fix: remove shared ref to static mut

[JyJyJcr]

Proposed changes

This PR removes shared references to mutable statics, which is announced to be treated as hard error in 2024 edition.
Link to the issue: rust-lang/rust#114447
While the compiler suggests to use addr_of!() or addr_of_mut!(), In most cases we use borrow() because we need not raw pointers but references.

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[bavshin-f5]

Thanks for the PR!

I don't believe std::borrow::Borrow is the right fix here. It silences the compiler diagnostic, but does not forbid creating shared references or mutating the global (I'm actually convinced it is a flaw in the diagnostic).

The proper way to address the UB could be a static with interior mutability (e.g. SyncUnsafeCell). This would require waiting for the stabilization of sync_unsafe_cell and implementing Sync for the ngx_module_t (which is safe, as the module declarations are only ever used from the main thread).
And while we're waiting for the API necessary for a proper fix, it's better to use a suggested mitigation method (&*addr_of!()) or recommend to disable the warning. Either way would keep the problematic constructions visible.

BTW, there's a proposal to remove static mut in the edition after 2024, so SyncUnsafeCell is also the most future-proof solution.

[JyJyJcr]

Thank you for the review!

Now I replaced borrow() to &*addr_of!() following the recommended migration. I agree that using borrow() is defective.

I had tried to use addr_of!() to deal with the warning, but I couldn't find the proper way. Thank you for teaching me the pattern working correctly.

[bavshin-f5]

Looks good, thanks!