feat(options): allow setting function values to vim.o/go/bo

Problem:
Assigning function values to options is tedious.

Solution:
Allow setting function values to options directly.

runtime/doc/lsp.txt

@@ -746,8 +746,11 @@ formatexpr({opts})                                      *vim.lsp.formatexpr()*
 
     Currently only supports a single client. This can be set via
     `setlocal formatexpr=v:lua.vim.lsp.formatexpr()` or (more typically) in
-    `on_attach` via
-    `vim.bo[bufnr].formatexpr = 'v:lua.vim.lsp.formatexpr(#{timeout_ms:250})'`.
+    `on_attach` via: >lua
+          vim.bo[bufnr].formatexpr = function()
+            return vim.lsp.formatexpr({ timeout_ms = 250 })
+          end
+<
 
     Parameters: ~
       • {opts}  (`table?`) A table with the following fields:

runtime/doc/lua.txt

@@ -1293,6 +1293,14 @@ window-scoped options. Note that this must NOT be confused with
 |local-options| and |:setlocal|. There is also |vim.go| that only accesses the
 global value of a |global-local| option, see |:setglobal|.
 
+Unlike |nvim_set_option_value()|, |vim.o|, |vim.go|, |vim.bo|
+and |vim.wo| can be assigned function values.
+
+Example: >lua
+    vim.bo.tagfunc = vim.lsp.tagfunc
+    vim.wo[1000].foldexpr = vim.treesitter.foldexpr
+<
+
 
                                                                *vim.opt_local*
                                                               *vim.opt_global*

runtime/lua/vim/_options.lua

@@ -134,6 +134,145 @@ local function get_options_info(name)
   return info
 end
 
+local func_opts = {
+  -- Options that can be set as a function
+  func = {
+    completefunc = true,
+    findfunc = true,
+    omnifunc = true,
+    operatorfunc = true,
+    quickfixtextfunc = true,
+    tagfunc = true,
+    thesaurusfunc = true,
+  },
+
+  -- Options that can be set as an expression
+  expr = {
+    diffexpr = true,
+    foldexpr = true,
+    formatexpr = true,
+    includeexpr = true,
+    indentexpr = true,
+    modelineexpr = true,
+    patchexpr = true,
+  },
+
+  -- Options that can be set as an expression with a prefix of '%!'
+  pct_bang_expr = {
+    statuscolumn = true,
+    statusline = true,
+    tabline = true,
+    winbar = true,
+  },
+
+  -- Options that can be set as an ex command if prefixed with ':'
+  ex_cmd_expr = {
+    keywordprg = true,
+  },
+}
+
+--- @param v integer?
+--- @return integer
+local function resolve_win(v)
+  return assert((not v or v == 0) and api.nvim_get_current_win() or v)
+end
+
+--- @type table<string,boolean>
+local all_func_opts = {}
+for _, v in pairs(func_opts) do
+  for k in pairs(v) do
+    all_func_opts[k] = true
+  end
+end
+
+--- @type table<string,function?>
+vim._func_opts = setmetatable({}, { __mode = 'v' })
+
+--- If the option name is a function option convert it to a string
+--- format and store the function value.
+--- @param name string
+--- @param value any
+--- @param info vim.api.keyset.get_option_info
+--- @param opts vim.api.keyset.option
+local function apply_func_opt(name, value, info, opts)
+  local idxs --- @type any[]
+
+  -- Find a table keep a references to the function value for an option.
+  -- We store a strong reference to the function in vim.g/b/w so it can be correctly garbage
+  -- collected when the buffer/window is closed. However because `v:lua` does not support
+  -- indexing with [], we need to store a weak reference to the function in another table under a
+  -- single string key.
+  if info.scope == 'global' or info.global_local and opts.scope == 'global' then
+    idxs = { 'g', '_func_opts' }
+  elseif info.scope == 'buf' then
+    idxs = { 'b', vim._resolve_bufnr(opts.buf), '_func_opts' }
+  else
+    assert(info.scope == 'win')
+    idxs = { 'w', resolve_win(opts.win), '_func_opts' }
+    if opts.scope == 'local' then
+      idxs[#idxs + 1] = vim._resolve_bufnr(opts.buf)
+    end
+  end
+
+  local fvalue = type(value) == 'function' and value or nil
+
+  -- If we have a function value, ensure the table for the strong references exists
+  if fvalue then
+    local t = vim --- @type table<string,any>
+    for _, k in ipairs(idxs) do
+      t[k] = t[k] or {}
+      t = t[k]
+    end
+  end
+
+  --- @type table<string,function?>
+  local t = vim.tbl_get(vim, unpack(idxs))
+  if t then
+    -- Note fvalue as nil is used to GC
+    t[name] = fvalue
+  end
+
+  if not fvalue then
+    return value
+  end
+
+  local vlua_key_parts = {} --- @type string[]
+  for _, k in ipairs(idxs) do
+    vlua_key_parts[#vlua_key_parts + 1] = k ~= '_func_opts' and k or nil
+  end
+  vlua_key_parts[#vlua_key_parts + 1] = name
+
+  local vlua_key = table.concat(vlua_key_parts, '_')
+
+  vim._func_opts[vlua_key] = fvalue
+
+  local expr_pfx = (
+    func_opts.pct_bang_expr[name] and '%!'
+    or func_opts.ex_cmd_expr[name] and ':call '
+    or ''
+  )
+
+  local call_sfx = func_opts.func[name] and '' or '()'
+
+  return ('%sv:lua.vim._func_opts.%s%s'):format(expr_pfx, vlua_key, call_sfx)
+end
+
+--- @param name string
+--- @param value any
+--- @param opts vim.api.keyset.option
+local function set_option_value(name, value, opts)
+  local info = api.nvim_get_option_info2(name, {})
+
+  -- Resolve name if it is a short name
+  name = info.name
+
+  if all_func_opts[name] then
+    value = apply_func_opt(name, value, info, opts)
+  end
+
+  api.nvim_set_option_value(name, value, opts)
+end
+
 --- Environment variables defined in the editor session.
 --- See |expand-env| and |:let-environment| for the Vimscript behavior.
 --- Invalid or unset key returns `nil`.
@@ -158,6 +297,7 @@ vim.env = setmetatable({}, {
   end,
 })
 
+--- @param bufnr? integer
 local function new_buf_opt_accessor(bufnr)
   return setmetatable({}, {
     __index = function(_, k)
@@ -167,8 +307,10 @@ local function new_buf_opt_accessor(bufnr)
       return api.nvim_get_option_value(k, { buf = bufnr or 0 })
     end,
 
+    --- @param k string
+    --- @param v any
     __newindex = function(_, k, v)
-      return api.nvim_set_option_value(k, v, { buf = bufnr or 0 })
+      return set_option_value(k, v, { buf = bufnr or 0 })
     end,
   })
 end
@@ -196,7 +338,7 @@ local function new_win_opt_accessor(winid, bufnr)
     end,
 
     __newindex = function(_, k, v)
-      return api.nvim_set_option_value(k, v, {
+      return set_option_value(k, v, {
         scope = bufnr and 'local' or nil,
         win = winid or 0,
       })
@@ -227,6 +369,14 @@ end
 --- window-scoped options. Note that this must NOT be confused with
 --- |local-options| and |:setlocal|. There is also |vim.go| that only accesses the
 --- global value of a |global-local| option, see |:setglobal|.
+---
+--- Unlike |nvim_set_option_value()|, |vim.o|, |vim.go|, |vim.bo|
+--- and |vim.wo| can be assigned function values.
+---
+--- Example: >lua
+---     vim.bo.tagfunc = vim.lsp.tagfunc
+---     vim.wo[1000].foldexpr = vim.treesitter.foldexpr
+--- <
 --- </pre>
 
 --- Get or set |options|. Like `:set`. Invalid key is an error.
@@ -245,8 +395,10 @@ vim.o = setmetatable({}, {
   __index = function(_, k)
     return api.nvim_get_option_value(k, {})
   end,
+  --- @param k string
+  --- @param v any
   __newindex = function(_, k, v)
-    return api.nvim_set_option_value(k, v, {})
+    return set_option_value(k, v, {})
   end,
 })
 
@@ -268,8 +420,10 @@ vim.go = setmetatable({}, {
   __index = function(_, k)
     return api.nvim_get_option_value(k, { scope = 'global' })
   end,
+  --- @param k string
+  --- @param v any
   __newindex = function(_, k, v)
-    return api.nvim_set_option_value(k, v, { scope = 'global' })
+    return set_option_value(k, v, { scope = 'global' })
   end,
 })
 

runtime/lua/vim/lsp.lua

@@ -1066,8 +1066,13 @@ end
 ---
 --- Currently only supports a single client. This can be set via
 --- `setlocal formatexpr=v:lua.vim.lsp.formatexpr()` or (more typically) in `on_attach`
---- via `vim.bo[bufnr].formatexpr = 'v:lua.vim.lsp.formatexpr(#{timeout_ms:250})'`.
+--- via:
 ---
+--- ```lua
+---   vim.bo[bufnr].formatexpr = function()
+---     return vim.lsp.formatexpr({ timeout_ms = 250 })
+---   end
+--- ```
 ---@param opts? vim.lsp.formatexpr.Opts
 function lsp.formatexpr(opts)
   opts = opts or {}

test/functional/lua/option_spec.lua

@@ -149,7 +149,7 @@ describe('lua stdlib', function()
         vim.api.nvim_buf_set_var(0, 'nullvar', vim.NIL)
         vim.api.nvim_buf_set_var(0, 'to_delete', { hello = 'world' })
         _G.BUF = vim.api.nvim_create_buf(false, true)
-        vim.api.nvim_buf_set_var(BUF, 'testing', 'bye')
+        vim.api.nvim_buf_set_var(_G.BUF, 'testing', 'bye')
       end)
 
       eq('hi', fn.luaeval 'vim.b.testing')
@@ -164,7 +164,10 @@ describe('lua stdlib', function()
         return { vim.b.nonexistent == vim.NIL, vim.b.nullvar == vim.NIL }
       end)
 
-      matches([[attempt to index .* nil value]], pcall_err(exec_lua, 'return vim.b[BUF][0].testing'))
+      matches(
+        [[attempt to index .* nil value]],
+        pcall_err(exec_lua, 'return vim.b[BUF][0].testing')
+      )
 
       eq({ hello = 'world' }, fn.luaeval 'vim.b.to_delete')
       exec_lua [[
@@ -174,12 +177,18 @@ describe('lua stdlib', function()
 
       exec_lua(function()
         local counter = 0
-        local function add_counter() counter = counter + 1 end
-        local function get_counter() return counter end
+        local function add_counter()
+          counter = counter + 1
+        end
+        local function get_counter()
+          return counter
+        end
         vim.b.AddCounter = add_counter
         vim.b.GetCounter = get_counter
-        vim.b.fn = {add = add_counter, get = get_counter}
-        vim.b.AddParens = function(s) return '(' .. s .. ')' end
+        vim.b.fn = { add = add_counter, get = get_counter }
+        vim.b.AddParens = function(s)
+          return '(' .. s .. ')'
+        end
       end)
 
       eq(0, eval('b:GetCounter()'))
@@ -199,12 +208,18 @@ describe('lua stdlib', function()
 
       exec_lua(function()
         local counter = 0
-        local function add_counter() counter = counter + 1 end
-        local function get_counter() return counter end
+        local function add_counter()
+          counter = counter + 1
+        end
+        local function get_counter()
+          return counter
+        end
         vim.api.nvim_buf_set_var(0, 'AddCounter', add_counter)
         vim.api.nvim_buf_set_var(0, 'GetCounter', get_counter)
-        vim.api.nvim_buf_set_var(0, 'fn', {add = add_counter, get = get_counter})
-        vim.api.nvim_buf_set_var(0, 'AddParens', function(s) return '(' .. s .. ')' end)
+        vim.api.nvim_buf_set_var(0, 'fn', { add = add_counter, get = get_counter })
+        vim.api.nvim_buf_set_var(0, 'AddParens', function(s)
+          return '(' .. s .. ')'
+        end)
       end)
 
       eq(0, eval('b:GetCounter()'))
@@ -261,7 +276,10 @@ describe('lua stdlib', function()
       eq(NIL, fn.luaeval 'vim.w.nonexistent')
       eq(NIL, fn.luaeval 'vim.w[WIN].nonexistent')
 
-      matches([[attempt to index .* nil value]], pcall_err(exec_lua, 'return vim.w[WIN][0].testing'))
+      matches(
+        [[attempt to index .* nil value]],
+        pcall_err(exec_lua, 'return vim.w[WIN][0].testing')
+      )
 
       eq({ hello = 'world' }, fn.luaeval 'vim.w.to_delete')
       exec_lua [[
@@ -456,7 +474,10 @@ describe('lua stdlib', function()
       eq({ 'one', 'two' }, eval('v:oldfiles'))
       exec_lua([[vim.v.oldfiles = {}]])
       eq({}, eval('v:oldfiles'))
-      eq('Setting v:oldfiles to value with wrong type', pcall_err(exec_lua, [[vim.v.oldfiles = 'a']]))
+      eq(
+        'Setting v:oldfiles to value with wrong type',
+        pcall_err(exec_lua, [[vim.v.oldfiles = 'a']])
+      )
       eq({}, eval('v:oldfiles'))
 
       feed('i foo foo foo<Esc>0/foo<CR>')
@@ -528,6 +549,13 @@ describe('lua stdlib', function()
         matches('Expected Lua string$', pcall_err(exec_lua, 'return vim.bo[0][0].autoread'))
         matches('Invalid buffer id: %-1$', pcall_err(exec_lua, 'return vim.bo[-1].filetype'))
       end)
+
+      it('can set function values', function()
+        eq_exec_lua('v:lua.vim._func_opts.b_1_tagfunc', function()
+          vim.bo.tagfunc = function() end
+          return vim.bo.tagfunc
+        end)
+      end)
     end)
 
     describe('vim.wo', function()
@@ -567,7 +595,26 @@ describe('lua stdlib', function()
 
       it('errors', function()
         matches('only bufnr=0 is supported', pcall_err(exec_lua, 'vim.wo[0][10].signcolumn = "no"'))
-        matches('only bufnr=0 is supported', pcall_err(exec_lua, 'local a = vim.wo[0][10].signcolumn'))
+        matches(
+          'only bufnr=0 is supported',
+          pcall_err(exec_lua, 'local a = vim.wo[0][10].signcolumn')
+        )
+      end)
+
+      it('can set function values', function()
+        eq_exec_lua({ '%!v:lua.vim._func_opts.w_1000_statusline()', 'HELLO' }, function()
+          vim.wo.statusline = function()
+            return 'HELLO'
+          end
+          return { vim.wo.statusline, vim.api.nvim_eval_statusline(vim.wo.statusline, {}).str }
+        end)
+
+        eq_exec_lua('v:lua.vim._func_opts.w_1000_1_foldexpr()', function()
+          vim.wo[0][0].foldexpr = function()
+            return 'HELLO'
+          end
+          return vim.wo[0].foldexpr
+        end)
       end)
     end)
 
@@ -1073,7 +1120,10 @@ describe('lua stdlib', function()
           )
           matches(
             "Invalid option type 'function' for 'listchars'",
-            pcall_err(exec_lua, [[vim.opt.listchars = function() return "eol:~,space:.,tab:>~" end]])
+            pcall_err(
+              exec_lua,
+              [[vim.opt.listchars = function() return "eol:~,space:.,tab:>~" end]]
+            )
           )
         end)
 
@@ -1198,5 +1248,4 @@ describe('lua stdlib', function()
       end)
     end)
   end)
-
 end)

test/functional/testnvim/exec_lua.lua

@@ -86,7 +86,7 @@ function M.handler(bytecode, upvalues, ...)
   set_upvalues(f, upvalues)
 
   -- Run in pcall so we can return any print messages
-  local ret = { pcall(f, ...) } --- @type any[]
+  local ret = { xpcall(f, debug.traceback, ...) } --- @type any[]
 
   _G.print = orig_print