Drop struct macro derive

A derive macro to free (drop) memory for structs that are used in the FFI.

Currently only c-strings (libc::c_char) and arrays (represented as a pointer and a length field) are supported.

Example:

```rust

[repr(C)]

[derive(DropStructMacro)]

pub struct FFIStagedSectorMetadata { pub sectoraccess: *const libc::cchar, pub sectorid: u64, pub pieceslen: libc::sizet, pub piecesptr: *const FFIPieceMetadata,

// must be one of: Pending, Failed, Sealing
pub seal_status_code: FFISealStatus,

// if sealing failed - here's the error
pub seal_error_msg: *const libc::c_char,

} ```

Will automatically create:

rust impl Drop for FFIStagedSectorMetadata { fn drop(&mut self) { unsafe { free_c_str(self.sector_access as *mut libc::c_char); drop(Vec::from_raw_parts( self.pieces_ptr as *mut FFIPieceMetadata, self.pieces_len, self.pieces_len, )); free_c_str(self.seal_error_msg as *mut libc::c_char); }; } }

To view the generated output after the macro was applied, you can use cargo-expand:

```console $ cd filecoin-proofs $ cargo expand --lib api::responses Checking filecoin-proofs v0.1.0 (/home/vmx/src/pl/filecoin/rust-fil-proofs/filecoin-proofs) Finished dev [unoptimized + debuginfo] target(s) in 0.70s

pub mod responses { use crate::api::sectorbuilder::errors::SectorBuilderErr; use crate::api::sectorbuilder::SectorBuilder; use crate::api::APIPOREPPROOFBYTES; use dropstructmacroderive::DropStructMacro; use failure::Error; … ```

License

MIT or Apache 2.0