I am also wondering how we should version the release, because at a guess we can't ask for master revision because the sha256 will fail. Right now my plan was to just periodically update the rev and sha, but there's gotta be a better waaaaaaaaaaay.

Perhaps you could do something similar to https://github.com/colemickens/nixpkgs-wayland

This version of clean-env.patch applies on emacs master:

Dump temacs in an empty environment to prevent -dev paths from ending
up in the dumped image.

diff --git a/src/Makefile.in b/src/Makefile.in
index fd05a45df5..13f529c253 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -570,7 +570,7 @@ emacs$(EXEEXT): temacs$(EXEEXT) \
                 lisp.mk $(etc)/DOC $(lisp) \
                 $(lispsource)/international/charprop.el ${charsets}
 ifeq ($(DUMPING),unexec)
-	LC_ALL=C $(RUN_TEMACS) -batch $(BUILD_DETAILS) -l loadup --temacs=dump
+	env -i LC_ALL=C $(RUN_TEMACS) -batch $(BUILD_DETAILS) -l loadup --temacs=dump
   ifneq ($(PAXCTL_dumped),)
 	      $(PAXCTL_dumped) emacs$(EXEEXT)
   endif

This expression works without duplicating the entire file:

{ pkgs ? import <nixpkgs> {}
, lib ? pkgs.lib
}:

let
  rev = "dfb0ba79b5f41ca6fed25a03d2a5cd6996ec4753";
  sha256 = "0cr8mlcxyib4rd8xj65hb1wxwlmppgg4823z7h3dbbxi273gz5b0";

in (pkgs.emacs.override { srcRepo = true; }).overrideAttrs(old: rec {
  name = "emacs-${version}";
  version = builtins.substring 0 7 rev;

  src = pkgs.fetchFromGitHub {
    owner = "emacs-mirror";
    repo = "emacs";
    inherit sha256 rev;
  };

  patches = [
    ./clean-env.patch
    (lib.elemAt old.patches 1)
  ];

})

FYI I've pushed an auto-updating emacs to mine and @etu`s overlay https://github.com/adisbladis/exwm-overlay.

@ar1a I'd actually like to close this PR, if a user wants bleeding edge emacs we now have the emacs overlay: https://github.com/nix-community/emacs-overlay.

Are you fine with this?

No love in emacs-overlay for macOS users :(

@rcorrear I happily accept patches for macOS in the overlay.
If you are thinking about emacsMacport upstream ( ftp://ftp.math.s.chiba-u.ac.jp/emacs/ ) does not yet have a mac port for Emacs 27.

@adisbladis I was hoping no one would comment before I was finished building it, but I'll take back my comment, I didn't understand overlays well enough :)

Took me a little while to figure out how to use them since the instructions (in your repo and almost everywhere else I found) are more tailored to NixOS users but not for (standalone) Nixpkgs users (which is what you get in macOS).

If I do find anything I can contribute to emacs-overlay I will, I'm slowly learning about Nix and overlays are something I just came across.

And yeah, I'm aware of emacsMacport not being ported yet to Emacs 27. Thanks for pointing that out though.