rustc_session: implement latent TODO

compiler/rustc_lint/src/context.rs

@@ -36,7 +36,7 @@
 use rustc_session::config::ExpectedValues;
 use rustc_session::lint::{BuiltinLintDiagnostics, LintExpectationId};
 use rustc_session::lint::{FutureIncompatibleInfo, Level, Lint, LintBuffer, LintId};
-use rustc_session::Session;
+use rustc_session::{LintStoreMarker, Session};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{BytePos, Span};
@@ -77,6 +77,8 @@ pub struct LintStore {
     lint_groups: FxHashMap<&'static str, LintGroup>,
 }
 
+impl LintStoreMarker for LintStore {}
+
 /// The target of the `by_name` map, which accounts for renaming/deprecation.
 #[derive(Debug)]
 enum TargetLint {

compiler/rustc_lint/src/late.rs

@@ -30,8 +30,9 @@
 use std::any::Any;
 use std::cell::Cell;
 
-/// Extract the `LintStore` from the query context.
-/// This function exists because we've erased `LintStore` as `dyn Any` in the session.
+/// Extract the [`LintStore`] from [`Session`].
+///
+/// This function exists because [`Session::lint_store`] is type-erased.
 pub fn unerased_lint_store(sess: &Session) -> &LintStore {
     let store: &Lrc<_> = sess.lint_store.as_ref().unwrap();
     let store: &dyn Any = &**store;

compiler/rustc_lint/src/lib.rs

@@ -39,6 +39,7 @@
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]
+#![cfg_attr(bootstrap, feature(trait_upcasting))]
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]

compiler/rustc_session/src/session.rs

@@ -139,6 +139,8 @@ pub struct CompilerIO {
     pub temps_dir: Option<PathBuf>,
 }
 
+pub trait LintStoreMarker: Any + DynSync + DynSend {}
+
 /// Represents the data associated with a compilation
 /// session for a single crate.
 pub struct Session {
@@ -171,10 +173,7 @@ pub struct Session {
     pub jobserver: Client,
 
     /// This only ever stores a `LintStore` but we don't want a dependency on that type here.
-    ///
-    /// FIXME(Centril): consider `dyn LintStoreMarker` once
-    /// we can upcast to `Any` for some additional type safety.
-    pub lint_store: Option<Lrc<dyn Any + DynSync + DynSend>>,
+    pub lint_store: Option<Lrc<dyn LintStoreMarker>>,
 
     /// Should be set if any lints are registered in `lint_store`.
     pub registered_lints: bool,