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| <h1 id="libraries-and-metadata"><a class="header" href="#libraries-and-metadata">Libraries and metadata</a></h1> |
| <p>When the compiler sees a reference to an external crate, it needs to load some |
| information about that crate. |
| This chapter gives an overview of that process, |
| and the supported file formats for crate libraries.</p> |
| <h2 id="libraries"><a class="header" href="#libraries">Libraries</a></h2> |
| <p>A crate dependency can be loaded from an <code>rlib</code>, <code>dylib</code>, or <code>rmeta</code> file. |
| A key point of these file formats is that they contain <code>rustc</code>-specific |
| <a href="#metadata"><em>metadata</em></a>. |
| This metadata allows the compiler to discover enough |
| information about the external crate to understand the items it contains, |
| which macros it exports, and <em>much</em> more.</p> |
| <h3 id="rlib"><a class="header" href="#rlib">rlib</a></h3> |
| <p>An <code>rlib</code> is an <a href="https://en.wikipedia.org/wiki/Ar_(Unix)">archive file</a>, which is similar to a tar file. |
| This file format is specific to <code>rustc</code>, and may change over time. |
| This file contains:</p> |
| <ul> |
| <li>Object code, which is the result of code generation. |
| This is used during regular linking. |
| There is a separate <code>.o</code> file for each <a href="../backend/codegen.html">codegen unit</a>. |
| The codegen step can be skipped with the <a href="https://doc.rust-lang.org/rustc/codegen-options/index.html#linker-plugin-lto"><code>-C linker-plugin-lto</code></a> CLI option, |
| which means each <code>.o</code> file will only contain LLVM bitcode.</li> |
| <li><a href="https://llvm.org/docs/BitCodeFormat.html">LLVM bitcode</a>, which is a binary representation of LLVM’s intermediate |
| representation, which is embedded as a section in the <code>.o</code> files. |
| This can be used for <a href="https://llvm.org/docs/LinkTimeOptimization.html">Link Time Optimization</a> (LTO). |
| This can be removed with the |
| <a href="https://doc.rust-lang.org/rustc/codegen-options/index.html#embed-bitcode"><code>-C embed-bitcode=no</code></a> CLI option to improve compile times |
| and reduce disk space if LTO is not needed.</li> |
| <li><code>rustc</code> <a href="#metadata">metadata</a>, in a file named <code>lib.rmeta</code>.</li> |
| <li>A symbol table, which is essentially a list of symbols with offsets to the |
| object files that contain that symbol. |
| This is pretty standard for archive files.</li> |
| </ul> |
| <h3 id="dylib"><a class="header" href="#dylib">dylib</a></h3> |
| <p>A <code>dylib</code> is a platform-specific shared library. |
| It includes the <code>rustc</code> <a href="#metadata">metadata</a> in a special link section called <code>.rustc</code>.</p> |
| <h3 id="rmeta"><a class="header" href="#rmeta">rmeta</a></h3> |
| <p>An <code>rmeta</code> file is a custom binary format that contains the <a href="#metadata">metadata</a> for the crate. |
| This file can be used for fast “checks” of a project by skipping all code |
| generation (as is done with <code>cargo check</code>), collecting enough information for |
| documentation (as is done with <code>cargo doc</code>), or for <a href="#pipelining">pipelining</a>. |
| This file is created if the <a href="https://doc.rust-lang.org/rustc/command-line-arguments.html#option-emit"><code>--emit=metadata</code></a> CLI option is used.</p> |
| <p><code>rmeta</code> files do not support linking, since they do not contain compiled object files.</p> |
| <h2 id="metadata"><a class="header" href="#metadata">Metadata</a></h2> |
| <p>The metadata contains a wide swath of different elements. |
| This guide will not go into detail about every field it contains. |
| You are encouraged to browse the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/struct.CrateRoot.html"><code>CrateRoot</code></a> definition to get a sense of the different elements it contains. |
| Everything about metadata encoding and decoding is in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/index.html"><code>rustc_metadata</code></a> package.</p> |
| <p>Here are a few highlights of things it contains:</p> |
| <ul> |
| <li>The version of the <code>rustc</code> compiler. |
| The compiler will refuse to load files from any other version.</li> |
| <li>The <a href="#strict-version-hash">Strict Version Hash</a> (SVH). |
| This helps ensure the correct dependency is loaded.</li> |
| <li>The <a href="#stable-crate-id">Stable Crate Id</a>. |
| This is a hash used to identify crates.</li> |
| <li>Information about all the source files in the library. |
| This can be used for a variety of things, such as diagnostics pointing to sources in a |
| dependency.</li> |
| <li>Information about exported macros, traits, types, and items. |
| Generally, |
| anything that’s needed to be known when a path references something inside a crate dependency.</li> |
| <li>Encoded <a href="../mir/index.html">MIR</a>. |
| This is optional, and only encoded if needed for code generation. |
| <code>cargo check</code> skips this for performance reasons.</li> |
| </ul> |
| <h3 id="strict-version-hash"><a class="header" href="#strict-version-hash">Strict Version Hash</a></h3> |
| <p>The Strict Version Hash (<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/svh/struct.Svh.html">SVH</a>, also known as the “crate hash”) is a 64-bit |
| hash that is used to ensure that the correct crate dependencies are loaded. |
| It is possible for a directory to contain multiple copies of the same dependency |
| built with different settings, or built from different sources. |
| The crate loader will skip any crates that have the wrong SVH.</p> |
| <p>The SVH is also used for the <a href="../queries/incremental-compilation.html">incremental compilation</a> session filename, |
| though that usage is mostly historic.</p> |
| <p>The hash includes a variety of elements:</p> |
| <ul> |
| <li>Hashes of the HIR nodes.</li> |
| <li>All of the upstream crate hashes.</li> |
| <li>All of the source filenames.</li> |
| <li>Hashes of certain command-line flags (like <code>-C metadata</code> via the <a href="#stable-crate-id">Stable |
| Crate Id</a>, and all CLI options marked with <code>[TRACKED]</code>).</li> |
| </ul> |
| <p>See <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/fn.compute_hir_hash.html"><code>compute_hir_hash</code></a> for where the hash is actually computed.</p> |
| <h3 id="stable-crate-id"><a class="header" href="#stable-crate-id">Stable Crate Id</a></h3> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/def_id/struct.StableCrateId.html"><code>StableCrateId</code></a> is a 64-bit hash used to identify different crates with |
| potentially the same name. |
| It is a hash of the crate name and all the |
| <a href="https://doc.rust-lang.org/rustc/codegen-options/index.html#metadata"><code>-C metadata</code></a> CLI options computed in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/def_id/struct.StableCrateId.html#method.new"><code>StableCrateId::new</code></a>. |
| It is used in a variety of places, such as symbol name mangling, crate loading, and |
| much more.</p> |
| <p>By default, all Rust symbols are mangled and incorporate the stable crate id. |
| This allows multiple versions of the same crate to be included together. |
| Cargo automatically generates <code>-C metadata</code> hashes based on a variety of factors, like |
| the package version, source, and target kind (a lib and test can have the same |
| crate name, so they need to be disambiguated).</p> |
| <h2 id="crate-loading"><a class="header" href="#crate-loading">Crate loading</a></h2> |
| <p>Crate loading can have quite a few subtle complexities. |
| During <a href="../name-resolution.html">name resolution</a>, when an external crate is referenced (via an <code>extern crate</code> or |
| path), the resolver uses the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/creader/struct.CStore.html"><code>CStore</code></a> which is responsible for finding |
| the crate libraries and loading the <a href="#metadata">metadata</a> for them. |
| After the dependency is loaded, the <code>CStore</code> will provide the information the resolver needs |
| to perform its job (such as expanding macros, resolving paths, etc.).</p> |
| <p>To load each external crate, the <code>CStore</code> uses a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/locator/struct.CrateLocator.html"><code>CrateLocator</code></a> to |
| actually find the correct files for one specific crate. |
| There is some great documentation in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/locator/index.html"><code>locator</code></a> module that goes into detail on how loading |
| works, and I strongly suggest reading it to get the full picture.</p> |
| <p>The location of a dependency can come from several different places. |
| Direct dependencies are usually passed with <code>--extern</code> flags, and the loader can look |
| at those directly. |
| Direct dependencies often have references to their own dependencies, which need to be loaded, too. |
| These are usually found by |
| scanning the directories passed with the <code>-L</code> flag for any file whose metadata |
| contains a matching crate name and <a href="#strict-version-hash">SVH</a>. |
| The loader will also look at the <a href="../building/bootstrapping/what-bootstrapping-does.html#what-is-a-sysroot">sysroot</a> to find dependencies.</p> |
| <p>As crates are loaded, they are kept in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/creader/struct.CStore.html"><code>CStore</code></a> with the crate metadata |
| wrapped in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/decoder/struct.CrateMetadata.html"><code>CrateMetadata</code></a> struct. |
| After resolution and expansion, the |
| <code>CStore</code> will make its way into the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.GlobalCtxt.html"><code>GlobalCtxt</code></a> for the rest of the compilation.</p> |
| <h2 id="pipelining"><a class="header" href="#pipelining">Pipelining</a></h2> |
| <p>One trick to improve compile times is to start building a crate as soon as the |
| metadata for its dependencies is available. |
| For a library, there is no need to wait for the code generation of dependencies to finish. |
| Cargo implements this technique by telling <code>rustc</code> to emit an <a href="#rmeta"><code>rmeta</code></a> file for each |
| dependency as well as an <a href="#rlib"><code>rlib</code></a>. |
| As early as it can, <code>rustc</code> will |
| save the <code>rmeta</code> file to disk before it continues to the code generation phase. |
| The compiler sends a JSON message to let the build tool know that it |
| can start building the next crate if possible.</p> |
| <p>The <a href="#crate-loading">crate loading</a> system is smart enough to know when it |
| sees an <code>rmeta</code> file to use that if the <code>rlib</code> is not there (or has only been partially written).</p> |
| <p>This pipelining isn’t possible for binaries, because the linking phase will |
| require the code generation of all its dependencies. |
| In the future, it may be |
| possible to further improve this scenario by splitting linking into a separate |
| command (see <a href="https://github.com/rust-lang/rust/issues/64191">#64191</a>).</p> |
| |
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