Replace &mut with & where we're not modifying the struct's state.

[kristof-mattei]

• fix: allow to run tests locally against modified version of netlink-sys
• fix: none of these methods modify the socket, they merely modify the kernel's state and the buffer's state
• fix: none of these methods modify the socket, they merely modify the kernel's state

[gemini-code-assist]

Summary of Changes

Hello @kristof-mattei, 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 focuses on enhancing the netlink-sys crate's API by refining its use of mutable references. The core change involves transitioning methods that do not inherently modify the socket's internal state (such as those interacting with the kernel or handling asynchronous I/O) from taking mutable references (&mut self) to immutable ones (&self). This improves the API's correctness and clarity, making it safer and easier to reason about the state of socket objects. Additionally, a configuration update has been included to facilitate local development and testing.

Highlights

• API Refinement: Immutable References: Refactored numerous methods across the AsyncSocket trait, its extension AsyncSocketExt, and concrete implementations (SmolSocket, TokioSocket), as well as core Socket configuration methods, to use immutable references (&self) instead of mutable ones (&mut self). This change clarifies that these operations interact with the kernel's state or manage asynchronous polling without directly modifying the Rust socket object itself.
• Examples and Tests Alignment: Updated example code for async_std and tokio, along with a test case in src/socket.rs, to remove unnecessary mut keywords from socket instance declarations, aligning with the new immutable reference pattern.
• Local Development Support: Added a [patch.crates-io] entry in Cargo.toml to allow for local testing against a modified version of the netlink-sys crate, streamlining development workflows.

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[gemini-code-assist]

Code Review

This pull request correctly removes unnecessary mutable borrows (&mut self) across the crate, replacing them with immutable borrows (&self). This is a good refactoring that improves the API ergonomics, especially for async code. The changes are consistent and well-justified by the fact that the modified methods only change kernel state or buffers, not the socket struct itself. The changes in src/socket.rs, src/async_socket.rs, src/async_socket_ext.rs, and the async backend implementations in src/smol.rs and src/tokio.rs are all correct. As a follow-up, you might consider removing the socket_mut() method from the AsyncSocket trait and its implementations, as it appears to be unused after these changes. Note that this would be a breaking change. I have one suggestion regarding the Cargo.toml file.

[cathay4t]

Nice patch!