timsaucer opened a new pull request, #848:
URL: https://github.com/apache/arrow-rs-object-store/pull/848
# Which issue does this PR close?
None — this is a one-line trait change, so I skipped filing an issue. Happy
to open one
for changelog purposes if maintainers prefer.
# Rationale for this change
Downstream crates that hold an `Arc<dyn ObjectStore>` sometimes need to
recover the
concrete store behind it. In DataFusion, stores are registered and later
retrieved from an
`ObjectStoreRegistry` as `Arc<dyn ObjectStore>`, and there is currently no
supported way to
get back to the concrete type once it has been erased. The alternatives — an
`as_any`
method every implementor has to write, or a downstream wrapper trait — both
add
boilerplate for something the language already gives us.
`ObjectStore` already requires `'static`, and `std::any::Any` is blanket
implemented for
every `?Sized + 'static` type, so adding `Any` as a supertrait requires no
changes from any
implementation, in-tree or downstream.
# What changes are included in this PR?
- Add `std::any::Any` to the `ObjectStore` supertrait list.
- Document the new capability under a `# Downcasting` heading on the trait,
including the
Rust 1.86 caveat below.
- Add `test_dyn_impl_is_any`, alongside the existing `test_dyn_impl` /
`test_generic_impl`
bound tests, as a regression guard so the supertrait is not silently
dropped later.
No implementation in this crate needed changing.
## A note on MSRV
**This PR does not change the MSRV.** Declaring the supertrait compiles on
1.85, and
`cargo check` (what the `msrv` CI job runs) passes on 1.85 with this change
— verified
locally with `cargo +1.85.0 check`.
However, the *trait object upcast* that makes downcasting possible —
`Arc<dyn ObjectStore>` → `Arc<dyn Any + Send + Sync>` — was stabilized in
Rust 1.86. So:
- On Rust >= 1.86, users get full `downcast` / `downcast_ref`.
- On Rust 1.85, users only get `Any::type_id` on a `dyn ObjectStore`.
The trait docs state this explicitly, and the upcast example is marked ````
```ignore ````
so it is not compiled at the current MSRV. The in-crate test asserts
`type_id` rather than
downcasting, for the same reason.
If maintainers would rather bump the MSRV to 1.86 and document/test the
downcast directly,
I'm happy to do that instead — say the word.
# Are there any user-facing changes?
Yes, additive:
- `dyn ObjectStore` can now be downcast to the concrete store type (Rust >=
1.86).
On breaking changes: adding a supertrait is technically a public API change,
but it should
not break anyone in practice. Every existing implementation already
satisfies `Any` via the
`'static` bound the trait has always required, so no downstream `impl
ObjectStore for ...`
needs to change. The only theoretical break is method resolution for
`type_id`, if a
downstream crate has some other trait with a `type_id` method in scope for
its store type.
Verified locally: `cargo fmt --all -- --check`, `cargo clippy --all-targets
--all-features`
(no new warnings), `cargo test --all-features --lib`, `cargo test
--all-features --doc`, and
`cargo +1.85.0 check`.
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