git-annex import --no-content from a directory special remote is implemented, but git-annex sync, when run without --content, does not operate on import/export special remotes. This is inconsistent, and it would be useful if it did.

https://git-annex.branchable.com/todo/importing_from_special_remote_without_downloading/#comment-e3db95e073f01a05b205e26f422f5bc5 describes a problem with doing that, involving merge conflicts. That should not actually happen with the directory special remote, because it generates the same key with importKey as git-annex import would. But other special remotes later using this interface might generate a key using a different hash than usual.

The suggestion there is that there could be a separate config that controls whether sync does a fast import or an import with content. Then when sync is run without --content, it can do a fast import. And when run with --content, it can do a fast import, followed by getting the content.

Or maybe that should just be what it always does, when a remote supports importKey? (If so, git-annex import should do the same.) Yeah, this seems better than a config. Look at it like this: The special remote makes pseudo "commits" when changes are made to it. And maybe it choses to use a different kind of key than the local repository would use. Same could happen when pulling from someone else's repo, if they've configured git-annex to use a different backend.

Except.. --no-content means annex.largefiles is not checked, so non-large files get added as annexed files. That's done because annex.largefiles can contain expressions that need to examine the content of the file. In particulat for mimetype and mimeencoding. So there would still be a conflict potential.

May be worth removing support for matching annex.largefiles when the expression needs the file content, when importing from a special remote. Or could detect when those are used, and only import with --content then.