note: cannot satisfy `_: DecoderRunnable<u8>` - rust

Question

On this simple sketch I made, I'm trying to do use a trait called Runnable to run an Arc<dyn LockableOption<T>>:
use std::sync::{Arc, LockResult, Mutex, MutexGuard, PoisonError};
pub type LockableArc<T> = Arc<Mutex<Option<T>>>;
pub struct MutexGuardOptionRef<'a, T: ?Sized> {
pub mutex_guard: MutexGuard<'a, Option<Box<T>>>,
}
pub trait LockableOption<T: ?Sized>: Send + Sync {
fn lock(&self) -> LockResult<MutexGuardOptionRef<T>>;
}
impl<T: ?Sized + Send> LockableOption<T> for LockableArc<Box<T>> {
fn lock(&self) -> LockResult<MutexGuardOptionRef<T>> {
unimplemented!()
}
}
pub trait Decoder<T>: Send {
}
pub struct FfmpegDecoder<T> {
x: T,
}
impl<T: 'static + Send> Decoder<T> for FfmpegDecoder<T> {
}
trait DecoderRunnable<T> {
fn run(s: Arc<dyn LockableOption<dyn Decoder<T>>>);
}
impl<T: 'static + Send> DecoderRunnable<T> for FfmpegDecoder<T> {
fn run(s_lockable: Arc<dyn LockableOption<dyn Decoder<T>>>) {
unimplemented!()
}
}
fn main() {
let r: LockableArc<Box<dyn Decoder<u8>>> = Arc::new(Mutex::new(Some(Box::new(FfmpegDecoder{x: 0u8}))));
let rr: Arc<dyn LockableOption<dyn Decoder<u8>>> = Arc::new(r);
DecoderRunnable::<u8>::run(rr.clone());
}
Playground
I get the error:
error[E0283]: type annotations needed
--> src/main.rs:42:5
|
30 | fn run(s: Arc<dyn LockableOption<dyn Decoder<T>>>);
| --------------------------------------------------- required by `DecoderRunnable::run`
...
42 | DecoderRunnable::<u8>::run(rr.clone());
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
|
= note: cannot satisfy `_: DecoderRunnable<u8>`
which I don't get. Why anything here should satisfy DecoderRunnable? DecoderRunnable is a trait that has a run function that expects Arc<dyn LockableOption<dyn Decoder<T>>> and I'm passing rr which is exactly that.

Answer 1

Traits have to be implemented types so the compiler can figure out which implementation to run. In your example, DecoderRunnable is only implemented for FfmpegDecoder<T> and you're trying to call it on an Arc<dyn LockableOption<dyn Decoder<u8>>>, which has no implementation.
You can always specify which implementation needs to be called by using this syntax:
<FfmpegDecoder<u8> as DecoderRunnable::<u8>>::run(rr);
Although it doesn't seem like what you're trying to do . It's not clear what you're trying to abstract, since you also have the decoder deeply nested inside LockableArc<T>.
If you just want to add convenience methods to LockableArc<Box<dyn Decoder<u8>>>, you can add an impl block for Arc<dyn ...> , and make the run method take &self instead of Arc<dyn ...> as its first parameter.