Expand description
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 multihash::derive::Multihash;
use multihash::MultihashDigest;
#[derive(Clone, Copy, Debug, Eq, Multihash, PartialEq)]
#[mh(alloc_size = 64)]
pub enum Code {
#[mh(code = 0x01, hasher = multihash::Sha2_256)]
Foo,
#[mh(code = 0x02, hasher = multihash::Sha2_512)]
Bar,
}
let hash = Code::Foo.digest(b"hello world!");
println!("{:02x?}", hash);