Rollup merge of #109198 - compiler-errors:new-rpitit-default-body, r=spastorino

Install projection from RPITIT to default trait method opaque correctly

1. For new lowering strategy `-Zlower-impl-trait-in-trait-to-assoc-ty`, install the correct default trait method projection predicates (RPITIT -> opaque). This makes default trait body tests pass!

2. Fix two WF-checking bugs -- first, we want to make sure that we're always looking for an opaque type in `check_return_position_impl_trait_in_trait_bounds`. That's because the RPITIT projections are normalized to opaques during wfcheck. Second, fix RPITIT's param-envs by not adding the projection predicates that we install on trait methods to make default RPITITs work -- I left a comment why.

3. Also, just a small drive-by for `rustc_on_unimplemented`. Not sure if it affects any tests, but can't hurt.

r? ````@spastorino,```` based off of #109140

compiler/rustc_hir_analysis/src/check/check.rs

@@ -557,7 +557,7 @@ fn check_item_type(tcx: TyCtxt<'_>, id: hir::ItemId) {
             check_opaque(tcx, id);
         }
         DefKind::ImplTraitPlaceholder => {
-            let parent = tcx.impl_trait_in_trait_parent(id.owner_id.to_def_id());
+            let parent = tcx.impl_trait_in_trait_parent_fn(id.owner_id.to_def_id());
             // Only check the validity of this opaque type if the function has a default body
             if let hir::Node::TraitItem(hir::TraitItem {
                 kind: hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(_)),

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1,7 +1,6 @@
 use crate::autoderef::Autoderef;
 use crate::constrained_generic_params::{identify_constrained_generic_params, Parameter};
 
-use hir::def::DefKind;
 use rustc_ast as ast;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
@@ -1548,16 +1547,27 @@ fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
     if let Some(assoc_item) = tcx.opt_associated_item(fn_def_id.to_def_id())
         && assoc_item.container == ty::AssocItemContainer::TraitContainer
     {
+        // FIXME(-Zlower-impl-trait-in-trait-to-assoc-ty): Even with the new lowering
+        // strategy, we can't just call `check_associated_item` on the new RPITITs,
+        // because tests like `tests/ui/async-await/in-trait/implied-bounds.rs` will fail.
+        // That's because we need to check that the bounds of the RPITIT hold using
+        // the special substs that we create during opaque type lowering, otherwise we're
+        // getting a bunch of early bound and free regions mixed up... Haven't looked too
+        // deep into this, though.
         for arg in fn_output.walk() {
             if let ty::GenericArgKind::Type(ty) = arg.unpack()
-                && let ty::Alias(ty::Opaque, proj) = ty.kind()
-                && tcx.def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder
-                && tcx.impl_trait_in_trait_parent(proj.def_id) == fn_def_id.to_def_id()
+                // RPITITs are always eagerly normalized into opaques, so always look for an
+                // opaque here.
+                && let ty::Alias(ty::Opaque, opaque_ty) = ty.kind()
+                && let Some(opaque_def_id) = opaque_ty.def_id.as_local()
+                && let opaque = tcx.hir().expect_item(opaque_def_id).expect_opaque_ty()
+                && let hir::OpaqueTyOrigin::FnReturn(source) | hir::OpaqueTyOrigin::AsyncFn(source) = opaque.origin
+                && source == fn_def_id
             {
-                let span = tcx.def_span(proj.def_id);
-                let bounds = wfcx.tcx().explicit_item_bounds(proj.def_id);
+                let span = tcx.def_span(opaque_ty.def_id);
+                let bounds = wfcx.tcx().explicit_item_bounds(opaque_ty.def_id);
                 let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
-                    let bound = ty::EarlyBinder(bound).subst(tcx, proj.substs);
+                    let bound = ty::EarlyBinder(bound).subst(tcx, opaque_ty.substs);
                     let normalized_bound = wfcx.normalize(span, None, bound);
                     traits::wf::predicate_obligations(
                         wfcx.infcx,

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -3,7 +3,7 @@ use crate::astconv::AstConv;
 use rustc_hir as hir;
 use rustc_infer::traits::util;
 use rustc_middle::ty::subst::InternalSubsts;
-use rustc_middle::ty::{self, ImplTraitInTraitData, Ty, TyCtxt};
+use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::def_id::DefId;
 use rustc_span::Span;
 
@@ -76,18 +76,26 @@ pub(super) fn explicit_item_bounds(
     tcx: TyCtxt<'_>,
     def_id: DefId,
 ) -> &'_ [(ty::Predicate<'_>, Span)] {
-    // If the def_id is about an RPITIT, delegate explicit_item_bounds to the opaque_def_id that
-    // generated the synthesized associate type.
-    let rpitit_info = if let Some(ImplTraitInTraitData::Trait { opaque_def_id, .. }) =
-        tcx.opt_rpitit_info(def_id)
-    {
-        Some(opaque_def_id)
-    } else {
-        None
-    };
+    match tcx.opt_rpitit_info(def_id) {
+        // RPITIT's bounds are the same as opaque type bounds, but with
+        // a projection self type.
+        Some(ty::ImplTraitInTraitData::Trait { opaque_def_id, .. }) => {
+            let item = tcx.hir().get_by_def_id(opaque_def_id.expect_local()).expect_item();
+            let opaque_ty = item.expect_opaque_ty();
+            return opaque_type_bounds(
+                tcx,
+                opaque_def_id,
+                opaque_ty.bounds,
+                tcx.mk_projection(def_id, ty::InternalSubsts::identity_for_item(tcx, def_id)),
+                item.span,
+            );
+        }
+        // These should have been fed!
+        Some(ty::ImplTraitInTraitData::Impl { .. }) => unreachable!(),
+        None => {}
+    }
 
-    let bounds_def_id = rpitit_info.unwrap_or(def_id);
-    let hir_id = tcx.hir().local_def_id_to_hir_id(bounds_def_id.expect_local());
+    let hir_id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
     match tcx.hir().get(hir_id) {
         hir::Node::TraitItem(hir::TraitItem {
             kind: hir::TraitItemKind::Type(bounds, _),
@@ -100,12 +108,12 @@ pub(super) fn explicit_item_bounds(
             ..
         }) => {
             let substs = InternalSubsts::identity_for_item(tcx, def_id);
-            let item_ty = if *in_trait || rpitit_info.is_some() {
+            let item_ty = if *in_trait && !tcx.lower_impl_trait_in_trait_to_assoc_ty() {
                 tcx.mk_projection(def_id, substs)
             } else {
                 tcx.mk_opaque(def_id, substs)
             };
-            opaque_type_bounds(tcx, bounds_def_id, bounds, item_ty, *span)
+            opaque_type_bounds(tcx, def_id, bounds, item_ty, *span)
         }
         _ => bug!("item_bounds called on {:?}", def_id),
     }

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -112,10 +112,14 @@ fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Varianc
         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
             match t.kind() {
                 ty::Alias(_, ty::AliasTy { def_id, substs, .. })
-                    if matches!(
-                        self.tcx.def_kind(*def_id),
-                        DefKind::OpaqueTy | DefKind::ImplTraitPlaceholder
-                    ) =>
+                    if matches!(self.tcx.def_kind(*def_id), DefKind::OpaqueTy) =>
+                {
+                    self.visit_opaque(*def_id, substs)
+                }
+                // FIXME(-Zlower-impl-trait-in-trait-to-assoc-ty) check whether this is necessary
+                // at all for RPITITs.
+                ty::Alias(_, ty::AliasTy { def_id, substs, .. })
+                    if self.tcx.is_impl_trait_in_trait(*def_id) =>
                 {
                     self.visit_opaque(*def_id, substs)
                 }