Add RSpec predicate matchers spec

spec/rspec/expectations/predicate_matchers_spec.cr

@@ -0,0 +1,86 @@
+require "../../spec_helper"
+
+# Examples taken from:
+# https://relishapp.com/rspec/rspec-expectations/v/3-8/docs/built-in-matchers/predicate-matchers
+# and modified to fit Spectator and Crystal.
+Spectator.describe "Predicate matchers" do
+  context "should be_zero (based on Int#zero?)" do
+    describe 0 do
+      it { is_expected.to be_zero }
+    end
+
+    describe 7 do
+      # deliberate failure
+      # TODO: Add support for expected failures.
+      xit { is_expected.to be_zero }
+    end
+  end
+
+  context "should_not be_empty (based on Array#empty?)" do
+    describe [1, 2, 3] do
+      it { is_expected.not_to be_empty }
+    end
+
+    describe [] of Int32 do
+      # deliberate failure
+      # TODO: Add support for expected failures.
+      xit { is_expected.not_to be_empty }
+    end
+  end
+
+  context "should have_key (based on Hash#has_key?)" do
+    describe Hash do
+      subject { {:foo => 7} }
+
+      it { is_expected.to have_key(:foo) }
+
+      # deliberate failure
+      # TODO: Add support for expected failures.
+      xit { is_expected.to have_key(:bar) }
+    end
+  end
+
+  context "should_not have_all_string_keys (based on custom #has_all_string_keys? method)" do
+    class ::Hash(K, V)
+      def has_all_string_keys?
+        keys.all? { |k| String === k }
+      end
+    end
+
+    describe Hash do
+      context "with symbol keys" do
+        subject { {:foo => 7, :bar => 5} }
+
+        it { is_expected.not_to have_all_string_keys }
+      end
+
+      context "with string keys" do
+        subject { {"foo" => 7, "bar" => 5} }
+
+        # deliberate failure
+        # TODO: Add support for expected failures.
+        xit { is_expected.not_to have_all_string_keys }
+      end
+    end
+  end
+
+  context "matcher arguments are passed on to the predicate method" do
+    struct ::Int
+      def multiple_of?(x)
+        (self % x).zero?
+      end
+    end
+
+    describe 12 do
+      it { is_expected.to be_multiple_of(3) }
+      it { is_expected.not_to be_multiple_of(7) }
+
+      # deliberate failures
+      # TODO: Add support for expected failures.
+      xit { is_expected.not_to be_multiple_of(4) }
+      xit { is_expected.to be_multiple_of(5) }
+    end
+  end
+
+  # The examples using private methods cause a compilation error in Crystal, and can't be used here.
+end