assert: Add Error and ErrorContains

sloggers/slogtest/assert/assert.go

@@ -1,7 +1,12 @@
 // Package assert is a helper package for test assertions.
+//
+// On failure, every assertion will fatal the test.
+//
+// The name argument is included in each assertion for easier debugging.
 package assert // import "cdr.dev/slog/sloggers/slogtest/assert"
 
 import (
+	"strings"
 	"testing"
 
 	"cdr.dev/slog"
@@ -24,8 +29,6 @@ func Equal(t testing.TB, exp, act interface{}, name string) {
 }
 
 // Success asserts err == nil.
-//
-// If err isn't nil, it will fatal the test with the error.
 func Success(t testing.TB, err error, name string) {
 	slog.Helper()
 	if err != nil {
@@ -37,9 +40,31 @@ func Success(t testing.TB, err error, name string) {
 }
 
 // True asserts act == true.
-//
-// If act isn't true, it will fatal the test.
 func True(t testing.TB, act bool, name string) {
 	slog.Helper()
 	Equal(t, true, act, name)
 }
+
+// Error asserts err != nil.
+func Error(t testing.TB, err error, name string) {
+	slog.Helper()
+	if err == nil {
+		slogtest.Fatal(t, "expected error",
+			slog.F("name", name),
+		)
+	}
+}
+
+// ErrorContains asserts err != nil and err.Error() contains sub.
+func ErrorContains(t testing.TB, err error, sub, name string) {
+	slog.Helper()
+	Error(t, err, name)
+	errs := err.Error()
+	if !strings.Contains(errs, sub) {
+		slogtest.Fatal(t, "unexpected error string",
+			slog.F("name", name),
+			slog.F("error_string", errs),
+			slog.F("expected_substring", sub),
+		)
+	}
+}