Auto merge of #121909 - Zoxc:drop-ast-task, r=petrochenkov

Drop AST on a separate thread and prefetch `hir_crate`

This drop AST on a separate thread and prefetches `hir_crate`.

A `spawn` function is added to the `parallel` module which spawn some work on the Rayon thread pool.

compiler/rustc_ast_lowering/src/lib.rs

@@ -49,6 +49,7 @@ use rustc_attr_parsing::{AttributeParser, OmitDoc};
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::spawn;
 use rustc_data_structures::tagged_ptr::TaggedRef;
 use rustc_errors::{DiagArgFromDisplay, DiagCtxtHandle, StashKey};
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
@@ -454,9 +455,14 @@ pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
         .lower_node(def_id);
     }
 
-    // Drop AST to free memory
     drop(ast_index);
-    sess.time("drop_ast", || drop(krate));
+
+    // Drop AST to free memory. It can be expensive so try to drop it on a separate thread.
+    let prof = sess.prof.clone();
+    spawn(move || {
+        let _timer = prof.verbose_generic_activity("drop_ast");
+        drop(krate);
+    });
 
     // Don't hash unless necessary, because it's expensive.
     let opt_hir_hash =