Return helpful error if subfolder syntax fails on Windows (#21813)

internal/data/tree.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"runtime"
 
 	"io"
 	"iter"
@@ -311,6 +312,9 @@ func FindTreeDirectory(ctx context.Context, repo restic.BlobLoader, id *restic.I
 			return nil, fmt.Errorf("path %s: %w", subfolder, err)
 		}
 		if node == nil {
+			if runtime.GOOS == "windows" && strings.Contains(dir, "\\") {
+				return nil, fmt.Errorf("path %s: not found; subfolder syntax currently requires forward slashes; check the output of `restic ls` for valid paths", subfolder)
+			}
 			return nil, fmt.Errorf("path %s: not found", subfolder)
 		}
 		if node.Type != NodeTypeDir || node.Subtree == nil {

internal/data/tree_test.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime"
 	"slices"
 	"strconv"
 	"strings"
@@ -401,6 +402,22 @@ func TestFindTreeDirectory(t *testing.T) {
 	rtest.Assert(t, err != nil, "missing error on null tree id")
 }
 
+func TestFindTreeDirectoryWindowsBackslashHint(t *testing.T) {
+	repo := repository.TestRepository(t)
+	sn := data.TestCreateSnapshot(t, repo, parseTimeUTC("2017-07-07 07:07:08"), 1)
+
+	_, err := data.FindTreeDirectory(context.TODO(), repo, sn.Tree, `missing\path`)
+	rtest.Assert(t, err != nil, "expected error")
+
+	if runtime.GOOS == "windows" {
+		rtest.Assert(t, strings.Contains(err.Error(), "forward slashes"),
+			"expected backslash hint on Windows, got %v", err)
+	} else {
+		rtest.Assert(t, err.Error() == `path missing\path: not found`,
+			"unexpected err: %v", err)
+	}
+}
+
 func TestDualTreeIterator(t *testing.T) {
 	testErr := errors.New("test error")