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src/serialization.md
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@ -1,9 +1,9 @@
# Serialization in Rustc # Serialization in Rustc
Rust's compiler has to [serialize] and deserialize various data during rustc has to [serialize] and deserialize various data during compilation.
compilation. Specifically: Specifically:
- Certain crate metadata, consisting mainly of query outputs, are serialized - "Crate metadata", consisting mainly of query outputs, are serialized
from a binary format into `rlib` and `rmeta` files that are output when from a binary format into `rlib` and `rmeta` files that are output when
compiling a library crate. These `rlib` and `rmeta` files are then compiling a library crate. These `rlib` and `rmeta` files are then
deserialized by the crates which depend on that library. deserialized by the crates which depend on that library.
@ -36,8 +36,8 @@ types, floating point types, `bool`, `char`, `str`, etc.
For types that are constructed from those types, `Encodable` and `Decodable` For types that are constructed from those types, `Encodable` and `Decodable`
are usually implemented by [derives]. These generate implementations that are usually implemented by [derives]. These generate implementations that
forward deserialization to the `field`s of the `struct` or `enum`. For a forward deserialization to the fields of the struct or enum. For a
`struct` those `impl`s look something like this: struct those impls look something like this:
```rust,ignore ```rust,ignore
#![feature(rustc_private)] #![feature(rustc_private)]
@ -73,14 +73,13 @@ impl<D: Decoder> Decodable<D> for MyStruct {
## Encoding and Decoding arena allocated types ## Encoding and Decoding arena allocated types
Rust's compiler has a lot of [arena allocated types]. Deserializing these types rustc has a lot of [arena allocated types].
isn't possible without access to the `arena` that they need to be allocated on. Deserializing these types isn't possible without access to the arena that they need to be allocated on.
The [`TyDecoder`] and [`TyEncoder`] `trait`s are supertraits of [`Decoder`] and The [`TyDecoder`] and [`TyEncoder`] traits are supertraits of [`Decoder`] and [`Encoder`] that allow access to a [`TyCtxt`].
[`Encoder`] that allow access to a [`TyCtxt`].
Types which contain `arena` allocated types can then bound the type parameter Types which contain `arena` allocated types can then bound the type parameter of their
of their [`Encodable`] and [`Decodable`] implementations with these `trait`s. For [`Encodable`] and [`Decodable`] implementations with these traits.
example For example
```rust,ignore ```rust,ignore
impl<'tcx, D: TyDecoder<'tcx>> Decodable<D> for MyStruct<'tcx> { impl<'tcx, D: TyDecoder<'tcx>> Decodable<D> for MyStruct<'tcx> {
@ -149,7 +148,7 @@ and `Encodable`.
`Ty` can be deeply recursive, if each `Ty` was encoded naively then crate `Ty` can be deeply recursive, if each `Ty` was encoded naively then crate
metadata would be very large. To handle this, each `TyEncoder` has a cache of metadata would be very large. To handle this, each `TyEncoder` has a cache of
locations in its output where it has serialized types. If a type being encoded locations in its output where it has serialized types. If a type being encoded
is in cache, then instead of serializing the type as usual, the byte offset is in the cache, then instead of serializing the type as usual, the byte offset
within the file being written is encoded instead. A similar scheme is used for within the file being written is encoded instead. A similar scheme is used for
`ty::Predicate`. `ty::Predicate`.