alamb commented on issue #1128:
URL: https://github.com/apache/arrow-rs/issues/1128#issuecomment-1004051178
So what I am really trying to do is to the following
## Initial state:
```rust
pub fn as_primitive_array<T>(arr: &ArrayRef) -> &PrimitiveArray<T>
where
T: ArrowPrimitiveType,
{
...
}
```
There are many calls in the code base that look like this:
```rust
let arr: ArrayRef = new_arr();
let foo = as_primitive_array::<Int8Type>(&arr);
```
I would like to re-use `as_primitive_array` in functions such as the compute
kernels that take a `&dyn Array` rather than `&ArrayRef`:
```rust
let arr_ref: &dyn Array = arr.as_ref();
let bar = as_primitive_array::<Int8Type>(arr_ref);
```
As written this results in an error:
```
let bar = as_primitive_array::<Int8Type>(arr_ref);
^^^^^^^ expected struct `Arc`,
found trait object `dyn Array`
```
Playground here:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3a7907b6e7f08eb3b510aaaa6edb1500
When I change the signature to my preferred one:
```rust
pub fn as_primitive_array<T>(arr: &dyn Array) -> &PrimitiveArray<T>
where
T: ArrowPrimitiveType,
{
...
}
```
Then the original calls error like this:
```
Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `Arc<(dyn Array + 'static)>: Array` is not
satisfied
--> src/main.rs:22:46
|
22 | let foo = as_primitive_array::<Int8Type>(&arr);
| ------------------------------ ^^^^ the trait `Array` is
not implemented for `Arc<(dyn Array + 'static)>`
| |
| required by a bound introduced by this call
|
= note: required for the cast to the object type `dyn Array`
```
Link to playground showing errors when I tried this:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=5ebd46406b1673e3bf80199b2cb5887e
When I tried using `impl`:
```rust
pub fn as_primitive_array<T>(arr: impl AsRef<dyn Array>) ->
&PrimitiveArray<T>
...
```
The compiler tells me it needs a named lifetime parameter:
```rust
Compiling playground v0.0.1 (/playground)
error[E0106]: missing lifetime specifier
--> src/main.rs:7:61
|
7 | pub fn as_primitive_array<T>(arr: impl AsRef<dyn Array>) ->
&PrimitiveArray<T>
| ^ expected
named lifetime parameter
```
But if I add a lifetime parameter, then all the existing calls of
`as_primitive_array::<Int8Type>(&arr);` need to be changed which is what I am
trying to avoid
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