feat: add `vim.filetype.get_option()`

8 files changed, 176 insertions, 5 deletions

diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 04825381ff..289788f036 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -1971,8 +1971,8 @@ nvim_get_option_value({name}, {*opts})               *nvim_get_option_value()*
                   Implies {scope} is "local".
                 • filetype: |filetype|. Used to get the default option for a
                   specific filetype. Cannot be used with any other option.
-                  Note: this is expensive, it is recommended to cache this
-                  value.
+                  Note: this will trigger |ftplugin| and all |FileType|
+                  autocommands for the corresponding filetype.
 
     Return: ~
         Option value
diff --git a/runtime/doc/lua.txt b/runtime/doc/lua.txt
index 611f2e0d32..e7dcc79f4a 100644
--- a/runtime/doc/lua.txt
+++ b/runtime/doc/lua.txt
@@ -2206,6 +2206,28 @@ add({filetypes})                                          *vim.filetype.add()*
       • {filetypes}  (table) A table containing new filetype maps (see
                      example).
 
+get_option({filetype}, {option})                   *vim.filetype.get_option()*
+    Get the default option value for a {filetype}.
+
+    The returned value is what would be set in a new buffer after 'filetype'
+    is set, meaning it should respect all FileType autocmds and ftplugin
+    files.
+
+    Example: >lua
+      vim.filetype.get_option('vim', 'commentstring')
+<
+
+    Note: this uses |nvim_get_option_value()| but caches the result. This
+    means |ftplugin| and |FileType| autocommands are only triggered once and
+    may not reflect later changes.
+
+    Parameters: ~
+      • {filetype}  string Filetype
+      • {option}    string Option name
+
+    Return: ~
+        string|boolean|integer: Option value
+
 match({args})                                           *vim.filetype.match()*
     Perform filetype detection.
 
diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index b7d5694802..5ac6db6f84 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -213,6 +213,9 @@ The following new APIs or features were added.
 • |nvim_get_option_value()| now has a `filetype` option so it can return the
   default option for a specific filetype.
 
+• |vim.filetype.get_option()| to get the default option value for a specific
+  filetype. This is a wrapper around |nvim_get_option_value()| with caching.
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changes*
 
diff --git a/runtime/lua/vim/filetype.lua b/runtime/lua/vim/filetype.lua
index cf813b19b1..69fcb01e8c 100644
--- a/runtime/lua/vim/filetype.lua
+++ b/runtime/lua/vim/filetype.lua
@@ -2631,4 +2631,24 @@ function M.match(args)
   end
 end
 
+--- Get the default option value for a {filetype}.
+---
+--- The returned value is what would be set in a new buffer after 'filetype'
+--- is set, meaning it should respect all FileType autocmds and ftplugin files.
+---
+--- Example:
+--- <pre>lua
+---   vim.filetype.get_option('vim', 'commentstring')
+--- </pre>
+---
+--- Note: this uses |nvim_get_option_value()| but caches the result.
+--- This means |ftplugin| and |FileType| autocommands are only
+--- triggered once and may not reflect later changes.
+--- @param filetype string Filetype
+--- @param option string Option name
+--- @return string|boolean|integer: Option value
+function M.get_option(filetype, option)
+  return require('vim.filetype.options').get_option(filetype, option)
+end
+
 return M
diff --git a/runtime/lua/vim/filetype/options.lua b/runtime/lua/vim/filetype/options.lua
new file mode 100644
index 0000000000..77a66991d5
--- /dev/null
+++ b/runtime/lua/vim/filetype/options.lua
@@ -0,0 +1,82 @@
+local api = vim.api
+
+local M = {}
+
+local function get_ftplugin_runtime(filetype)
+  return api.nvim__get_runtime({
+    string.format('ftplugin/%s.vim', filetype),
+    string.format('ftplugin/%s_*.vim', filetype),
+    string.format('ftplugin/%s/*.vim', filetype),
+    string.format('ftplugin/%s.lua', filetype),
+    string.format('ftplugin/%s_*.lua', filetype),
+    string.format('ftplugin/%s/*.lua', filetype),
+  }, true, {}) --[[@as string[] ]]
+end
+
+-- Keep track of ftplugin files
+local ftplugin_cache = {} ---@type table<string,table<string,integer>>
+
+-- Keep track of total number of FileType autocmds
+local ft_autocmd_num ---@type integer?
+
+-- Keep track of filetype options
+local ft_option_cache = {} ---@type table<string,table<string,any>>
+
+--- @param path string
+--- @return integer
+local function hash(path)
+  local mtime0 = vim.loop.fs_stat(path).mtime
+  return mtime0.sec * 1000000000 + mtime0.nsec
+end
+
+--- Only update the cache on changes to the number of FileType autocmds
+--- and changes to any ftplugin/ file. This isn't guaranteed to catch everything
+--- but should be good enough.
+--- @param filetype string
+local function update_ft_option_cache(filetype)
+  local new_ftautos = #api.nvim_get_autocmds({ event = 'FileType' })
+  if new_ftautos ~= ft_autocmd_num then
+    -- invalidate
+    ft_option_cache[filetype] = nil
+    ft_autocmd_num = new_ftautos
+  end
+
+  local files = get_ftplugin_runtime(filetype)
+
+  ftplugin_cache[filetype] = ftplugin_cache[filetype] or {}
+
+  if #files ~= #vim.tbl_keys(ftplugin_cache[filetype]) then
+    -- invalidate
+    ft_option_cache[filetype] = nil
+    ftplugin_cache[filetype] = {}
+  end
+
+  for _, f in ipairs(files) do
+    -- VIMRUNTIME should be static so shouldn't need to worry about it changing
+    if not vim.startswith(f, vim.env.VIMRUNTIME) then
+      local mtime = hash(f)
+      if ftplugin_cache[filetype][f] ~= mtime then
+        -- invalidate
+        ft_option_cache[filetype] = nil
+        ftplugin_cache[filetype][f] = mtime
+      end
+    end
+  end
+end
+
+--- @private
+--- @param filetype string Filetype
+--- @param option string Option name
+--- @return string|integer|boolean
+function M.get_option(filetype, option)
+  update_ft_option_cache(filetype)
+
+  if not ft_option_cache[filetype] or not ft_option_cache[filetype][option] then
+    ft_option_cache[filetype] = ft_option_cache[filetype] or {}
+    ft_option_cache[filetype][option] = api.nvim_get_option_value(option, { filetype = filetype })
+  end
+
+  return ft_option_cache[filetype][option]
+end
+
+return M
diff --git a/src/nvim/api/options.c b/src/nvim/api/options.c
index a0351cc6cd..7aef6e6146 100644
--- a/src/nvim/api/options.c
+++ b/src/nvim/api/options.c
@@ -114,7 +114,11 @@ static buf_T *do_ft_buf(char *filetype, aco_save_T *aco, Error *err)
 
   ftbuf->b_p_ft = xstrdup(filetype);
 
-  apply_autocmds(EVENT_FILETYPE, ftbuf->b_p_ft, ftbuf->b_fname, true, ftbuf);
+  TRY_WRAP({
+    try_start();
+    apply_autocmds(EVENT_FILETYPE, ftbuf->b_p_ft, ftbuf->b_fname, true, ftbuf);
+    try_end(err);
+  });
 
   return ftbuf;
 }
@@ -133,8 +137,8 @@ static buf_T *do_ft_buf(char *filetype, aco_save_T *aco, Error *err)
 ///                         Implies {scope} is "local".
 ///                  - filetype: |filetype|. Used to get the default option for a
 ///                    specific filetype. Cannot be used with any other option.
-///                    Note: this is expensive, it is recommended to cache this
-///                    value.
+///                    Note: this will trigger |ftplugin| and all |FileType|
+///                    autocommands for the corresponding filetype.
 /// @param[out] err  Error details, if any
 /// @return          Option value
 Object nvim_get_option_value(String name, Dict(option) *opts, Error *err)
diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index 2af041c706..ff1bfef591 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -1518,6 +1518,31 @@ describe('API', function()
       nvim('get_option_value', 'filetype', {buf = buf})
       eq({1, 9}, nvim('win_get_cursor', win))
     end)
+
+    it('can get default option values for filetypes', function()
+      command('filetype plugin on')
+      for ft, opts in pairs {
+        lua = { commentstring = '-- %s' },
+        vim = { commentstring = '"%s' },
+        man = { tagfunc = 'v:lua.require\'man\'.goto_tag' },
+        xml = { formatexpr = 'xmlformat#Format()' }
+      } do
+        for option, value in pairs(opts) do
+          eq(value, nvim('get_option_value', option, { filetype = ft }))
+        end
+      end
+
+      command'au FileType lua setlocal commentstring=NEW\\ %s'
+
+      eq('NEW %s', nvim('get_option_value', 'commentstring', { filetype = 'lua' }))
+    end)
+
+    it('errors for bad FileType autocmds', function()
+      command'au FileType lua setlocal commentstring=BAD'
+      eq([[FileType Autocommands for "lua": Vim(setlocal):E537: 'commentstring' must be empty or contain %s: commentstring=BAD]],
+         pcall_err(nvim, 'get_option_value', 'commentstring', { filetype = 'lua' }))
+    end)
+
   end)
 
   describe('nvim_{get,set}_current_buf, nvim_list_bufs', function()
diff --git a/test/functional/lua/filetype_spec.lua b/test/functional/lua/filetype_spec.lua
index add69235b6..540eae1c9b 100644
--- a/test/functional/lua/filetype_spec.lua
+++ b/test/functional/lua/filetype_spec.lua
@@ -114,6 +114,21 @@ describe('vim.filetype', function()
     ]])
   end)
 
+  it('can get default option values for filetypes via vim.filetype.get_option()', function()
+    command('filetype plugin on')
+
+    for ft, opts in pairs {
+      lua = { commentstring = '-- %s' },
+      vim = { commentstring = '"%s' },
+      man = { tagfunc = 'v:lua.require\'man\'.goto_tag' },
+      xml = { formatexpr = 'xmlformat#Format()' }
+    } do
+      for option, value in pairs(opts) do
+        eq(value, exec_lua([[ return vim.filetype.get_option(...) ]], ft, option))
+      end
+    end
+
+  end)
 end)
 
 describe('filetype.lua', function()