"mir" and "unity8" snap interfaces are not available

The unity8-session snap does provide that interface. But it is not installed by default.

Agreed that since the unity8-desktop-session deb provides the Mir socket, it would be nice if it could expose that fact to the snappy world by providing a "mir" slot.

From a recent email thread

On a snap based system, mir-libs snap will provide the content interface to access the libraries. Both client and server bits reside there.

This is largely done. mir-libs has been updated to match zesty's version (0.26.1) and several apps and the unity8-session snap itself are now using it. The desktop-launch helper script uses mir-libs if it is connected.
The snapd content interface for mir-libs is not autoconnected yet. But that is mitigated by the fact that any canonical-owned app will autoconnect and the unity8-snap-install script will also manually connect the core apps. In future, once we are more comfortable with the mir-libs snap and think the libraries are ready for wide use, we can ask the snapd team to turn on autoconnection.

Ubuntu-app-platform will no longer include the mir client library.

This is still to be published

On a classic system, we need an implicit mir interface so that confined snaps can use mir as provided by debs in the base system.

This needs work.

Workaround, based on Michael's comment and guidance over IRC:

1) install the unity8-session snap
2) remove the unity8-session-snap debian package, if you have it installed
3) if you have already some unity8 apps installed as a snap, connect their "mir" interface manually