Warning added when dependency crate has async drop types, and the feature is disabled

Author: azhogin

compiler/rustc_interface/src/passes.rs

@@ -282,6 +282,7 @@ fn configure_and_expand(
     resolver.resolve_crate(&krate);
 
     CStore::from_tcx(tcx).report_incompatible_target_modifiers(tcx, &krate);
+    CStore::from_tcx(tcx).report_incompatible_async_drop_feature(tcx, &krate);
     krate
 }
 

compiler/rustc_metadata/messages.ftl

@@ -1,6 +1,10 @@
 metadata_as_needed_compatibility =
     linking modifier `as-needed` is only compatible with `dylib` and `framework` linking kinds
 
+metadata_async_drop_types_in_dependency =
+    found async drop types in dependency `{$extern_crate}`, but async_drop feature is disabled for `{$local_crate}`
+    .help = if async drop type will be dropped in a crate without `feature(async_drop)`, sync Drop will be used
+
 metadata_bad_panic_strategy =
     the linked panic runtime `{$runtime}` is not compiled with this crate's panic strategy `{$strategy}`
 

compiler/rustc_metadata/src/creader.rs

@@ -473,6 +473,27 @@ impl CStore {
         }
     }
 
+    // Report about async drop types in dependency if async drop feature is disabled
+    pub fn report_incompatible_async_drop_feature(&self, tcx: TyCtxt<'_>, krate: &Crate) {
+        if tcx.features().async_drop() {
+            return;
+        }
+        for (_cnum, data) in self.iter_crate_data() {
+            if data.is_proc_macro_crate() {
+                continue;
+            }
+            if data.has_async_drops() {
+                let extern_crate = data.name();
+                let local_crate = tcx.crate_name(LOCAL_CRATE);
+                tcx.dcx().emit_warn(errors::AsyncDropTypesInDependency {
+                    span: krate.spans.inner_span.shrink_to_lo(),
+                    extern_crate,
+                    local_crate,
+                });
+            }
+        }
+    }
+
     pub fn new(metadata_loader: Box<MetadataLoaderDyn>) -> CStore {
         CStore {
             metadata_loader,

compiler/rustc_metadata/src/errors.rs

@@ -811,3 +811,13 @@ pub struct UnknownTargetModifierUnsafeAllowed {
     pub span: Span,
     pub flag_name: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(metadata_async_drop_types_in_dependency)]
+#[help]
+pub struct AsyncDropTypesInDependency {
+    #[primary_span]
+    pub span: Span,
+    pub extern_crate: Symbol,
+    pub local_crate: Symbol,
+}

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1984,6 +1984,10 @@ impl CrateMetadata {
         self.root.header.hash
     }
 
+    pub(crate) fn has_async_drops(&self) -> bool {
+        self.root.tables.adt_async_destructor.len > 0
+    }
+
     fn num_def_ids(&self) -> usize {
         self.root.tables.def_keys.size()
     }

tests/ui/async-await/async-drop/dependency-dropped.rs

@@ -1,9 +1,12 @@
 //@ run-pass
 //@ check-run-results
+//@ revisions: with_feature without_feature
 //@ aux-build:async-drop-dep.rs
 //@ edition:2021
 
-#![feature(async_drop)]
+#![cfg_attr(with_feature, feature(async_drop))]
+//[without_feature]~^ WARN found async drop types in dependency `async_drop_dep`, but async_drop feature is disabled for `dependency_dropped`
+
 #![allow(incomplete_features)]
 
 extern crate async_drop_dep;

tests/ui/async-await/async-drop/dependency-dropped.run.stdout renamed to tests/ui/async-await/async-drop/dependency-dropped.with_feature.run.stdout

@@ -0,0 +1 @@
+Sync drop

tests/ui/async-await/async-drop/dependency-dropped.without_feature.run.stdout

@@ -0,0 +1,10 @@
+warning: found async drop types in dependency `async_drop_dep`, but async_drop feature is disabled for `dependency_dropped`
+  --> $DIR/dependency-dropped.rs:7:1
+   |
+LL | #![cfg_attr(with_feature, feature(async_drop))]
+   | ^
+   |
+   = help: if async drop type will be dropped in a crate without `feature(async_drop)`, sync Drop will be used
+
+warning: 1 warning emitted
+