Deprecate default_formula Requirement DSL

This has been a nightmare in terms of the complexity to our dependency
system and the whack-a-mole required on bugs. If a Requirement resolves
to a Formula it should just use `depends_on "formula"` instead. This
matches the effective behaviour all users of bottles (the vast majority
of users and installs) and what we're doing in Homebrew/homebrew-core.

Library/Homebrew/build.rb

@@ -48,9 +48,6 @@ class Build
         Requirement.prune
       elsif req.build? && dependent != formula
         Requirement.prune
-      elsif req.satisfied? && (dep = req.to_dependency) && dep.installed?
-        deps << dep
-        Requirement.prune
       end
     end
   end

Library/Homebrew/cmd/deps.rb

@@ -94,26 +94,14 @@ module Homebrew
     if ARGV.include?("--include-requirements")
       deps
     else
-      deps.map do |dep|
+      deps.select { |dep| dep.is_a? Dependency }
-        if dep.is_a? Dependency
-          dep
-        elsif dep.default_formula?
-          dep.to_dependency
-        end
-      end.compact
     end
   end
 
   def dep_display_name(dep)
     str = if dep.is_a? Requirement
       if ARGV.include?("--include-requirements")
-        if dep.default_formula?
+        ":#{dep.display_s}"
-          ":#{dep.display_s} (#{dep_display_name(dep.to_dependency)})"
-        else
-          ":#{dep.display_s}"
-        end
-      elsif dep.default_formula?
-        dep_display_name(dep.to_dependency)
       else
         # This shouldn't happen, but we'll put something here to help debugging
         "::#{dep.name}"
@@ -207,7 +195,7 @@ module Homebrew
     max = dependables.length - 1
     @dep_stack.push f.name
     dependables.each_with_index do |dep, i|
-      next if !ARGV.include?("--include-requirements") && dep.is_a?(Requirement) && !dep.default_formula?
+      next if !ARGV.include?("--include-requirements") && dep.is_a?(Requirement)
       tree_lines = if i == max
         "└──"
       else
@@ -223,9 +211,6 @@ module Homebrew
       else
         "│   "
       end
-      if dep.is_a?(Requirement) && dep.default_formula?
-        recursive_deps_tree(Formulary.factory(dep.to_dependency.name), prefix + prefix_addition, true)
-      end
       if dep.is_a? Dependency
         recursive_deps_tree(Formulary.factory(dep.name), prefix + prefix_addition, true)
       end

Library/Homebrew/cmd/uses.rb

@@ -113,9 +113,7 @@ module Homebrew
             end
           end
 
-          reqs.any? do |req|
+          reqs.any? { |req| req.name == ff.name }
-            req.name == ff.name || [ff.name, ff.full_name].include?(req.default_formula)
-          end
         rescue FormulaUnavailableError
           # Silently ignore this case as we don't care about things used in
           # taps that aren't currently tapped.

Library/Homebrew/formula.rb

@@ -1486,15 +1486,10 @@ class Formula
   # Returns a list of Dependency objects that are required at runtime.
   # @private
   def runtime_dependencies
-    runtime_dependencies = recursive_dependencies do |_, dependency|
+    recursive_dependencies do |_, dependency|
       Dependency.prune if dependency.build?
       Dependency.prune if !dependency.required? && build.without?(dependency)
     end
-    runtime_requirement_deps = recursive_requirements do |_, requirement|
-      Requirement.prune if requirement.build?
-      Requirement.prune if !requirement.required? && build.without?(requirement)
-    end.map(&:to_dependency).compact
-    runtime_dependencies + runtime_requirement_deps
   end
 
   # Returns a list of formulae depended on by this formula that aren't
@@ -1552,7 +1547,6 @@ class Formula
     hsh["requirements"] = requirements.map do |req|
       {
         "name" => req.name,
-        "default_formula" => req.default_formula,
         "cask" => req.cask,
         "download" => req.download,
       }

Library/Homebrew/formula_installer.rb

@@ -415,16 +415,6 @@ class FormulaInstaller
     raise UnsatisfiedRequirements, fatals
   end
 
-  def install_requirement_formula?(req_dependency, req, dependent, install_bottle_for_dependent)
-    return false unless req_dependency
-    return false if req.build? && dependent.installed?
-    return true unless req.satisfied?
-    return false if req.run?
-    return true if build_bottle?
-    return true if req.satisfied_by_formula?
-    install_bottle_for_dependent
-  end
-
   def runtime_requirements(formula)
     runtime_deps = formula.runtime_dependencies.map(&:to_formula)
     recursive_requirements = formula.recursive_requirements do |dependent, _|
@@ -443,17 +433,9 @@ class FormulaInstaller
       f.recursive_requirements do |dependent, req|
         build = effective_build_options_for(dependent)
         install_bottle_for_dependent = install_bottle_for?(dependent, build)
-        use_default_formula = install_bottle_for_dependent || build_bottle?
-        req_dependency = req.to_dependency(use_default_formula: use_default_formula)
 
         if (req.optional? || req.recommended?) && build.without?(req)
           Requirement.prune
-        elsif req.build? && use_default_formula && req_dependency&.installed?
-          Requirement.prune
-        elsif install_requirement_formula?(req_dependency, req, dependent, install_bottle_for_dependent)
-          deps.unshift(req_dependency)
-          formulae.unshift(req_dependency.to_formula)
-          Requirement.prune
         elsif req.satisfied?
           Requirement.prune
         elsif !runtime_requirements.include?(req) && install_bottle_for_dependent

Library/Homebrew/os/mac/linkage_checker.rb

@@ -62,8 +62,7 @@ class LinkageChecker
       formula.build.without?(dep)
     end
     declared_deps = formula.deps.reject { |dep| filter_out.call(dep) }.map(&:name)
-    declared_requirement_deps = formula.requirements.reject { |req| filter_out.call(req) }.map(&:default_formula).compact
+    declared_dep_names = declared_deps.map { |dep| dep.split("/").last }
-    declared_dep_names = (declared_deps + declared_requirement_deps).map { |dep| dep.split("/").last }
     undeclared_deps = @brewed_dylibs.keys.reject do |full_name|
       name = full_name.split("/").last
       next true if name == formula.name

Library/Homebrew/requirement.rb

@@ -9,13 +9,11 @@ require "build_environment"
 class Requirement
   include Dependable
 
-  attr_reader :tags, :name, :cask, :download, :default_formula
+  attr_reader :tags, :name, :cask, :download
 
   def initialize(tags = [])
-    @default_formula = self.class.default_formula
     @cask ||= self.class.cask
     @download ||= self.class.download
-    @formula = nil
     tags.each do |tag|
       next unless tag.is_a? Hash
       @cask ||= tag[:cask]
@@ -56,12 +54,6 @@ class Requirement
     result = self.class.satisfy.yielder { |p| instance_eval(&p) }
     @satisfied_result = result
     return false unless result
-
-    if parent = satisfied_result_parent
-      parent.to_s =~ %r{(#{Regexp.escape(HOMEBREW_CELLAR)}|#{Regexp.escape(HOMEBREW_PREFIX)}/opt)/([\w+-.@]+)}
-      @formula = Regexp.last_match(2)
-    end
-
     true
   end
 
@@ -71,10 +63,6 @@ class Requirement
     self.class.fatal || false
   end
 
-  def default_formula?
-    self.class.default_formula || false
-  end
-
   def satisfied_result_parent
     return unless @satisfied_result.is_a?(Pathname)
     parent = @satisfied_result.resolved_path.parent
@@ -124,24 +112,6 @@ class Requirement
     "#<#{self.class.name}: #{name.inspect} #{tags.inspect}>"
   end
 
-  def formula
-    @formula || self.class.default_formula
-  end
-
-  def satisfied_by_formula?
-    !@formula.nil?
-  end
-
-  def to_dependency(use_default_formula: false)
-    if use_default_formula && default_formula?
-      Dependency.new(self.class.default_formula, tags, method(:modify_build_environment), name)
-    elsif formula =~ HOMEBREW_TAP_FORMULA_REGEX
-      TapDependency.new(formula, tags, method(:modify_build_environment), name)
-    elsif formula
-      Dependency.new(formula, tags, method(:modify_build_environment), name)
-    end
-  end
-
   def display_s
     name
   end
@@ -167,8 +137,11 @@ class Requirement
     include BuildEnvironment::DSL
 
     attr_reader :env_proc, :build
-    attr_rw :fatal, :default_formula
+    attr_rw :fatal, :cask, :download
-    attr_rw :cask, :download
+
+    def default_formula(val = nil)
+      # odeprecated "Requirement.default_formula"
+    end
 
     def satisfy(options = nil, &block)
       return @satisfied if options.nil? && !block_given?

Library/Homebrew/test/formula_installer_spec.rb

@@ -132,64 +132,4 @@ describe FormulaInstaller do
       fi.check_install_sanity
     }.to raise_error(CannotInstallFormulaError)
   end
-
-  describe "#install_requirement_formula?", :needs_compat do
-    before do
-      @requirement = Python3Requirement.new
-      @requirement_dependency = @requirement.to_dependency
-      @install_bottle_for_dependent = false
-      allow(@requirement).to receive(:satisfied?).and_return(satisfied?)
-      allow(@requirement).to receive(:satisfied_by_formula?).and_return(satisfied_by_formula?)
-      allow(@requirement).to receive(:build?).and_return(build?)
-      @dependent = formula do
-        url "foo"
-        version "0.1"
-        depends_on :python3
-      end
-      allow(@dependent).to receive(:installed?).and_return(installed?)
-      @fi = FormulaInstaller.new(@dependent)
-    end
-
-    subject { @fi.install_requirement_formula?(@requirement_dependency, @requirement, @dependent, @install_bottle_for_dependent) }
-
-    context "it returns false when requirement is satisfied" do
-      let(:satisfied?) { true }
-      let(:satisfied_by_formula?) { false }
-      let(:build?) { false }
-      let(:installed?) { false }
-      it { is_expected.to be false }
-    end
-
-    context "it returns false when requirement is satisfied but default formula is installed" do
-      let(:satisfied?) { true }
-      let(:satisfied_by_formula?) { false }
-      let(:build?) { false }
-      let(:installed?) { false }
-      it { is_expected.to be false }
-    end
-
-    context "it returns false when requirement is :build and dependent is installed" do
-      let(:satisfied?) { false }
-      let(:satisfied_by_formula?) { false }
-      let(:build?) { true }
-      let(:installed?) { true }
-      it { is_expected.to be false }
-    end
-
-    context "it returns true when requirement isn't satisfied" do
-      let(:satisfied?) { false }
-      let(:satisfied_by_formula?) { false }
-      let(:build?) { false }
-      let(:installed?) { false }
-      it { is_expected.to be true }
-    end
-
-    context "it returns true when requirement is satisfied by a formula" do
-      let(:satisfied?) { true }
-      let(:satisfied_by_formula?) { true }
-      let(:build?) { false }
-      let(:installed?) { false }
-      it { is_expected.to be true }
-    end
-  end
 end

Library/Homebrew/test/requirement_spec.rb

@@ -2,7 +2,6 @@ require "extend/ENV"
 require "requirement"
 
 describe Requirement do
-  alias_matcher :have_a_default_formula, :be_a_default_formula
   alias_matcher :be_a_build_requirement, :be_a_build
 
   subject { klass.new }
@@ -173,60 +172,6 @@ describe Requirement do
     its(:option_names) { are_expected.to eq(["foo"]) }
   end
 
-  describe "#default_formula?" do
-    context "#default_formula specified" do
-      let(:klass) do
-        Class.new(described_class) do
-          default_formula "foo"
-        end
-      end
-
-      it { is_expected.to have_a_default_formula }
-    end
-
-    context "#default_formula omitted" do
-      it { is_expected.not_to have_a_default_formula }
-    end
-  end
-
-  describe "#to_dependency" do
-    let(:klass) do
-      Class.new(described_class) do
-        default_formula "foo"
-      end
-    end
-
-    it "returns a Dependency for its default Formula" do
-      expect(subject.to_dependency).to eq(Dependency.new("foo"))
-    end
-
-    context "#modify_build_environment" do
-      context "with error" do
-        let(:klass) do
-          Class.new(described_class) do
-            class ModifyBuildEnvironmentError < StandardError; end
-
-            default_formula "foo"
-
-            satisfy do
-              true
-            end
-
-            env do
-              raise ModifyBuildEnvironmentError
-            end
-          end
-        end
-
-        it "raises an error" do
-          expect {
-            subject.to_dependency.modify_build_environment
-          }.to raise_error(klass.const_get(:ModifyBuildEnvironmentError))
-        end
-      end
-    end
-  end
-
   describe "#modify_build_environment" do
     context "without env proc" do
       let(:klass) { Class.new(described_class) }

docs/Building-Against-Non-Homebrew-Dependencies.md

@@ -0,0 +1,11 @@
+# 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.

docs/README.md

@@ -33,6 +33,7 @@
 - [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)