It’s somewhat annoying that you can’t take an arbitrary expression and prefix something to it with constant &&& expression. This would require that the expression have the same input type as constant, which if constant is a const-word jet, is unit.
This also means that the source type of an expression created by the Rust scribe method (and presumably the Haskell one as well) is nonobvious – it will be unit if any constant-word jets were used but it will be free otherwise.
It’s easy enough to work around this by composing with unit, as (unit; constant) &&& expression but it’s a minor papercut.