Is it possible to use type `char` in an account field / as funciton arguments?

Question

The goal is to use to use the char type.

I couldn't find a way or ressource on how to solve to borsh serialization errors that get thrown when trying to store a char type in an #[account] struct field.

Other types or enums won't make problems.

A reproduction is as simple as taking any anchor project and defining one of the fileds of an account to be type of char. E.g. when using one of the example projects from the anchor repo and adding a char field it will produce an error:

#[account]
pub struct Counter {
    pub authority: Pubkey,
    pub count: u64,
    pub something: char, // <- will produce error
}

To take an example from one of the projects I'm working on:

Error msg:

rustc: the trait bound `char: BorshSerialize` is not satisfied
the following other types implement trait `BorshSerialize`:
  &T
  ()
  (T0, T1)
  (T0, T1, T2)
  (T0, T1, T2, T3)
  (T0, T1, T2, T3, T4)
  (T0, T1, T2, T3, T4, T5)
  (T0, T1, T2, T3, T4, T5, T6)
and 185 others
see issue #48214

Also trying to use a char as a function argument throws errors for the #[program] macro.

Project in screenshots: https://github.com/tobealive/anchor-solana-twitter/

Answer 1

A char in rust is not considered serializable by the Borsh spec which anchor uses. This is because a char is really an array of 4 bytes with many invalid values.

Depending on your use case you can use a string that you then take the first character of only or store a 4 byte array that you utf8 encode into and out of. It's very likely though that you really need a char stored. If you elaborate on the use case there may be a better solution.

A more extreme option would be to extend borsh yourself by storing a wrapped value and implementing BorshSerialize for that. You will need to manually deserialize it on the client side and anchor doesn't usually like custom types.