Function handle_panic

src/function.rs:245–267

This only needs `&dyn Any`, but we keep a `Box` all the way to avoid the risk of a subtle bug in the caller where `&Box ` is coerced to `&dyn Any` by unsizing with another layer of vtable and wide pointer, instead of the expected auto-deref.

(_py: Python, panic: Box<dyn any::Any>)

// `&dyn Any` by unsizing with another layer of vtable and wide pointer,
// instead of the expected auto-deref.
fn handle_panic(_py: Python, panic: Box<dyn any::Any>) {
    let panic_str = if let Some(s) = panic.downcast_ref::<String>() {
        Some(s.as_str())
    } else if let Some(s) = panic.downcast_ref::<&'static str>() {
        Some(*s)
    } else {
        None
    };
    let panic_cstring = panic_str.and_then(|s| {
        let result = CString::new(format!("Rust panic: {}", s));
        // Give up on representing the panic payload if it contains a null byte
        // TODO: use PyErr_SetObject instead, so a `char*` string isn’t needed?
        result.ok()
    });
    let msg = if let Some(s) = &panic_cstring {
        s.as_c_str()
    } else {
        cstr!("Rust panic")
    };
    unsafe {
        ffi::PyErr_SetString(ffi::PyExc_SystemError, msg.as_ptr());
    }
}

pub struct AbortOnDrop<'a>(pub &'a str);