"sub X by NULL" is (sadly) acceptable

[simoncozens]

There's a terrible hack that's not in the FEA specification, but is used by a number of fonts. It's possible to use multiple substitution to delete a glyph; this is supported by the spec, in fonttools and makeotf.

Because you can do sub X by NULL you can also do sub a X' by NULL; but currently fea-rs pulls a face at this:

error: Expected glyph class features.fea at 4004:22
     |
4004 | sub @All.Medis r0' by NULL;

[cmyr]

okay I think this is just an oversight in the parsing of contextual lookups, we do support the sub X by NULL syntax in non-contextual... contexts. Will take a look. :)

[cmyr]

I guess this means we should also support,

sub @someclass a';

to be an implied by NULL?

[simoncozens]

I don't think we need an implied one, no - the syntax for deleting is sub <single glyph> by NULL, nothing else. But we need both x by NULL; and <context> x' <context> by NULL;

[cmyr]

I was just thinking of how in various places the spec says, "Omitting the by clause is equivalent to adding by NULL".

[cmyr]

oh also: this repo has moved into fontc, I'll see about moving this issue over there.

[simoncozens]

Maybe archive it, then?