age: add NoIdentityMatchError.StanzaTypes field

For https://github.com/FiloSottile/age/issues/56#issuecomment-1962622903.

age.go

@@ -204,6 +204,9 @@ type NoIdentityMatchError struct {
 	// Errors is a slice of all the errors returned to Decrypt by the Unwrap
 	// calls it made. They all wrap [ErrIncorrectIdentity].
 	Errors []error
+	// StanzaTypes are the first argument of each recipient stanza in the
+	// encrypted file's header.
+	StanzaTypes []string
 }
 
 func (*NoIdentityMatchError) Error() string {
@@ -265,10 +268,11 @@ func decryptHdr(hdr *format.Header, identities ...Identity) ([]byte, error) {
 	})
 
 	stanzas := make([]*Stanza, 0, len(hdr.Recipients))
+	errNoMatch := &NoIdentityMatchError{}
 	for _, s := range hdr.Recipients {
+		errNoMatch.StanzaTypes = append(errNoMatch.StanzaTypes, s.Type)
 		stanzas = append(stanzas, (*Stanza)(s))
 	}
-	errNoMatch := &NoIdentityMatchError{}
 	var fileKey []byte
 	for _, id := range identities {
 		var err error

age_test.go

@@ -6,10 +6,12 @@ package age_test
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"log"
 	"os"
+	"slices"
 	"strings"
 	"testing"
 
@@ -329,6 +331,54 @@ func TestDecryptNativeIdentitiesFirst(t *testing.T) {
 	}
 }
 
+type stanzaTypeRecipient string
+
+func (s stanzaTypeRecipient) Wrap(fileKey []byte) ([]*age.Stanza, error) {
+	return []*age.Stanza{{Type: string(s)}}, nil
+}
+
+func TestNoIdentityMatchErrorStanzaTypes(t *testing.T) {
+	a, err := age.GenerateX25519Identity()
+	if err != nil {
+		t.Fatal(err)
+	}
+	b, err := age.GenerateX25519Identity()
+	if err != nil {
+		t.Fatal(err)
+	}
+	wrong, err := age.GenerateX25519Identity()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	buf := &bytes.Buffer{}
+	w, err := age.Encrypt(buf, a.Recipient(), stanzaTypeRecipient("other"), b.Recipient())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := io.WriteString(w, helloWorld); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = age.Decrypt(bytes.NewReader(buf.Bytes()), wrong)
+	if err == nil {
+		t.Fatal("expected decryption to fail")
+	}
+
+	var noMatch *age.NoIdentityMatchError
+	if !errors.As(err, &noMatch) {
+		t.Fatalf("expected NoIdentityMatchError, got %T: %v", err, err)
+	}
+
+	want := []string{"X25519", "other", "X25519"}
+	if !slices.Equal(noMatch.StanzaTypes, want) {
+		t.Errorf("StanzaTypes = %v, want %v", noMatch.StanzaTypes, want)
+	}
+}
+
 func TestDetachedHeader(t *testing.T) {
 	i, err := age.GenerateX25519Identity()
 	if err != nil {