chore - Bump MSRV/toolchain to stable 1.93 and fix strict clippy

[mingley]

Summary

This PR upgrades the repository toolchain/MSRV to Rust 1.93.0 and applies strict-clippy compatibility fixes while preserving runtime behavior and public API compatibility.

Quality objective:

• keep the codebase clean under -D warnings,
• avoid unintended semantic changes,
• maintain Error::GrpcAPI usability for downstream code.

Rationale

Problem statement

Moving to Rust 1.93 and strict clippy surfaced new lints in generated/protocol and request/transaction paths. A prior approach introduced awkward conditional typing for Error::GrpcAPI; this PR keeps the API representation stable and solves lint pressure in a clearer way.

Design changes

• Toolchain/MSRV:
  • Cargo.toml: rust-version = "1.93.0"
  • rust-toolchain.toml: channel 1.93.0
• Error::GrpcAPI compatibility:
  • keep payload as tonic::Status (single representation),
  • keep From<tonic::Status> for Error conversion path,
  • remove cfg(clippy) payload-type split.
• Lint strategy for large error payload:
  • targeted #![allow(clippy::result_large_err)] in src/lib.rs and examples/raw.rs.
• Clippy cleanup (behavior-preserving simplifications):
  • remove redundant conversions,
  • replace repeat(...).take(...) with repeat_n(...),
  • narrow generated-code dead-code handling in src/proto.rs.

Risk analysis

• Primary risk: accidental API/behavior drift during lint cleanup.
• Mitigation:
  • explicit tests for error conversion and variant behavior,
  • full format/clippy/test gates.

File Scope

• Cargo.toml
• rust-toolchain.toml
• src/common/errors.rs
• src/lib.rs
• examples/raw.rs
• src/proto.rs
• src/pd/retry.rs
• src/store/request.rs
• src/transaction/requests.rs
• src/transaction/transaction.rs
• tests/integration_tests.rs

Testing Done

Executed locally:

1. cargo fmt -> pass
2. cargo clippy --all-targets --all-features -- -D warnings -> pass
3. cargo test -> pass

Added/maintained regression coverage in src/common/errors.rs:

• from_tonic_status_produces_grpc_api_error
• grpc_api_variant_accepts_tonic_status_payload
• from_proto_region_error_wraps_region_error
• from_proto_key_error_wraps_key_error

Compatibility

• Public Error::GrpcAPI construction/pattern behavior remains compatible (tonic::Status payload).
• No intentional runtime behavior change beyond lint/toolchain conformance.

Summary by CodeRabbit

• Chores

  • Updated the Rust toolchain to 1.93.0 and set the project’s minimum supported Rust version.

• Improvements

  • Improved gRPC error handling consistency by adjusting how gRPC status errors are converted.
  • Reduced unnecessary per-item conversions in request and transaction read paths.
  • Updated lint allowances/documentation to better match Rust 1.93 clippy behavior.

• Tests

  • Added unit coverage for the updated gRPC status-to-error conversion behavior.

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📝 Walkthrough

Walkthrough

Upgrade Rust toolchain to 1.93.0 via Cargo.toml and rust-toolchain.toml with clippy allowances for compatibility. Refactor Error::GrpcAPI to hold plain tonic::Status with explicit From<tonic::Status> impl and tests. Update request dispatch error handling and simplify iterators by replacing repeat().take() with repeat_n(). Remove redundant per-element Into::into conversions across transaction and store code.

Changes

Toolchain upgrade and compatibility

Layer / File(s)	Summary
Manifest toolchain declarations Cargo.toml, rust-toolchain.toml	Rust version constraint added to Cargo.toml and toolchain channel updated to 1.93.0 in rust-toolchain.toml.
Clippy allowances for 1.93.0 compatibility src/lib.rs, examples/raw.rs, src/proto.rs	Added result_large_err allowance in crate root and example file; added dead_code allowance for protobuf-generated code to suppress Rust 1.93 clippy warnings.

Error type restructuring

Layer / File(s)	Summary
GrpcAPI variant and From impl src/common/errors.rs	Removed #[from] attribute from GrpcAPI variant and added explicit impl From<tonic::Status> for Error that constructs Error::GrpcAPI(status) with tests validating the conversion and payload preservation.
Error handling in request dispatch src/store/request.rs	Updated store request dispatch to map gRPC errors via Error::from instead of the removed Error::GrpcAPI derive-from behavior.

Iterator and type conversion simplifications

Layer / File(s)	Summary
Iterator API updates src/transaction/requests.rs, tests/integration_tests.rs	Replaced iter::repeat().take() with iter::repeat_n() in prewrite request construction and integration test batch_scan invocation.
Removal of redundant Into conversions src/transaction/requests.rs, src/transaction/transaction.rs, src/pd/retry.rs	Removed per-element Into::into conversions from transaction request building, retry response mapping, and scan lock merging; direct collection or assignment used instead.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 The toolchain hops to 1.93's gate,
Error conversions now explicit, not late.
Repeat_n simplifies, Into we drop,
Clippy is happy—the warnings do stop! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main changes: upgrading the Rust toolchain to 1.93 and fixing clippy issues.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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[mingley]

@coderabbitai review

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Review triggered.

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[mingley]

Addressed local review findings #2 and #3 in cfad559:

• Removed crate-wide/example clippy::result_large_err suppressions.
• Replaced Error::GrpcAPI(tonic::Status) with boxed storage + explicit From<tonic::Status> so API signatures remain unchanged while reducing error size pressure.
• Dropped generated-file rewrite logic from proto-build/src/main.rs.
• Moved dead-code suppression to generated module boundary in src/proto.rs with explicit “why now vs before” context comment.

Also added a dedicated "Follow-on PR Callout: Dependency Modernization" section to the PR description with major version gaps and rationale.

[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In `@src/common/errors.rs`:
- Around line 202-211: The test grpc_api_variant_accepts_tonic_status_payload is
gated with #[cfg(not(clippy))], causing it to be skipped under cargo clippy;
remove the #[cfg(not(clippy))] attribute so the test always runs, and ensure it
still passes after applying the unconditional boxing change mentioned in the
surrounding comments (the test constructs Error::GrpcAPI from tonic::Status and
asserts status.code() and status.message()).
- Around line 11-14: The conditional type alias GrpcApiErrorPayload causes
divergent in-memory representations under clippy vs normal builds; make the
payload unconditionally boxed by changing the alias to "type GrpcApiErrorPayload
= Box<tonic::Status>;" so Error::GrpcAPI has a consistent, smaller enum variant
across all builds, then remove the #[cfg(not(clippy))] gating from tests that
construct Error::GrpcAPI (the test that conditionally constructs GrpcAPI can now
use Error::GrpcAPI(status) directly) and consider removing the now-redundant
#[allow(clippy::large_enum_variant)] if no longer needed.