Provide "Did you mean? task_name" suggestions if task was not found

lib/rake/task_manager.rb

@@ -59,9 +59,29 @@ def [](task_name, scopes=nil)
       self.lookup(task_name, scopes) or
         enhance_with_matching_rule(task_name) or
         synthesize_file_task(task_name) or
-        fail "Don't know how to build task '#{task_name}'"
+        build_task_fail(task_name)
     end
 
+    # If a task match wasn't found, try to offer suggestions.
+    # The task name must be at least 2 characters.
+    def build_task_fail(task_name)
+      msg       = "Don't know how to build task '#{task_name}'"
+      threshold = (task_name.size * 0.3).ceil
+
+      matches = @tasks.keys.
+        map {|name| [levenshtein_distance(name.downcase, task_name), name] }.
+        select {|distance, _| distance <= threshold }.
+        sort.
+        map(&:last)
+
+      if matches.size > 0
+        msg += ".\nDid you mean one of these? #{matches.join(", ")}"
+      end
+
+      fail msg
+    end
+    private :build_task_fail
+
     def synthesize_file_task(task_name) # :nodoc:
       return nil unless File.exist?(task_name)
       define_task(Rake::FileTask, task_name)
@@ -301,6 +321,36 @@ def get_description(task)
       desc
     end
 
+    # Return a value representing the "cost" of transforming str1 into str2
+    def levenshtein_distance(str1, str2)
+      n = str1.length
+      m = str2.length
+      return m if n.zero?
+      return n if m.zero?
+
+      d = (0..m).to_a
+      x = nil
+
+      str1.each_char.each_with_index do |char1,i|
+        e = i+1
+
+        str2.each_char.each_with_index do |char2,j|
+          cost = (char1 == char2) ? 0 : 1
+          x = [
+               d[j+1] + 1, # insertion
+               e + 1,      # deletion
+               d[j] + cost # substitution
+              ].min
+          d[j] = e
+          e = x
+        end
+
+        d[m] = x
+      end
+
+      x
+    end
+
     class << self
       attr_accessor :record_task_metadata # :nodoc:
       TaskManager.record_task_metadata = false

test/test_rake_task.rb

@@ -148,6 +148,14 @@ def test_find
     assert_equal "Don't know how to build task 'leaves'", ex.message
   end
 
+  def test_find_with_alts
+    task :tfind
+    ex = Task[:tfindx] rescue $!
+
+    assert_match /Don\'t know how to build task \'tfindx\'/, ex.message
+    assert_match /Did you mean one of these\? tfind/, ex.message
+  end
+
   def test_defined
     assert ! Task.task_defined?(:a)
     task :a