feat: add preview functionality to user commands

Adds a Lua-only `preview` flag to user commands which allows the command to be incrementally previewed like `:substitute` when 'inccommand' is set.

1 files changed, 28 insertions, 3 deletions

diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index a826dd07d3..deff2347e0 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -1840,11 +1840,12 @@ cleanup:
   xfree(info);
 }
 
-void nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap)
+/// @param preview Invoke the callback as a |:command-preview| handler.
+int nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap, bool preview)
 {
   lua_State *const lstate = global_lstate;
 
-  nlua_pushref(lstate, cmd->uc_luaref);
+  nlua_pushref(lstate, preview ? cmd->uc_preview_luaref : cmd->uc_luaref);
 
   lua_newtable(lstate);
   lua_pushboolean(lstate, eap->forceit == 1);
@@ -1969,7 +1970,31 @@ void nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap)
 
   lua_setfield(lstate, -2, "smods");
 
-  if (nlua_pcall(lstate, 1, 0)) {
+  if (preview) {
+    lua_pushinteger(lstate, cmdpreview_get_ns());
+
+    handle_T cmdpreview_bufnr = cmdpreview_get_bufnr();
+    if (cmdpreview_bufnr != 0) {
+      lua_pushinteger(lstate, cmdpreview_bufnr);
+    } else {
+      lua_pushnil(lstate);
+    }
+  }
+
+  if (nlua_pcall(lstate, preview ? 3 : 1, preview ? 1 : 0)) {
     nlua_error(lstate, _("Error executing Lua callback: %.*s"));
+    return 0;
   }
+
+  int retv = 0;
+
+  if (preview) {
+    if (lua_isnumber(lstate, -1) && (retv = (int)lua_tointeger(lstate, -1)) >= 0 && retv <= 2) {
+      lua_pop(lstate, 1);
+    } else {
+      retv = 0;
+    }
+  }
+
+  return retv;
 }