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<ol class="chapter"><li class="chapter-item affix "><a href="about-this-guide.html">About this guide</a></li><li class="chapter-item affix "><a href="getting-started.html">Getting Started</a></li><li class="spacer"></li><li class="chapter-item affix "><li class="part-title">Building and debugging rustc</li><li class="chapter-item "><a href="building/how-to-build-and-run.html"><strong aria-hidden="true">1.</strong> How to Build and Run the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="building/prerequisites.html"><strong aria-hidden="true">1.1.</strong> Prerequisites</a></li><li class="chapter-item "><a href="building/suggested.html"><strong aria-hidden="true">1.2.</strong> Suggested Workflows</a></li><li class="chapter-item "><a href="building/build-install-distribution-artifacts.html"><strong aria-hidden="true">1.3.</strong> Distribution artifacts</a></li><li class="chapter-item "><a href="building/compiler-documenting.html"><strong aria-hidden="true">1.4.</strong> Documenting Compiler</a></li><li class="chapter-item "><a href="rustdoc.html"><strong aria-hidden="true">1.5.</strong> Rustdoc overview</a></li><li class="chapter-item "><a href="building/new-target.html"><strong aria-hidden="true">1.6.</strong> Adding a new target</a></li></ol></li><li class="chapter-item "><a href="tests/intro.html"><strong aria-hidden="true">2.</strong> The compiler testing framework</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="tests/running.html"><strong aria-hidden="true">2.1.</strong> Running tests</a></li><li class="chapter-item "><a href="tests/adding.html"><strong aria-hidden="true">2.2.</strong> Adding new tests</a></li><li class="chapter-item "><a href="compiletest.html"><strong aria-hidden="true">2.3.</strong> Using compiletest commands to control test execution</a></li></ol></li><li class="chapter-item "><a href="compiler-debugging.html"><strong aria-hidden="true">3.</strong> Debugging the Compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="tracing.html"><strong aria-hidden="true">3.1.</strong> Using the tracing/logging instrumentation</a></li></ol></li><li class="chapter-item "><a href="profiling.html"><strong aria-hidden="true">4.</strong> Profiling the compiler</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="profiling/with_perf.html"><strong aria-hidden="true">4.1.</strong> with the linux perf tool</a></li><li class="chapter-item "><a href="profiling/wpa_profiling.html"><strong aria-hidden="true">4.2.</strong> with Windows Performance Analyzer</a></li></ol></li><li class="chapter-item "><a href="crates-io.html"><strong aria-hidden="true">5.</strong> crates.io Dependencies</a></li><li class="chapter-item affix "><li class="part-title">Contributing to Rust</li><li class="chapter-item "><a href="contributing.html"><strong aria-hidden="true">6.</strong> Introduction</a></li><li class="chapter-item "><a href="compiler-team.html"><strong aria-hidden="true">7.</strong> About the compiler team</a></li><li class="chapter-item "><a href="git.html"><strong aria-hidden="true">8.</strong> Using Git</a></li><li class="chapter-item "><a href="rustbot.html"><strong aria-hidden="true">9.</strong> Mastering @rustbot</a></li><li class="chapter-item "><a href="walkthrough.html"><strong aria-hidden="true">10.</strong> Walkthrough: a typical contribution</a></li><li class="chapter-item "><a href="bug-fix-procedure.html"><strong aria-hidden="true">11.</strong> Bug Fix Procedure</a></li><li class="chapter-item "><a href="implementing_new_features.html"><strong aria-hidden="true">12.</strong> Implementing new features</a></li><li class="chapter-item "><a href="stability.html"><strong aria-hidden="true">13.</strong> Stability attributes</a></li><li class="chapter-item "><a href="stabilization_guide.html"><strong aria-hidden="true">14.</strong> Stabilizing Features</a></li><li class="chapter-item "><a href="feature-gates.html"><strong aria-hidden="true">15.</strong> Feature Gates</a></li><li class="chapter-item "><a href="conventions.html"><strong aria-hidden="true">16.</strong> Coding conventions</a></li><li class="chapter-item "><a href="notification-groups/about.html"><strong aria-hidden="true">17.</strong> Notification groups</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="notification-groups/arm.html"><strong aria-hidden="true">17.1.</strong> ARM</a></li><li class="chapter-item "><a href="notification-groups/cleanup-crew.html"><strong aria-hidden="true">17.2.</strong> Cleanup Crew</a></li><li class="chapter-item "><a href="notification-groups/llvm.html"><strong aria-hidden="true">17.3.</strong> LLVM</a></li><li class="chapter-item "><a href="notification-groups/risc-v.html"><strong aria-hidden="true">17.4.</strong> RISC-V</a></li><li class="chapter-item "><a href="notification-groups/windows.html"><strong aria-hidden="true">17.5.</strong> Windows</a></li></ol></li><li class="chapter-item "><a href="licenses.html"><strong aria-hidden="true">18.</strong> Licenses</a></li><li class="chapter-item affix "><li class="part-title">High-level Compiler Architecture</li><li class="chapter-item "><a href="part-2-intro.html"><strong aria-hidden="true">19.</strong> Prologue</a></li><li class="chapter-item "><a href="overview.html"><strong aria-hidden="true">20.</strong> Overview of the Compiler</a></li><li class="chapter-item "><a href="compiler-src.html"><strong aria-hidden="true">21.</strong> The compiler source code</a></li><li class="chapter-item "><a href="building/bootstrapping.html"><strong aria-hidden="true">22.</strong> Bootstrapping</a></li><li class="chapter-item "><a href="query.html"><strong aria-hidden="true">23.</strong> Queries: demand-driven compilation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="queries/query-evaluation-model-in-detail.html"><strong aria-hidden="true">23.1.</strong> The Query Evaluation Model in Detail</a></li><li class="chapter-item "><a href="queries/incremental-compilation.html"><strong aria-hidden="true">23.2.</strong> Incremental compilation</a></li><li class="chapter-item "><a href="queries/incremental-compilation-in-detail.html"><strong aria-hidden="true">23.3.</strong> Incremental compilation In Detail</a></li><li class="chapter-item "><a href="incrcomp-debugging.html"><strong aria-hidden="true">23.4.</strong> Debugging and Testing</a></li><li class="chapter-item "><a href="salsa.html"><strong aria-hidden="true">23.5.</strong> Salsa</a></li></ol></li><li class="chapter-item "><a href="memory.html"><strong aria-hidden="true">24.</strong> Memory Management in Rustc</a></li><li class="chapter-item expanded "><a href="serialization.html" class="active"><strong aria-hidden="true">25.</strong> Serialization in Rustc</a></li><li class="chapter-item "><a href="parallel-rustc.html"><strong aria-hidden="true">26.</strong> Parallel Compilation</a></li><li class="chapter-item "><a href="rustdoc-internals.html"><strong aria-hidden="true">27.</strong> Rustdoc internals</a></li><li class="chapter-item affix "><li class="part-title">Source Code Representation</li><li class="chapter-item "><a href="part-3-intro.html"><strong aria-hidden="true">28.</strong> Prologue</a></li><li class="chapter-item "><a href="cli.html"><strong aria-hidden="true">29.</strong> Command-line arguments</a></li><li class="chapter-item "><a href="rustc-driver.html"><strong aria-hidden="true">30.</strong> The Rustc Driver and Interface</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="rustc-driver-interacting-with-the-ast.html"><strong aria-hidden="true">30.1.</strong> Ex: Type checking through rustc_interface</a></li><li class="chapter-item "><a href="rustc-driver-getting-diagnostics.html"><strong aria-hidden="true">30.2.</strong> Ex: Getting diagnostics through rustc_interface</a></li></ol></li><li class="chapter-item "><a href="syntax-intro.html"><strong aria-hidden="true">31.</strong> Syntax and the AST</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="the-parser.html"><strong aria-hidden="true">31.1.</strong> Lexing and Parsing</a></li><li class="chapter-item "><a href="macro-expansion.html"><strong aria-hidden="true">31.2.</strong> Macro expansion</a></li><li class="chapter-item "><a href="name-resolution.html"><strong aria-hidden="true">31.3.</strong> Name resolution</a></li><li class="chapter-item "><a href="test-implementation.html"><strong aria-hidden="true">31.4.</strong> #[test] Implementation</a></li><li class="chapter-item "><a href="panic-implementation.html"><strong aria-hidden="true">31.5.</strong> Panic Implementation</a></li><li class="chapter-item "><a href="ast-validation.html"><strong aria-hidden="true">31.6.</strong> AST Validation</a></li><li class="chapter-item "><a href="feature-gate-ck.html"><strong aria-hidden="true">31.7.</strong> Feature Gate Checking</a></li><li class="chapter-item "><a href="lang-items.html"><strong aria-hidden="true">31.8.</strong> Lang Items</a></li></ol></li><li class="chapter-item "><a href="hir.html"><strong aria-hidden="true">32.</strong> The HIR (High-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="lowering.html"><strong aria-hidden="true">32.1.</strong> Lowering AST to HIR</a></li><li class="chapter-item "><a href="hir-debugging.html"><strong aria-hidden="true">32.2.</strong> Debugging</a></li></ol></li><li class="chapter-item "><a href="thir.html"><strong aria-hidden="true">33.</strong> The THIR (Typed High-level IR)</a></li><li class="chapter-item "><a href="mir/index.html"><strong aria-hidden="true">34.</strong> The MIR (Mid-level IR)</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="mir/construction.html"><strong aria-hidden="true">34.1.</strong> MIR construction</a></li><li class="chapter-item "><a href="mir/visitor.html"><strong aria-hidden="true">34.2.</strong> MIR visitor and traversal</a></li><li class="chapter-item "><a href="mir/passes.html"><strong aria-hidden="true">34.3.</strong> MIR passes: getting the MIR for a function</a></li></ol></li><li class="chapter-item "><a href="identifiers.html"><strong aria-hidden="true">35.</strong> Identifiers in the Compiler</a></li><li class="chapter-item "><a href="closure.html"><strong aria-hidden="true">36.</strong> Closure expansion</a></li><li class="chapter-item affix "><li class="part-title">Analysis</li><li class="chapter-item "><a href="part-4-intro.html"><strong aria-hidden="true">37.</strong> Prologue</a></li><li class="chapter-item "><a href="ty.html"><strong aria-hidden="true">38.</strong> The ty module: representing types</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="generics.html"><strong aria-hidden="true">38.1.</strong> Generics and substitutions</a></li><li class="chapter-item "><a href="ty-fold.html"><strong aria-hidden="true">38.2.</strong> TypeFolder and TypeFoldable</a></li><li class="chapter-item "><a href="generic_arguments.html"><strong aria-hidden="true">38.3.</strong> Generic arguments</a></li><li class="chapter-item "><a href="constants.html"><strong aria-hidden="true">38.4.</strong> Constants in the type system</a></li></ol></li><li class="chapter-item "><a href="type-inference.html"><strong aria-hidden="true">39.</strong> Type inference</a></li><li class="chapter-item "><a href="traits/resolution.html"><strong aria-hidden="true">40.</strong> Trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="early-late-bound.html"><strong aria-hidden="true">40.1.</strong> Early and Late Bound Parameters</a></li><li class="chapter-item "><a href="traits/hrtb.html"><strong aria-hidden="true">40.2.</strong> Higher-ranked trait bounds</a></li><li class="chapter-item "><a href="traits/caching.html"><strong aria-hidden="true">40.3.</strong> Caching subtleties</a></li><li class="chapter-item "><a href="traits/specialization.html"><strong aria-hidden="true">40.4.</strong> Specialization</a></li><li class="chapter-item "><a href="traits/chalk.html"><strong aria-hidden="true">40.5.</strong> Chalk-based trait solving</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="traits/lowering-to-logic.html"><strong aria-hidden="true">40.5.1.</strong> Lowering to logic</a></li><li class="chapter-item "><a href="traits/goals-and-clauses.html"><strong aria-hidden="true">40.5.2.</strong> Goals and clauses</a></li><li class="chapter-item "><a href="traits/canonical-queries.html"><strong aria-hidden="true">40.5.3.</strong> Canonical queries</a></li></ol></li></ol></li><li class="chapter-item "><a href="type-checking.html"><strong aria-hidden="true">41.</strong> Type checking</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="method-lookup.html"><strong aria-hidden="true">41.1.</strong> Method Lookup</a></li><li class="chapter-item "><a href="variance.html"><strong aria-hidden="true">41.2.</strong> Variance</a></li><li class="chapter-item "><a href="opaque-types-type-alias-impl-trait.html"><strong aria-hidden="true">41.3.</strong> Opaque Types</a></li></ol></li><li class="chapter-item "><a href="pat-exhaustive-checking.html"><strong aria-hidden="true">42.</strong> Pattern and Exhaustiveness Checking</a></li><li class="chapter-item "><a href="mir/dataflow.html"><strong aria-hidden="true">43.</strong> MIR dataflow</a></li><li class="chapter-item "><a href="borrow_check.html"><strong aria-hidden="true">44.</strong> The borrow checker</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization.html"><strong aria-hidden="true">44.1.</strong> Tracking moves and initialization</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/moves_and_initialization/move_paths.html"><strong aria-hidden="true">44.1.1.</strong> Move paths</a></li></ol></li><li class="chapter-item "><a href="borrow_check/type_check.html"><strong aria-hidden="true">44.2.</strong> MIR type checker</a></li><li class="chapter-item "><a href="borrow_check/region_inference.html"><strong aria-hidden="true">44.3.</strong> Region inference</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="borrow_check/region_inference/constraint_propagation.html"><strong aria-hidden="true">44.3.1.</strong> Constraint propagation</a></li><li class="chapter-item "><a href="borrow_check/region_inference/lifetime_parameters.html"><strong aria-hidden="true">44.3.2.</strong> Lifetime parameters</a></li><li class="chapter-item "><a href="borrow_check/region_inference/member_constraints.html"><strong aria-hidden="true">44.3.3.</strong> Member constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/placeholders_and_universes.html"><strong aria-hidden="true">44.3.4.</strong> Placeholders and universes</a></li><li class="chapter-item "><a href="borrow_check/region_inference/closure_constraints.html"><strong aria-hidden="true">44.3.5.</strong> Closure constraints</a></li><li class="chapter-item "><a href="borrow_check/region_inference/error_reporting.html"><strong aria-hidden="true">44.3.6.</strong> Error reporting</a></li></ol></li><li class="chapter-item "><a href="borrow_check/two_phase_borrows.html"><strong aria-hidden="true">44.4.</strong> Two-phase-borrows</a></li></ol></li><li class="chapter-item "><a href="param_env.html"><strong aria-hidden="true">45.</strong> Parameter Environments</a></li><li class="chapter-item "><a href="diagnostics.html"><strong aria-hidden="true">46.</strong> Errors and Lints</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="diagnostics/sessiondiagnostic.html"><strong aria-hidden="true">46.1.</strong> Creating Errors With SessionDiagnostic</a></li><li class="chapter-item "><a href="diagnostics/lintstore.html"><strong aria-hidden="true">46.2.</strong> LintStore</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-codes.html"><strong aria-hidden="true">46.3.</strong> Diagnostic Codes</a></li><li class="chapter-item "><a href="diagnostics/diagnostic-items.html"><strong aria-hidden="true">46.4.</strong> Diagnostic Items</a></li></ol></li><li class="chapter-item "><li class="part-title">MIR to Binaries</li><li class="chapter-item "><a href="part-5-intro.html"><strong aria-hidden="true">47.</strong> Prologue</a></li><li class="chapter-item "><a href="mir/optimizations.html"><strong aria-hidden="true">48.</strong> MIR optimizations</a></li><li class="chapter-item "><a href="mir/debugging.html"><strong aria-hidden="true">49.</strong> Debugging</a></li><li class="chapter-item "><a href="const-eval.html"><strong aria-hidden="true">50.</strong> Constant evaluation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="miri.html"><strong aria-hidden="true">50.1.</strong> miri const evaluator</a></li></ol></li><li class="chapter-item "><a href="backend/monomorph.html"><strong aria-hidden="true">51.</strong> Monomorphization</a></li><li class="chapter-item "><a href="backend/lowering-mir.html"><strong aria-hidden="true">52.</strong> Lowering MIR</a></li><li class="chapter-item "><a href="backend/codegen.html"><strong aria-hidden="true">53.</strong> Code Generation</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="backend/updating-llvm.html"><strong aria-hidden="true">53.1.</strong> Updating LLVM</a></li><li class="chapter-item "><a href="backend/debugging.html"><strong aria-hidden="true">53.2.</strong> Debugging LLVM</a></li><li class="chapter-item "><a href="backend/backend-agnostic.html"><strong aria-hidden="true">53.3.</strong> Backend Agnostic Codegen</a></li><li class="chapter-item "><a href="backend/implicit-caller-location.html"><strong aria-hidden="true">53.4.</strong> Implicit Caller Location</a></li></ol></li><li class="chapter-item "><a href="backend/libs-and-metadata.html"><strong aria-hidden="true">54.</strong> Libraries and Metadata</a></li><li class="chapter-item "><a href="profile-guided-optimization.html"><strong aria-hidden="true">55.</strong> Profile-guided Optimization</a></li><li class="chapter-item "><a href="llvm-coverage-instrumentation.html"><strong aria-hidden="true">56.</strong> LLVM Source-Based Code Coverage</a></li><li class="chapter-item "><a href="sanitizers.html"><strong aria-hidden="true">57.</strong> Sanitizers Support</a></li><li class="chapter-item "><a href="debugging-support-in-rustc.html"><strong aria-hidden="true">58.</strong> Debugging Support in the Rust Compiler</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="appendix/background.html">Appendix A: Background topics</a></li><li class="chapter-item affix "><a href="appendix/glossary.html">Appendix B: Glossary</a></li><li class="chapter-item affix "><a href="appendix/code-index.html">Appendix C: Code Index</a></li><li class="chapter-item affix "><a href="appendix/compiler-lecture.html">Appendix D: Compiler Lecture Series</a></li><li class="chapter-item affix "><a href="appendix/bibliography.html">Appendix E: Bibliography</a></li><li class="chapter-item affix "><a href="appendix/humorust.html">Appendix Z: HumorRust</a></li><li 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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>&quot;Crate metadata&quot;, mainly query outputs, are serialized in a binary
format into <code>rlib</code> and <code>rmeta</code> files that are output when compiling a library
crate, these are then deserialized by crates that 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>The <code>-Z ast-json</code> and <code>-Z ast-json-noexpand</code> flags serialize the <a href="the-parser.html">AST</a> to json
and output the result to stdout.</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 json when the <code>-Z no-link</code> flag is used, and
deserialized from json 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&lt;S: Encoder&gt; {
fn encode(&amp;self, s: &amp;mut S) -&gt; Result&lt;(), S::Error&gt;;
}
pub trait Decodable&lt;D: Decoder&gt;: Sized {
fn decode(d: &amp;mut D) -&gt; Result&lt;Self, D::Error&gt;;
}
</code></pre>
<p>It also defines implementations of these for integer types, floating point
types, <code>bool</code>, <code>char</code>, <code>str</code> and various common standard library types.</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&lt;E: Encoder&gt; Encodable&lt;E&gt; for MyStruct {
fn encode(&amp;self, s: &amp;mut E) -&gt; Result&lt;(), E::Error&gt; {
s.emit_struct(&quot;MyStruct&quot;, 2, |s| {
s.emit_struct_field(&quot;int&quot;, 0, |s| self.int.encode(s))?;
s.emit_struct_field(&quot;float&quot;, 1, |s| self.float.encode(s))
})
}
}
impl&lt;D: Decoder&gt; Decodable&lt;D&gt; for MyStruct {
fn decode(s: &amp;mut D) -&gt; Result&lt;MyStruct, D::Error&gt; {
s.read_struct(&quot;MyStruct&quot;, 2, |d| {
let int = d.read_struct_field(&quot;int&quot;, 0, Decodable::decode)?;
let float = d.read_struct_field(&quot;float&quot;, 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.TyEncoder.html"><code>TyDecoder</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.TyDecoder.html"><code>TyEncoder</code></a> traits are supertraits of <code>Decoder</code> and
<code>Encoder</code> that allow access to a <code>TyCtxt</code>.</p>
<p>Types which contain arena allocated types can then bound the type parameter of
their <code>Encodable</code> and <code>Decodable</code> implementations with these traits. For
example</p>
<pre><code class="language-rust ignore">impl&lt;'tcx, D: TyDecoder&lt;'tcx&gt;&gt; Decodable&lt;D&gt; for MyStruct&lt;'tcx&gt; {
/* ... */
}
</code></pre>
<p>The <code>TyEncodable</code> and <code>TyDecodable</code> <a href="#derive-macros">derive macros</a> will expand to such
an implementation.</p>
<p>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 <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 <code>rustc_middle</code>. 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 <code>ty::Predicate</code> 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 <code>rustc_macros</code> 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 <code>rustc_middle</code>, or that have to be serialized by a type that does
not implement <code>TyEncoder</code>.</li>
<li><code>MetadataEncodable</code> and <code>MetadataDecodable</code> 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 <code>rustc_metadata::rmeta::Lazy</code>.</li>
<li><code>TyEncodable</code> and <code>TyDecoder</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="lazyt"><a class="header" href="#lazyt"><code>Lazy&lt;T&gt;</code></a></h2>
<p>Crate metadata is initially loaded before the <code>TyCtxt&lt;'tcx&gt;</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.Lazy.html"><code>Lazy&lt;T&gt;</code></a> type wraps the (relative) offset in the crate metadata where a
<code>T</code> has been serialized.</p>
<p>The <code>Lazy&lt;[T]&gt;</code> and <code>Lazy&lt;Table&lt;I, T&gt;&gt;</code> type provide some functionality over
<code>Lazy&lt;Vec&lt;T&gt;&gt;</code> and <code>Lazy&lt;HashMap&lt;I, T&gt;&gt;</code>:</p>
<ul>
<li>It's possible to encode a <code>Lazy&lt;[T]&gt;</code> directly from an iterator, without
first collecting into a <code>Vec&lt;T&gt;</code>.</li>
<li>Indexing into a <code>Lazy&lt;Table&lt;I, T&gt;&gt;</code> does not require decoding entries other
than the one being read.</li>
</ul>
<p><strong>note</strong>: <code>Lazy&lt;T&gt;</code> does not cache its value after being deserialized the first
time. Instead the query system 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:
<code>DefId</code> has a <code>default</code> implementation of <code>Encodable&lt;E&gt;</code> and a specialized one
for <code>Encodable&lt;CacheEncoder&gt;</code>.</p>
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