Add contain matcher

src/spectator/dsl/matcher_dsl.cr

@@ -327,5 +327,22 @@ module Spectator::DSL
     macro end_with(expected)
       ::Spectator::Matchers::EndWithMatcher.new({{expected.stringify}}, {{expected}})
     end
+
+    # Indicates that some value or set should contain another value.
+    # This is typically used on a `String` or `Array` (any `Enumerable` works).
+    # The `expected` argument can be a `String` or `Char`
+    # when the actual type (being comapred against) is a `String`.
+    # For `Enumerable` types, each item is inspected until one matches.
+    # In both cases, the `includes?` method is used.
+    #
+    # Examples:
+    # ```
+    # expect("foobar").to contain("foo")
+    # expect("foobar").to contain('o')
+    # expect(%i[a b c]).to contain(:b)
+    # ```
+    macro contain(expected)
+      ::Spectator::Matchers::ContainMatcher.new({{expected.stringify}}, {{expected}})
+    end
   end
 end

src/spectator/matchers/contain_matcher.cr

@@ -0,0 +1,25 @@
+require "./value_matcher"
+
+module Spectator::Matchers
+  # Matcher that tests whether a value, such as a `String` or `Array`, contains a value.
+  # The values are checked with the `includes?` operator.
+  struct ContainMatcher(ExpectedType) < ValueMatcher(ExpectedType)
+    # Determines whether the matcher is satisfied with the value given to it.
+    # True is returned if the match was successful, false otherwise.
+    def match?(partial)
+      partial.actual.includes?(expected)
+    end
+
+    # Describes the condition that satisfies the matcher.
+    # This is informational and displayed to the end-user.
+    def message(partial)
+      "Expected #{partial.label} to contain #{label}"
+    end
+
+    # Describes the condition that won't satsify the matcher.
+    # This is informational and displayed to the end-user.
+    def negated_message(partial)
+      "Expected #{partial.label} to not contain #{label}"
+    end
+  end
+end