fix(#2157): use stdpath cache for nvim-tree-on-attach.lua (#2159)

* fix(#2157): use stdpath cache for my_on_attach.lua

* fix(#2157): my_on_attach.lua -> nvim-tree-on-attach.lua

doc/nvim-tree-lua.txt

@@ -194,9 +194,10 @@ Requirements
 
 *:NvimTreeGenerateOnAttach*
 
-    Creates and opens a new file `/tmp/my_on_attach.lua` containing an
-    |nvim-tree.on_attach| function based on your |nvim-tree.view.mappings|,
-    |nvim-tree.remove_keymaps| as well as the defaults.
+    Creates and opens a new file `nvim-tree-on-attach.lua` in |stdpath| `"cache"`,
+    usually `$XDG_CACHE_HOME/nvim`. Contains |nvim-tree.on_attach| function based
+    on your |nvim-tree.view.mappings|, |nvim-tree.remove_keymaps| as well as the
+    defaults.
     See https://github.com/nvim-tree/nvim-tree.lua/wiki/Migrating-To-on_attach
 
 ==============================================================================
@@ -1219,7 +1220,8 @@ In the event of a problem please disable the experiment and raise an issue.
 Configuration for diagnostic logging.
 
     *nvim-tree.log.enable*
-    Enable logging to a file `$XDG_CACHE_HOME/nvim/nvim-tree.log`
+    Enable logging to a file `nvim-tree.log` in |stdpath| `"cache"`, usually
+    `$XDG_CACHE_HOME/nvim`
       Type: `boolean`, Default: `false`
 
     *nvim-tree.log.truncate*

lua/nvim-tree/keymap-legacy.lua

@@ -2,6 +2,7 @@ local api = require "nvim-tree.api"
 local open_file = require "nvim-tree.actions.node.open-file"
 local keymap = require "nvim-tree.keymap"
 local notify = require "nvim-tree.notify"
+local utils = require "nvim-tree.utils"
 
 local M = {
   -- only populated when legacy mappings active
@@ -401,7 +402,7 @@ function M.cmd_generate_on_attach()
     return
   end
 
-  local name = "/tmp/my_on_attach.lua"
+  local name = utils.path_join { vim.fn.stdpath "cache", "nvim-tree-on-attach.lua" }
   local file = io.output(name)
   io.write(M.on_attach_lua)
   io.close(file)