execute-extended-command') that is written in Lisp. It is no longer written in C. This should make it easy for users to extend and do new things with this command (execute command name at point, display prefix arguments numerically, ...) by easily redefining it. Although, it was possible to "advise" the C implementation before given Emacs good advising system with `
I had talked about Coding Emacs's M-x in Lisp on this blog over 3 years ago. It was accepted in Emacs after I finally got around to posting the code to gnu-emacs-sources last year.
Thanks to Stefan Monnier, one of the lead maintainers, for doing most of the leg work of merging these bits into Emacs.