Add struct for dynamic creation of diagnostics

[gavrilikhin-d]

It would be cool to have builtin way to create dynamic diagnostics in miette, instead of writing your own

#[derive(Debug, Error, PartialEq, Eq, Clone)]
pub struct CustomError {
    pub severity: Severity,
    pub message: String,
    pub code: Option<String>,
    pub help: Option<String>,
    pub labels: Option<Vec<LabeledSpan>>,
    pub url: Option<String>,
}

impl Display for CustomError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.message)
    }
}

impl Diagnostic for CustomError {
    fn code<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
        self.code.as_ref().map(|s| Box::new(s) as Box<dyn Display>)
    }
    fn help<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
        self.help.as_ref().map(|s| Box::new(s) as Box<dyn Display>)
    }
    fn labels(&self) -> Option<Box<dyn Iterator<Item = miette::LabeledSpan> + '_>> {
        self.labels
            .as_ref()
            .map(|labels| Box::new(labels.iter().cloned()) as Box<dyn Iterator<Item = LabeledSpan>>)
    }
    fn severity(&self) -> Option<miette::Severity> {
        Some(self.severity)
    }
    fn url<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
        self.url.as_ref().map(|s| Box::new(s) as Box<dyn Display>)
    }
}

[zkat]

I've actually talked with folks in the past about including something like this as a convenience! I'd totally take this as a PR :)

(you'll need to manually implement std::error::Error, ofc. derive(Error) is a thiserror thing and I don't want that dependency)

[gavrilikhin-d]

What name should it have? My suggestions are:

• DynamicDiagnostic
• CustomDiagnostic
• MietteDiagnostic