Add conflicts_with DSL method

conflicts_with is a thin wrapper around Requirement which simplifies
marking conflicts between formulae.

Closes Homebrew/homebrew#13687.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Library/Homebrew/cmd/info.rb

@@ -68,6 +68,11 @@ module Homebrew extend self
     end
 
     puts "Depends on: #{f.deps*', '}" unless f.deps.empty?
+    conflicts = []
+    f.external_deps.each do |dep|
+      conflicts << dep.formula if dep.is_a? ConflictRequirement
+    end
+    puts "Conflicts with: #{conflicts*', '}" unless conflicts.empty?
 
     if f.rack.directory?
       kegs = f.rack.children

Library/Homebrew/dependencies.rb

@@ -287,3 +287,23 @@ class MPIDependency < Requirement
   end
 
 end
+
+class ConflictRequirement < Requirement
+  attr_reader :formula
+
+  def initialize formula, message
+    @formula = formula
+    @message = message
+  end
+
+  def message; @message; end
+
+  def satisfied?
+    keg = Formula.factory(@formula).prefix
+    not keg.exist? && Keg.new(keg).linked?
+  end
+
+  def fatal?
+    not ARGV.force?
+  end
+end

Library/Homebrew/formula.rb

@@ -626,6 +626,21 @@ private
       dependencies.add(dep)
     end
 
+    def conflicts_with formula, opts={}
+      message = <<-EOS.undent
+      #{formula} cannot be installed alongside #{name.downcase}.
+      EOS
+      message << "This is because #{opts[:reason]}\n" if opts[:reason]
+      if !ARGV.force? then message << <<-EOS.undent
+      Please `brew unlink` or `brew uninstall` #{formula} before continuing.
+      To install anyway, use:
+        brew install --force
+        EOS
+      end
+
+      dependencies.add ConflictRequirement.new(formula, message)
+    end
+
     def skip_clean paths
       if paths == :all
         @skip_clean_all = true