fix(api): deprecate nvim_subscribe, nvim_unsubscribe #30456

Problem:
- nvim_subscribe, nvim_unsubscribe were deprecated in
  aec4938a21a02d279d13a9eb64ef3b7cc592c374 but this wasn't set in the
  API metadata.
- The function annotations
  ```
  FUNC_API_SINCE(1) FUNC_API_REMOTE_ONLY FUNC_API_DEPRECATED_SINCE(13)
  ```
  cause this test to fail:
   ```
   RUN T3 api metadata functions are compatible with old metadata or have new level: 3.00 ms ERR
   test/functional/api/version_spec.lua:135: function vim_subscribe was removed but exists in level 0 which nvim should be compatible with
   stack traceback:
     test/functional/api/version_spec.lua:135: in function <test/functional/api/version_spec.lua:128>
   ```

Solution:
- Set the API metadata.
- Rearrange the annotations so that FUNC_API_DEPRECATED_SINCE is 2nd:
  ```
  FUNC_API_SINCE(1) FUNC_API_DEPRECATED_SINCE(13) FUNC_API_REMOTE_ONLY
  ```

4 files changed, 27 insertions, 16 deletions

diff --git a/src/nvim/api/deprecated.c b/src/nvim/api/deprecated.c
index 493856905f..51ed6eaa41 100644
--- a/src/nvim/api/deprecated.c
+++ b/src/nvim/api/deprecated.c
@@ -798,7 +798,8 @@ theend:
 /// @param channel_id Channel id (passed automatically by the dispatcher)
 /// @param event      Event type string
 void nvim_subscribe(uint64_t channel_id, String event)
-  FUNC_API_SINCE(1) FUNC_API_REMOTE_ONLY
+// XXX: c_grammar.lua is order-sensitive.
+  FUNC_API_SINCE(1) FUNC_API_DEPRECATED_SINCE(13) FUNC_API_REMOTE_ONLY
 {
   // Does nothing. `rpcnotify(0,…)` broadcasts to all channels, there are no "subscriptions".
 }
@@ -808,7 +809,8 @@ void nvim_subscribe(uint64_t channel_id, String event)
 /// @param channel_id Channel id (passed automatically by the dispatcher)
 /// @param event      Event type string
 void nvim_unsubscribe(uint64_t channel_id, String event)
-  FUNC_API_SINCE(1) FUNC_API_REMOTE_ONLY
+// XXX: c_grammar.lua is order-sensitive.
+  FUNC_API_SINCE(1) FUNC_API_DEPRECATED_SINCE(13) FUNC_API_REMOTE_ONLY
 {
   // Does nothing. `rpcnotify(0,…)` broadcasts to all channels, there are no "subscriptions".
 }
diff --git a/src/nvim/generators/c_grammar.lua b/src/nvim/generators/c_grammar.lua
index 9b1c284c1e..890c260843 100644
--- a/src/nvim/generators/c_grammar.lua
+++ b/src/nvim/generators/c_grammar.lua
@@ -257,6 +257,11 @@ if arg[1] == '--test' then
     'char *xstpcpy(char *restrict dst, const char *restrict src) {}',
     'bool try_leave(const TryState *const tstate, Error *const err) {}',
     'void api_set_error(ErrorType errType) {}',
+    {
+      'void nvim_subscribe(uint64_t channel_id, String event)',
+      'FUNC_API_SINCE(1) FUNC_API_DEPRECATED_SINCE(13) FUNC_API_REMOTE_ONLY',
+      '{}',
+    },
 
     -- Do not consume leading preproc statements
     {
diff --git a/src/nvim/generators/gen_api_dispatch.lua b/src/nvim/generators/gen_api_dispatch.lua
index 402382acd2..c987037324 100644
--- a/src/nvim/generators/gen_api_dispatch.lua
+++ b/src/nvim/generators/gen_api_dispatch.lua
@@ -1,3 +1,10 @@
+-- Example (manual) invocation:
+--
+--    make
+--    cp build/nvim_version.lua src/nvim/
+--    cd src/nvim
+--    nvim -l generators/gen_api_dispatch.lua "../../build/src/nvim/auto/api/private/dispatch_wrappers.generated.h" "../../build/src/nvim/auto/api/private/api_metadata.generated.h" "../../build/funcs_metadata.mpack" "../../build/src/nvim/auto/lua_api_c_bindings.generated.h" "../../build/src/nvim/auto/keysets_defs.generated.h" "../../build/ui_metadata.mpack" "../../build/cmake.config/auto/versiondef_git.h" "./api/autocmd.h" "./api/buffer.h" "./api/command.h" "./api/deprecated.h" "./api/extmark.h" "./api/keysets_defs.h" "./api/options.h" "./api/tabpage.h" "./api/ui.h" "./api/vim.h" "./api/vimscript.h" "./api/win_config.h" "./api/window.h" "../../build/include/api/autocmd.h.generated.h" "../../build/include/api/buffer.h.generated.h" "../../build/include/api/command.h.generated.h" "../../build/include/api/deprecated.h.generated.h" "../../build/include/api/extmark.h.generated.h" "../../build/include/api/options.h.generated.h" "../../build/include/api/tabpage.h.generated.h" "../../build/include/api/ui.h.generated.h" "../../build/include/api/vim.h.generated.h" "../../build/include/api/vimscript.h.generated.h" "../../build/include/api/win_config.h.generated.h" "../../build/include/api/window.h.generated.h"
+
 local mpack = vim.mpack
 
 local hashy = require 'generators.hashy'
@@ -8,7 +15,7 @@ assert(#arg >= pre_args)
 local dispatch_outputf = arg[1]
 -- output h file with packed metadata (api_metadata.generated.h)
 local api_metadata_outputf = arg[2]
--- output metadata mpack file, for use by other build scripts (api_metadata.mpack)
+-- output metadata mpack file, for use by other build scripts (funcs_metadata.mpack)
 local mpack_outputf = arg[3]
 local lua_c_bindings_outputf = arg[4] -- lua_api_c_bindings.generated.c
 local keysets_outputf = arg[5] -- keysets_defs.generated.h
@@ -235,7 +242,7 @@ for x in string.gmatch(ui_options_text, '"([a-z][a-z_]+)"') do
   table.insert(ui_options, x)
 end
 
-local version = require 'nvim_version'
+local version = require 'nvim_version' -- `build/nvim_version.lua` file.
 local git_version = io.open(git_version_inputf):read '*a'
 local version_build = string.match(git_version, '#define NVIM_VERSION_BUILD "([^"]+)"') or vim.NIL
 
@@ -266,10 +273,7 @@ fixdict(1 + #version)
 for _, item in ipairs(version) do
   -- NB: all items are mandatory. But any error will be less confusing
   -- with placeholder vim.NIL (than invalid mpack data)
-  local val = item[2]
-  if val == nil then
-    val = vim.NIL
-  end
+  local val = item[2] == nil and vim.NIL or item[2]
   put(item[1], val)
 end
 put('build', version_build)
diff --git a/test/functional/api/version_spec.lua b/test/functional/api/version_spec.lua
index 617786eb2d..72b5e307ee 100644
--- a/test/functional/api/version_spec.lua
+++ b/test/functional/api/version_spec.lua
@@ -93,28 +93,28 @@ describe('api metadata', function()
   local function clean_level_0(metadata)
     for _, f in ipairs(metadata.functions) do
       f.can_fail = nil
-      f.async = nil
+      f.async = nil -- XXX: renamed to "fast".
       f.receives_channel_id = nil
       f.since = 0
     end
   end
 
-  local api_info, compat, stable, api_level
+  local api_info --[[@type table]]
+  local compat --[[@type integer]]
+  local stable --[[@type integer]]
+  local api_level --[[@type integer]]
   local old_api = {}
   setup(function()
     clear() -- Ensure a session before requesting api_info.
+    --[[@type { version: {api_compatible: integer, api_level: integer, api_prerelease: boolean} }]]
     api_info = api.nvim_get_api_info()[2]
     compat = api_info.version.api_compatible
     api_level = api_info.version.api_level
-    if api_info.version.api_prerelease then
-      stable = api_level - 1
-    else
-      stable = api_level
-    end
+    stable = api_info.version.api_prerelease and api_level - 1 or api_level
 
     for level = compat, stable do
       local path = ('test/functional/fixtures/api_level_' .. tostring(level) .. '.mpack')
-      old_api[level] = read_mpack_file(path)
+      old_api[level] = read_mpack_file(path) --[[@type table]]
       if old_api[level] == nil then
         local errstr = 'missing metadata fixture for stable level ' .. level .. '. '
         if level == api_level and not api_info.version.api_prerelease then