Macro http_error 

macro_rules! http_error {
    ($status_code:ident, $reason:literal) => { ... };
    ($status_code:ident $(, source = $src:expr)? $(, reason = $($arg:tt)*)?) => { ... };
    ($status_code:ident $(, $($arg:tt)*)?) => { ... };
}

Construct an ad-hoc Error from a status code, optional source error and formatted reason.

fn foo() -> anyhow::Result<()> {
    const CODE: i32 = 1234;
    Err(http_error!(BAD_REQUEST, "invalid payload, code {}", CODE))?;

    // with source
    let source = anyhow!("source error");
    Err(http_error!(BAD_REQUEST, source = source, reason = "invalid payload, code {}", CODE))?;
    Ok(())
}