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-rw-r--r--docs/Building-Against-Non-Homebrew-Dependencies.md11
-rw-r--r--docs/README.md1
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+# Building Against Non-Homebrew Dependencies
+
+## History
+
+Originally Homebrew was a build-from-source package manager and all user environment variables and non-Homebrew-installed software were available to builds. Since then Homebrew added `Requirement`s to specify dependencies on non-Homebrew software (such as those provided by `brew cask` like X11/XQuartz), the `superenv` build system to strip out unspecified dependencies, environment filtering to stop the user environment leaking into Homebrew builds and `default_formula` to specify that a `Requirement` can be satisifed by a particular formula.
+
+As Homebrew became primarily a binary package manager, most users were fulfilling `Requirement`s with the `default_formula`, not with arbitrary alternatives. To improve quality and reduce variation, Homebrew now exclusively supports using the default formula, as an ordinary dependency, and no longer supports using arbitrary alternatives.
+
+## Today
+
+If you wish to build against custom non-Homebrew dependencies that are provided by Homebrew (e.g. a non-Homebrew, non-macOS `ruby`) then you must [create and maintain your own tap](How-to-Create-and-Maintain-a-Tap.md) as these formulae will not be accepted in Homebrew/homebrew-core. Once you have done that you can specify `env :std` in the formula which will allow a e.g. `which ruby` to access your existing `PATH` variable and allow compilation to link against this Ruby.
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- [Python for Formula Authors](Python-for-Formula-Authors.md)
- [Migrating A Formula To A Tap](Migrating-A-Formula-To-A-Tap.md)
- [Rename A Formula](Rename-A-Formula.md)
+- [Building Against Non-Homebrew Dependencies](Building-Against-Non-Homebrew-Dependencies.md)
- [How To Create (And Maintain) A Tap](How-to-Create-and-Maintain-a-Tap.md)
- [Brew Test Bot](Brew-Test-Bot.md)
- [Prose Style Guidelines](Prose-Style-Guidelines.md)