Add some documentation

src/librustc/lint/context.rs

@@ -155,23 +155,31 @@ impl LintStore {
             .collect()
     }
 
-    pub fn register_early_pass(&mut self,
-        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync) {
+    pub fn register_early_pass(
+        &mut self,
+        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync
+    ) {
         self.early_passes.push(Box::new(pass));
     }
 
-    pub fn register_pre_expansion_pass(&mut self,
-        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync) {
+    pub fn register_pre_expansion_pass(
+        &mut self,
+        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync,
+    ) {
         self.pre_expansion_passes.push(Box::new(pass));
     }
 
-    pub fn register_late_pass(&mut self,
-        pass: impl Fn() -> LateLintPassObject + 'static + sync::Send + sync::Sync) {
+    pub fn register_late_pass(
+        &mut self,
+        pass: impl Fn() -> LateLintPassObject + 'static + sync::Send + sync::Sync,
+    ) {
         self.late_passes.push(Box::new(pass));
     }
 
-    pub fn register_late_mod_pass(&mut self,
-        pass: impl Fn() -> LateLintPassObject + 'static + sync::Send + sync::Sync) {
+    pub fn register_late_mod_pass(
+        &mut self,
+        pass: impl Fn() -> LateLintPassObject + 'static + sync::Send + sync::Sync,
+    ) {
         self.late_module_passes.push(Box::new(pass));
     }
 

src/librustc_driver/lib.rs

@@ -106,6 +106,7 @@ pub fn abort_on_err<T>(result: Result<T, ErrorReported>, sess: &Session) -> T {
 pub trait Callbacks {
     /// Called before creating the compiler instance
     fn config(&mut self, _config: &mut interface::Config) {}
+    /// Called early during compilation to allow other drivers to easily register lints.
     fn extra_lints(&mut self, _ls: &mut lint::LintStore) {}
     /// Called after parsing. Return value instructs the compiler whether to
     /// continue the compilation afterwards (defaults to `Compilation::Continue`)

src/librustc_interface/interface.rs

@@ -82,6 +82,11 @@ pub struct Config {
     pub crate_name: Option<String>,
     pub lint_caps: FxHashMap<lint::LintId, lint::Level>,
 
+    /// This is a callback from the driver that is called when we're registering lints;
+    /// it is called during plugin registration when we have the LintStore in a non-shared state.
+    ///
+    /// Note that if you find a Some here you probably want to call that function in the new
+    /// function being registered.
     pub register_lints: Option<Box<dyn Fn(&Session, &mut lint::LintStore) + Send + Sync>>,
 }
 

src/librustc_plugin/registry.rs

@@ -25,6 +25,7 @@ pub struct Registry<'a> {
     /// from the plugin registrar.
     pub sess: &'a Session,
 
+    /// The `LintStore` allows plugins to register new lints.
     pub lint_store: &'a mut LintStore,
 
     #[doc(hidden)]