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| <h1 id="lints"><a class="header" href="#lints">Lints</a></h1> |
| <p>This page documents some of the machinery around lint registration and how we |
| run lints in the compiler.</p> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LintStore.html"><code>LintStore</code></a> is the central piece of infrastructure, around which |
| everything rotates. The <code>LintStore</code> is held as part of the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/struct.Session.html"><code>Session</code></a>, and it |
| gets populated with the list of lints shortly after the <code>Session</code> is created.</p> |
| <h2 id="lints-vs-lint-passes"><a class="header" href="#lints-vs-lint-passes">Lints vs. lint passes</a></h2> |
| <p>There are two parts to the linting mechanism within the compiler: lints and |
| lint passes. Unfortunately, a lot of the documentation we have refers to both |
| of these as just "lints."</p> |
| <p>First, we have the lint declarations themselves, |
| and this is where the name and default lint level and other metadata come from. |
| These are normally defined by way of the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/macro.declare_lint.html"><code>declare_lint!</code></a> macro, |
| which boils down to a static with type <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint_defs/struct.Lint.html"><code>&rustc_lint_defs::Lint</code></a> |
| (although this may change in the future, |
| as the macro is somewhat unwieldy to add new fields to, |
| like all macros).</p> |
| <p>As of <!-- date-check --> Aug 2022, |
| we lint against direct declarations without the use of the macro.</p> |
| <p>Lint declarations don't carry any "state" - they are merely global identifiers |
| and descriptions of lints. We assert at runtime that they are not registered |
| twice (by lint name).</p> |
| <p>Lint passes are the meat of any lint. Notably, there is not a one-to-one |
| relationship between lints and lint passes; a lint might not have any lint pass |
| that emits it, it could have many, or just one -- the compiler doesn't track |
| whether a pass is in any way associated with a particular lint, and frequently |
| lints are emitted as part of other work (e.g., type checking, etc.).</p> |
| <h2 id="registration"><a class="header" href="#registration">Registration</a></h2> |
| <h3 id="high-level-overview"><a class="header" href="#high-level-overview">High-level overview</a></h3> |
| <p>In <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/index.html#reexport.run_compiler"><code>rustc_interface::run_compiler</code></a>, |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LintStore.html"><code>LintStore</code></a> is created, |
| and all lints are registered.</p> |
| <p>There are three 'sources' of lints:</p> |
| <ul> |
| <li>internal lints: lints only used by the rustc codebase</li> |
| <li>builtin lints: lints built into the compiler and not provided by some outside |
| source</li> |
| <li><code>rustc_interface::Config</code><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/interface/struct.Config.html#structfield.register_lints"><code>register_lints</code></a>: lints passed into the compiler |
| during construction</li> |
| </ul> |
| <p>Lints are registered via the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LintStore.html#method.register_lints"><code>LintStore::register_lint</code></a> function. This should |
| happen just once for any lint, or an ICE will occur.</p> |
| <p>Once the registration is complete, we "freeze" the lint store by placing it in |
| an <code>Arc</code>.</p> |
| <p>Lint passes are registered separately into one of the categories |
| (pre-expansion, early, late, late module). Passes are registered as a closure |
| -- i.e., <code>impl Fn() -> Box<dyn X></code>, where <code>dyn X</code> is either an early or late |
| lint pass trait object. When we run the lint passes, we run the closure and |
| then invoke the lint pass methods. The lint pass methods take <code>&mut self</code> so |
| they can keep track of state internally.</p> |
| <h4 id="internal-lints"><a class="header" href="#internal-lints">Internal lints</a></h4> |
| <p>These are lints used just by the compiler or drivers like <code>clippy</code>. They can be |
| found in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/internal/index.html"><code>rustc_lint::internal</code></a>.</p> |
| <p>An example of such a lint is the check that lint passes are implemented using |
| the <code>declare_lint_pass!</code> macro and not by hand. This is accomplished with the |
| <code>LINT_PASS_IMPL_WITHOUT_MACRO</code> lint.</p> |
| <p>Registration of these lints happens in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/fn.register_internals.html"><code>rustc_lint::register_internals</code></a> |
| function which is called when constructing a new lint store inside |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/fn.new_lint_store.html"><code>rustc_lint::new_lint_store</code></a>.</p> |
| <h4 id="builtin-lints"><a class="header" href="#builtin-lints">Builtin Lints</a></h4> |
| <p>These are primarily described in two places, |
| <code>rustc_lint_defs::builtin</code> and <code>rustc_lint::builtin</code>. |
| Often the first provides the definitions for the lints themselves, |
| and the latter provides the lint pass definitions (and implementations), |
| but this is not always true.</p> |
| <p>The builtin lint registration happens in |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/fn.register_builtins.html"><code>rustc_lint::register_builtins</code></a> function. |
| Just like with internal lints, |
| this happens inside of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/fn.new_lint_store.html"><code>rustc_lint::new_lint_store</code></a>.</p> |
| <h4 id="driver-lints"><a class="header" href="#driver-lints">Driver lints</a></h4> |
| <p>These are the lints provided by drivers via the <code>rustc_interface::Config</code> |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_interface/interface/struct.Config.html#structfield.register_lints"><code>register_lints</code></a> field, which is a callback. Drivers should, if finding it |
| already set, call the function currently set within the callback they add. The |
| best way for drivers to get access to this is by overriding the |
| <code>Callbacks::config</code> function which gives them direct access to the <code>Config</code> |
| structure.</p> |
| <h2 id="compiler-lint-passes-are-combined-into-one-pass"><a class="header" href="#compiler-lint-passes-are-combined-into-one-pass">Compiler lint passes are combined into one pass</a></h2> |
| <p>Within the compiler, for performance reasons, we usually do not register dozens |
| of lint passes. Instead, we have a single lint pass of each variety (e.g., |
| <code>BuiltinCombinedModuleLateLintPass</code>) which will internally call all of the |
| individual lint passes; this is because then we get the benefits of static over |
| dynamic dispatch for each of the (often empty) trait methods.</p> |
| <p>Ideally, we'd not have to do this, since it adds to the complexity of |
| understanding the code. However, with the current type-erased lint store |
| approach, it is beneficial to do so for performance reasons.</p> |
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