linhr opened a new issue, #11100:
URL: https://github.com/apache/datafusion/issues/11100
### Is your feature request related to a problem or challenge?
I'm trying to implement a helper function like this in my own project, but
it cannot compile.
```rust
fn expr_to_func_impl<T: ScalarUDFImpl + 'static>(expr: &Expr) -> Option<&T> {
match expr {
Expr::ScalarFunction(ScalarFunction { func, .. }) => {
func.inner().as_any().downcast_ref::<T>()
}
_ => None,
}
}
```
The compiler gives the following error.
```text
| func.inner().as_any().downcast_ref::<T>()
| ------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| returns a value referencing data owned by the current function
| temporary value created here
```
The issue is that `inner()` is defined to return a clone of `Arc` whose
lifetime is not associated with `&self`. Therefore, it's inconvenient if
someone has a reference to an outer `struct` and would like to cast the inner
`Arc<dyn MyTrait>` object to `&MyTraitImpl` where `MyTraitImpl` implements
`MyTrait`.
```rust
impl ScalarUDF {
pub fn inner(&self) -> Arc<dyn ScalarUDFImpl> {
self.inner.clone()
}
}
```
### Describe the solution you'd like
We should return `&Arc` to inner trait objects for `&self` methods. Many
data structures in DataFusion already follow this pattern, and we could simply
fix the few remaining places, such as the `ScalarUDF::inner()` method discussed
above.
### Describe alternatives you've considered
N/A
### Additional context
N/A
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