342 lines
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342 lines
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<main>
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<h1 id="serialization-in-rustc"><a class="header" href="#serialization-in-rustc">Serialization in rustc</a></h1>
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<p>rustc has to <a href="https://en.wikipedia.org/wiki/Serialization">serialize</a> and deserialize various data during compilation.
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Specifically:</p>
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<ul>
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<li>"Crate metadata", consisting mainly of query outputs, are serialized
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from a binary format into <code>rlib</code> and <code>rmeta</code> files that are output when
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compiling a library crate. These <code>rlib</code> and <code>rmeta</code> files are then
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deserialized by the crates which depend on that library.</li>
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<li>Certain query outputs are serialized in a binary format to
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<a href="queries/incremental-compilation-in-detail.html#the-real-world-how-persistence-makes-everything-complicated">persist incremental compilation results</a>.</li>
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<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
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deserialized from <code>JSON</code> when the <code>-Z link-only</code> flag is used.</li>
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</ul>
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<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>
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<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>
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<pre><code class="language-rust ignore">pub trait Encodable<S: Encoder> {
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fn encode(&self, s: &mut S) -> Result<(), S::Error>;
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}
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pub trait Decodable<D: Decoder>: Sized {
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fn decode(d: &mut D) -> Result<Self, D::Error>;
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}</code></pre>
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<p>It also defines implementations of these for various common standard library
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<a href="https://doc.rust-lang.org/std/#primitives">primitive types</a> such as integer
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types, floating point types, <code>bool</code>, <code>char</code>, <code>str</code>, etc.</p>
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<p>For types that are constructed from those types, <code>Encodable</code> and <code>Decodable</code>
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are usually implemented by <a href="#derive-macros">derives</a>. These generate implementations that
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forward deserialization to the fields of the struct or enum. For a
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struct those impls look something like this:</p>
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<pre><code class="language-rust ignore">#![feature(rustc_private)]
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extern crate rustc_serialize;
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use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
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struct MyStruct {
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int: u32,
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float: f32,
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}
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impl<E: Encoder> Encodable<E> for MyStruct {
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fn encode(&self, s: &mut E) -> Result<(), E::Error> {
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s.emit_struct("MyStruct", 2, |s| {
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s.emit_struct_field("int", 0, |s| self.int.encode(s))?;
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s.emit_struct_field("float", 1, |s| self.float.encode(s))
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})
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}
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}
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impl<D: Decoder> Decodable<D> for MyStruct {
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fn decode(s: &mut D) -> Result<MyStruct, D::Error> {
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s.read_struct("MyStruct", 2, |d| {
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let int = d.read_struct_field("int", 0, Decodable::decode)?;
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let float = d.read_struct_field("float", 1, Decodable::decode)?;
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Ok(MyStruct { int, float })
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})
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}
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}</code></pre>
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<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>
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<p>rustc has a lot of <a href="memory.html">arena allocated types</a>.
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Deserializing these types isn't possible without access to the arena that they need to be allocated on.
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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>
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<p>Types which contain <code>arena</code> allocated types can then bound the type parameter of their
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<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.
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For example</p>
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<pre><code class="language-rust ignore">impl<'tcx, D: TyDecoder<'tcx>> Decodable<D> for MyStruct<'tcx> {
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/* ... */
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}</code></pre>
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<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
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an implementation.</p>
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<p>Decoding the actual <code>arena</code> allocated type is harder, because some of the
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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,
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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
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implemented for any type. The <code>TyDecodable</code> macro will call <code>RefDecodable</code> to
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decode references, but various generic code needs types to actually be
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<code>Decodable</code> with a specific decoder.</p>
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<p>For interned types instead of manually implementing <code>RefDecodable</code>, using a new
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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
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<code>Decodable</code> may be simpler.</p>
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<h2 id="derive-macros"><a class="header" href="#derive-macros">Derive macros</a></h2>
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<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>
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and <code>Encodable</code>.</p>
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<ul>
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<li>The <code>Encodable</code> and <code>Decodable</code> macros generate implementations that apply to
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all <code>Encoders</code> and <code>Decoders</code>. These should be used in crates that don't
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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
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not implement <code>TyEncoder</code>.</li>
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<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
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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
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<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
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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>
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<li><code>TyEncodable</code> and <code>TyDecodable</code> generate implementation that apply to any
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<code>TyEncoder</code> or <code>TyDecoder</code>. These should be used for types that are only
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serialized in crate metadata and/or the incremental cache, which is most
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serializable types in <code>rustc_middle</code>.</li>
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</ul>
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<h2 id="shorthands"><a class="header" href="#shorthands">Shorthands</a></h2>
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<p><code>Ty</code> can be deeply recursive, if each <code>Ty</code> was encoded naively then crate
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metadata would be very large. To handle this, each <code>TyEncoder</code> has a cache of
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locations in its output where it has serialized types. If a type being encoded
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is in the cache, then instead of serializing the type as usual, the byte offset
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within the file being written is encoded instead. A similar scheme is used for
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<code>ty::Predicate</code>.</p>
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<h2 id="lazyvaluet"><a class="header" href="#lazyvaluet"><code>LazyValue<T></code></a></h2>
|
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<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.
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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
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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>
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|
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
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|
<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>
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|
<li>Indexing into a <code>LazyTable<I, T></code> does not require decoding entries other
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|
than the one being read.</li>
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|
</ul>
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|
<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>
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|
<h2 id="specialization"><a class="header" href="#specialization">Specialization</a></h2>
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|
<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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<script src="mermaid.min.js"></script>
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<script src="mermaid-init.js"></script>
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</div>
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</body>
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</html>
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