Rollup merge of #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:
* https://github.com/rust-lang/rust/issues/133119

compiler/rustc_trait_selection/src/traits/project.rs

@@ -7,8 +7,8 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
 use rustc_hir::lang_items::LangItem;
-use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
+use rustc_infer::infer::{DefineOpaqueTypes, RegionVariableOrigin};
 use rustc_infer::traits::{ObligationCauseCode, PredicateObligations};
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::traits::{BuiltinImplSource, ImplSource, ImplSourceUserDefinedData};
@@ -18,6 +18,7 @@ use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{self, Term, Ty, TyCtxt, TypingMode, Upcast};
 use rustc_middle::{bug, span_bug};
 use rustc_span::symbol::sym;
+use thin_vec::thin_vec;
 use tracing::{debug, instrument};
 
 use super::{
@@ -61,6 +62,9 @@ enum ProjectionCandidate<'tcx> {
     /// Bounds specified on an object type
     Object(ty::PolyProjectionPredicate<'tcx>),
 
+    /// Built-in bound for a dyn async fn in trait
+    ObjectRpitit,
+
     /// From an "impl" (or a "pseudo-impl" returned by select)
     Select(Selection<'tcx>),
 }
@@ -827,6 +831,17 @@ fn assemble_candidates_from_object_ty<'cx, 'tcx>(
         env_predicates,
         false,
     );
+
+    // `dyn Trait` automagically project their AFITs to `dyn* Future`.
+    if tcx.is_impl_trait_in_trait(obligation.predicate.def_id)
+        && let Some(out_trait_def_id) = data.principal_def_id()
+        && let rpitit_trait_def_id = tcx.parent(obligation.predicate.def_id)
+        && tcx
+            .supertrait_def_ids(out_trait_def_id)
+            .any(|trait_def_id| trait_def_id == rpitit_trait_def_id)
+    {
+        candidate_set.push_candidate(ProjectionCandidate::ObjectRpitit);
+    }
 }
 
 #[instrument(
@@ -1247,6 +1262,8 @@ fn confirm_candidate<'cx, 'tcx>(
         ProjectionCandidate::Select(impl_source) => {
             confirm_select_candidate(selcx, obligation, impl_source)
         }
+
+        ProjectionCandidate::ObjectRpitit => confirm_object_rpitit_candidate(selcx, obligation),
     };
 
     // When checking for cycle during evaluation, we compare predicates with
@@ -2034,6 +2051,45 @@ fn confirm_impl_candidate<'cx, 'tcx>(
     }
 }
 
+fn confirm_object_rpitit_candidate<'cx, 'tcx>(
+    selcx: &mut SelectionContext<'cx, 'tcx>,
+    obligation: &ProjectionTermObligation<'tcx>,
+) -> Progress<'tcx> {
+    let tcx = selcx.tcx();
+    let mut obligations = thin_vec![];
+
+    // Compute an intersection lifetime for all the input components of this GAT.
+    let intersection =
+        selcx.infcx.next_region_var(RegionVariableOrigin::MiscVariable(obligation.cause.span));
+    for component in obligation.predicate.args {
+        match component.unpack() {
+            ty::GenericArgKind::Lifetime(lt) => {
+                obligations.push(obligation.with(tcx, ty::OutlivesPredicate(lt, intersection)));
+            }
+            ty::GenericArgKind::Type(ty) => {
+                obligations.push(obligation.with(tcx, ty::OutlivesPredicate(ty, intersection)));
+            }
+            ty::GenericArgKind::Const(_ct) => {
+                // Consts have no outlives...
+            }
+        }
+    }
+
+    Progress {
+        term: Ty::new_dynamic(
+            tcx,
+            tcx.item_bounds_to_existential_predicates(
+                obligation.predicate.def_id,
+                obligation.predicate.args,
+            ),
+            intersection,
+            ty::DynStar,
+        )
+        .into(),
+        obligations,
+    }
+}
+
 // Get obligations corresponding to the predicates from the where-clause of the
 // associated type itself.
 fn assoc_ty_own_obligations<'cx, 'tcx>(