Allow clippy annotations in extern "Rust" blocks

[daira]

The code generated for a function in an extern "Rust" block strips any annotations, including clippy annotations. So for example,

#[cxx::bridge(namespace = "sapling")]
mod ffi {
    extern "Rust" {
        #![allow(clippy::too_many_arguments)]
        fn check_spend(
            &mut self,
            cv: &[u8; 32],
            anchor: &[u8; 32],
            nullifier: &[u8; 32],
            rk: &[u8; 32],
            zkproof: &[u8; 192],
            spend_auth_sig: &[u8; 64],
            sighash_value: &[u8; 32],
        ) -> bool;
    }
}

will still produce a too_many_arguments lint.

Workaround: put the clippy annotation on the whole module:

// This is added because `check_spend` takes several arguments over FFI. This
// annotation gets removed by the cxx procedural macro so it needs to be enabled
// on the entire module.
#![allow(clippy::too_many_arguments)]

[dtolnay]

Fixed in 1.0.70.