Use serde

[gavrilikhin-d]

Could you make your SourceOffset, SourceSpan, Severity, LabledSpan and other types to implement Serialize and Deserialize?
Now you need to use builtin types instead of them to have serialization autoimplemented:

#[derive(Debug, Error, Diagnostic, Serialize, Deserialize, PartialEq, Eq, Clone)]
#[error("expected '{expected}'")]
pub struct Expected {
    pub expected: String,
    #[label("{expected}")]
    pub at: usize, // This is auto serialized/deserialized
}

So you can't just write:

#[derive(Debug, Error, Diagnostic, Serialize, Deserialize, PartialEq, Eq, Clone)]
#[error("expected '{expected}'")]
pub struct Expected {
    pub expected: String,
    #[label("{expected}")]
    pub at: SourceOffset, // Doesn't work. Need to implement manually
}

[kleinesfilmroellchen]

Since serde is a large dependency, this should most definitely be an optional or opt-out dependency for anyone who doesn't need it.

[zkat]

This seems reasonable, as long as it's hidden behind a non-default serde flag. I can't really do a lot of stuff myself right now, but I probably have time to review/merge a PR if someone contributes it.

[gavrilikhin-d]

Ok, I will create a PR on these weekends

[gavrilikhin-d]

This one looks related: #245