refactor!: replace lazy_static with std::sync::OnceLock

[erenatas]

This PR

This change enables Webassembly compatibility by replacing lazy_static with syd::sync:OnceLock and tokio feature changes. It should be considered breaking due MSRV change.

This change doesn't have any performance impact on the library itself. tokio's sync feature provides RwLock which works identically regardless of runtime. Users of this library can enable tokio's multi-threaded runtime if needed.

Changes in summary:

• Replace lazy_static crate with std::sync::OnceLock for singleton
• Bump MSRV from 1.77.0 to 1.80.1
• Remove lazy_static dependency
• Split tokio features: sync for library, rt-multi-thread/macros for tests
• Update doc comment to reference public API methods

Related Issues

This change is the first follow up from comments over Slack regarding web assembly compatibility requests

[gemini-code-assist]

Summary of Changes

Hello @erenatas, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request modernizes the library's singleton initialization mechanism by migrating from the lazy_static crate to std::sync::OnceLock, a change crucial for achieving WebAssembly compatibility. It also refines Tokio feature usage for better modularity and updates the minimum supported Rust version, which is a significant breaking change for consumers of the library.

Highlights

• WebAssembly Compatibility: The primary goal of this change is to enable WebAssembly compatibility for the library.
• Singleton Implementation: Replaced the lazy_static crate with Rust's standard library std::sync::OnceLock for managing the OpenFeature singleton instance.
• Minimum Supported Rust Version (MSRV): Bumped the MSRV from 1.77.0 to 1.80.1, which is considered a breaking change.
• Tokio Feature Management: Refactored Tokio features, specifying sync for the library and sync, rt-multi-thread, and macros for tests to provide more granular control and reduce unnecessary dependencies.
• Documentation Updates: Updated doc comments to reflect the new singleton access methods.

Changelog

• Cargo.toml
  • Updated the rust-version from 1.77.0 to 1.80.1.
  • Removed the lazy_static dependency.
  • Modified tokio dependency features from full to sync for the main library.
  • Added a separate tokio dependency for development/test purposes with sync, rt-multi-thread, and macros features.
• src/api/api.rs
  • Replaced lazy_static import with std::sync::OnceLock.
  • Changed the SINGLETON static variable from lazy_static! macro to OnceLock<RwLock<OpenFeature>>.
  • Introduced a new private function singleton_lock() to initialize and retrieve the RwLock<OpenFeature> instance using OnceLock.
  • Updated OpenFeature::singleton() and OpenFeature::singleton_mut() methods to use the new singleton_lock() function.
  • Adjusted doc comments to reference the new singleton access pattern.

Activity

• This change is a direct follow-up to discussions on Slack regarding WebAssembly compatibility requests.

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[gemini-code-assist]

Code Review

This pull request correctly replaces lazy_static with std::sync::OnceLock to improve WebAssembly compatibility and modernize the codebase. The changes also include a sensible split of tokio features between library dependencies and development dependencies, which is a great improvement. The implementation of the singleton pattern using OnceLock is correct and thread-safe. I have one suggestion to improve the naming of a helper function for better clarity.

[jonathannorris]

Looks good to me, scoping tokio should help with WASM support.