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| <h1 id="serialization-in-rustc"><a class="header" href="#serialization-in-rustc">Serialization in rustc</a></h1> |
| <p>rustc has to <a href="https://en.wikipedia.org/wiki/Serialization">serialize</a> and deserialize various data during compilation. |
| Specifically:</p> |
| <ul> |
| <li>"Crate metadata", consisting mainly of query outputs, are serialized |
| from a binary format into <code>rlib</code> and <code>rmeta</code> files that are output when |
| compiling a library crate. These <code>rlib</code> and <code>rmeta</code> files are then |
| deserialized by the crates which depend on that library.</li> |
| <li>Certain query outputs are serialized in a binary format to |
| <a href="queries/incremental-compilation-in-detail.html#the-real-world-how-persistence-makes-everything-complicated">persist incremental compilation results</a>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/struct.CrateInfo.html"><code>CrateInfo</code></a> is serialized to <code>JSON</code> when the <code>-Z no-link</code> flag is used, and |
| deserialized from <code>JSON</code> when the <code>-Z link-only</code> flag is used.</li> |
| </ul> |
| <h2 id="the-encodable-and-decodable-traits"><a class="header" href="#the-encodable-and-decodable-traits">The <code>Encodable</code> and <code>Decodable</code> traits</a></h2> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/index.html"><code>rustc_serialize</code></a> crate defines two traits for types which can be serialized:</p> |
| <pre><code class="language-rust ignore">pub trait Encodable<S: Encoder> { |
| fn encode(&self, s: &mut S) -> Result<(), S::Error>; |
| } |
| |
| pub trait Decodable<D: Decoder>: Sized { |
| fn decode(d: &mut D) -> Result<Self, D::Error>; |
| }</code></pre> |
| <p>It also defines implementations of these for various common standard library |
| <a href="https://doc.rust-lang.org/std/#primitives">primitive types</a> such as integer |
| types, floating point types, <code>bool</code>, <code>char</code>, <code>str</code>, etc.</p> |
| <p>For types that are constructed from those types, <code>Encodable</code> and <code>Decodable</code> |
| are usually implemented by <a href="#derive-macros">derives</a>. These generate implementations that |
| forward deserialization to the fields of the struct or enum. For a |
| struct those impls look something like this:</p> |
| <pre><code class="language-rust ignore">#![feature(rustc_private)] |
| extern crate rustc_serialize; |
| use rustc_serialize::{Decodable, Decoder, Encodable, Encoder}; |
| |
| struct MyStruct { |
| int: u32, |
| float: f32, |
| } |
| |
| impl<E: Encoder> Encodable<E> for MyStruct { |
| fn encode(&self, s: &mut E) -> Result<(), E::Error> { |
| s.emit_struct("MyStruct", 2, |s| { |
| s.emit_struct_field("int", 0, |s| self.int.encode(s))?; |
| s.emit_struct_field("float", 1, |s| self.float.encode(s)) |
| }) |
| } |
| } |
| |
| impl<D: Decoder> Decodable<D> for MyStruct { |
| fn decode(s: &mut D) -> Result<MyStruct, D::Error> { |
| 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 }) |
| }) |
| } |
| }</code></pre> |
| <h2 id="encoding-and-decoding-arena-allocated-types"><a class="header" href="#encoding-and-decoding-arena-allocated-types">Encoding and Decoding arena allocated types</a></h2> |
| <p>rustc has a lot of <a href="memory.html">arena allocated types</a>. |
| Deserializing these types isn't possible without access to the arena that they need to be allocated on. |
| The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.TyDecoder.html"><code>TyDecoder</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.TyEncoder.html"><code>TyEncoder</code></a> traits are supertraits of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/trait.Decoder.html"><code>Decoder</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/trait.Encoder.html"><code>Encoder</code></a> that allow access to a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html"><code>TyCtxt</code></a>.</p> |
| <p>Types which contain <code>arena</code> allocated types can then bound the type parameter of their |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/trait.Encodable.html"><code>Encodable</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/trait.Decodable.html"><code>Decodable</code></a> implementations with these traits. |
| For example</p> |
| <pre><code class="language-rust ignore">impl<'tcx, D: TyDecoder<'tcx>> Decodable<D> for MyStruct<'tcx> { |
| /* ... */ |
| }</code></pre> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_macros/derive.TyEncodable.html"><code>TyEncodable</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_macros/derive.TyDecodable.html"><code>TyDecodable</code></a> <a href="#derive-macros">derive macros</a> will expand to such |
| an implementation.</p> |
| <p>Decoding the actual <code>arena</code> allocated type is harder, because some of the |
| implementations can't be written due to the <a href="https://doc.rust-lang.org/reference/items/implementations.html#orphan-rules">orphan rules</a>. To work around this, |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.RefDecodable.html"><code>RefDecodable</code></a> trait is defined in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_type_ir/codec.rs.html#21"><code>rustc_middle</code></a>. This can then be |
| implemented for any type. The <code>TyDecodable</code> macro will call <code>RefDecodable</code> to |
| decode references, but various generic code needs types to actually be |
| <code>Decodable</code> with a specific decoder.</p> |
| <p>For interned types instead of manually implementing <code>RefDecodable</code>, using a new |
| type wrapper, like <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/predicate/struct.Predicate.html"><code>ty::Predicate</code></a> and manually implementing <code>Encodable</code> and |
| <code>Decodable</code> may be simpler.</p> |
| <h2 id="derive-macros"><a class="header" href="#derive-macros">Derive macros</a></h2> |
| <p>The <a href="https://github.com/rust-lang/rust/tree/master/compiler/rustc_macros"><code>rustc_macros</code></a> crate defines various derives to help implement <code>Decodable</code> |
| and <code>Encodable</code>.</p> |
| <ul> |
| <li>The <code>Encodable</code> and <code>Decodable</code> macros generate implementations that apply to |
| all <code>Encoders</code> and <code>Decoders</code>. These should be used in crates that don't |
| depend on <a href="https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_type_ir/codec.rs.html#21"><code>rustc_middle</code></a>, or that have to be serialized by a type that does |
| not implement <code>TyEncoder</code>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_macros/derive.MetadataEncodable.html"><code>MetadataEncodable</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_macros/derive.MetadataDecodable.html"><code>MetadataDecodable</code></a> generate implementations that |
| only allow decoding by <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/encoder/struct.EncodeContext.html"><code>rustc_metadata::rmeta::encoder::EncodeContext</code></a> and |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/decoder/struct.DecodeContext.html"><code>rustc_metadata::rmeta::decoder::DecodeContext</code></a>. These are used for types |
| that contain <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/index.html"><code>rustc_metadata::rmeta::</code></a><code>Lazy*</code>.</li> |
| <li><code>TyEncodable</code> and <code>TyDecodable</code> generate implementation that apply to any |
| <code>TyEncoder</code> or <code>TyDecoder</code>. These should be used for types that are only |
| serialized in crate metadata and/or the incremental cache, which is most |
| serializable types in <code>rustc_middle</code>.</li> |
| </ul> |
| <h2 id="shorthands"><a class="header" href="#shorthands">Shorthands</a></h2> |
| <p><code>Ty</code> can be deeply recursive, if each <code>Ty</code> was encoded naively then crate |
| metadata would be very large. To handle this, each <code>TyEncoder</code> has a cache of |
| locations in its output where it has serialized types. If a type being encoded |
| 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 |
| <code>ty::Predicate</code>.</p> |
| <h2 id="lazyvaluet"><a class="header" href="#lazyvaluet"><code>LazyValue<T></code></a></h2> |
| <p>Crate metadata is initially loaded before the <code>TyCtxt<'tcx></code> is created, so |
| some deserialization needs to be deferred from the initial loading of metadata. |
| The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/struct.LazyValue.html"><code>LazyValue<T></code></a> type wraps the (relative) offset in the crate metadata |
| where a <code>T</code> has been serialized. There are also some variants, <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/struct.LazyValue.html"><code>LazyArray<T></code></a> |
| and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/struct.LazyValue.html"><code>LazyTable<I, T></code></a>.</p> |
| <p>The <code>LazyArray<[T]></code> and <code>LazyTable<I, T></code> types provide some functionality over |
| <code>Lazy<Vec<T>></code> and <code>Lazy<HashMap<I, T>></code>:</p> |
| <ul> |
| <li>It's possible to encode a <code>LazyArray<T></code> directly from an <code>Iterator</code>, without |
| first collecting into a <code>Vec<T></code>.</li> |
| <li>Indexing into a <code>LazyTable<I, T></code> does not require decoding entries other |
| than the one being read.</li> |
| </ul> |
| <p><strong>note</strong>: <code>LazyValue<T></code> does not cache its value after being deserialized the |
| first time. Instead the query system itself is the main way of caching these |
| results.</p> |
| <h2 id="specialization"><a class="header" href="#specialization">Specialization</a></h2> |
| <p>A few types, most notably <code>DefId</code>, need to have different implementations for |
| different <code>Encoder</code>s. This is currently handled by ad-hoc specializations, for |
| example: <code>DefId</code> has a <code>default</code> implementation of <code>Encodable<E></code> and a |
| specialized one for <code>Encodable<CacheEncoder></code>.</p> |
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