vim-patch:9.1.0547: No way to get the arity of a Vim function (#29638)

Problem:  No way to get the arity of a Vim function
          (Austin Ziegler)
Solution: Enhance get() Vim script function to return the function
          argument info using get(func, "arity") (LemonBoy)

fixes: vim/vim#15097
closes: vim/vim#15109

https://github.com/vim/vim/commit/48b7d05a4f88c4326bd5d7a73a523f2d953b3e51

Co-authored-by: LemonBoy <thatlemon@gmail.com>

8 files changed, 168 insertions, 26 deletions

diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 648598fb3f..334531cec7 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -2028,17 +2028,17 @@ garbagecollect([{atexit}])                                    *garbagecollect()*
 		it's safe to perform.  This is when waiting for the user to
 		type a character.
 
-get({list}, {idx} [, {default}])                                         *get()*
+get({list}, {idx} [, {default}])                              *get()* *get()-list*
 		Get item {idx} from |List| {list}.  When this item is not
 		available return {default}.  Return zero when {default} is
 		omitted.
 
-get({blob}, {idx} [, {default}])
+get({blob}, {idx} [, {default}])                                    *get()-blob*
 		Get byte {idx} from |Blob| {blob}.  When this byte is not
 		available return {default}.  Return -1 when {default} is
 		omitted.
 
-get({dict}, {key} [, {default}])
+get({dict}, {key} [, {default}])                                    *get()-dict*
 		Get item with key {key} from |Dictionary| {dict}.  When this
 		item is not available return {default}.  Return zero when
 		{default} is omitted.  Useful example: >vim
@@ -2046,13 +2046,26 @@ get({dict}, {key} [, {default}])
 <		This gets the value of g:var_name if it exists, and uses
 		"default" when it does not exist.
 
-get({func}, {what})
-		Get item {what} from Funcref {func}.  Possible values for
+get({func}, {what})                                                 *get()-func*
+		Get item {what} from |Funcref| {func}.  Possible values for
 		{what} are:
-			"name"	The function name
-			"func"	The function
-			"dict"	The dictionary
-			"args"	The list with arguments
+		  "name"    The function name
+		  "func"    The function
+		  "dict"    The dictionary
+		  "args"    The list with arguments
+		  "arity"   A dictionary with information about the number of
+			    arguments accepted by the function (minus the
+			    {arglist}) with the following fields:
+				required    the number of positional arguments
+				optional    the number of optional arguments,
+					    in addition to the required ones
+				varargs     |TRUE| if the function accepts a
+					    variable number of arguments |...|
+
+				Note: There is no error, if the {arglist} of
+				the Funcref contains more arguments than the
+				Funcref expects, it's not validated.
+
 		Returns zero on error.
 
 getbufinfo([{buf}])                                               *getbufinfo()*
diff --git a/runtime/lua/vim/_meta/vimfn.lua b/runtime/lua/vim/_meta/vimfn.lua
index 34a3addb57..d9bd683bbb 100644
--- a/runtime/lua/vim/_meta/vimfn.lua
+++ b/runtime/lua/vim/_meta/vimfn.lua
@@ -2523,12 +2523,25 @@ function vim.fn.get(blob, idx, default) end
 --- @return any
 function vim.fn.get(dict, key, default) end
 
---- Get item {what} from Funcref {func}.  Possible values for
+--- Get item {what} from |Funcref| {func}.  Possible values for
 --- {what} are:
----   "name"  The function name
----   "func"  The function
----   "dict"  The dictionary
----   "args"  The list with arguments
+---   "name"    The function name
+---   "func"    The function
+---   "dict"    The dictionary
+---   "args"    The list with arguments
+---   "arity"   A dictionary with information about the number of
+---       arguments accepted by the function (minus the
+---       {arglist}) with the following fields:
+---     required    the number of positional arguments
+---     optional    the number of optional arguments,
+---           in addition to the required ones
+---     varargs     |TRUE| if the function accepts a
+---           variable number of arguments |...|
+---
+---     Note: There is no error, if the {arglist} of
+---     the Funcref contains more arguments than the
+---     Funcref expects, it's not validated.
+---
 --- Returns zero on error.
 ---
 --- @param func function
diff --git a/scripts/gen_eval_files.lua b/scripts/gen_eval_files.lua
index 76092f8b39..b490e7b480 100755
--- a/scripts/gen_eval_files.lua
+++ b/scripts/gen_eval_files.lua
@@ -432,14 +432,15 @@ local function render_eval_meta(f, fun, write)
 end
 
 --- @param name string
+--- @param name_tag boolean
 --- @param fun vim.EvalFn
 --- @param write fun(line: string)
-local function render_sig_and_tag(name, fun, write)
+local function render_sig_and_tag(name, name_tag, fun, write)
   if not fun.signature then
     return
   end
 
-  local tags = { '*' .. name .. '()*' }
+  local tags = name_tag and { '*' .. name .. '()*' } or {}
 
   if fun.tags then
     for _, t in ipairs(fun.tags) do
@@ -447,6 +448,11 @@ local function render_sig_and_tag(name, fun, write)
     end
   end
 
+  if #tags == 0 then
+    write(fun.signature)
+    return
+  end
+
   local tag = table.concat(tags, ' ')
   local siglen = #fun.signature
   local conceal_offset = 2 * (#tags - 1)
@@ -472,11 +478,7 @@ local function render_eval_doc(f, fun, write)
     return
   end
 
-  if f:find('__%d+$') then
-    write(fun.signature)
-  else
-    render_sig_and_tag(fun.name or f, fun, write)
-  end
+  render_sig_and_tag(fun.name or f, not f:find('__%d+$'), fun, write)
 
   if not fun.desc then
     return
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 0e5503b829..72a3246da5 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -3141,6 +3141,7 @@ M.funcs = {
     name = 'get',
     params = { { 'list', 'any[]' }, { 'idx', 'integer' }, { 'default', 'any' } },
     signature = 'get({list}, {idx} [, {default}])',
+    tags = { 'get()-list' },
   },
   get__1 = {
     args = { 2, 3 },
@@ -3153,6 +3154,7 @@ M.funcs = {
     name = 'get',
     params = { { 'blob', 'string' }, { 'idx', 'integer' }, { 'default', 'any' } },
     signature = 'get({blob}, {idx} [, {default}])',
+    tags = { 'get()-blob' },
   },
   get__2 = {
     args = { 2, 3 },
@@ -3168,23 +3170,38 @@ M.funcs = {
     name = 'get',
     params = { { 'dict', 'table<string,any>' }, { 'key', 'string' }, { 'default', 'any' } },
     signature = 'get({dict}, {key} [, {default}])',
+    tags = { 'get()-dict' },
   },
   get__3 = {
     args = { 2, 3 },
     base = 1,
     desc = [=[
-      Get item {what} from Funcref {func}.  Possible values for
+      Get item {what} from |Funcref| {func}.  Possible values for
       {what} are:
-      	"name"	The function name
-      	"func"	The function
-      	"dict"	The dictionary
-      	"args"	The list with arguments
+        "name"    The function name
+        "func"    The function
+        "dict"    The dictionary
+        "args"    The list with arguments
+        "arity"   A dictionary with information about the number of
+      	    arguments accepted by the function (minus the
+      	    {arglist}) with the following fields:
+      		required    the number of positional arguments
+      		optional    the number of optional arguments,
+      			    in addition to the required ones
+      		varargs     |TRUE| if the function accepts a
+      			    variable number of arguments |...|
+
+      		Note: There is no error, if the {arglist} of
+      		the Funcref contains more arguments than the
+      		Funcref expects, it's not validated.
+
       Returns zero on error.
     ]=],
     name = 'get',
     params = { { 'func', 'function' }, { 'what', 'string' } },
     returns = 'any',
     signature = 'get({func}, {what})',
+    tags = { 'get()-func' },
   },
   getbufinfo = {
     args = { 0, 1 },
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index c319bd8214..22e7f383a5 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -2372,6 +2372,33 @@ static void f_get(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
         for (int i = 0; i < pt->pt_argc; i++) {
           tv_list_append_tv(rettv->vval.v_list, &pt->pt_argv[i]);
         }
+      } else if (strcmp(what, "arity") == 0) {
+        int required = 0;
+        int optional = 0;
+        bool varargs = false;
+        const char *name = partial_name(pt);
+
+        get_func_arity(name, &required, &optional, &varargs);
+
+        rettv->v_type = VAR_DICT;
+        tv_dict_alloc_ret(rettv);
+        dict_T *dict = rettv->vval.v_dict;
+
+        // Take into account the arguments of the partial, if any.
+        // Note that it is possible to supply more arguments than the function
+        // accepts.
+        if (pt->pt_argc >= required + optional) {
+          required = optional = 0;
+        } else if (pt->pt_argc > required) {
+          optional -= pt->pt_argc - required;
+          required = 0;
+        } else {
+          required -= pt->pt_argc;
+        }
+
+        tv_dict_add_nr(dict, S_LEN("required"), required);
+        tv_dict_add_nr(dict, S_LEN("optional"), optional);
+        tv_dict_add_bool(dict, S_LEN("varargs"), varargs);
       } else {
         semsg(_(e_invarg2), what);
       }
diff --git a/src/nvim/eval/userfunc.c b/src/nvim/eval/userfunc.c
index 122a1ac8ab..42391ecec7 100644
--- a/src/nvim/eval/userfunc.c
+++ b/src/nvim/eval/userfunc.c
@@ -642,6 +642,44 @@ static char *fname_trans_sid(const char *const name, char *const fname_buf, char
   return fname;
 }
 
+int get_func_arity(const char *name, int *required, int *optional, bool *varargs)
+{
+  int argcount = 0;
+  int min_argcount = 0;
+
+  const EvalFuncDef *fdef = find_internal_func(name);
+  if (fdef != NULL) {
+    argcount = fdef->max_argc;
+    min_argcount = fdef->min_argc;
+    *varargs = false;
+  } else {
+    char fname_buf[FLEN_FIXED + 1];
+    char *tofree = NULL;
+    int error = FCERR_NONE;
+
+    // May need to translate <SNR>123_ to K_SNR.
+    char *fname = fname_trans_sid(name, fname_buf, &tofree, &error);
+    ufunc_T *ufunc = NULL;
+    if (error == FCERR_NONE) {
+      ufunc = find_func(fname);
+    }
+    xfree(tofree);
+
+    if (ufunc == NULL) {
+      return FAIL;
+    }
+
+    argcount = ufunc->uf_args.ga_len;
+    min_argcount = ufunc->uf_args.ga_len - ufunc->uf_def_args.ga_len;
+    *varargs = ufunc->uf_varargs;
+  }
+
+  *required = min_argcount;
+  *optional = argcount - min_argcount;
+
+  return OK;
+}
+
 /// Find a function by name, return pointer to it in ufuncs.
 ///
 /// @return  NULL for unknown function.
diff --git a/test/old/testdir/test_getvar.vim b/test/old/testdir/test_getvar.vim
index 1e29f49fb4..56f737ab9c 100644
--- a/test/old/testdir/test_getvar.vim
+++ b/test/old/testdir/test_getvar.vim
@@ -142,21 +142,31 @@ func Test_get_func()
   let l:F = function('tr')
   call assert_equal('tr', get(l:F, 'name'))
   call assert_equal(l:F, get(l:F, 'func'))
+  call assert_equal({'required': 3, 'optional': 0, 'varargs': v:false},
+      \ get(l:F, 'arity'))
 
   let Fb_func = function('s:FooBar')
   call assert_match('<SNR>\d\+_FooBar', get(Fb_func, 'name'))
+  call assert_equal({'required': 0, 'optional': 0, 'varargs': v:false},
+      \ get(Fb_func, 'arity'))
   let Fb_ref = funcref('s:FooBar')
   call assert_match('<SNR>\d\+_FooBar', get(Fb_ref, 'name'))
+  call assert_equal({'required': 0, 'optional': 0, 'varargs': v:false},
+      \ get(Fb_ref, 'arity'))
 
   call assert_equal({'func has': 'no dict'}, get(l:F, 'dict', {'func has': 'no dict'}))
   call assert_equal(0, get(l:F, 'dict'))
   call assert_equal([], get(l:F, 'args'))
+
   " Nvim doesn't have null functions
   " let NF = test_null_function()
   " call assert_equal('', get(NF, 'name'))
   " call assert_equal(NF, get(NF, 'func'))
   " call assert_equal(0, get(NF, 'dict'))
   " call assert_equal([], get(NF, 'args'))
+  " call assert_equal({'required': 0, 'optional': 0, 'varargs': v:false}, get(NF, 'arity'))
 endfunc
 
 " get({partial}, {what} [, {default}]) - in test_partial.vim
+
+" vim: shiftwidth=2 sts=2 expandtab
diff --git a/test/old/testdir/test_partial.vim b/test/old/testdir/test_partial.vim
index 8b62e4a0e5..d049cc9e4b 100644
--- a/test/old/testdir/test_partial.vim
+++ b/test/old/testdir/test_partial.vim
@@ -284,6 +284,11 @@ func Test_auto_partial_rebind()
 endfunc
 
 func Test_get_partial_items()
+  func s:Qux(x, y, z=3, w=1, ...)
+  endfunc
+  func s:Qux1(x, y)
+  endfunc
+
   let dict = {'name': 'hello'}
   let args = ["foo", "bar"]
   let Func = function('MyDictFunc')
@@ -304,6 +309,23 @@ func Test_get_partial_items()
   let dict = {'partial has': 'no dict'}
   call assert_equal(dict, get(P, 'dict', dict))
   call assert_equal(0, get(l:P, 'dict'))
+
+  call assert_equal({'required': 2, 'optional': 2, 'varargs': v:true},
+      \ get(funcref('s:Qux', []), 'arity'))
+  call assert_equal({'required': 1, 'optional': 2, 'varargs': v:true},
+      \ get(funcref('s:Qux', [1]), 'arity'))
+  call assert_equal({'required': 0, 'optional': 2, 'varargs': v:true},
+      \ get(funcref('s:Qux', [1, 2]), 'arity'))
+  call assert_equal({'required': 0, 'optional': 1, 'varargs': v:true},
+      \ get(funcref('s:Qux', [1, 2, 3]), 'arity'))
+  call assert_equal({'required': 0, 'optional': 0, 'varargs': v:true},
+      \ get(funcref('s:Qux', [1, 2, 3, 4]), 'arity'))
+  " More args than expected is not an error
+  call assert_equal({'required': 0, 'optional': 0, 'varargs': v:false},
+      \ get(funcref('s:Qux1', [1, 2, 3, 4]), 'arity'))
+
+  delfunc s:Qux
+  delfunc s:Qux1
 endfunc
 
 func Test_compare_partials()