Cannot use self in field &convert expressions

[bbannier]

It looks like self cannot be used in field &convert expressions; using self in a unit &convert attribute like mentioned in the docs works though.

# foo.spicy
module foo;

public type X = unit {
	len: uint8;
	x: bytes &eod &convert=self.len;
	# x: bytes &eod &convert=f($$, self.len); # Originally intended use.
};

function f(x: bytes, len: uint8): string {
	return "";
}

$ spicyc -j foo.spicy
[error] foo.spicy:5:25: unknown ID 'self'
[error] spicyc: aborting after errors