Add shorter and more direct error for dyn AsyncFn

Fix #132713

Author: cramertj

compiler/rustc_error_codes/src/error_codes/E0802.md

@@ -0,0 +1,30 @@
+The `Async{Fn, FnMut, FnOnce}` traits are not yet `dyn`-compatible.
+This error indicates that you attempted to use `dyn AsyncFn`,
+`dyn AsyncFnMut`, or `dyn AsyncFnOnce`.
+
+This is not yet possible because the `Async...` traits internally return
+a concrete `Future` associated type. For dynamic callsites, it is impossible
+to know the size of the returned `Future` object since different
+`Async...` implementations may return differently-sized `Future` objects.
+
+This is analogous to the more general issue of creating a `dyn` type without
+specifying associated types, e.g. `dyn Iterator` as opposed to
+`dyn Iterator<Item = SomeItemType>`.
+
+Erroneous code example:
+
+```compile_fail,E0802
+async fn call_async_fn_twice(some_fn: &dyn AsyncFn()) {
+  some_fn().await;
+  some_fn().await;
+}
+```
+
+One workaround to this issue is to use `impl Async...` instead:
+
+```
+async fn call_async_fn_twice(some_fn: &impl AsyncFn()) {
+  some_fn().await;
+  some_fn().await;
+}
+```

compiler/rustc_error_codes/src/lib.rs

@@ -541,6 +541,7 @@ E0798: 0798,
 E0799: 0799,
 E0800: 0800,
 E0801: 0801,
+E0802: 0802,
         );
     )
 }

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -103,17 +103,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // most importantly, that the supertraits don't contain `Self`,
         // to avoid ICEs.
         for item in &regular_traits {
-            let violations =
-                hir_ty_lowering_dyn_compatibility_violations(tcx, item.trait_ref().def_id());
+            let item_def_id = item.trait_ref().def_id();
+            let violations = hir_ty_lowering_dyn_compatibility_violations(tcx, item_def_id);
             if !violations.is_empty() {
-                let reported = report_dyn_incompatibility(
-                    tcx,
-                    span,
-                    Some(hir_id),
-                    item.trait_ref().def_id(),
-                    &violations,
-                )
-                .emit();
+                let reported =
+                    report_dyn_incompatibility(tcx, span, Some(hir_id), item_def_id, &violations)
+                        .emit();
                 return Ty::new_error(tcx, reported);
             }
         }

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -733,6 +733,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 (span, def_ids.into_iter().map(|did| tcx.associated_item(did)).collect())
             })
             .collect();
+
         let mut names: FxIndexMap<String, Vec<Symbol>> = Default::default();
         let mut names_len = 0;
 

compiler/rustc_middle/src/traits/mod.rs

@@ -712,6 +712,22 @@ pub enum DynCompatibilityViolation {
 
     /// GAT
     GAT(Symbol, Span),
+
+    /// Async{Fn, FnMut, FnOnce}
+    ///
+    /// `fn_trait` is the name of the `AsyncFn...` trait,
+    AsyncFnTrait {
+        /// The `AsyncFn...` trait referenced.
+        ///
+        /// This is useful for better error reporting in cases where the
+        /// `dyn`-incompatible trait inherits from `Async...`.
+        //
+        // FIXME(cramertj): I'd love for this to be a DefId for proper comparison
+        // in the error reporting stage, but sadly this isn't possible because
+        // DefIds cannot be stored at this stage. Is there a better way to handle
+        // catching the supertrait case than string comparison?
+        fn_trait: Symbol,
+    },
 }
 
 impl DynCompatibilityViolation {
@@ -779,14 +795,18 @@ impl DynCompatibilityViolation {
             DynCompatibilityViolation::GAT(name, _) => {
                 format!("it contains the generic associated type `{name}`").into()
             }
+            DynCompatibilityViolation::AsyncFnTrait { .. } => {
+                "`async` function traits are not yet `dyn`-compatible".into()
+            }
         }
     }
 
     pub fn solution(&self) -> DynCompatibilityViolationSolution {
         match self {
             DynCompatibilityViolation::SizedSelf(_)
             | DynCompatibilityViolation::SupertraitSelf(_)
-            | DynCompatibilityViolation::SupertraitNonLifetimeBinder(..) => {
+            | DynCompatibilityViolation::SupertraitNonLifetimeBinder(..)
+            | DynCompatibilityViolation::AsyncFnTrait { .. } => {
                 DynCompatibilityViolationSolution::None
             }
             DynCompatibilityViolation::Method(

compiler/rustc_middle/src/ty/context.rs

@@ -560,6 +560,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.trait_is_alias(trait_def_id)
     }
 
+    fn trait_is_async_fn(self, trait_def_id: DefId) -> bool {
+        self.trait_is_async_fn(trait_def_id)
+    }
+
     fn trait_is_dyn_compatible(self, trait_def_id: DefId) -> bool {
         self.is_dyn_compatible(trait_def_id)
     }

compiler/rustc_middle/src/ty/mod.rs

@@ -1830,6 +1830,17 @@ impl<'tcx> TyCtxt<'tcx> {
         self.trait_def(trait_def_id).has_auto_impl
     }
 
+    /// Returns `true` if this is an `AsyncFn`, `AsyncFnMut`, or `AsyncFnOnce` trait.
+    pub fn trait_is_async_fn(self, trait_def_id: DefId) -> bool {
+        let lang_items = self.lang_items();
+        [
+            lang_items.async_fn_trait(),
+            lang_items.async_fn_mut_trait(),
+            lang_items.async_fn_once_trait(),
+        ]
+        .contains(&Some(trait_def_id))
+    }
+
     /// Returns `true` if this is coinductive, either because it is
     /// an auto trait or because it has the `#[rustc_coinductive]` attribute.
     pub fn trait_is_coinductive(self, trait_def_id: DefId) -> bool {

compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs

@@ -7,7 +7,7 @@ pub mod suggestions;
 use std::{fmt, iter};
 
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
-use rustc_errors::{Applicability, Diag, E0038, E0276, MultiSpan, struct_span_code_err};
+use rustc_errors::{Applicability, Diag, E0038, E0276, E0802, MultiSpan, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::{self as hir, LangItem};
@@ -17,7 +17,7 @@ use rustc_infer::traits::{
 };
 use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_span::{ErrorGuaranteed, ExpnKind, Span};
+use rustc_span::{ErrorGuaranteed, ExpnKind, Span, Symbol};
 use tracing::{info, instrument};
 
 pub use self::overflow::*;
@@ -428,10 +428,50 @@ pub fn report_dyn_incompatibility<'tcx>(
     violations: &[DynCompatibilityViolation],
 ) -> Diag<'tcx> {
     let trait_str = tcx.def_path_str(trait_def_id);
+
+    // Avoid errors diving into the details of the `AsyncFn` traits if this is
+    // a straightforward "`AsyncFn` is not yet `dyn`-compatible" error.
+    if tcx.trait_is_async_fn(trait_def_id) {
+        let fn_trait: Symbol = violations
+            .iter()
+            .find_map(|violation| {
+                // Try to find the original trait that caused the violation.
+                match *violation {
+                    DynCompatibilityViolation::AsyncFnTrait { fn_trait } => Some(fn_trait),
+                    _ => None,
+                }
+            })
+            .expect("AsyncFn traits should report a corresponding DynCompatibilityViolation");
+        let mut err = struct_span_code_err!(
+            tcx.dcx(),
+            span,
+            E0802,
+            "the trait `{}` is not yet `dyn`-compatible",
+            fn_trait
+        );
+        // Note: this check is quite imprecise.
+        // Comparing the DefIds or similar would be better, but we can't store
+        // DefIds in `DynCompatibilityViolation`.
+        if fn_trait.as_str() == trait_str {
+            err.span_label(span, format!("`{fn_trait}` is not yet `dyn`-compatible"));
+        } else {
+            let trait_str = tcx.def_path_str(trait_def_id);
+            err.span_label(
+                span,
+                format!(
+                    "`{trait_str}` inherits from `{fn_trait}` which is not yet `dyn`-compatible'"
+                ),
+            );
+        }
+        attempt_dyn_to_impl_suggestion(tcx, hir_id, &mut err);
+        return err;
+    }
+
     let trait_span = tcx.hir().get_if_local(trait_def_id).and_then(|node| match node {
         hir::Node::Item(item) => Some(item.ident.span),
         _ => None,
     });
+
     let mut err = struct_span_code_err!(
         tcx.dcx(),
         span,
@@ -441,24 +481,8 @@ pub fn report_dyn_incompatibility<'tcx>(
     );
     err.span_label(span, format!("`{trait_str}` cannot be made into an object"));
 
-    if let Some(hir_id) = hir_id
-        && let hir::Node::Ty(ty) = tcx.hir_node(hir_id)
-        && let hir::TyKind::TraitObject([trait_ref, ..], ..) = ty.kind
-    {
-        let mut hir_id = hir_id;
-        while let hir::Node::Ty(ty) = tcx.parent_hir_node(hir_id) {
-            hir_id = ty.hir_id;
-        }
-        if tcx.parent_hir_node(hir_id).fn_sig().is_some() {
-            // Do not suggest `impl Trait` when dealing with things like super-traits.
-            err.span_suggestion_verbose(
-                ty.span.until(trait_ref.span),
-                "consider using an opaque type instead",
-                "impl ",
-                Applicability::MaybeIncorrect,
-            );
-        }
-    }
+    attempt_dyn_to_impl_suggestion(tcx, hir_id, &mut err);
+
     let mut reported_violations = FxIndexSet::default();
     let mut multi_span = vec![];
     let mut messages = vec![];
@@ -583,3 +607,23 @@ pub fn report_dyn_incompatibility<'tcx>(
 
     err
 }
+
+fn attempt_dyn_to_impl_suggestion(tcx: TyCtxt<'_>, hir_id: Option<hir::HirId>, err: &mut Diag<'_>) {
+    let Some(hir_id) = hir_id else { return };
+    let hir::Node::Ty(ty) = tcx.hir_node(hir_id) else { return };
+    let hir::TyKind::TraitObject([trait_ref, ..], ..) = ty.kind else { return };
+    let mut hir_id = hir_id;
+    while let hir::Node::Ty(ty) = tcx.parent_hir_node(hir_id) {
+        hir_id = ty.hir_id;
+    }
+    if tcx.parent_hir_node(hir_id).fn_sig().is_none() {
+        // Do not suggest `impl Trait` when dealing with things like super-traits.
+        return;
+    }
+    err.span_suggestion_verbose(
+        ty.span.until(trait_ref.span),
+        "consider using an opaque type instead",
+        "impl ",
+        Applicability::MaybeIncorrect,
+    );
+}

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -39,6 +39,16 @@ pub fn hir_ty_lowering_dyn_compatibility_violations(
     trait_def_id: DefId,
 ) -> Vec<DynCompatibilityViolation> {
     debug_assert!(tcx.generics_of(trait_def_id).has_self);
+
+    // Check for `AsyncFn` traits first to avoid reporting various other
+    // errors about the details of why these traits aren't object-safe.
+    for supertrait in tcx.supertrait_def_ids(trait_def_id) {
+        if tcx.trait_is_async_fn(supertrait) {
+            let fn_trait = tcx.item_name(supertrait);
+            return vec![DynCompatibilityViolation::AsyncFnTrait { fn_trait }];
+        }
+    }
+
     tcx.supertrait_def_ids(trait_def_id)
         .map(|def_id| predicates_reference_self(tcx, def_id, true))
         .filter(|spans| !spans.is_empty())
@@ -53,6 +63,17 @@ fn dyn_compatibility_violations(
     debug_assert!(tcx.generics_of(trait_def_id).has_self);
     debug!("dyn_compatibility_violations: {:?}", trait_def_id);
 
+    // Check for `AsyncFn` traits first to avoid reporting various other
+    // errors about the details of why these traits aren't object-safe.
+    for supertrait in tcx.supertrait_def_ids(trait_def_id) {
+        if tcx.trait_is_async_fn(supertrait) {
+            let fn_trait = tcx.item_name(supertrait);
+            return core::slice::from_ref(
+                tcx.arena.alloc(DynCompatibilityViolation::AsyncFnTrait { fn_trait }),
+            );
+        }
+    }
+
     tcx.arena.alloc_from_iter(
         tcx.supertrait_def_ids(trait_def_id)
             .flat_map(|def_id| dyn_compatibility_violations_for_trait(tcx, def_id)),

compiler/rustc_type_ir/src/interner.rs

@@ -262,6 +262,8 @@ pub trait Interner:
 
     fn trait_is_alias(self, trait_def_id: Self::DefId) -> bool;
 
+    fn trait_is_async_fn(self, trait_def_id: Self::DefId) -> bool;
+
     fn trait_is_dyn_compatible(self, trait_def_id: Self::DefId) -> bool;
 
     fn trait_is_fundamental(self, def_id: Self::DefId) -> bool;

tests/ui/async-await/async-closures/dyn.rs

@@ -0,0 +1,39 @@
+// Test the diagnostic output (error descriptions) resulting from `dyn AsyncFn{,Mut,Once}`.
+//@ edition:2018
+
+#![feature(async_closure)]
+
+use core::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
+
+// --- Explicit `dyn` ---
+
+fn takes_async_fn(_: &dyn AsyncFn()) {}
+//~^ ERROR the trait `AsyncFn` is not yet `dyn`-compatible
+
+fn takes_async_fn_mut(_: &mut dyn AsyncFnMut()) {}
+//~^ ERROR the trait `AsyncFnMut` is not yet `dyn`-compatible
+
+fn takes_async_fn_once(_: Box<dyn AsyncFnOnce()>) {}
+//~^ ERROR the trait `AsyncFnOnce` is not yet `dyn`-compatible
+
+// --- Non-explicit `dyn` ---
+
+#[allow(bare_trait_objects)]
+fn takes_async_fn_implicit_dyn(_: &AsyncFn()) {}
+//~^ ERROR the trait `AsyncFn` is not yet `dyn`-compatible
+
+#[allow(bare_trait_objects)]
+fn takes_async_fn_mut_implicit_dyn(_: &mut AsyncFnMut()) {}
+//~^ ERROR the trait `AsyncFnMut` is not yet `dyn`-compatible
+
+#[allow(bare_trait_objects)]
+fn takes_async_fn_once_implicit_dyn(_: Box<AsyncFnOnce()>) {}
+//~^ ERROR the trait `AsyncFnOnce` is not yet `dyn`-compatible
+
+// --- Supertrait ---
+
+trait SubAsyncFn: AsyncFn() {}
+fn takes_sub_async_fn(_: &dyn SubAsyncFn) {}
+//~^ ERROR the trait `SubAsyncFn` cannot be made into an object
+
+fn main() {}