This proc macro derives a custom Multihash code table from a list of hashers. It also generates a public type called Multihash which corresponds to the specified alloc_size.

The digests are stack allocated with a fixed size. That size needs to be big enough to hold any of the specified hash digests. This cannot be determined reliably on compile-time, hence it needs to set manually via the alloc_size attribute. Also you might want to set it to bigger sizes then necessarily needed for backwards/forward compatibility.

If you set #mh(alloc_size = …) to a too low value, you will get compiler errors. Please note the the sizes are checked only on a syntactic level and not on the type level. This means that digest need to have a size const generic, which is a valid usize, for example 32 or 64.

You can disable those compiler errors with setting the no_alloc_size_errors attribute. This can be useful if you e.g. have specified type aliases for your hash digests and you are sure you use the correct value for alloc_size.

Example

use sp_multihash::derive::Multihash;
use sp_multihash::MultihashDigest;

#[derive(Clone, Copy, Debug, Eq, Multihash, PartialEq)]
#[mh(alloc_size = 64)]
pub enum Code {
    #[mh(code = 0x01, hasher = sp_multihash::Sha2_256, digest = sp_multihash::Sha2Digest<32>)]
    Foo,
    #[mh(code = 0x02, hasher = sp_multihash::Sha2_512, digest = sp_multihash::Sha2Digest<64>)]
    Bar,
}

let hash = Code::Foo.digest(b"hello world!");
println!("{:02x?}", hash);