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| <h1 id="cfgbootstrap-in-compiler-dependencies"><a class="header" href="#cfgbootstrap-in-compiler-dependencies"><code>cfg(bootstrap)</code> in compiler dependencies</a></h1> |
| <p>The rust compiler uses some external crates that can run into cyclic dependencies with the compiler itself: the compiler needs an updated crate to build, but the crate needs an updated compiler. This page describes how <code>#[cfg(bootstrap)]</code> can be used to break this cycle.</p> |
| <h2 id="enabling-cfgbootstrap"><a class="header" href="#enabling-cfgbootstrap">Enabling <code>#[cfg(bootstrap)]</code></a></h2> |
| <p>Usually the use of <code>#[cfg(bootstrap)]</code> in an external crate causes a warning:</p> |
| <pre><code>warning: unexpected `cfg` condition name: `bootstrap` |
| --> src/main.rs:1:7 |
| | |
| 1 | #[cfg(bootstrap)] |
| | ^^^^^^^^^ |
| | |
| = help: expected names are: `docsrs`, `feature`, and `test` and 31 more |
| = help: consider using a Cargo feature instead |
| = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint: |
| [lints.rust] |
| unexpected_cfgs = { level = "warn", check-cfg = ['cfg(bootstrap)'] } |
| = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(bootstrap)");` to the top of the `build.rs` |
| = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration |
| = note: `#[warn(unexpected_cfgs)]` on by default |
| </code></pre> |
| <p>This warning can be silenced by adding these lines to the project's <code>Cargo.toml</code>:</p> |
| <pre><code class="language-toml">[lints.rust] |
| unexpected_cfgs = { level = "warn", check-cfg = ['cfg(bootstrap)'] } |
| </code></pre> |
| <p>Now <code>#[cfg(bootstrap)]</code> can be used in the crate just like it can be in the compiler: when the bootstrap compiler is used, code annotated with <code>#[cfg(bootstrap)]</code> is compiled, otherwise code annotated with <code>#[cfg(not(bootstrap))]</code> is compiled.</p> |
| <h2 id="the-update-dance"><a class="header" href="#the-update-dance">The update dance</a></h2> |
| <p>As a concrete example we'll use a change where the <code>#[naked]</code> attribute was made into an unsafe attribute, which caused a cyclic dependency with the <code>compiler-builtins</code> crate.</p> |
| <h3 id="step-1-accept-the-new-behavior-in-the-compiler-139797"><a class="header" href="#step-1-accept-the-new-behavior-in-the-compiler-139797">Step 1: accept the new behavior in the compiler (<a href="https://github.com/rust-lang/rust/pull/139797">#139797</a>)</a></h3> |
| <p>In this example it is possible to accept both the old and new behavior at the same time by disabling an error.</p> |
| <h3 id="step-2-update-the-crate-821"><a class="header" href="#step-2-update-the-crate-821">Step 2: update the crate (<a href="https://github.com/rust-lang/compiler-builtins/pull/821">#821</a>)</a></h3> |
| <p>Now in the crate, use <code>#[cfg(bootstrap)]</code> to use the old behavior, or <code>#[cfg(not(bootstrap))]</code> to use the new behavior.</p> |
| <h3 id="step-3-update-the-crate-version-used-by-the-compiler-139934"><a class="header" href="#step-3-update-the-crate-version-used-by-the-compiler-139934">Step 3: update the crate version used by the compiler (<a href="https://github.com/rust-lang/rust/pull/139934">#139934</a>)</a></h3> |
| <p>For <code>compiler-builtins</code> this meant a version bump, in other cases it may be a git submodule update.</p> |
| <h3 id="step-4-remove-the-old-behavior-from-the-compiler-139753"><a class="header" href="#step-4-remove-the-old-behavior-from-the-compiler-139753">Step 4: remove the old behavior from the compiler (<a href="https://github.com/rust-lang/rust/pull/139753">#139753</a>)</a></h3> |
| <p>The updated crate can now be used. In this example that meant that the old behavior could be removed.</p> |
| |
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