-Zlink-pub-async-impls flag added to monomorphize pub async fn implementation coroutine (func::{closure#0}), when func itself is monomorphized

Author: azhogin

compiler/rustc_monomorphize/src/collector.rs

@@ -1510,7 +1510,20 @@ impl<'v> RootCollector<'_, 'v> {
     fn process_nested_body(&mut self, def_id: LocalDefId) {
         match self.tcx.def_kind(def_id) {
             DefKind::Closure => {
-                if self.strategy == MonoItemCollectionStrategy::Eager
+                // for 'pub async fn foo(..)' also trying to monomorphize foo::{closure}
+                let is_pub_fn_coroutine = self.tcx.sess.opts.unstable_opts.link_pub_async_impls
+                    && match *self.tcx.type_of(def_id).instantiate_identity().kind() {
+                        ty::Coroutine(cor_id, _args) => {
+                            let tcx = self.tcx;
+                            let parent_id = tcx.parent(cor_id);
+                            tcx.def_kind(parent_id) == DefKind::Fn
+                                && tcx.asyncness(parent_id).is_async()
+                                && tcx.visibility(parent_id).is_public()
+                        }
+                        ty::Closure(..) | ty::CoroutineClosure(..) => false,
+                        _ => unreachable!(),
+                    };
+                if (self.strategy == MonoItemCollectionStrategy::Eager || is_pub_fn_coroutine)
                     && !self
                         .tcx
                         .generics_of(self.tcx.typeck_root_def_id(def_id.to_def_id()))

compiler/rustc_session/src/options.rs

@@ -2281,6 +2281,10 @@ options! {
         "link native libraries in the linker invocation (default: yes)"),
     link_only: bool = (false, parse_bool, [TRACKED],
         "link the `.rlink` file generated by `-Z no-link` (default: no)"),
+    link_pub_async_impls: bool = (false, parse_bool, [TRACKED],
+        "monomorphize pub async function implementation coroutine, func::{closure}, \
+        when func itself is monomorphized (default: no)
+        Should optimize compilation time for projects with async fn lib dependencies"),
     linker_features: LinkerFeaturesCli = (LinkerFeaturesCli::default(), parse_linker_features, [UNTRACKED],
         "a comma-separated list of linker features to enable (+) or disable (-): `lld`"),
     lint_llvm_ir: bool = (false, parse_bool, [TRACKED],

tests/codegen-units/item-collection/async-fn-impl.rs

@@ -0,0 +1,11 @@
+//@ edition: 2024
+// When pub async fn is monomorphized, its implementation coroutine is also monomorphized
+// (with -Zlink-pub-async-impls)
+//@ compile-flags: -Zlink-pub-async-impls --crate-type=lib
+
+//~ MONO_ITEM fn async_fn @@
+//~ MONO_ITEM fn async_fn::{closure#0} @@
+#[unsafe(no_mangle)]
+pub async fn async_fn(x: u64) -> bool {
+    true
+}