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Feature Request: downcasting Log, but for real #666
Description
Seems #399 was already closed (4.5 years ago, for bad usecase & maybe backwards compatibility reasons?), but I have an actual use!
My logger writes a message on panic, but does so with a custom format different to anything you can write with the Log trait. So I need to be able to downcast the &dyn Log into my own logger from the panic handler. I could probably use a OnceLock to store if my logger's been initialized but log already does that with the (set_)logger methods, so it feels redundant.
For now my current way of doing this is this very cursed set of functions:
fn as_dyn_ref(logger: *const Logger) -> *const dyn Log {
// split into one function to always attach the same metadata
logger as *const dyn Log
}
fn upcast_log(logger: &'static Logger) -> &'static dyn Log {
// SAFETY: as_dyn_ref returns a reference to the same object as passed in
unsafe { &*as_dyn_ref(logger) }
}
fn downcast_log(log: &'static dyn Log) -> Option<&'static Logger> {
// horribly cursed implementation to fetch a reference to the installed logger
let (logger_ptr, logger_meta) = (&raw const *log).to_raw_parts();
let (_, fake_logger_meta) = as_dyn_ref(ptr::null::<Logger>()).to_raw_parts();
(logger_meta == fake_logger_meta).then(|| {
// SAFETY: v-tables match so it's probably ours!
unsafe { &*logger_ptr.cast::<Logger>() }
})
}but that's a lot of unsafe, and nightly-only.
To prevent needing to add a + 'static bound on Log (which probably breaks compatibility somehow), yandros suggested (in rplcs #dark-arts) adding something like:
fn type_id(&self) -> TypeId
where Self: 'static
{
TypeId::of<Self>()
}as an automatic implementation on the Log trait.
Activity
KodrAus commentedon May 14, 2025
Hi @1e1001 👋
This is something I'd be open to exploring a bit more. We'd need to be careful adding a
type_idmethod to theLogtrait, because it would be unsound to override. TheErrortrait for instance defines its squirrel-type-id method like this to try get around that1e1001 commentedon May 21, 2025
Ah yeah that makes sense, in which case I'm not quite sure how one would implement that without some sort of
+ 'staticor+ Anybound on theLogtrait itself (although I'm curious on how much real code that would actually break, given that the library only ever asks for&'static dyn Logs).djugei commentedon Sep 19, 2026
I am the author of indicatif-log-bridge and i would like the same feature, downcasting the
&'static dyn Logto LogWrapper (the specific logger) to access the interalMultiProgress.i might be misinformed but i think this would be as simple as adding a simple function with a default implementation to Log
due to the default implementation it would be backwards compatible.