LGTM.

So I would change this to:

break :fits (from_int.positive or from_int.eqZero() or int.key.is_signed) and 
int.key.bit_count >= from_int.bitCountTwosComp(); 

Does that look right?

Think you'd want something more like this:

if (!int.key.is_signed and !from_int.positive) {
    break :fits false;
}

const req_bits = from_int.bitCountTwosComp() + @boolToInt(from_int.positive and int.key.is_signed);
break :fits int.key.bit_count >= req_bits

Signed never fits in an unsigned target and we need one extra bit for a positive source to a signed target.

With the special-cased zero fitting in any target:

if (from_int.eqZero()) {
    break :fits true;
}

It may make sense to add a fits function to Int itself for this purpose since this seems pretty general. It'd pretty much be like to except we would bother returning the converted value.