Enable unused_crate_dependencies Rust lint, remove unused dependencies

[findepi]

Which issue does this PR close?

Fixes #6796

Rationale for this change

Removing unused dependencies is expected to improve compile times.

What changes are included in this PR?

• enable unused_crate_dependencies Rust lint for all crates
• remove unused dependencies flagged by that lint

Are there any user-facing changes?

no

[findepi]

The PR is expected to fail because (1) i didn't remove all the dependencies flagged by the lint and (2) there are false-positives.
However, it failed in an unexpected way https://github.com/apache/arrow-rs/actions/runs/12027104363/job/33527467696?pr=6804

Run cargo clippy -- -D warnings
  cargo clippy -- -D warnings
  shell: sh -e {0}
  env:
    RUSTFLAGS: -C debuginfo=1
    RUST_BACKTRACE: 1
error: failed to parse manifest at `/__w/arrow-rs/arrow-rs/object_store/Cargo.toml`

Caused by:
  error inheriting `lints` from workspace root manifest's `workspace.lints`

Caused by:
  failed to find a workspace root

Any idea why failed to find a workspace root? i am not getting this error when i run

cargo clean
RUSTFLAGS="-C debuginfo=1" RUST_BACKTRACE=1 cargo +stable clippy -- -D warnings

locally

[tustvold]

Any idea why failed to find a workspace root? i am not getting this error when i run

object_store is not a member of the crate workspace as it is released separately

[findepi]

Thanks, that explains why it fails on CI.
I missed the working-directory: object_store when replying CI steps locally.

[tustvold]

Marking this as a draft as there appear to still be unresolved issues, feel free to mark ready for review when you would like me (or someone else) to take another look

[findepi]

I changed the approach. Instead of enabling lint in the cargo.toml, i switched to enaling it on specific cargo invocations as part of CI. Why? Because it feels cargo lacks necessary information to make this check work well in all cases. It apparently does not distinguishes dependencies when compiling tests and examples, or tests with different feature flags. I couldn't find a way to make it work, without adding a lot of suppressions (#allow or fake imports). Moreover, being very strict about dev/test dependencies isn't probably useful, as it doesn't benefit Arrow's users in any way.

[findepi]

cc @alamb

[alamb]

Thank you @findepi

Sorry for the delay in reviewing

I reviewed the CI job output and it looks good to me (all the jobs ran well):
https://github.com/apache/arrow-rs/actions/runs/12116683077/job/33777545429?pr=6804

[alamb]

Nit- the names are slightly inconsistent:

Suggested change

	- name: Clippy arrow-schema
	- name: Clippy arrow-schema with all features

[findepi]

I'd go opposite direction, ie remove "with all features" as now this tests various combinations: "all features", "default features", "no features"

[findepi]

done

[alamb]

💯

[alamb]

I'll plan to merge this tomorrow unless there are any other comments.

I hope to make the RC around Thursday

[alamb]

Thanks again @findepi