feat: add IsError and NotIsError helpers

As Go's error wrapping has become more and more common, these types of
tests are becoming ubiquitous.

.golangci.yml

@@ -45,6 +45,13 @@ linters:
     - gomoddirectives
     - thelper
     - varnamelen
+    - depguard
+    - exhaustruct
+    - nonamedreturns
+    - varcheck
+    - structcheck
+    - deadcode
+    - nosnakecase
 
 linters-settings:
   govet:

README.md

@@ -64,6 +64,12 @@ func Error(t testing.TB, err error, msgAndArgs ...interface{})
 // NoError asserts that an error is nil.
 func NoError(t testing.TB, err error, msgAndArgs ...interface{})
 
+// IsError asserts than any error in "err"'s tree matches "target".
+func IsError(t testing.TB, err, target error, msgAndArgs ...interface{})
+
+// NotIsError asserts than no error in "err"'s tree matches "target".
+func NotIsError(t testing.TB, err, target error, msgAndArgs ...interface{})
+
 // Panics asserts that the given function panics.
 func Panics(t testing.TB, fn func(), msgAndArgs ...interface{})
 

assert.go

@@ -3,6 +3,7 @@ package assert
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"reflect"
 	"strconv"
@@ -124,6 +125,24 @@ func EqualError(t testing.TB, err error, errString string, msgAndArgs ...interfa
 	}
 }
 
+// IsError asserts than any error in "err"'s tree matches "target".
+func IsError(t testing.TB, err, target error, msgAndArgs ...interface{}) {
+	if errors.Is(err, target) {
+		return
+	}
+	t.Helper()
+	t.Fatal(formatMsgAndArgs(fmt.Sprintf("Error tree %q should contain error %q", err, target), msgAndArgs...))
+}
+
+// NotIsError asserts than no error in "err"'s tree matches "target".
+func NotIsError(t testing.TB, err, target error, msgAndArgs ...interface{}) {
+	if !errors.Is(err, target) {
+		return
+	}
+	t.Helper()
+	t.Fatal(formatMsgAndArgs(fmt.Sprintf("Error tree %q should NOT contain error %q", err, target), msgAndArgs...))
+}
+
 // Error asserts that an error is not nil.
 func Error(t testing.TB, err error, msgAndArgs ...interface{}) {
 	if err != nil {
@@ -185,15 +204,15 @@ func NotPanics(t testing.TB, fn func(), msgAndArgs ...interface{}) {
 	fn()
 }
 
-func diff[T any](lhs, rhs T) string {
+func diff[T any](before, after T) string {
 	var lhss, rhss string
 	// Special case strings so we get nice diffs.
-	if l, ok := any(lhs).(string); ok {
+	if l, ok := any(before).(string); ok {
 		lhss = l
-		rhss = any(rhs).(string)
+		rhss = any(after).(string)
 	} else {
-		lhss = repr.String(lhs, repr.Indent("  ")) + "\n"
-		rhss = repr.String(rhs, repr.Indent("  ")) + "\n"
+		lhss = repr.String(before, repr.Indent("  ")) + "\n"
+		rhss = repr.String(after, repr.Indent("  ")) + "\n"
 	}
 	edits := myers.ComputeEdits("a.txt", lhss, rhss)
 	lines := strings.Split(fmt.Sprint(gotextdiff.ToUnified("expected.txt", "actual.txt", lhss, edits)), "\n")

assert_test.go

@@ -2,6 +2,7 @@ package assert
 
 import (
 	"fmt"
+	"os"
 	"testing"
 )
 
@@ -123,6 +124,24 @@ func TestNotZero(t *testing.T) {
 	})
 }
 
+func TestIsError(t *testing.T) {
+	assertOk(t, "SameError", func(t testing.TB) {
+		IsError(t, fmt.Errorf("os error: %w", os.ErrClosed), os.ErrClosed)
+	})
+	assertFail(t, "DifferentError", func(t testing.TB) {
+		IsError(t, fmt.Errorf("not an os error"), os.ErrClosed)
+	})
+}
+
+func TestNotIsError(t *testing.T) {
+	assertFail(t, "SameError", func(t testing.TB) {
+		NotIsError(t, fmt.Errorf("os error: %w", os.ErrClosed), os.ErrClosed)
+	})
+	assertOk(t, "DifferentError", func(t testing.TB) {
+		NotIsError(t, fmt.Errorf("not an os error"), os.ErrClosed)
+	})
+}
+
 type testTester struct {
 	*testing.T
 	failed string

bin/go

@@ -1 +1 @@
-.go-1.19.1.pkg
+.go-1.20.5.pkg

bin/gofmt

@@ -1 +1 @@
-.go-1.19.1.pkg
+.go-1.20.5.pkg

bin/golangci-lint

@@ -1 +1 @@
-.golangci-lint-1.45.2.pkg
+.golangci-lint-1.53.2.pkg