add Len matcher (#368)

This matcher will check the length with any type that allows this
behavior via reflection. Implementation inspired by #189.

gomock/matchers.go

@@ -176,6 +176,24 @@ func (am allMatcher) String() string {
 	return strings.Join(ss, "; ")
 }
 
+type lenMatcher struct {
+	i int
+}
+
+func (m lenMatcher) Matches(x interface{}) bool {
+	v := reflect.ValueOf(x)
+	switch v.Kind() {
+	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
+		return v.Len() == m.i
+	default:
+		return false
+	}
+}
+
+func (m lenMatcher) String() string {
+	return fmt.Sprintf("has length %d", m.i)
+}
+
 // Constructors
 
 // All returns a composite Matcher that returns true if and only all of the
@@ -192,6 +210,12 @@ func Any() Matcher { return anyMatcher{} }
 //   Eq(5).Matches(4) // returns false
 func Eq(x interface{}) Matcher { return eqMatcher{x} }
 
+// Len returns a matcher that matches on length. This matcher returns false if
+// is compared to a type that is not an array, chan, map, slice, or string.
+func Len(i int) Matcher {
+	return lenMatcher{i}
+}
+
 // Nil returns a matcher that matches if the received value is nil.
 //
 // Example usage:

gomock/matchers_test.go

@@ -38,6 +38,10 @@ func TestMatchers(t *testing.T) {
 			[]e{"", 0, make(chan bool), errors.New("err"), new(int)}},
 		{"test Not", gomock.Not(gomock.Eq(4)), []e{3, "blah", nil, int64(4)}, []e{4}},
 		{"test All", gomock.All(gomock.Any(), gomock.Eq(4)), []e{4}, []e{3, "blah", nil, int64(4)}},
+		{"test Len", gomock.Len(2),
+			[]e{[]int{1, 2}, "ab", map[string]int{"a": 0, "b": 1}, [2]string{"a", "b"}},
+			[]e{[]int{1}, "a", 42, 42.0, false, [1]string{"a"}},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {