Auto merge of #102334 - compiler-errors:rpitit-substs-issue, r=cjgillot

Fix subst issues with return-position `impl Trait` in trait

1. Fix an issue where we were rebase impl substs onto trait method substs, instead of trait substs
2. Fix an issue where early-bound regions aren't being mapped correctly for RPITIT hidden types

Fixes #102301
Fixes #102310
Fixes #102334
Fixes #102918

5 files changed, 117 insertions, 23 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/compare_method.rs b/compiler/rustc_hir_analysis/src/check/compare_method.rs
index 5e5dbedb4bd..60eaad9b498 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_method.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_method.rs
@@ -465,30 +465,30 @@ pub fn collect_trait_impl_trait_tys<'tcx>(
     let ocx = ObligationCtxt::new(infcx);
 
     let norm_cause = ObligationCause::misc(return_span, impl_m_hir_id);
-    let impl_return_ty = ocx.normalize(
+    let impl_sig = ocx.normalize(
         norm_cause.clone(),
         param_env,
-        infcx
-            .replace_bound_vars_with_fresh_vars(
-                return_span,
-                infer::HigherRankedType,
-                tcx.fn_sig(impl_m.def_id),
-            )
-            .output(),
+        infcx.replace_bound_vars_with_fresh_vars(
+            return_span,
+            infer::HigherRankedType,
+            tcx.fn_sig(impl_m.def_id),
+        ),
     );
+    let impl_return_ty = impl_sig.output();
 
     let mut collector = ImplTraitInTraitCollector::new(&ocx, return_span, param_env, impl_m_hir_id);
-    let unnormalized_trait_return_ty = tcx
+    let unnormalized_trait_sig = tcx
         .liberate_late_bound_regions(
             impl_m.def_id,
             tcx.bound_fn_sig(trait_m.def_id).subst(tcx, trait_to_placeholder_substs),
         )
-        .output()
         .fold_with(&mut collector);
-    let trait_return_ty =
-        ocx.normalize(norm_cause.clone(), param_env, unnormalized_trait_return_ty);
+    let trait_sig = ocx.normalize(norm_cause.clone(), param_env, unnormalized_trait_sig);
+    let trait_return_ty = trait_sig.output();
 
-    let wf_tys = FxHashSet::from_iter([unnormalized_trait_return_ty, trait_return_ty]);
+    let wf_tys = FxHashSet::from_iter(
+        unnormalized_trait_sig.inputs_and_output.iter().chain(trait_sig.inputs_and_output.iter()),
+    );
 
     match infcx.at(&cause, param_env).eq(trait_return_ty, impl_return_ty) {
         Ok(infer::InferOk { value: (), obligations }) => {
@@ -521,6 +521,26 @@ pub fn collect_trait_impl_trait_tys<'tcx>(
         }
     }
 
+    // Unify the whole function signature. We need to do this to fully infer
+    // the lifetimes of the return type, but do this after unifying just the
+    // return types, since we want to avoid duplicating errors from
+    // `compare_predicate_entailment`.
+    match infcx
+        .at(&cause, param_env)
+        .eq(tcx.mk_fn_ptr(ty::Binder::dummy(trait_sig)), tcx.mk_fn_ptr(ty::Binder::dummy(impl_sig)))
+    {
+        Ok(infer::InferOk { value: (), obligations }) => {
+            ocx.register_obligations(obligations);
+        }
+        Err(terr) => {
+            let guar = tcx.sess.delay_span_bug(
+                return_span,
+                format!("could not unify `{trait_sig}` and `{impl_sig}`: {terr:?}"),
+            );
+            return Err(guar);
+        }
+    }
+
     // Check that all obligations are satisfied by the implementation's
     // RPITs.
     let errors = ocx.select_all_or_error();
@@ -551,15 +571,40 @@ pub fn collect_trait_impl_trait_tys<'tcx>(
                 let id_substs = InternalSubsts::identity_for_item(tcx, def_id);
                 debug!(?id_substs, ?substs);
                 let map: FxHashMap<ty::GenericArg<'tcx>, ty::GenericArg<'tcx>> =
-                    substs.iter().enumerate().map(|(index, arg)| (arg, id_substs[index])).collect();
+                    std::iter::zip(substs, id_substs).collect();
                 debug!(?map);
 
+                // NOTE(compiler-errors): RPITITs, like all other RPITs, have early-bound
+                // region substs that are synthesized during AST lowering. These are substs
+                // that are appended to the parent substs (trait and trait method). However,
+                // we're trying to infer the unsubstituted type value of the RPITIT inside
+                // the *impl*, so we can later use the impl's method substs to normalize
+                // an RPITIT to a concrete type (`confirm_impl_trait_in_trait_candidate`).
+                //
+                // Due to the design of RPITITs, during AST lowering, we have no idea that
+                // an impl method corresponds to a trait method with RPITITs in it. Therefore,
+                // we don't have a list of early-bound region substs for the RPITIT in the impl.
+                // Since early region parameters are index-based, we can't just rebase these
+                // (trait method) early-bound region substs onto the impl, and there's no
+                // guarantee that the indices from the trait substs and impl substs line up.
+                // So to fix this, we subtract the number of trait substs and add the number of
+                // impl substs to *renumber* these early-bound regions to their corresponding
+                // indices in the impl's substitutions list.
+                //
+                // Also, we only need to account for a difference in trait and impl substs,
+                // since we previously enforce that the trait method and impl method have the
+                // same generics.
+                let num_trait_substs = trait_to_impl_substs.len();
+                let num_impl_substs = tcx.generics_of(impl_m.container_id(tcx)).params.len();
                 let ty = tcx.fold_regions(ty, |region, _| {
-                    if let ty::ReFree(_) = region.kind() {
-                        map[&region.into()].expect_region()
-                    } else {
-                        region
-                    }
+                    let ty::ReFree(_) = region.kind() else { return region; };
+                    let ty::ReEarlyBound(e) = map[&region.into()].expect_region().kind()
+                        else { bug!("expected ReFree to map to ReEarlyBound"); };
+                    tcx.mk_region(ty::ReEarlyBound(ty::EarlyBoundRegion {
+                        def_id: e.def_id,
+                        name: e.name,
+                        index: (e.index as usize - num_trait_substs + num_impl_substs) as u32,
+                    }))
                 });
                 debug!(%ty);
                 collected_tys.insert(def_id, ty);
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 36eb2ab5157..e552d3f1cc5 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -606,9 +606,21 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
         #[cold]
         #[inline(never)]
-        fn region_param_out_of_range(data: ty::EarlyBoundRegion) -> ! {
+        fn region_param_out_of_range(data: ty::EarlyBoundRegion, substs: &[GenericArg<'_>]) -> ! {
             bug!(
-                "Region parameter out of range when substituting in region {} (index={})",
+                "Region parameter out of range when substituting in region {} (index={}, substs = {:?})",
+                data.name,
+                data.index,
+                substs,
+            )
+        }
+
+        #[cold]
+        #[inline(never)]
+        fn region_param_invalid(data: ty::EarlyBoundRegion, other: GenericArgKind<'_>) -> ! {
+            bug!(
+                "Unexpected parameter {:?} when substituting in region {} (index={})",
+                other,
                 data.name,
                 data.index
             )
@@ -624,7 +636,8 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
                 let rk = self.substs.get(data.index as usize).map(|k| k.unpack());
                 match rk {
                     Some(GenericArgKind::Lifetime(lt)) => self.shift_region_through_binders(lt),
-                    _ => region_param_out_of_range(data),
+                    Some(other) => region_param_invalid(data, other),
+                    None => region_param_out_of_range(data, self.substs),
                 }
             }
             _ => r,
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 1ca9a1c1994..693c1728931 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -2254,7 +2254,10 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
     }
 
     let impl_fn_def_id = leaf_def.item.def_id;
-    let impl_fn_substs = obligation.predicate.substs.rebase_onto(tcx, trait_fn_def_id, data.substs);
+    // Rebase from {trait}::{fn}::{opaque} to {impl}::{fn}::{opaque},
+    // since `data.substs` are the impl substs.
+    let impl_fn_substs =
+        obligation.predicate.substs.rebase_onto(tcx, tcx.parent(trait_fn_def_id), data.substs);
 
     let cause = ObligationCause::new(
         obligation.cause.span,
diff --git a/src/test/ui/async-await/in-trait/issue-102310.rs b/src/test/ui/async-await/in-trait/issue-102310.rs
new file mode 100644
index 00000000000..49c3e9feeb4
--- /dev/null
+++ b/src/test/ui/async-await/in-trait/issue-102310.rs
@@ -0,0 +1,15 @@
+// check-pass
+// edition:2021
+
+#![feature(async_fn_in_trait)]
+#![allow(incomplete_features)]
+
+pub trait SpiDevice {
+    async fn transaction<F, R>(&mut self);
+}
+
+impl SpiDevice for () {
+    async fn transaction<F, R>(&mut self) {}
+}
+
+fn main() {}
diff --git a/src/test/ui/impl-trait/in-trait/issue-102301.rs b/src/test/ui/impl-trait/in-trait/issue-102301.rs
new file mode 100644
index 00000000000..a93714a658e
--- /dev/null
+++ b/src/test/ui/impl-trait/in-trait/issue-102301.rs
@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+trait Foo<T> {
+    fn foo<F2: Foo<T>>(self) -> impl Foo<T>;
+}
+
+struct Bar;
+
+impl Foo<u8> for Bar {
+    fn foo<F2: Foo<u8>>(self) -> impl Foo<u8> {
+        self
+    }
+}
+
+fn main() {}