Rollup merge of rust-lang#133122 - compiler-errors:afidt, r=oli-obk

Add unpolished, experimental support for AFIDT (async fn in dyn trait)

This allows us to begin messing around `async fn` in `dyn Trait`. Calling an async fn from a trait object always returns a `dyn* Future<Output = ...>`.

To make it work, Implementations are currently required to return something that can be coerced to a `dyn* Future` (see the example in `tests/ui/async-await/dyn/works.rs`). If it's not the right size, then it'll raise an error at the coercion site (see the example in `tests/ui/async-await/dyn/wrong-size.rs`). Currently the only practical way of doing this is wrapping the body in `Box::pin(async move { .. })`.

This PR does not implement a helper type like a "`Boxing`"[^boxing] adapter, and I'll probably follow-up with another PR to improve the error message for the `PointerLike` trait (something that explains in just normal prose what is happening here, rather than a trait error).
[^boxing]: https://rust-lang.github.io/async-fundamentals-initiative/explainer/user_guide_future.html#the-boxing-adapter

This PR also does not implement new trait solver support for AFIDT; I'll need to think how best to integrate it into candidate assembly, and that's a bit of a matter of taste, but I don't think it will be difficult to do.

This could also be generalized:
* To work on functions that are `-> impl Future` (soon).
* To work on functions that are `-> impl Iterator` and other "dyn rpitit safe" traits. We still need to nail down exactly what is needed for this to be okay (not soon).

Tracking:
* rust-lang#133119

Author: fmease

compiler/rustc_feature/src/unstable.rs

@@ -390,6 +390,8 @@ declare_features! (
     (unstable, associated_type_defaults, "1.2.0", Some(29661)),
     /// Allows `async || body` closures.
     (unstable, async_closure, "1.37.0", Some(62290)),
+    /// Allows async functions to be called from `dyn Trait`.
+    (incomplete, async_fn_in_dyn_trait, "CURRENT_RUSTC_VERSION", Some(133119)),
     /// Allows `#[track_caller]` on async functions.
     (unstable, async_fn_track_caller, "1.73.0", Some(110011)),
     /// Allows `for await` loops.

compiler/rustc_middle/src/ty/instance.rs

@@ -677,23 +677,26 @@ impl<'tcx> Instance<'tcx> {
                 //
                 // 1) The underlying method expects a caller location parameter
                 // in the ABI
-                if resolved.def.requires_caller_location(tcx)
-                        // 2) The caller location parameter comes from having `#[track_caller]`
-                        // on the implementation, and *not* on the trait method.
-                        && !tcx.should_inherit_track_caller(def)
-                        // If the method implementation comes from the trait definition itself
-                        // (e.g. `trait Foo { #[track_caller] my_fn() { /* impl */ } }`),
-                        // then we don't need to generate a shim. This check is needed because
-                        // `should_inherit_track_caller` returns `false` if our method
-                        // implementation comes from the trait block, and not an impl block
-                        && !matches!(
-                            tcx.opt_associated_item(def),
-                            Some(ty::AssocItem {
-                                container: ty::AssocItemContainer::Trait,
-                                ..
-                            })
-                        )
-                {
+                let needs_track_caller_shim = resolved.def.requires_caller_location(tcx)
+                    // 2) The caller location parameter comes from having `#[track_caller]`
+                    // on the implementation, and *not* on the trait method.
+                    && !tcx.should_inherit_track_caller(def)
+                    // If the method implementation comes from the trait definition itself
+                    // (e.g. `trait Foo { #[track_caller] my_fn() { /* impl */ } }`),
+                    // then we don't need to generate a shim. This check is needed because
+                    // `should_inherit_track_caller` returns `false` if our method
+                    // implementation comes from the trait block, and not an impl block
+                    && !matches!(
+                        tcx.opt_associated_item(def),
+                        Some(ty::AssocItem {
+                            container: ty::AssocItemContainer::Trait,
+                            ..
+                        })
+                    );
+                // We also need to generate a shim if this is an AFIT.
+                let needs_rpitit_shim =
+                    tcx.return_position_impl_trait_in_trait_shim_data(def).is_some();
+                if needs_track_caller_shim || needs_rpitit_shim {
                     if tcx.is_closure_like(def) {
                         debug!(
                             " => vtable fn pointer created for closure with #[track_caller]: {:?} for method {:?} {:?}",

compiler/rustc_middle/src/ty/mod.rs

@@ -146,6 +146,7 @@ mod opaque_types;
 mod parameterized;
 mod predicate;
 mod region;
+mod return_position_impl_trait_in_trait;
 mod rvalue_scopes;
 mod structural_impls;
 #[allow(hidden_glob_reexports)]

compiler/rustc_middle/src/ty/return_position_impl_trait_in_trait.rs

@@ -0,0 +1,95 @@
+use rustc_hir::def_id::DefId;
+
+use crate::ty::{self, ExistentialPredicateStableCmpExt, TyCtxt};
+
+impl<'tcx> TyCtxt<'tcx> {
+    /// Given a `def_id` of a trait or impl method, compute whether that method needs to
+    /// have an RPITIT shim applied to it for it to be object safe. If so, return the
+    /// `def_id` of the RPITIT, and also the args of trait method that returns the RPITIT.
+    ///
+    /// NOTE that these args are not, in general, the same as than the RPITIT's args. They
+    /// are a subset of those args,  since they do not include the late-bound lifetimes of
+    /// the RPITIT. Depending on the context, these will need to be dealt with in different
+    /// ways -- in codegen, it's okay to fill them with ReErased.
+    pub fn return_position_impl_trait_in_trait_shim_data(
+        self,
+        def_id: DefId,
+    ) -> Option<(DefId, ty::EarlyBinder<'tcx, ty::GenericArgsRef<'tcx>>)> {
+        let assoc_item = self.opt_associated_item(def_id)?;
+
+        let (trait_item_def_id, opt_impl_def_id) = match assoc_item.container {
+            ty::AssocItemContainer::Impl => {
+                (assoc_item.trait_item_def_id?, Some(self.parent(def_id)))
+            }
+            ty::AssocItemContainer::Trait => (def_id, None),
+        };
+
+        let sig = self.fn_sig(trait_item_def_id);
+
+        // Check if the trait returns an RPITIT.
+        let ty::Alias(ty::Projection, ty::AliasTy { def_id, .. }) =
+            *sig.skip_binder().skip_binder().output().kind()
+        else {
+            return None;
+        };
+        if !self.is_impl_trait_in_trait(def_id) {
+            return None;
+        }
+
+        let args = if let Some(impl_def_id) = opt_impl_def_id {
+            // Rebase the args from the RPITIT onto the impl trait ref, so we can later
+            // substitute them with the method args of the *impl* method, since that's
+            // the instance we're building a vtable shim for.
+            ty::GenericArgs::identity_for_item(self, trait_item_def_id).rebase_onto(
+                self,
+                self.parent(trait_item_def_id),
+                self.impl_trait_ref(impl_def_id)
+                    .expect("expected impl trait ref from parent of impl item")
+                    .instantiate_identity()
+                    .args,
+            )
+        } else {
+            // This is when we have a default trait implementation.
+            ty::GenericArgs::identity_for_item(self, trait_item_def_id)
+        };
+
+        Some((def_id, ty::EarlyBinder::bind(args)))
+    }
+
+    /// Given a `DefId` of an RPITIT and its args, return the existential predicates
+    /// that corresponds to the RPITIT's bounds with the self type erased.
+    pub fn item_bounds_to_existential_predicates(
+        self,
+        def_id: DefId,
+        args: ty::GenericArgsRef<'tcx>,
+    ) -> &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>> {
+        let mut bounds: Vec<_> = self
+            .item_super_predicates(def_id)
+            .iter_instantiated(self, args)
+            .filter_map(|clause| {
+                clause
+                    .kind()
+                    .map_bound(|clause| match clause {
+                        ty::ClauseKind::Trait(trait_pred) => Some(ty::ExistentialPredicate::Trait(
+                            ty::ExistentialTraitRef::erase_self_ty(self, trait_pred.trait_ref),
+                        )),
+                        ty::ClauseKind::Projection(projection_pred) => {
+                            Some(ty::ExistentialPredicate::Projection(
+                                ty::ExistentialProjection::erase_self_ty(self, projection_pred),
+                            ))
+                        }
+                        ty::ClauseKind::TypeOutlives(_) => {
+                            // Type outlives bounds don't really turn into anything,
+                            // since we must use an intersection region for the `dyn*`'s
+                            // region anyways.
+                            None
+                        }
+                        _ => unreachable!("unexpected clause in item bounds: {clause:?}"),
+                    })
+                    .transpose()
+            })
+            .collect();
+        bounds.sort_by(|a, b| a.skip_binder().stable_cmp(self, &b.skip_binder()));
+        self.mk_poly_existential_predicates(&bounds)
+    }
+}

compiler/rustc_mir_transform/src/shim.rs

@@ -9,6 +9,7 @@ use rustc_index::{Idx, IndexVec};
 use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::*;
 use rustc_middle::query::Providers;
+use rustc_middle::ty::adjustment::PointerCoercion;
 use rustc_middle::ty::{
     self, CoroutineArgs, CoroutineArgsExt, EarlyBinder, GenericArgs, Ty, TyCtxt,
 };
@@ -710,6 +711,13 @@ fn build_call_shim<'tcx>(
     };
 
     let def_id = instance.def_id();
+
+    let rpitit_shim = if let ty::InstanceKind::ReifyShim(..) = instance {
+        tcx.return_position_impl_trait_in_trait_shim_data(def_id)
+    } else {
+        None
+    };
+
     let sig = tcx.fn_sig(def_id);
     let sig = sig.map_bound(|sig| tcx.instantiate_bound_regions_with_erased(sig));
 
@@ -765,9 +773,34 @@ fn build_call_shim<'tcx>(
     let mut local_decls = local_decls_for_sig(&sig, span);
     let source_info = SourceInfo::outermost(span);
 
+    let mut destination = Place::return_place();
+    if let Some((rpitit_def_id, fn_args)) = rpitit_shim {
+        let rpitit_args =
+            fn_args.instantiate_identity().extend_to(tcx, rpitit_def_id, |param, _| {
+                match param.kind {
+                    ty::GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
+                    ty::GenericParamDefKind::Type { .. }
+                    | ty::GenericParamDefKind::Const { .. } => {
+                        unreachable!("rpitit should have no addition ty/ct")
+                    }
+                }
+            });
+        let dyn_star_ty = Ty::new_dynamic(
+            tcx,
+            tcx.item_bounds_to_existential_predicates(rpitit_def_id, rpitit_args),
+            tcx.lifetimes.re_erased,
+            ty::DynStar,
+        );
+        destination = local_decls.push(local_decls[RETURN_PLACE].clone()).into();
+        local_decls[RETURN_PLACE].ty = dyn_star_ty;
+        let mut inputs_and_output = sig.inputs_and_output.to_vec();
+        *inputs_and_output.last_mut().unwrap() = dyn_star_ty;
+        sig.inputs_and_output = tcx.mk_type_list(&inputs_and_output);
+    }
+
     let rcvr_place = || {
         assert!(rcvr_adjustment.is_some());
-        Place::from(Local::new(1 + 0))
+        Place::from(Local::new(1))
     };
     let mut statements = vec![];
 
@@ -854,7 +887,7 @@ fn build_call_shim<'tcx>(
         TerminatorKind::Call {
             func: callee,
             args,
-            destination: Place::return_place(),
+            destination,
             target: Some(BasicBlock::new(1)),
             unwind: if let Some(Adjustment::RefMut) = rcvr_adjustment {
                 UnwindAction::Cleanup(BasicBlock::new(3))
@@ -882,7 +915,24 @@ fn build_call_shim<'tcx>(
         );
     }
     // BB #1/#2 - return
-    block(&mut blocks, vec![], TerminatorKind::Return, false);
+    // NOTE: If this is an RPITIT in dyn, we also want to coerce
+    // the return type of the function into a `dyn*`.
+    let stmts = if rpitit_shim.is_some() {
+        vec![Statement {
+            source_info,
+            kind: StatementKind::Assign(Box::new((
+                Place::return_place(),
+                Rvalue::Cast(
+                    CastKind::PointerCoercion(PointerCoercion::DynStar, CoercionSource::Implicit),
+                    Operand::Move(destination),
+                    sig.output(),
+                ),
+            ))),
+        }]
+    } else {
+        vec![]
+    };
+    block(&mut blocks, stmts, TerminatorKind::Return, false);
     if let Some(Adjustment::RefMut) = rcvr_adjustment {
         // BB #3 - drop if closure panics
         block(

compiler/rustc_monomorphize/src/lib.rs

@@ -42,7 +42,10 @@ fn custom_coerce_unsize_info<'tcx>(
             ..
         })) => Ok(tcx.coerce_unsized_info(impl_def_id)?.custom_kind.unwrap()),
         impl_source => {
-            bug!("invalid `CoerceUnsized` impl_source: {:?}", impl_source);
+            bug!(
+                "invalid `CoerceUnsized` from {source_ty} to {target_ty}: impl_source: {:?}",
+                impl_source
+            );
         }
     }
 }

compiler/rustc_span/src/symbol.rs

@@ -461,6 +461,7 @@ symbols! {
         async_drop_slice,
         async_drop_surface_drop_in_place,
         async_fn,
+        async_fn_in_dyn_trait,
         async_fn_in_trait,
         async_fn_kind_helper,
         async_fn_kind_upvars,

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -11,6 +11,7 @@ use rustc_abi::BackendRepr;
 use rustc_errors::FatalError;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_middle::bug;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{
     self, EarlyBinder, ExistentialPredicateStableCmpExt as _, GenericArgs, Ty, TyCtxt,
@@ -901,23 +902,59 @@ fn contains_illegal_impl_trait_in_trait<'tcx>(
     fn_def_id: DefId,
     ty: ty::Binder<'tcx, Ty<'tcx>>,
 ) -> Option<MethodViolationCode> {
-    // This would be caught below, but rendering the error as a separate
-    // `async-specific` message is better.
+    let ty = tcx.liberate_late_bound_regions(fn_def_id, ty);
+
     if tcx.asyncness(fn_def_id).is_async() {
-        return Some(MethodViolationCode::AsyncFn);
+        // FIXME(async_fn_in_dyn_trait): Think of a better way to unify these code paths
+        // to issue an appropriate feature suggestion when users try to use AFIDT.
+        // Obviously we must only do this once AFIDT is finished enough to actually be usable.
+        if tcx.features().async_fn_in_dyn_trait() {
+            let ty::Alias(ty::Projection, proj) = *ty.kind() else {
+                bug!("expected async fn in trait to return an RPITIT");
+            };
+            assert!(tcx.is_impl_trait_in_trait(proj.def_id));
+
+            // FIXME(async_fn_in_dyn_trait): We should check that this bound is legal too,
+            // and stop relying on `async fn` in the definition.
+            for bound in tcx.item_bounds(proj.def_id).instantiate(tcx, proj.args) {
+                if let Some(violation) = bound
+                    .visit_with(&mut IllegalRpititVisitor { tcx, allowed: Some(proj) })
+                    .break_value()
+                {
+                    return Some(violation);
+                }
+            }
+
+            None
+        } else {
+            // Rendering the error as a separate `async-specific` message is better.
+            Some(MethodViolationCode::AsyncFn)
+        }
+    } else {
+        ty.visit_with(&mut IllegalRpititVisitor { tcx, allowed: None }).break_value()
     }
+}
+
+struct IllegalRpititVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    allowed: Option<ty::AliasTy<'tcx>>,
+}
+
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IllegalRpititVisitor<'tcx> {
+    type Result = ControlFlow<MethodViolationCode>;
 
-    // FIXME(RPITIT): Perhaps we should use a visitor here?
-    ty.skip_binder().walk().find_map(|arg| {
-        if let ty::GenericArgKind::Type(ty) = arg.unpack()
-            && let ty::Alias(ty::Projection, proj) = ty.kind()
-            && tcx.is_impl_trait_in_trait(proj.def_id)
+    fn visit_ty(&mut self, ty: Ty<'tcx>) -> Self::Result {
+        if let ty::Alias(ty::Projection, proj) = *ty.kind()
+            && Some(proj) != self.allowed
+            && self.tcx.is_impl_trait_in_trait(proj.def_id)
         {
-            Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.def_id)))
+            ControlFlow::Break(MethodViolationCode::ReferencesImplTraitInTrait(
+                self.tcx.def_span(proj.def_id),
+            ))
         } else {
-            None
+            ty.super_visit_with(self)
         }
-    })
+    }
 }
 
 pub(crate) fn provide(providers: &mut Providers) {