It's the Be + Infinitive Construction, if you need a name. The construction is not limited to infinitive use; i.e, it's not just to be to + Infinitive. It can also be used in the present or past tense.
- He is to leave tomorrow.
- He was to leave yesterday, but the flight was cancelled.
A bare infinitive in a strange place, like after an auxiliary verb (e.g, be or have), is bound to be an idiom, and usually has a modal meaning:
- He has to leave. (He must leave)
- He is to leave (He will leave ~ He is expected/scheduled to leave)
just like relative infinitives (infinitive clauses modifying nouns) always have a modal meaning:
- He is the man to see about this. (He is the man that one should see)
- He is the man to do the job. (He is the man who should do it)