Add primary key/unique constraint using prefix columns of an index

Now that there are index only scans, there is a use case for having a
composite index which has the primary key or a unique key as the
prefix column(s) but with extra columns after that.  Currently you
would also need another index with exactly the primary/unique key,
which seems like a waste of storage and maintenance.

Should there be a way to declare a "unique" index with the unique
property applying to a prefix of the indexed columns/expression?  And
having that, a way to turn that prefix into a primary key constraint?

Of course this is easier said then done, but is there some reason for
it not to be a to-do item?

Thanks,

Jeff