Lints

This page documents some of the machinery around lint registration and how we run lints in the compiler.

The LintStore is the central piece of infrastructure, around which everything rotates. The LintStore is held as part of the Session, and it gets populated with the list of lints shortly after the Session is created.

Lints vs. lint passes

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."

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 declare_lint! macro, which boils down to a static with type &rustc_lint_defs::Lint (although this may change in the future, as the macro is somewhat unwieldy to add new fields to, like all macros).

As of Aug 2022, we lint against direct declarations without the use of the macro.

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).

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.).

Registration

High-level overview

In rustc_interface::run_compiler, the LintStore is created, and all lints are registered.

There are three 'sources' of lints:

• internal lints: lints only used by the rustc codebase
• builtin lints: lints built into the compiler and not provided by some outside source
• rustc_interface::Configregister_lints: lints passed into the compiler during construction

Lints are registered via the LintStore::register_lint function. This should happen just once for any lint, or an ICE will occur.

Once the registration is complete, we "freeze" the lint store by placing it in an Lrc.

Lint passes are registered separately into one of the categories (pre-expansion, early, late, late module). Passes are registered as a closure -- i.e., impl Fn() -> Box<dyn X>, where dyn X 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 &mut self so they can keep track of state internally.

Internal lints

These are lints used just by the compiler or drivers like clippy. They can be found in rustc_lint::internal.

An example of such a lint is the check that lint passes are implemented using the declare_lint_pass! macro and not by hand. This is accomplished with the LINT_PASS_IMPL_WITHOUT_MACRO lint.

Registration of these lints happens in the rustc_lint::register_internals function which is called when constructing a new lint store inside rustc_lint::new_lint_store.

Builtin Lints

These are primarily described in two places, rustc_lint_defs::builtin and rustc_lint::builtin. 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.

The builtin lint registration happens in the rustc_lint::register_builtins function. Just like with internal lints, this happens inside of rustc_lint::new_lint_store.

Driver lints

These are the lints provided by drivers via the rustc_interface::Config register_lints 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 Callbacks::config function which gives them direct access to the Config structure.

Compiler lint passes are combined into one pass

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., BuiltinCombinedModuleLateLintPass) 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.

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.