How can I merge a TargetEntry with a tuple?

Hi,

I started working again on my
IDENTITY/GENERATED patch.
My question is $SUBJECT.
This code is in rewriteTargetlist():

new_attr = build_column_default()
new_tle = makeTargetEntry((Expr *) new_expr, ...)

Now, in ExecInsert() I have to compute
the default for IDENTITY/GENERATED
between ExecConstraints() and heap_insert().

How can I create a Datum out of either an Expr
or a TargetEntry (that contains the computed
constant out of the default expression) so I can
use it after I did an heap_deform_tuple()?

Best regards,
Zoltán Böszörményi