eval: add api_info()

Previously, the api metadata was only accessible frow within nvim as
msgpackparse(systemlist('nvim --api-info'))[0]

runtime/doc/eval.txt

@@ -1783,6 +1783,7 @@ abs({expr})			Float or Number  absolute value of {expr}
 acos({expr})			Float	arc cosine of {expr}
 add({list}, {item})		List	append {item} to |List| {list}
 and({expr}, {expr})		Number  bitwise AND
+api_info()			Dict	api metadata
 append({lnum}, {string})	Number	append {string} below line {lnum}
 append({lnum}, {list})		Number	append lines {list} below line {lnum}
 argc()				Number	number of files in the argument list
@@ -2196,6 +2197,11 @@ and({expr}, {expr})					*and()*
 			:let flag = and(bits, 0x80)
 
 
+api_info()						*api_info()*
+		Return Dictionary containing api metadata.
+		See |api-metadata|.
+
+
 append({lnum}, {expr})					*append()*
 		When {expr} is a |List|: Append each item of the |List| as a
 		text line below line {lnum} in the current buffer.

runtime/doc/msgpack_rpc.txt

@@ -45,7 +45,7 @@ msgpack-rpc details from application developers. The wrappers can be
 automatically generated by reading bundled API metadata from a compiled Nvim
 instance.
 
-There are two ways to obtain API metadata:
+There are three ways to obtain API metadata:
 
   1. Connect to a running Nvim instance and call `vim_get_api_info` via
      msgpack-rpc. This is best for clients written in dynamic languages which
@@ -55,8 +55,10 @@ There are two ways to obtain API metadata:
      of msgpack metadata to standard output. This is useful for clients
      written in statically-compiled languages.
 
+3. In vimscript the metadata is available as |api_info()|.
+
 To get a human-readable list of API functions: >
-    :new|put =map(msgpackparse(systemlist('nvim --api-info'))[0].functions, 'v:val.name._VAL[0]')
+    :new|put =map(api_info().functions, 'v:val.name')
 <
 To get a formatted dump of the API using python (requires the `pyyaml` and
 `msgpack-python` packages): >

src/nvim/eval.c

@@ -6684,6 +6684,7 @@ static struct fst {
   { "acos",              1, 1, f_acos },  // WJMc
   { "add",               2, 2, f_add },
   { "and",               2, 2, f_and },
+  { "api_info",          0, 0, f_api_info },
   { "append",            2, 2, f_append },
   { "argc",              0, 0, f_argc },
   { "argidx",            0, 0, f_argidx },
@@ -7466,6 +7467,15 @@ static void f_and(typval_T *argvars, typval_T *rettv)
                          & get_tv_number_chk(&argvars[1], NULL);
 }
 
+
+/// "api_info()" function
+static void f_api_info(typval_T *argvars, typval_T *rettv)
+{
+  Dictionary metadata = api_metadata();
+  object_to_vim(DICTIONARY_OBJ(metadata), rettv, NULL);
+  api_free_dictionary(metadata);
+}
+
 /*
  * "append(lnum, string/list)" function
  */

test/functional/viml/function_spec.lua

@@ -2,6 +2,7 @@ local helpers = require('test.functional.helpers')(after_each)
 
 local clear = helpers.clear
 local eq = helpers.eq
+local eval = helpers.eval
 local exc_exec = helpers.exc_exec
 
 describe('Up to MAX_FUNC_ARGS arguments are handled by', function()
@@ -27,3 +28,11 @@ describe('Up to MAX_FUNC_ARGS arguments are handled by', function()
     eq('Vim(call):E740: Too many arguments for function rpcnotify', ret)
   end)
 end)
+
+describe('api_info()', function()
+  before_each(clear)
+  it('has the right keys', function()
+    local api_keys = eval("sort(keys(api_info()))")
+    eq({'error_types', 'functions', 'types'}, api_keys)
+  end)
+end)