feat(api): experimental nvim__exec_lua_fast #35758

Problem:
Remote UIs can't execute lua code when a blocking prompt is waiting for
input. This is needed when implementing IME pre-edit for example.

Solution:
Add an `nvim__exec_lua_fast` experimental API function, which is allowed
to run instead of being queued until after the message has been shown.

runtime/doc/api.txt

@@ -1674,6 +1674,30 @@ nvim__complete_set({index}, {opts})                     *nvim__complete_set()*
         • winid: (number) floating window id
         • bufnr: (number) buffer id in floating window
 
+nvim__exec_lua_fast({code}, {args})                    *nvim__exec_lua_fast()*
+    WARNING: This feature is experimental/unstable.
+
+    EXPERIMENTAL: this API may change or be removed in the future.
+
+    Like |nvim_exec_lua()|, but can be called during |api-fast| contexts.
+
+    Execute Lua code. Parameters (if any) are available as `...` inside the
+    chunk. The chunk can return a value.
+
+    Only statements are executed. To evaluate an expression, prefix it with
+    `return`: return my_function(...)
+
+    Attributes: ~
+        |api-fast|
+        |RPC| only
+
+    Parameters: ~
+      • {code}  (`string`) Lua code to execute
+      • {args}  (`any[]`) Arguments to the code
+
+    Return: ~
+        (`any`) Return value of Lua code if present or NIL.
+
 nvim__get_runtime({pat}, {all}, {opts})                  *nvim__get_runtime()*
     Find files in runtime directories
 

runtime/doc/news.txt

@@ -156,6 +156,8 @@ API
 • |nvim_echo()| can create |Progress| messages
 • |nvim_open_win()| floating windows can show a 'statusline'. Plugins can use
   `style='minimal'` or `:setlocal statusline=` to hide the statusline.
+• Added experimental |nvim__exec_lua_fast()| to allow remote API clients to
+  execute code while nvim is blocking for input.
 
 BUILD
 

src/nvim/api/vim.c

@@ -517,6 +517,30 @@ Object nvim_exec_lua(String code, Array args, Arena *arena, Error *err)
   return nlua_exec(code, NULL, args, kRetObject, arena, err);
 }
 
+/// EXPERIMENTAL: this API may change or be removed in the future.
+///
+/// Like |nvim_exec_lua()|, but can be called during |api-fast| contexts.
+///
+/// Execute Lua code. Parameters (if any) are available as `...` inside the
+/// chunk. The chunk can return a value.
+///
+/// Only statements are executed. To evaluate an expression, prefix it
+/// with `return`: return my_function(...)
+///
+/// @param code       Lua code to execute
+/// @param args       Arguments to the code
+/// @param[out] err   Details of an error encountered while parsing
+///                   or executing the Lua code.
+///
+/// @return           Return value of Lua code if present or NIL.
+Object nvim__exec_lua_fast(String code, Array args, Arena *arena, Error *err)
+  FUNC_API_SINCE(14)
+  FUNC_API_REMOTE_ONLY
+  FUNC_API_FAST
+{
+  return nvim_exec_lua(code, args, arena, err);
+}
+
 /// Calculates the number of display cells occupied by `text`.
 /// Control characters including [<Tab>] count as one cell.
 ///