Some fonts use FEA include for arbitrary string concatenation

[cmyr]

That is, either or both of the including/included files are not valid FEA independently, and are only valid if they are parsed after they are concatenated.

For instance the Yantramanav family seems to have some FEA that looks something like this:

features.fea

feature derp {
    sub a by b;
    include(my_include.fea);

sub x by y;
} derp;

This is not well handled by fea-rs, because fea-rs attempts to parse each fea file independently, and errors because in features.fea the feature is never closed.

This is not a common pattern, and the spec is not clear as to whether it should be supported or not; all it really says is,

An include directive is valid in any context that otherwise contains statements ending in semicolons: “Top-level” statements; feature, lookup, table, cvParameter, and AxisValue blocks; and name groups. (An implementation that processes include statements at the token level is not required to enforce these restrictions.)

– (3. Including files)