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
  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 `Arc`.

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.