Add slog compatibility

CHANGELOG.md

@@ -24,6 +24,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   }
   ```
 
+- Add `LogAttr` function to log any error with log/slog with the key "error".
+  If the provided error is wrapped with errtrace, it will log the full trace.
+  Otherwise, the original error message will be logged.
 - cmd/errtrace:
   Add `-no-wrapn` option to disable wrapping with generic `WrapN` functions.
   This is only useful for toolexec mode due to tooling limitations.
@@ -38,6 +41,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 - Update `go` directive in go.mod to 1.21, and drop compatibility with Go 1.20 and earlier.
+- Errors wrapped with errtrace are now compatible with log/slog-based loggers,
+  and will report the full error trace when logged.
 
 ### Fixed
 - cmd/errtrace: Don't exit with a non-zero status when `-h` is used.

errtrace.go

@@ -56,6 +56,7 @@ package errtrace
 import (
 	"fmt"
 	"io"
+	"log/slog"
 	"strings"
 )
 
@@ -123,6 +124,11 @@ func (e *errTrace) Format(s fmt.State, verb rune) {
 	fmt.Fprintf(s, fmt.FormatString(s, verb), e.err)
 }
 
+// LogValue implements the [slog.LogValuer] interface.
+func (e *errTrace) LogValue() slog.Value {
+	return slog.StringValue(FormatString(e))
+}
+
 // TracePC returns the program counter for the location
 // in the frame that the error originated with.
 //

example_trace_test.go

@@ -3,6 +3,7 @@ package errtrace_test
 import (
 	"errors"
 	"fmt"
+	"log/slog"
 	"runtime"
 	"strings"
 
@@ -80,3 +81,37 @@ func ExampleUnwrapFrame() {
 	//braces.dev/errtrace_test.f3
 	//	/path/to/errtrace/example_trace_test.go:3
 }
+
+func Example_logWithSlog() {
+	// This example demonstrates how to log an errtrace-wrapped error
+	// with the slog package.
+	// Unlike LogAttr, we're able to use any key name here.
+	logger, printLogOutput := newExampleLogger()
+
+	if err := f1(); err != nil {
+		logger.Error("f1 failed", "my-error", err)
+	}
+
+	printLogOutput()
+
+	// Output:
+	// {"level":"ERROR","msg":"f1 failed","my-error":"failed\n\nbraces.dev/errtrace_test.f3\n\t/path/to/errtrace/example_trace_test.go:3\nbraces.dev/errtrace_test.f2\n\t/path/to/errtrace/example_trace_test.go:2\nbraces.dev/errtrace_test.f1\n\t/path/to/errtrace/example_trace_test.go:1\n"}
+}
+
+// newExampleLogger creates a new slog.Logger for use in examples.
+// It omits timestamps from the output to allow for output matching,
+// and cleans paths in trace output to make them environment-agnostic.
+func newExampleLogger() (logger *slog.Logger, printOutput func()) {
+	var buf strings.Builder
+	return slog.New(slog.NewJSONHandler(&buf, &slog.HandlerOptions{
+			ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
+				if len(groups) == 0 && a.Key == slog.TimeKey {
+					return slog.Attr{}
+				}
+				return a
+			},
+		})), func() {
+			fmt.Println(tracetest.MustClean(buf.String()))
+			buf.Reset()
+		}
+}