appendIdentStr

fn appendIdentStr(zg: *ZonGen, ident_token: Ast.TokenIndex) error{ OutOfMemory, BadString }!u32 {
    const gpa = zg.gpa;
    const tree = zg.tree;
    assert(tree.tokenTag(ident_token) == .identifier);
    const ident_name = tree.tokenSlice(ident_token);
    if (!mem.startsWith(u8, ident_name, "@")) {
        const start = zg.string_bytes.items.len;
        try zg.string_bytes.appendSlice(gpa, ident_name);
        return @intCast(start);
    }
    const offset = 1;
    const start: u32 = @intCast(zg.string_bytes.items.len);
    const raw_string = zg.tree.tokenSlice(ident_token)[offset..];
    try zg.string_bytes.ensureUnusedCapacity(gpa, raw_string.len);
    const result = r: {
        var aw: Writer.Allocating = .fromArrayList(gpa, &zg.string_bytes);
        defer zg.string_bytes = aw.toArrayList();
        break :r std.zig.string_literal.parseWrite(&aw.writer, raw_string) catch |err| switch (err) {
            error.WriteFailed => return error.OutOfMemory,
        };
    };
    switch (result) {
        .success => {},
        .failure => |err| {
            try zg.lowerStrLitError(err, ident_token, raw_string, offset);
            return error.BadString;
        },
    }

    const slice = zg.string_bytes.items[start..];
    if (mem.findScalar(u8, slice, 0) != null) {
        try zg.addErrorTok(ident_token, "identifier cannot contain null bytes", .{});
        return error.BadString;
    } else if (slice.len == 0) {
        try zg.addErrorTok(ident_token, "identifier cannot be empty", .{});
        return error.BadString;
    }
    return start;
}