HaveExistingField matcher (#553)

- implements new HaveExistingField matcher, fixing #548.
- modifies existing extractField helper from HaveField for reuse with HaveExistingField
- adds new unit tests for HaveExistingField matcher
- updates documentation

Author: thediveo

docs/index.md

@@ -889,7 +889,7 @@ succeeds if the capacity of `ACTUAL` is `INT`. `ACTUAL` must be of type `array`,
 or
 
 ```go
-Ω(ACTUAL).Should(ContainElement(ELEMENT, <Pointer>))
+Ω(ACTUAL).Should(ContainElement(ELEMENT, <POINTER>))
 ```
 
 
@@ -901,7 +901,7 @@ By default `ContainElement()` uses the `Equal()` matcher under the hood to asser
 Ω([]string{"Foo", "FooBar"}).Should(ContainElement(ContainSubstring("Bar")))
 ```
 
-In addition, there are occasions when you need to grab (all) matching contained elements, for instance, to make several assertions against the matching contained elements. To do this, you can ask the `ContainElement` matcher for the matching contained elements by passing it a pointer to a variable of the appropriate type. If multiple matching contained elements are expected, then a pointer to either a slice or a map should be passed (but not a pointer to an array), otherwise a pointer to a scalar (non-slice, non-map):
+In addition, there are occasions when you need to grab (all) matching contained elements, for instance, to make several assertions against the matching contained elements. To do this, you can ask the `ContainElement()` matcher for the matching contained elements by passing it a pointer to a variable of the appropriate type. If multiple matching contained elements are expected, then a pointer to either a slice or a map should be passed (but not a pointer to an array), otherwise a pointer to a scalar (non-slice, non-map):
 
 ```go
 var findings []string
@@ -1085,6 +1085,22 @@ and an instance book `var book = Book{...}` - you can use `HaveField` to make as
 
 If you want to make lots of complex assertions against the fields of a struct take a look at the [`gstruct`package](#gstruct-testing-complex-data-types) package documented below.  
 
+#### HaveExistingField(field interface{})
+
+While `HaveField()` considers a missing field to be an error (instead of non-success), combining it with `HaveExistingField()` allows `HaveField()` to be reused in test contexts other than assertions: for instance, as filters to [`ContainElement(ELEMENT, <POINTER>)`](#containelementelement-interface) or in detecting resource leaks (like leaked file descriptors).
+
+```go
+Ω(ACTUAL).Should(HaveExistingField(FIELD))
+```
+
+succeeds if `ACTUAL` is a struct with a field `FIELD`, regardless of this field's value. It is an error for `ACTUAL` to not be a `struct`. Like `HaveField()`, `HaveExistingField()` supports accessing nested structs using the `.` delimiter. Methods on the struct are invoked by adding a `()` suffix to the `FIELD` - these methods must take no arguments and return exactly one value.
+
+To assert a particular field value, but only if such a field exists in an `ACTUAL` struct, use the composing [`And`](#andmatchers-gomegamatcher) matcher:
+
+```go
+Ω(ACTUAL).Should(And(HaveExistingField(FIELD), HaveField(FIELD, VALUE)))
+```
+
 ### Working with Numbers and Times
 
 #### BeNumerically(comparator string, compareTo ...interface{})

matchers.go

@@ -404,6 +404,19 @@ func HaveField(field string, expected interface{}) types.GomegaMatcher {
 	}
 }
 
+// HaveExistingField succeeds if actual is a struct and the specified field
+// exists.
+//
+// HaveExistingField can be combined with HaveField in order to cover use cases
+// with optional fields. HaveField alone would trigger an error in such situations.
+//
+//     Expect(MrHarmless).NotTo(And(HaveExistingField("Title"), HaveField("Title", "Supervillain")))
+func HaveExistingField(field string) types.GomegaMatcher {
+	return &matchers.HaveExistingFieldMatcher{
+		Field: field,
+	}
+}
+
 // HaveValue applies the given matcher to the value of actual, optionally and
 // repeatedly dereferencing pointers or taking the concrete value of interfaces.
 // Thus, the matcher will always be applied to non-pointer and non-interface

matchers/have_existing_field_matcher.go

@@ -0,0 +1,36 @@
+package matchers
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/onsi/gomega/format"
+)
+
+type HaveExistingFieldMatcher struct {
+	Field string
+}
+
+func (matcher *HaveExistingFieldMatcher) Match(actual interface{}) (success bool, err error) {
+	// we don't care about the field's actual value, just about any error in
+	// trying to find the field (or method).
+	_, err = extractField(actual, matcher.Field, "HaveExistingField")
+	if err == nil {
+		return true, nil
+	}
+	var mferr missingFieldError
+	if errors.As(err, &mferr) {
+		// missing field errors aren't errors in this context, but instead
+		// unsuccessful matches.
+		return false, nil
+	}
+	return false, err
+}
+
+func (matcher *HaveExistingFieldMatcher) FailureMessage(actual interface{}) (message string) {
+	return fmt.Sprintf("Expected\n%s\nto have field '%s'", format.Object(actual, 1), matcher.Field)
+}
+
+func (matcher *HaveExistingFieldMatcher) NegatedFailureMessage(actual interface{}) (message string) {
+	return fmt.Sprintf("Expected\n%s\nnot to have field '%s'", format.Object(actual, 1), matcher.Field)
+}

matchers/have_existing_field_matcher_test.go

@@ -0,0 +1,84 @@
+package matchers_test
+
+import (
+	"time"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("HaveExistingField", func() {
+
+	var book Book
+	BeforeEach(func() {
+		book = Book{
+			Title: "Les Miserables",
+			Author: person{
+				FirstName: "Victor",
+				LastName:  "Hugo",
+				DOB:       time.Date(1802, 2, 26, 0, 0, 0, 0, time.UTC),
+			},
+			Pages: 2783,
+			Sequel: &Book{
+				Title: "Les Miserables 2",
+			},
+		}
+	})
+
+	DescribeTable("traversing the struct works",
+		func(field string) {
+			Ω(book).Should(HaveExistingField(field))
+		},
+		Entry("Top-level field", "Title"),
+		Entry("Nested field", "Author.FirstName"),
+		Entry("Top-level method", "AuthorName()"),
+		Entry("Nested method", "Author.DOB.Year()"),
+		Entry("Traversing past a method", "AbbreviatedAuthor().FirstName"),
+		Entry("Traversing a pointer", "Sequel.Title"),
+	)
+
+	DescribeTable("negation works",
+		func(field string) {
+			Ω(book).ShouldNot(HaveExistingField(field))
+		},
+		Entry("Top-level field", "Class"),
+		Entry("Nested field", "Author.Class"),
+		Entry("Top-level method", "ClassName()"),
+		Entry("Nested method", "Author.DOB.BOT()"),
+		Entry("Traversing past a method", "AbbreviatedAuthor().LastButOneName"),
+		Entry("Traversing a pointer", "Sequel.Titles"),
+	)
+
+	It("errors appropriately", func() {
+		success, err := HaveExistingField("Pages.Count").Match(book)
+		Ω(success).Should(BeFalse())
+		Ω(err.Error()).Should(Equal("HaveExistingField encountered:\n    <int>: 2783\nWhich is not a struct."))
+
+		success, err = HaveExistingField("Prequel.Title").Match(book)
+		Ω(success).Should(BeFalse())
+		Ω(err.Error()).Should(ContainSubstring("HaveExistingField encountered nil while dereferencing a pointer of type *matchers_test.Book."))
+
+		success, err = HaveExistingField("HasArg()").Match(book)
+		Ω(success).Should(BeFalse())
+		Ω(err.Error()).Should(ContainSubstring("HaveExistingField found an invalid method named 'HasArg()' in struct of type matchers_test.Book.\nMethods must take no arguments and return exactly one value."))
+	})
+
+	It("renders failure messages", func() {
+		matcher := HaveExistingField("Turtle")
+		success, err := matcher.Match(book)
+		Ω(success).Should(BeFalse())
+		Ω(err).ShouldNot(HaveOccurred())
+
+		msg := matcher.FailureMessage(book)
+		Ω(msg).Should(MatchRegexp(`(?s)Expected\n\s+<matchers_test\.Book>: .*\nto have field 'Turtle'`))
+
+		matcher = HaveExistingField("Title")
+		success, err = matcher.Match(book)
+		Ω(success).Should(BeTrue())
+		Ω(err).ShouldNot(HaveOccurred())
+
+		msg = matcher.NegatedFailureMessage(book)
+		Ω(msg).Should(MatchRegexp(`(?s)Expected\n\s+<matchers_test\.Book>: .*\nnot to have field 'Title'`))
+	})
+
+})

matchers/have_field.go

@@ -8,19 +8,28 @@ import (
 	"github.com/onsi/gomega/format"
 )
 
-func extractField(actual interface{}, field string) (interface{}, error) {
+// missingFieldError represents a missing field extraction error that
+// HaveExistingFieldMatcher can ignore, as opposed to other, sever field
+// extraction errors, such as nil pointers, et cetera.
+type missingFieldError string
+
+func (e missingFieldError) Error() string {
+	return string(e)
+}
+
+func extractField(actual interface{}, field string, matchername string) (interface{}, error) {
 	fields := strings.SplitN(field, ".", 2)
 	actualValue := reflect.ValueOf(actual)
 
 	if actualValue.Kind() == reflect.Ptr {
 		actualValue = actualValue.Elem()
 	}
 	if actualValue == (reflect.Value{}) {
-		return nil, fmt.Errorf("HaveField encountered nil while dereferencing a pointer of type %T.", actual)
+		return nil, fmt.Errorf("%s encountered nil while dereferencing a pointer of type %T.", matchername, actual)
 	}
 
 	if actualValue.Kind() != reflect.Struct {
-		return nil, fmt.Errorf("HaveField encountered:\n%s\nWhich is not a struct.", format.Object(actual, 1))
+		return nil, fmt.Errorf("%s encountered:\n%s\nWhich is not a struct.", matchername, format.Object(actual, 1))
 	}
 
 	var extractedValue reflect.Value
@@ -31,24 +40,24 @@ func extractField(actual interface{}, field string) (interface{}, error) {
 			extractedValue = actualValue.Addr().MethodByName(strings.TrimSuffix(fields[0], "()"))
 		}
 		if extractedValue == (reflect.Value{}) {
-			return nil, fmt.Errorf("HaveField could not find method named '%s' in struct of type %T.", fields[0], actual)
+			return nil, missingFieldError(fmt.Sprintf("%s could not find method named '%s' in struct of type %T.", matchername, fields[0], actual))
 		}
 		t := extractedValue.Type()
 		if t.NumIn() != 0 || t.NumOut() != 1 {
-			return nil, fmt.Errorf("HaveField found an invalid method named '%s' in struct of type %T.\nMethods must take no arguments and return exactly one value.", fields[0], actual)
+			return nil, fmt.Errorf("%s found an invalid method named '%s' in struct of type %T.\nMethods must take no arguments and return exactly one value.", matchername, fields[0], actual)
 		}
 		extractedValue = extractedValue.Call([]reflect.Value{})[0]
 	} else {
 		extractedValue = actualValue.FieldByName(fields[0])
 		if extractedValue == (reflect.Value{}) {
-			return nil, fmt.Errorf("HaveField could not find field named '%s' in struct:\n%s", fields[0], format.Object(actual, 1))
+			return nil, missingFieldError(fmt.Sprintf("%s could not find field named '%s' in struct:\n%s", matchername, fields[0], format.Object(actual, 1)))
 		}
 	}
 
 	if len(fields) == 1 {
 		return extractedValue.Interface(), nil
 	} else {
-		return extractField(extractedValue.Interface(), fields[1])
+		return extractField(extractedValue.Interface(), fields[1], matchername)
 	}
 }
 
@@ -61,7 +70,7 @@ type HaveFieldMatcher struct {
 }
 
 func (matcher *HaveFieldMatcher) Match(actual interface{}) (success bool, err error) {
-	matcher.extractedField, err = extractField(actual, matcher.Field)
+	matcher.extractedField, err = extractField(actual, matcher.Field, "HaveField")
 	if err != nil {
 		return false, err
 	}