Enable the deny(unsafe_op_in_unsafe_fn) lint everywhere in zcash-light-client-ffi we define unsafe functions

[daira]

From https://github.com/zcash-hackworks/zcash-light-client-ffi/pull/32/files#r1022962018 :

A side-effect of declaring a function unsafe is that unsafe operations in its body are no longer required to be in unsafe blocks (see Rust RFC 2585 for a proposal to change this, not yet adopted). Ideally, we should enable the deny(unsafe_op_in_unsafe_fn) lint everywhere we define unsafe functions, since the style we write FFI functions in is mostly already consistent with that lint.

[daira]

This can be enabled crate-wide by adding

[build]
rustflags = ["-Dunsafe_op_in_unsafe_fn"]

in .cargo/config.

[sellout]

This should also update all extern functions with safety requirements on callers to unsafe extern.

[nuttycom]

I mentioned this in a comment, but has @str4d chimed in on this change yet? He had suggested to me that he preferred that the function not be labeled unsafe, but instead that the unsafe operations be explicitly demarcated by unsafe blocks internal to the function, so that it's easier to identify which parts are actually performing unsafe operations.

[sellout]

Yeah, that’s the primary portion of this issue – using deny(unsafe_op_in_unsafe_fn) so that we can have both.

And get back to vacation!

[daira]

This will be fixed by #66 (which includes #67).