smart diff to testutil.GatherAndCompare

go.mod

@@ -3,8 +3,10 @@ module github.com/prometheus/client_golang
 require (
 	github.com/beorn7/perks v1.0.1
 	github.com/cespare/xxhash/v2 v2.1.2
+	github.com/davecgh/go-spew v1.1.1
 	github.com/golang/protobuf v1.5.2
 	github.com/json-iterator/go v1.1.12
+	github.com/pmezard/go-difflib v1.0.0
 	github.com/prometheus/client_model v0.2.0
 	github.com/prometheus/common v0.32.1
 	github.com/prometheus/procfs v0.7.3

prometheus/testutil/testutil.go

@@ -41,7 +41,10 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"reflect"
 
+	"github.com/davecgh/go-spew/spew"
+	"github.com/pmezard/go-difflib/difflib"
 	"github.com/prometheus/common/expfmt"
 
 	dto "github.com/prometheus/client_model/go"
@@ -211,18 +214,71 @@ func compare(got, want []*dto.MetricFamily) error {
 			return fmt.Errorf("encoding expected metrics failed: %s", err)
 		}
 	}
+	if diffErr := diff(wantBuf, gotBuf); diffErr != "" {
+		return fmt.Errorf(diffErr)
+	}
+	return nil
+}
 
-	if wantBuf.String() != gotBuf.String() {
-		return fmt.Errorf(`
-metric output does not match expectation; want:
+// diff returns a diff of both values as long as both are of the same type and
+// are a struct, map, slice, array or string. Otherwise it returns an empty string.
+func diff(expected interface{}, actual interface{}) string {
+	if expected == nil || actual == nil {
+		return ""
+	}
 
-%s
-got:
+	et, ek := typeAndKind(expected)
+	at, _ := typeAndKind(actual)
+	if et != at {
+		return ""
+	}
 
-%s`, wantBuf.String(), gotBuf.String())
+	if ek != reflect.Struct && ek != reflect.Map && ek != reflect.Slice && ek != reflect.Array && ek != reflect.String {
+		return ""
+	}
 
+	var e, a string
+	c := spew.ConfigState{
+		Indent:                  " ",
+		DisablePointerAddresses: true,
+		DisableCapacities:       true,
+		SortKeys:                true,
 	}
-	return nil
+	if et != reflect.TypeOf("") {
+		e = c.Sdump(expected)
+		a = c.Sdump(actual)
+	} else {
+		e = reflect.ValueOf(expected).String()
+		a = reflect.ValueOf(actual).String()
+	}
+
+	diff, _ := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
+		A:        difflib.SplitLines(e),
+		B:        difflib.SplitLines(a),
+		FromFile: "metric output does not match expectation; want",
+		FromDate: "",
+		ToFile:   "got:",
+		ToDate:   "",
+		Context:  1,
+	})
+
+	if diff == "" {
+		return ""
+	}
+
+	return "\n\nDiff:\n" + diff
+}
+
+// typeAndKind returns the type and kind of the given interface{}
+func typeAndKind(v interface{}) (reflect.Type, reflect.Kind) {
+	t := reflect.TypeOf(v)
+	k := t.Kind()
+
+	if k == reflect.Ptr {
+		t = t.Elem()
+		k = t.Kind()
+	}
+	return t, k
 }
 
 func filterMetrics(metrics []*dto.MetricFamily, names []string) []*dto.MetricFamily {

prometheus/testutil/testutil_test.go

@@ -284,17 +284,18 @@ func TestMetricNotFound(t *testing.T) {
 	`
 
 	expectedError := `
-metric output does not match expectation; want:
 
-# HELP some_other_metric A value that represents a counter.
-# TYPE some_other_metric counter
-some_other_metric{label1="value1"} 1
-
-got:
-
-# HELP some_total A value that represents a counter.
-# TYPE some_total counter
-some_total{label1="value1"} 1
+Diff:
+--- metric output does not match expectation; want
++++ got:
+@@ -1,4 +1,4 @@
+-(bytes.Buffer) # HELP some_other_metric A value that represents a counter.
+-# TYPE some_other_metric counter
+-some_other_metric{label1="value1"} 1
++(bytes.Buffer) # HELP some_total A value that represents a counter.
++# TYPE some_total counter
++some_total{label1="value1"} 1
+
 `
 
 	err := CollectAndCompare(c, strings.NewReader(metadata+expected))