Run early lint checks in the background

src/librustc_driver/lib.rs

@@ -391,7 +391,7 @@ pub fn run_compiler(
                 })?;
                 None
             } else {
-                // Drop AST after creating GlobalCtxt to free memory
+                // Drop a reference to the AST
                 let prof = sess.prof.clone();
                 let ast = queries.expansion()?.take().0;
                 Some(Future::spawn(move || {
@@ -400,6 +400,11 @@ pub fn run_compiler(
                 }))
             };
 
+            queries.global_ctxt()?;
+
+            // Drop a reference to the AST by waiting on the lint future.
+            queries.lower_to_hir()?.take().1.join();
+
             queries.global_ctxt()?.peek_mut().enter(|tcx| tcx.analysis(LOCAL_CRATE))?;
 
             // Ensure the AST is dropped by this point.
@@ -410,6 +415,7 @@ pub fn run_compiler(
             }
 
             if sess.opts.debugging_opts.save_analysis {
+                // Drop AST to free memory
                 mem::drop(queries.expansion()?.take());
             }
 

src/librustc_interface/passes.rs

@@ -435,16 +435,16 @@ fn configure_and_expand_inner<'a>(
 }
 
 pub fn lower_to_hir<'res, 'tcx>(
-    sess: &'tcx Session,
-    lint_store: &LintStore,
+    sess: Lrc<Session>,
+    lint_store: Lrc<LintStore>,
     resolver: &'res mut Resolver<'_>,
     dep_graph: &'res DepGraph,
-    krate: &'res ast::Crate,
+    krate: Lrc<ast::Crate>,
     arena: &'tcx Arena<'tcx>,
-) -> Result<map::Forest<'tcx>> {
+) -> Result<(map::Forest<'tcx>, Future<'static, ()>)> {
     // Lower AST to HIR.
     let hir_crate = rustc_ast_lowering::lower_crate(
-        sess,
+        &sess,
         &dep_graph,
         &krate,
         resolver,
@@ -458,23 +458,27 @@ pub fn lower_to_hir<'res, 'tcx>(
 
     let hir_forest = map::Forest::new(hir_crate, &dep_graph);
 
-    sess.time("early_lint_checks", || {
-        rustc_lint::check_ast_crate(
-            sess,
-            lint_store,
-            &krate,
-            false,
-            Some(std::mem::take(resolver.lint_buffer())),
-            rustc_lint::BuiltinCombinedEarlyLintPass::new(),
-        )
-    });
+    let lint_buffer = std::mem::take(resolver.lint_buffer());
 
     // Discard hygiene data, which isn't required after lowering to HIR.
     if !sess.opts.debugging_opts.keep_hygiene_data {
         rustc_span::hygiene::clear_syntax_context_map();
     }
 
-    Ok(hir_forest)
+    let lints = Future::spawn(move || {
+        sess.time("early_lint_checks", || {
+            rustc_lint::check_ast_crate(
+                &sess,
+                &lint_store,
+                &krate,
+                false,
+                Some(lint_buffer),
+                rustc_lint::BuiltinCombinedEarlyLintPass::new(),
+            )
+        })
+    });
+
+    Ok((hir_forest, lints))
 }
 
 // Returns all the paths that correspond to generated files.

src/librustc_interface/queries.rs

@@ -77,9 +77,9 @@ pub struct Queries<'tcx> {
         Steal<(ast::Crate, Lrc<LintStore>)>,
         Steal<Future<'static, Option<DepGraphFuture>>>,
     )>,
-    expansion: Query<(ast::Crate, Steal<Rc<RefCell<BoxedResolver>>>, Lrc<LintStore>)>,
+    expansion: Query<(Lrc<ast::Crate>, Steal<Rc<RefCell<BoxedResolver>>>, Lrc<LintStore>)>,
     dep_graph: Query<DepGraph>,
-    lower_to_hir: Query<(&'tcx map::Forest<'tcx>, Steal<ResolverOutputs>)>,
+    lower_to_hir: Query<(&'tcx map::Forest<'tcx>, Future<'static, ()>, Steal<ResolverOutputs>)>,
     prepare_outputs: Query<OutputFilenames>,
     global_ctxt: Query<QueryContext<'tcx>>,
     ongoing_codegen: Query<Box<dyn Any>>,
@@ -163,7 +163,7 @@ impl<'tcx> Queries<'tcx> {
 
     pub fn expansion(
         &self,
-    ) -> Result<&Query<(ast::Crate, Steal<Rc<RefCell<BoxedResolver>>>, Lrc<LintStore>)>> {
+    ) -> Result<&Query<(Lrc<ast::Crate>, Steal<Rc<RefCell<BoxedResolver>>>, Lrc<LintStore>)>> {
         self.expansion.compute(|| {
             let crate_name = self.crate_name()?.peek().clone();
             let (krate, lint_store) = self.register_plugins()?.peek().0.steal();
@@ -176,7 +176,7 @@ impl<'tcx> Queries<'tcx> {
                 &crate_name,
             )
             .map(|(krate, resolver)| {
-                (krate, Steal::new(Rc::new(RefCell::new(resolver))), lint_store)
+                (Lrc::new(krate), Steal::new(Rc::new(RefCell::new(resolver))), lint_store)
             })
         })
     }
@@ -204,25 +204,26 @@ impl<'tcx> Queries<'tcx> {
 
     pub fn lower_to_hir(
         &'tcx self,
-    ) -> Result<&Query<(&'tcx map::Forest<'tcx>, Steal<ResolverOutputs>)>> {
+    ) -> Result<&Query<(&'tcx map::Forest<'tcx>, Future<'static, ()>, Steal<ResolverOutputs>)>>
+    {
         self.lower_to_hir.compute(|| {
             let expansion_result = self.expansion()?;
             let peeked = expansion_result.peek();
-            let krate = &peeked.0;
+            let krate = peeked.0.clone();
             let resolver = peeked.1.steal();
-            let lint_store = &peeked.2;
-            let hir = resolver.borrow_mut().access(|resolver| {
+            let lint_store = peeked.2.clone();
+            let (hir, lints) = resolver.borrow_mut().access(|resolver| {
                 passes::lower_to_hir(
-                    self.session(),
+                    self.session().clone(),
                     lint_store,
                     resolver,
                     &*self.dep_graph()?.peek(),
-                    &krate,
+                    krate,
                     &self.arena,
                 )
             })?;
             let hir = self.arena.alloc(hir);
-            Ok((hir, Steal::new(BoxedResolver::to_resolver_outputs(resolver))))
+            Ok((hir, lints, Steal::new(BoxedResolver::to_resolver_outputs(resolver))))
         })
     }
 
@@ -248,7 +249,7 @@ impl<'tcx> Queries<'tcx> {
             let outputs = self.prepare_outputs()?.peek().clone();
             let lint_store = self.expansion()?.peek().2.clone();
             let hir = self.lower_to_hir()?.peek();
-            let (ref hir_forest, ref resolver_outputs) = &*hir;
+            let (ref hir_forest, _, ref resolver_outputs) = &*hir;
             let _timer = self.session().timer("create_global_ctxt");
             Ok(passes::create_global_ctxt(
                 self.compiler,
@@ -338,6 +339,12 @@ impl Compiler {
             });
         });
 
+        // Join the early lint check future if has started, but haven't been stolen yet.
+        let _join_lint_future = OnDrop(|| {
+            let result = queries.lower_to_hir.result.borrow_mut().take();
+            result.map(|result| result.map(|result| result.1.join()));
+        });
+
         let ret = f(&queries);
 
         if self.session().opts.debugging_opts.query_stats {
@@ -369,6 +376,7 @@ impl Compiler {
             queries.global_ctxt()?;
 
             // Drop AST after creating GlobalCtxt to free memory.
+            queries.lower_to_hir()?.take().1.join();
             mem::drop(queries.expansion()?.take());
 
             queries.ongoing_codegen()?;