     elseif vim.fn.executable('xdg-open') == 1 then
-    cmd = { 'xdg-open', path }
+      cmd = { 'xdg-open', uri }
       opts.stdout = false
       opts.stderr = false
     else
@@ -167,6 +164,48 @@ function M.open(path)
     return vim.system(cmd, opts), nil
   end
 
+  local function do_open2(uri)
+    local cmd, err = do_open(uri)
+    -- wait() terminates the process if necessary (avoids stale processes), and allows us to show an
+    -- error message if needed.
+    local rv = cmd and cmd:wait(1000) or nil
+    if cmd and rv and rv.code ~= 0 then
+      err = ('vim.ui.open: command %s (%d): %s'):format(
+        (rv.code == 124 and 'timeout' or 'failed'),
+        rv.code,
+        vim.inspect(cmd.cmd)
+      )
+    end
+    return err
+  end
+
+  if path then
+    local is_uri = path:match('%w+:')
+    if not is_uri then
+      path = vim.fs.normalize(path)
+    end
+    return do_open(path)
+  else -- DWIM mode: get the URL or path from the cursor position, and show error.
+    local visual = not not vim.fn.mode():match('[vV\22]')
+    if visual then
+      local lines =
+        vim.fn.getregion(vim.fn.getpos('.'), vim.fn.getpos('v'), { type = vim.fn.mode() })
+      -- Trim whitespace on each line and concatenate.
+      local err = do_open2(table.concat(vim.iter(lines):map(vim.trim):totable()))
+      if err then
+        vim.notify(err, vim.log.levels.ERROR)
+      end
+    else
+      for _, url in ipairs(M._get_urls()) do
+        local err = do_open2(url)
+        if err then
+          vim.notify(err, vim.log.levels.ERROR)
+        end
+      end
+    end
+  end
+end
+
 --- Returns all URLs at cursor, if any.
 --- @return string[]
 function M._get_urls()