AsyncDrop trait without sync Drop generates an error

Author: azhogin

compiler/rustc_hir_analysis/messages.ftl

@@ -46,6 +46,9 @@ hir_analysis_associated_type_trait_uninferred_generic_params = cannot use the {$
 
 hir_analysis_associated_type_trait_uninferred_generic_params_multipart_suggestion = use a fully qualified path with explicit lifetimes
 
+hir_analysis_async_drop_without_sync_drop = `AsyncDrop` impl without `Drop` impl
+    .help = type implementing `AsyncDrop` trait must also implement `Drop` trait to be used in sync context and unwinds
+
 hir_analysis_auto_deref_reached_recursion_limit = reached the recursion limit while auto-dereferencing `{$ty}`
     .label = deref recursion limit reached
     .help = consider increasing the recursion limit by adding a `#![recursion_limit = "{$suggested_limit}"]` attribute to your crate (`{$crate_name}`)

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -113,7 +113,15 @@ pub fn provide(providers: &mut Providers) {
 }
 
 fn adt_destructor(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::Destructor> {
-    tcx.calculate_dtor(def_id, always_applicable::check_drop_impl)
+    let dtor = tcx.calculate_dtor(def_id, always_applicable::check_drop_impl);
+    if dtor.is_none() && tcx.features().async_drop() {
+        if let Some(async_dtor) = adt_async_destructor(tcx, def_id) {
+            // When type has AsyncDrop impl, but doesn't have Drop impl, generate error
+            let span = tcx.def_span(async_dtor.impl_did);
+            tcx.dcx().emit_err(errors::AsyncDropWithoutSyncDrop { span });
+        }
+    }
+    dtor
 }
 
 fn adt_async_destructor(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::AsyncDestructor> {

compiler/rustc_hir_analysis/src/errors.rs

@@ -1712,3 +1712,11 @@ pub(crate) struct AbiCustomClothedFunction {
     )]
     pub naked_span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_async_drop_without_sync_drop)]
+#[help]
+pub(crate) struct AsyncDropWithoutSyncDrop {
+    #[primary_span]
+    pub span: Span,
+}

tests/ui/async-await/async-drop/async-without-sync.rs

@@ -0,0 +1,19 @@
+//@ edition: 2024
+#![feature(async_drop)]
+#![allow(incomplete_features)]
+#![crate_type = "lib"]
+
+use std::future::AsyncDrop;
+use std::pin::Pin;
+
+async fn foo() {
+    let _st = St;
+}
+
+struct St;
+
+impl AsyncDrop for St { //~ ERROR: `AsyncDrop` impl without `Drop` impl
+    async fn drop(self: Pin<&mut Self>) {
+        println!("123");
+    }
+}

tests/ui/async-await/async-drop/async-without-sync.stderr

@@ -0,0 +1,10 @@
+error: `AsyncDrop` impl without `Drop` impl
+  --> $DIR/async-without-sync.rs:15:1
+   |
+LL | impl AsyncDrop for St {
+   | ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: type implementing `AsyncDrop` trait must also implement `Drop` trait to be used in sync context and unwinds
+
+error: aborting due to 1 previous error
+