Desirability of client-side expressions in psql?

Hello devs,

This is a discussion without actual patch intended for pg12, to be added
to CF 2018-09. The expected end result is either "returned with feedback",
meaning proceed to send some big patch(es), or "rejected", meaning the
project does not want this, no point in submitting something.

Client "psql" has an "\if" which can test a boolean value and has
":"-prefixed variables, including special presets such as ":VERSION_NUM"
and ":SERVER_VERSION_NUM".

The features are already usable because one can do server-side expressions
(if connected), which is a little cumbersome and ugly but nevertheless
functional, eg:

SELECT :VERSION_NUM = :SERVER_VERSION_NUM AS "same_version" \gset
\if :same_version
...

However, when the "\if" patch was discussed, there was the underlying idea
to extend psql so as to add client-side expression. That would allow
things like:

\let i <some arithmetic or logical expression...>
\if :VERSION_NUM = :SERVER_VERSION_NUM
...

Before eventually starting on this subject with a loose objective of
targeting 12.0, I would like to ascertain, especially from committers, but
also from others, that:

(1) the objective is desirable (i.e. avoid ending with "we do not want
this feature on principle, the cost-benefit balance is not good
enough").

(2) maybe have a feedback on the proposed changes (not necessarily
distinct patches, opinions are welcome), which would be to:
(a) extend pgbench expressions so that they can handle what
psql can do (eg variable-exists test which are available in psql)
(b) maybe do some preliminary refactoring (eg create
"pgbench/expression.c", "pgbench/variable.c")
(c) move the pgbench expression engine to fe-utils
(lexer, parser, execution...),
(d) do some renaming (eg some "feex" prefix for "front-end
expressions" to the various functions & types?),
(e) abstract pgbench and psql variables so that they can be used
transparently by expressions (i.e. some API alignment)
(f) connect the engine to "psql"
(g) create a shared documentation about these expressions,
referenced from both psql and pgbench documentations.
(h) provide non-regression tests on psql side as well.

The overall transformation would be quite large (about 2000 lines moved
around) but "quite" simple (it is moving an existing, working and tested
feature to allow using it elsewhere), not a lot of new code per se.

--
Fabien.