Make the error message more useful when using match_array

* Now the error message will let you know if you're trying to match an
  array against something that isn't an array-like thing.
* Closes #186
* Fixes #185

Changelog.md

@@ -8,6 +8,9 @@ Enhancements
   matcher (Myron Marston)
 * Print a warning when users accidentally pass a non-string argument
   as an expectation message (Sam Phippen)
+* `=~` and `match_array` matchers output a more useful error message when
+  the actual value is not an array (or an object that responds to `#to_ary`)
+  (Sam Phippen)
 
 Bug fixes
 

lib/rspec/matchers/built_in/match_array.rb

@@ -3,16 +3,22 @@ module Matchers
     module BuiltIn
       class MatchArray < BaseMatcher
         def match(expected, actual)
+          return false unless actual.respond_to? :to_ary
           @extra_items = difference_between_arrays(actual, expected)
           @missing_items = difference_between_arrays(expected, actual)
           @extra_items.empty? & @missing_items.empty?
         end
 
         def failure_message_for_should
-          message =  "expected collection contained:  #{safe_sort(expected).inspect}\n"
-          message += "actual collection contained:    #{safe_sort(actual).inspect}\n"
-          message += "the missing elements were:      #{safe_sort(@missing_items).inspect}\n" unless @missing_items.empty?
-          message += "the extra elements were:        #{safe_sort(@extra_items).inspect}\n"   unless @extra_items.empty?
+          if actual.respond_to? :to_ary
+            message =  "expected collection contained:  #{safe_sort(expected).inspect}\n"
+            message += "actual collection contained:    #{safe_sort(actual).inspect}\n"
+            message += "the missing elements were:      #{safe_sort(@missing_items).inspect}\n" unless @missing_items.empty?
+            message += "the extra elements were:        #{safe_sort(@extra_items).inspect}\n"   unless @extra_items.empty?
+          else
+            message = "expected an array, actual collection was #{actual.inspect}"
+          end
+
           message
         end
 
@@ -31,8 +37,8 @@ def safe_sort(array)
         end
 
         def difference_between_arrays(array_1, array_2)
-          difference = array_1.dup
-          array_2.each do |element|
+          difference = array_1.to_ary.dup
+          array_2.to_ary.each do |element|
             if index = difference.index(element)
               difference.delete_at(index)
             end

spec/rspec/matchers/match_array_spec.rb

@@ -122,3 +122,17 @@ def ==(other)
     }.should fail_with(/Matcher does not support should_not/)
   end
 end
+
+describe "matching against things that aren't arrays" do
+  it "fails with nil and the expected error message is given" do
+    expect { nil.should match_array([1,2,3]) }.to fail_with(/expected an array/)
+  end
+
+  it "fails with a float and the expected error message is given" do
+    expect { (3.7).should match_array([1,2,3]) }.to fail_with(/expected an array/)
+  end
+
+  it "fails with a string and the expected error message is given" do
+    expect { "I like turtles".should match_array([1,2,3]) }.to fail_with(/expected an array/)
+  end
+end