s.read_struct("MyStruct", 2, |d| {
            let int = d.read_struct_field("int", 0, Decodable::decode)?;
            let float = d.read_struct_field("float", 1, Decodable::decode)?;

            Ok(MyStruct { int, float })
        })
    }
}
```

## Encoding and Decoding arena allocated types

rustc has a lot of [arena allocated types].
Deserializing these types isn't possible without access to the arena that they need to be allocated on.
The [`TyDecoder`] and [`TyEncoder`] traits are supertraits of [`Decoder`] and [`Encoder`] that allow access to a [`TyCtxt`].

Types which contain `arena` allocated types can then bound the type parameter of their
[`Encodable`] and [`Decodable`] implementations with these traits.
For example

```rust,ignore
impl<'tcx, D: TyDecoder<'tcx>> Decodable<D> for MyStruct<'tcx> {
    /* ... */
}
```

The [`TyEncodable`] and [`TyDecodable`] [derive macros][derives] will expand to such
an implementation.

Decoding the actual `arena` allocated type is harder, because some of the
implementations can't be written due to the [orphan rules]. To work around this,
the [`RefDecodable`] trait is defined in [`rustc_middle`]. This can then be
implemented for any type. The `TyDecodable` macro will call `RefDecodable` to
decode references, but various generic code needs types to actually be
`Decodable` with a specific decoder.

For interned types instead of manually implementing `RefDecodable`, using a new
type wrapper, like [`ty::Predicate`] and manually implementing `Encodable` and
`Decodable` may be simpler.


## Derive macros

The [`rustc_macros`] crate defines various derives to help implement `Decodable`
and `Encodable`.

- The `Encodable` and `Decodable` macros generate implementations that apply to
  all `Encoders` and `Decoders`. These should be used in crates that don't
  depend on [`rustc_middle`], or that have to be serialized by a type that does
  not implement `TyEncoder`.
- [`MetadataEncodable`] and [`MetadataDecodable`] generate implementations that
  only allow decoding by [`rustc_metadata::rmeta::encoder::EncodeContext`] and