[perf] alternative approach to #117703

compiler/rustc_error_codes/src/error_codes/E0733.md

@@ -13,7 +13,7 @@ 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:
 
-```edition2018,compile_fail,E0720
+```edition2018,compile_fail,E0733
 use std::future::Future;
 fn foo_desugared(n: usize) -> impl Future<Output = ()> {
     async move {
@@ -41,4 +41,18 @@ fn foo_recursive(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
 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 recursive call-site can be boxed:
+
+```edition2018
+use std::future::Future;
+use std::pin::Pin;
+fn foo_recursive(n: usize) -> impl Future<Output = ()> {
+    async move {
+        if n > 0 {
+            Box::pin(foo_recursive(n - 1)).await;
+        }
+    }
+}
+```
+
 [`async`]: https://doc.rust-lang.org/std/keyword.async.html

compiler/rustc_hir/src/hir.rs

@@ -1494,6 +1494,15 @@ pub enum CoroutineKind {
     Coroutine,
 }
 
+impl CoroutineKind {
+    pub fn is_fn_like(self) -> bool {
+        matches!(
+            self,
+            CoroutineKind::Async(CoroutineSource::Fn) | CoroutineKind::Gen(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

@@ -216,13 +216,12 @@ fn check_opaque(tcx: TyCtxt<'_>, id: hir::ItemId) {
         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;
     }
 
@@ -233,19 +232,16 @@ fn check_opaque(tcx: TyCtxt<'_>, id: hir::ItemId) {
 pub(super) fn check_opaque_for_cycles<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
-    args: GenericArgsRef<'tcx>,
     span: Span,
-    origin: &hir::OpaqueTyOrigin,
 ) -> Result<(), ErrorGuaranteed> {
+    let args = GenericArgs::identity_for_item(tcx, def_id);
+
     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 reported = opaque_type_cycle_error(tcx, def_id, span);
+        return Err(reported);
     }
+
+    Ok(())
 }
 
 /// Check that the concrete type behind `impl Trait` actually implements `Trait`.
@@ -1324,16 +1320,6 @@ pub(super) fn check_mod_item_types(tcx: TyCtxt<'_>, module_def_id: LocalModDefId
     }
 }
 
-fn async_opaque_type_cycle_error(tcx: TyCtxt<'_>, span: Span) -> ErrorGuaranteed {
-    struct_span_err!(tcx.sess, span, E0733, "recursion in an `async fn` requires boxing")
-        .span_label(span, "recursive `async fn`")
-        .note("a recursive `async fn` must be rewritten to return a boxed `dyn Future`")
-        .note(
-            "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
@@ -1514,5 +1500,12 @@ pub(super) fn check_coroutine_obligations(tcx: TyCtxt<'_>, def_id: LocalDefId) {
     debug!(?errors);
     if !errors.is_empty() {
         infcx.err_ctxt().report_fulfillment_errors(errors);
+        return;
+    }
+
+    if let Err(&LayoutError::Cycle(_guar)) =
+        tcx.layout_of(param_env.and(tcx.type_of(def_id).instantiate_identity()))
+    {
+        debug!("layout error in coroutine")
     }
 }

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

@@ -1393,6 +1393,8 @@ 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

@@ -701,22 +701,6 @@ impl<'tcx> TyCtxt<'tcx> {
             .map(|decl| ty::EarlyBinder::bind(decl.ty))
     }
 
-    /// Normalizes all opaque types in the given value, replacing them
-    /// with their underlying types.
-    pub fn expand_opaque_types(self, val: Ty<'tcx>) -> Ty<'tcx> {
-        let mut visitor = OpaqueTypeExpander {
-            seen_opaque_tys: FxHashSet::default(),
-            expanded_cache: FxHashMap::default(),
-            primary_def_id: None,
-            found_recursion: false,
-            found_any_recursion: false,
-            check_recursion: false,
-            expand_coroutines: false,
-            tcx: self,
-        };
-        val.fold_with(&mut visitor)
-    }
-
     /// Expands the given impl trait type, stopping if the type is recursive.
     #[instrument(skip(self), level = "debug", ret)]
     pub fn try_expand_impl_trait_type(
@@ -731,7 +715,6 @@ impl<'tcx> TyCtxt<'tcx> {
             found_recursion: false,
             found_any_recursion: false,
             check_recursion: true,
-            expand_coroutines: true,
             tcx: self,
         };
 
@@ -749,9 +732,13 @@ impl<'tcx> TyCtxt<'tcx> {
         match def_kind {
             DefKind::AssocFn if self.associated_item(def_id).fn_has_self_parameter => "method",
             DefKind::Coroutine => match self.coroutine_kind(def_id).unwrap() {
-                rustc_hir::CoroutineKind::Async(..) => "async closure",
-                rustc_hir::CoroutineKind::Coroutine => "coroutine",
-                rustc_hir::CoroutineKind::Gen(..) => "gen closure",
+                hir::CoroutineKind::Async(hir::CoroutineSource::Fn) => "async fn",
+                hir::CoroutineKind::Async(hir::CoroutineSource::Block) => "async block",
+                hir::CoroutineKind::Async(hir::CoroutineSource::Closure) => "async closure",
+                hir::CoroutineKind::Gen(hir::CoroutineSource::Fn) => "gen fn",
+                hir::CoroutineKind::Gen(hir::CoroutineSource::Block) => "gen block",
+                hir::CoroutineKind::Gen(hir::CoroutineSource::Closure) => "gen closure",
+                hir::CoroutineKind::Coroutine => "coroutine",
             },
             _ => def_kind.descr(def_id),
         }
@@ -767,9 +754,9 @@ impl<'tcx> TyCtxt<'tcx> {
         match def_kind {
             DefKind::AssocFn if self.associated_item(def_id).fn_has_self_parameter => "a",
             DefKind::Coroutine => match self.coroutine_kind(def_id).unwrap() {
-                rustc_hir::CoroutineKind::Async(..) => "an",
-                rustc_hir::CoroutineKind::Coroutine => "a",
-                rustc_hir::CoroutineKind::Gen(..) => "a",
+                hir::CoroutineKind::Async(..) => "an",
+                hir::CoroutineKind::Coroutine => "a",
+                hir::CoroutineKind::Gen(..) => "a",
             },
             _ => def_kind.article(),
         }
@@ -808,7 +795,6 @@ struct OpaqueTypeExpander<'tcx> {
     primary_def_id: Option<DefId>,
     found_recursion: bool,
     found_any_recursion: bool,
-    expand_coroutines: bool,
     /// Whether or not to check for recursive opaque types.
     /// This is `true` when we're explicitly checking for opaque type
     /// recursion, and 'false' otherwise to avoid unnecessary work.
@@ -845,37 +831,6 @@ impl<'tcx> OpaqueTypeExpander<'tcx> {
             None
         }
     }
-
-    fn expand_coroutine(&mut self, def_id: DefId, args: GenericArgsRef<'tcx>) -> Option<Ty<'tcx>> {
-        if self.found_any_recursion {
-            return None;
-        }
-        let args = args.fold_with(self);
-        if !self.check_recursion || self.seen_opaque_tys.insert(def_id) {
-            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);
-                    }
-                    let expanded_ty = Ty::new_coroutine_witness(self.tcx, def_id, args);
-                    self.expanded_cache.insert((def_id, args), expanded_ty);
-                    expanded_ty
-                }
-            };
-            if self.check_recursion {
-                self.seen_opaque_tys.remove(&def_id);
-            }
-            Some(expanded_ty)
-        } else {
-            // If another opaque type that we contain is recursive, then it
-            // will report the error, so we don't have to.
-            self.found_any_recursion = true;
-            self.found_recursion = def_id == *self.primary_def_id.as_ref().unwrap();
-            None
-        }
-    }
 }
 
 impl<'tcx> TypeFolder<TyCtxt<'tcx>> for OpaqueTypeExpander<'tcx> {
@@ -884,22 +839,20 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for OpaqueTypeExpander<'tcx> {
     }
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
-        let mut t = if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) = *t.kind() {
+        if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) = *t.kind() {
             self.expand_opaque_ty(def_id, args).unwrap_or(t)
-        } else if t.has_opaque_types() || t.has_coroutines() {
+        } else if t.has_opaque_types() {
             t.super_fold_with(self)
         } else {
             t
-        };
-        if self.expand_coroutines {
-            if let ty::CoroutineWitness(def_id, args) = *t.kind() {
-                t = self.expand_coroutine(def_id, args).unwrap_or(t);
-            }
         }
-        t
     }
 
     fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        if !p.has_opaque_types() {
+            return p;
+        }
+
         if let ty::PredicateKind::Clause(clause) = p.kind().skip_binder()
             && let ty::ClauseKind::Projection(projection_pred) = clause
         {
@@ -1433,7 +1386,6 @@ pub fn reveal_opaque_types_in_bounds<'tcx>(
         found_recursion: false,
         found_any_recursion: false,
         check_recursion: false,
-        expand_coroutines: false,
         tcx,
     };
     val.fold_with(&mut visitor)