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This magic is called type coercion. You can read about it here or here. Basically what happens is an implicit conversion of Rc<&'a mut [u8]>> into &mut [u8]. The way Rust does it is as following:

1. Auto-dereference when use . operator - Rc<...> is dereferenced into RefCell<> when you call .borrow_mut().
2. borrow_mut returns a value of type RefMut, which implements trait DerefMut.
3. The method .serialize takes &mut [u8] as argument, so you cannot call it with just &mut data.borrow_mut() as it has the type of &mut RefMut<&mut [u8]>.
4. Adding additional &mut before RefMut allows Rust compiler to use the type coercion. Seeing the type &mut &mut RefMut<&mut [u8]> it can automatically convert inner &mut RefMut into just &mut T, so you get &mut &mut &mut [u8]. Then it can repeat the process twice, each time turning &mut &mut into &mut. So in the end you get &mut [u8].

The more explicit way to do the same thing is to derefrence RefMut:

implement the serialize and deserialize in the mod jwt_numeric_date like this:

Your from_string module as written only supports values with T: std::str::FromStr,. Option never implements the FromStr trait, not even if the inner value implements it.

There are multiple options how to fix that. You can change the from_string module to return a Result, D::Error>. In the module, you deserialize a Option and parse it as needed. To make the field truly optional, you might want to add #[serde(default)] on that field too, since the field is not automatically default if you use the with attribute.

You can also use crate which already implement this composition of custom behavior, for example using serde_with::DisplayFromStr. You could write this instead:

I already stored the config with AdHoch::config() but to retrieve it within the guard I need to use request.rocket().state::(). The corrected source code is below:

The default size of a message produced in Kafka is 1 MB. If the message is greater than the size you can incur data loss.

To increase the size of your message, set the following configuration: message.max.bytes and allocate the size of message you want to produce and consume.

Also, set the following configuration max.partition.fetch.bytes in your consumer to fetch the size of messages you want to receive.

You can also read more about these configurations in this link: https://kafka.apache.org/documentation/#brokerconfigs