Technically, env() should not be an ASF. (😱) This is why some tests
still fail - env() as specced is expected to have its syntax checked
fully at parse-time, whereas ASFs are not properly syntax-checked until
later. However, I think this approach was worth doing for a few reasons:
- env() behaves like an ASF otherwise. (It is replaced with a set of
arbitrary component-values that are not known until computed-value
time.)
- env() was defined before the ASF concept existed, so I strongly
suspect it will be updated in the future to match that definition,
with a couple of adjustments. (eg, env() is allowed in some extra
places compared to var() and attr().)
- This was much quicker and easier to implement (under 3 hours in total)
compared to the greater amount of work to implement a whole separate
system just for env().
- Most of these tests are marked tentative, and the spec definition of
env() is still somewhat in flux, so failing some is not a huge deal.
If in the future I turn out to be wrong on this, we can convert it to
its own special thing.
"Arbitrary substitution functions" are a family of functions that
includes var() and attr(). All of them resolve to an arbitrary set of
component values that are not known at parse-time, so they have to be
substituted at computed-value time.
Besides it being nice to follow the spec closely, this means we'll be
able to implement the others (such as `if()` and `inherit()`) more
easily.
The main omission here is the new "spread syntax", which can be
implemented in the future.
