Support async recursive calls (as long as they have indirection)

compiler/rustc_error_codes/src/error_codes/E0733.md

@@ -10,35 +10,31 @@ async fn foo(n: usize) {
 }
 ```
 
-To perform async recursion, the `async fn` needs to be desugared such that the
-`Future` is explicit in the return type:
+The recursive invocation can be boxed:
 
-```edition2018,compile_fail,E0720
-use std::future::Future;
-fn foo_desugared(n: usize) -> impl Future<Output = ()> {
-    async move {
-        if n > 0 {
-            foo_desugared(n - 1).await;
-        }
+```edition2018
+async fn foo(n: usize) {
+    if n > 0 {
+        Box::pin(foo(n - 1)).await;
     }
 }
 ```
 
-Finally, the future is wrapped in a pinned box:
+The `Box<...>` ensures that the result is of known size, and the pin is
+required to keep it in the same place in memory.
+
+Alternatively, the body can be boxed:
 
 ```edition2018
 use std::future::Future;
 use std::pin::Pin;
-fn foo_recursive(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
+fn foo(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
     Box::pin(async move {
         if n > 0 {
-            foo_recursive(n - 1).await;
+            foo(n - 1).await;
         }
     })
 }
 ```
 
-The `Box<...>` ensures that the result is of known size, and the pin is
-required to keep it in the same place in memory.
-
 [`async`]: https://doc.rust-lang.org/std/keyword.async.html

compiler/rustc_hir/src/hir.rs

@@ -1361,6 +1361,12 @@ impl CoroutineKind {
     }
 }
 
+impl CoroutineKind {
+    pub fn is_fn_like(self) -> bool {
+        matches!(self, CoroutineKind::Desugared(_, CoroutineSource::Fn))
+    }
+}
+
 impl fmt::Display for CoroutineKind {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {

compiler/rustc_hir_analysis/src/check/check.rs

@@ -17,7 +17,7 @@ use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::traits::{DefiningAnchor, ObligationCauseCode};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
-use rustc_middle::ty::util::{Discr, IntTypeExt};
+use rustc_middle::ty::util::{Discr, InspectCoroutineFields, IntTypeExt};
 use rustc_middle::ty::GenericArgKind;
 use rustc_middle::ty::{
     AdtDef, ParamEnv, RegionKind, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
@@ -213,13 +213,12 @@ fn check_opaque(tcx: TyCtxt<'_>, def_id: LocalDefId) {
         return;
     }
 
-    let args = GenericArgs::identity_for_item(tcx, item.owner_id);
     let span = tcx.def_span(item.owner_id.def_id);
 
     if tcx.type_of(item.owner_id.def_id).instantiate_identity().references_error() {
         return;
     }
-    if check_opaque_for_cycles(tcx, item.owner_id.def_id, args, span, origin).is_err() {
+    if check_opaque_for_cycles(tcx, item.owner_id.def_id, span).is_err() {
         return;
     }
 
@@ -230,19 +229,36 @@ fn check_opaque(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 pub(super) fn check_opaque_for_cycles<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
-    args: GenericArgsRef<'tcx>,
     span: Span,
-    origin: &hir::OpaqueTyOrigin,
 ) -> Result<(), ErrorGuaranteed> {
-    if tcx.try_expand_impl_trait_type(def_id.to_def_id(), args).is_err() {
-        let reported = match origin {
-            hir::OpaqueTyOrigin::AsyncFn(..) => async_opaque_type_cycle_error(tcx, span),
-            _ => opaque_type_cycle_error(tcx, def_id, span),
-        };
-        Err(reported)
-    } else {
-        Ok(())
+    let args = GenericArgs::identity_for_item(tcx, def_id);
+
+    // First, try to look at any opaque expansion cycles, considering coroutine fields
+    // (even though these aren't necessarily true errors).
+    if tcx
+        .try_expand_impl_trait_type(def_id.to_def_id(), args, InspectCoroutineFields::Yes)
+        .is_err()
+    {
+        // Look for true opaque expansion cycles, but ignore coroutines.
+        // This will give us any true errors. Coroutines are only problematic
+        // if they cause layout computation errors.
+        if tcx
+            .try_expand_impl_trait_type(def_id.to_def_id(), args, InspectCoroutineFields::No)
+            .is_err()
+        {
+            let reported = opaque_type_cycle_error(tcx, def_id, span);
+            return Err(reported);
+        }
+
+        // And also look for cycle errors in the layout of coroutines.
+        if let Err(&LayoutError::Cycle(guar)) =
+            tcx.layout_of(tcx.param_env(def_id).and(Ty::new_opaque(tcx, def_id.to_def_id(), args)))
+        {
+            return Err(guar);
+        }
     }
+
+    Ok(())
 }
 
 /// Check that the concrete type behind `impl Trait` actually implements `Trait`.
@@ -1300,16 +1316,6 @@ pub(super) fn check_type_params_are_used<'tcx>(
     }
 }
 
-fn async_opaque_type_cycle_error(tcx: TyCtxt<'_>, span: Span) -> ErrorGuaranteed {
-    struct_span_err!(tcx.dcx(), span, E0733, "recursion in an `async fn` requires boxing")
-        .span_label_mv(span, "recursive `async fn`")
-        .note_mv("a recursive `async fn` must be rewritten to return a boxed `dyn Future`")
-        .note_mv(
-            "consider using the `async_recursion` crate: https://crates.io/crates/async_recursion",
-        )
-        .emit()
-}
-
 /// Emit an error for recursive opaque types.
 ///
 /// If this is a return `impl Trait`, find the item's return expressions and point at them. For

compiler/rustc_middle/src/query/keys.rs

@@ -40,7 +40,7 @@ pub trait Key: Sized {
         None
     }
 
-    fn ty_adt_id(&self) -> Option<DefId> {
+    fn ty_def_id(&self) -> Option<DefId> {
         None
     }
 }
@@ -406,9 +406,10 @@ impl<'tcx> Key for Ty<'tcx> {
         DUMMY_SP
     }
 
-    fn ty_adt_id(&self) -> Option<DefId> {
-        match self.kind() {
+    fn ty_def_id(&self) -> Option<DefId> {
+        match *self.kind() {
             ty::Adt(adt, _) => Some(adt.did()),
+            ty::Coroutine(def_id, ..) => Some(def_id),
             _ => None,
         }
     }
@@ -452,6 +453,10 @@ impl<'tcx, T: Key> Key for ty::ParamEnvAnd<'tcx, T> {
     fn default_span(&self, tcx: TyCtxt<'_>) -> Span {
         self.value.default_span(tcx)
     }
+
+    fn ty_def_id(&self) -> Option<DefId> {
+        self.value.ty_def_id()
+    }
 }
 
 impl Key for Symbol {
@@ -550,7 +555,7 @@ impl<'tcx> Key for (ValidityRequirement, ty::ParamEnvAnd<'tcx, Ty<'tcx>>) {
         DUMMY_SP
     }
 
-    fn ty_adt_id(&self) -> Option<DefId> {
+    fn ty_def_id(&self) -> Option<DefId> {
         match self.1.value.kind() {
             ty::Adt(adt, _) => Some(adt.did()),
             _ => None,

compiler/rustc_middle/src/query/mod.rs

@@ -1387,6 +1387,8 @@ rustc_queries! {
     ) -> Result<ty::layout::TyAndLayout<'tcx>, &'tcx ty::layout::LayoutError<'tcx>> {
         depth_limit
         desc { "computing layout of `{}`", key.value }
+        // we emit our own error during query cycle handling
+        cycle_delay_bug
     }
 
     /// Compute a `FnAbi` suitable for indirect calls, i.e. to `fn` pointers.

compiler/rustc_middle/src/query/plumbing.rs

@@ -53,7 +53,7 @@ pub struct DynamicQuery<'tcx, C: QueryCache> {
         fn(tcx: TyCtxt<'tcx>, key: &C::Key, index: SerializedDepNodeIndex) -> bool,
     pub hash_result: HashResult<C::Value>,
     pub value_from_cycle_error:
-        fn(tcx: TyCtxt<'tcx>, cycle: &[QueryInfo], guar: ErrorGuaranteed) -> C::Value,
+        fn(tcx: TyCtxt<'tcx>, cycle_error: &CycleError, guar: ErrorGuaranteed) -> C::Value,
     pub format_value: fn(&C::Value) -> String,
 }
 

compiler/rustc_middle/src/ty/util.rs

@@ -702,6 +702,7 @@ impl<'tcx> TyCtxt<'tcx> {
         self,
         def_id: DefId,
         args: GenericArgsRef<'tcx>,
+        inspect_coroutine_fields: InspectCoroutineFields,
     ) -> Result<Ty<'tcx>, Ty<'tcx>> {
         let mut visitor = OpaqueTypeExpander {
             seen_opaque_tys: FxHashSet::default(),
@@ -712,6 +713,7 @@ impl<'tcx> TyCtxt<'tcx> {
             check_recursion: true,
             expand_coroutines: true,
             tcx: self,
+            inspect_coroutine_fields,
         };
 
         let expanded_type = visitor.expand_opaque_ty(def_id, args).unwrap();
@@ -729,16 +731,43 @@ impl<'tcx> TyCtxt<'tcx> {
             DefKind::AssocFn if self.associated_item(def_id).fn_has_self_parameter => "method",
             DefKind::Closure if let Some(coroutine_kind) = self.coroutine_kind(def_id) => {
                 match coroutine_kind {
-                    hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _) => {
-                        "async closure"
-                    }
-                    hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::AsyncGen, _) => {
-                        "async gen closure"
-                    }
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::Async,
+                        hir::CoroutineSource::Fn,
+                    ) => "async fn",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::Async,
+                        hir::CoroutineSource::Block,
+                    ) => "async block",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::Async,
+                        hir::CoroutineSource::Closure,
+                    ) => "async closure",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::AsyncGen,
+                        hir::CoroutineSource::Fn,
+                    ) => "async gen fn",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::AsyncGen,
+                        hir::CoroutineSource::Block,
+                    ) => "async gen block",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::AsyncGen,
+                        hir::CoroutineSource::Closure,
+                    ) => "async gen closure",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::Gen,
+                        hir::CoroutineSource::Fn,
+                    ) => "gen fn",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::Gen,
+                        hir::CoroutineSource::Block,
+                    ) => "gen block",
+                    hir::CoroutineKind::Desugared(
+                        hir::CoroutineDesugaring::Gen,
+                        hir::CoroutineSource::Closure,
+                    ) => "gen closure",
                     hir::CoroutineKind::Coroutine(_) => "coroutine",
-                    hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _) => {
-                        "gen closure"
-                    }
                 }
             }
             _ => def_kind.descr(def_id),
@@ -858,6 +887,13 @@ struct OpaqueTypeExpander<'tcx> {
     /// recursion, and 'false' otherwise to avoid unnecessary work.
     check_recursion: bool,
     tcx: TyCtxt<'tcx>,
+    inspect_coroutine_fields: InspectCoroutineFields,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum InspectCoroutineFields {
+    No,
+    Yes,
 }
 
 impl<'tcx> OpaqueTypeExpander<'tcx> {
@@ -899,9 +935,11 @@ impl<'tcx> OpaqueTypeExpander<'tcx> {
             let expanded_ty = match self.expanded_cache.get(&(def_id, args)) {
                 Some(expanded_ty) => *expanded_ty,
                 None => {
-                    for bty in self.tcx.coroutine_hidden_types(def_id) {
-                        let hidden_ty = bty.instantiate(self.tcx, args);
-                        self.fold_ty(hidden_ty);
+                    if matches!(self.inspect_coroutine_fields, InspectCoroutineFields::Yes) {
+                        for bty in self.tcx.coroutine_hidden_types(def_id) {
+                            let hidden_ty = bty.instantiate(self.tcx, args);
+                            self.fold_ty(hidden_ty);
+                        }
                     }
                     let expanded_ty = Ty::new_coroutine_witness(self.tcx, def_id, args);
                     self.expanded_cache.insert((def_id, args), expanded_ty);
@@ -1479,6 +1517,7 @@ pub fn reveal_opaque_types_in_bounds<'tcx>(
         check_recursion: false,
         expand_coroutines: false,
         tcx,
+        inspect_coroutine_fields: InspectCoroutineFields::No,
     };
     val.fold_with(&mut visitor)
 }