TL;DR:

• Download here: https://github.com/jimeh/emacs-builds/releases/tag/Emacs-28.0.90-pretest
• System Requirements:
  • macOS 10.15.x or later (uses Rosetta2 on Apple Silicon machines).
  • Xcode Command Line Tools (macOS will prompt to install them on first use of native compilation).
• All dependencies, including libgccjit, are bundled into Emacs.app, and have been tested to work with native compilation on fresh macOS VMs with nothing installed except Xcode Command Line Tools.
• Native Apple Silicon builds are not yet available, and depend on GitHub-hosted Action runners adding Apple Silicon hardware. See Issue #4 for details. But the Intel-based builds have been reported to work fine on Apple Silicon via Rosetta2.
• The application is signed and notarized by Apple via my personal Apple developer account, so there's none of the hassle needed to launch unsigned applications.

Longer story:

I've posted before about my emacs-builds project (https://github.com/jimeh/emacs-builds), which has primarily been aimed at nightly builds. Earlier today I added support for it to properly detect and handle pretest and release candidate git tags.

There's no brew cask for the pretest build yet though, and I'm actually undecided if one is really needed considering how short-lived its usefulness would be. But I'm happy to provide one if there's demand for it.

Also this past weekend, I finally figured out and fixed the long-standing bug with my builds which was causing native compilation to fail unless the gcc Homebrew formula was installed. So any builds produced after 26th of November are now fully self-contained with all dependencies, shared libraries, libgccjit, and gcc libraries, etc. all bundled into the Emacs.app bundle.

With all the shared libraries and all *.eln files from a full ahead-of-time native compile being bundled into Emacs.app itself, it yields a 570MB application. But this allows native-compilation to work without libgccjit, GCC, or even Homebrew being installed on the system, and also means end-users don't spend any CPU cycles on native-compiling parts of Emacs itself.

If you prefer building Emacs from source yourself, the build script that's used to make these builds is available as a separate project here: https://github.com/jimeh/build-emacs-for-macos

Edit: Added TL;DR bullet point about the app being signed and notarized.

Edit 2: Thank you kind internet stranger for the gold :)