Swallow panic when calling String or Error (#221)

If a panic occurs while calling String or Error,
the reporter recovers from it and ignores it, proceeding with
its usual functionality for formatting a value.

cmp/compare_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"crypto/md5"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -874,6 +875,18 @@ func reporterTests() []test {
 	)
 
 	return []test{{
+		label:     label + "/PanicStringer",
+		x:         struct{ X fmt.Stringer }{struct{ fmt.Stringer }{nil}},
+		y:         struct{ X fmt.Stringer }{bytes.NewBuffer(nil)},
+		wantEqual: false,
+		reason:    "panic from fmt.Stringer should not crash the reporter",
+	}, {
+		label:     label + "/PanicError",
+		x:         struct{ X error }{struct{ error }{nil}},
+		y:         struct{ X error }{errors.New("")},
+		wantEqual: false,
+		reason:    "panic from error should not crash the reporter",
+	}, {
 		label:     label + "/AmbiguousType",
 		x:         foo1.Bar{},
 		y:         foo2.Bar{},

cmp/report_reflect.go

@@ -125,12 +125,18 @@ func (opts formatOptions) FormatValue(v reflect.Value, parentKind reflect.Kind,
 		// implementations crash when doing so.
 		if (t.Kind() != reflect.Ptr && t.Kind() != reflect.Interface) || !v.IsNil() {
 			var prefix, strVal string
-			switch v := v.Interface().(type) {
-			case error:
-				prefix, strVal = "e", v.Error()
-			case fmt.Stringer:
-				prefix, strVal = "s", v.String()
-			}
+			func() {
+				// Swallow and ignore any panics from String or Error.
+				defer func() { recover() }()
+				switch v := v.Interface().(type) {
+				case error:
+					strVal = v.Error()
+					prefix = "e"
+				case fmt.Stringer:
+					strVal = v.String()
+					prefix = "s"
+				}
+			}()
 			if prefix != "" {
 				maxLen := len(strVal)
 				if opts.LimitVerbosity {

cmp/testdata/diffs

@@ -252,6 +252,18 @@
   	})),
   }
 >>> TestDiff/Transformer/AcyclicString
+<<< TestDiff/Reporter/PanicStringer
+  struct{ X fmt.Stringer }{
+- 	X: struct{ fmt.Stringer }{},
++ 	X: s"",
+  }
+>>> TestDiff/Reporter/PanicStringer
+<<< TestDiff/Reporter/PanicError
+  struct{ X error }{
+- 	X: struct{ error }{},
++ 	X: e"",
+  }
+>>> TestDiff/Reporter/PanicError
 <<< TestDiff/Reporter/AmbiguousType
   interface{}(
 - 	"github.com/google/go-cmp/cmp/internal/teststructs/foo1".Bar{},