Add brew command 'uses'

'uses' shows the formulas that depend on a formula given on the
command-line.

Library/Homebrew/formula.rb

@@ -201,6 +201,22 @@ class Formula
     name.capitalize.gsub(/[-_.\s]([a-zA-Z0-9])/) { $1.upcase } \
                    .gsub('+', 'x')
   end
+  
+  def self.get_used_by
+    used_by = {}
+    Formulary.read_all do |name, klass|
+      deps = klass.deps
+      next if deps == nil
+
+      deps.each do |dep|
+        _deps = used_by[dep] || []
+        _deps << name unless _deps.include? name
+        used_by[dep] = _deps
+      end
+    end
+    
+    return used_by
+  end
 
   def self.factory name
     return name if name.kind_of? Formula

bin/brew

@@ -226,6 +226,24 @@ begin
   when 'log'
     Dir.chdir HOMEBREW_REPOSITORY
     exec "git", "log", ARGV.formulae.first.path, *ARGV.options
+    
+  when 'uses'
+    # For each formula given, show which other formulas depend on it.
+    # We only go one level up, direct dependencies.
+    require 'formula'
+    require 'utils'
+    
+    deps = Formula.get_used_by
+    
+    ARGV.formulae.each do |f|
+      name = f.name
+      our_deps = deps[name]
+      if our_deps == nil
+        puts "#{name} is not a dependency."
+      else
+        puts "#{name} is a dependency for #{our_deps.join(', ')}."
+      end
+    end
 
   when 'pull', 'push', 'checkout', 'branch'
     onoe "Unknown command: #{arg} (did you mean 'git #{arg}'?)"