Rollup merge of #114267 - compiler-errors:rpitit-opaque-bounds, r=spastorino

Map RPITIT's opaque type bounds back from projections to opaques

An RPITIT in a program's AST is eventually translated into both a projection GAT and an opaque. The opaque is used for default trait methods, like:

```
trait Foo {
  fn bar() -> impl Sized { 0i32 }
}
```

The item bounds for both the projection and opaque are identical, and both have a *projection* self ty. This is mostly okay, since we can normalize this projection within the default trait method body to the opaque, but it does two things:
1. it leads to bugs in places where we don't normalize item bounds, like `deduce_future_output_from_obligations`
2. it leads to extra match arms that are both suspicious looking and also easy to miss

This PR maps the opaque type bounds of the RPITIT's *opaque* back to the opaque's self type to avoid this quirk. Then we can fix the UI test for #108304 (1.) and also remove a bunch of match arms (2.).

Fixes #108304

r? `@spastorino`

compiler/rustc_hir_analysis/src/check/check.rs

@@ -576,7 +576,6 @@ fn find_and_apply_rpit_args<'tcx>(
     struct Visitor<'tcx> {
         tcx: TyCtxt<'tcx>,
         opaque: DefId,
-        function: DefId,
         seen: FxHashSet<DefId>,
     }
     impl<'tcx> ty::TypeVisitor<TyCtxt<'tcx>> for Visitor<'tcx> {
@@ -601,19 +600,6 @@ fn find_and_apply_rpit_args<'tcx>(
                         }
                     }
                 }
-                ty::Alias(ty::Projection, alias) => {
-                    if self.tcx.is_impl_trait_in_trait(alias.def_id)
-                        && self.tcx.impl_trait_in_trait_parent_fn(alias.def_id) == self.function
-                    {
-                        // If we're lowering to associated item, install the opaque type which is just
-                        // the `type_of` of the trait's associated item. If we're using the old lowering
-                        // strategy, then just reinterpret the associated type like an opaque :^)
-                        self.tcx
-                            .type_of(alias.def_id)
-                            .instantiate(self.tcx, alias.args)
-                            .visit_with(self)?;
-                    }
-                }
                 ty::Alias(ty::Weak, alias) => {
                     self.tcx
                         .type_of(alias.def_id)
@@ -627,7 +613,7 @@ fn find_and_apply_rpit_args<'tcx>(
         }
     }
     if let ControlFlow::Break(args) =
-        ret.visit_with(&mut Visitor { tcx, function, opaque, seen: Default::default() })
+        ret.visit_with(&mut Visitor { tcx, opaque, seen: Default::default() })
     {
         trace!(?args);
         trace!("expected: {hidden_ty:#?}");

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

@@ -3,7 +3,7 @@ use crate::astconv::{AstConv, PredicateFilter};
 use rustc_hir as hir;
 use rustc_infer::traits::util;
 use rustc_middle::ty::GenericArgs;
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeFolder};
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::Span;
 
@@ -113,7 +113,7 @@ pub(super) fn explicit_item_bounds(
             ..
         }) => associated_type_bounds(tcx, def_id, bounds, *span),
         hir::Node::Item(hir::Item {
-            kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }),
+            kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, in_trait: false, .. }),
             span,
             ..
         }) => {
@@ -121,6 +121,27 @@ pub(super) fn explicit_item_bounds(
             let item_ty = Ty::new_opaque(tcx, def_id.to_def_id(), args);
             opaque_type_bounds(tcx, def_id, bounds, item_ty, *span)
         }
+        // Since RPITITs are astconv'd as projections in `ast_ty_to_ty`, when we're asking
+        // for the item bounds of the *opaques* in a trait's default method signature, we
+        // need to map these projections back to opaques.
+        hir::Node::Item(hir::Item {
+            kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, in_trait: true, origin, .. }),
+            span,
+            ..
+        }) => {
+            let (hir::OpaqueTyOrigin::FnReturn(fn_def_id)
+            | hir::OpaqueTyOrigin::AsyncFn(fn_def_id)) = *origin
+            else {
+                bug!()
+            };
+            let args = GenericArgs::identity_for_item(tcx, def_id);
+            let item_ty = Ty::new_opaque(tcx, def_id.to_def_id(), args);
+            tcx.arena.alloc_slice(
+                &opaque_type_bounds(tcx, def_id, bounds, item_ty, *span)
+                    .to_vec()
+                    .fold_with(&mut AssocTyToOpaque { tcx, fn_def_id: fn_def_id.to_def_id() }),
+            )
+        }
         hir::Node::Item(hir::Item { kind: hir::ItemKind::TyAlias(..), .. }) => &[],
         _ => bug!("item_bounds called on {:?}", def_id),
     };
@@ -135,3 +156,26 @@ pub(super) fn item_bounds(
         tcx.mk_clauses_from_iter(util::elaborate(tcx, bounds.iter().map(|&(bound, _span)| bound)))
     })
 }
+
+struct AssocTyToOpaque<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    fn_def_id: DefId,
+}
+
+impl<'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTyToOpaque<'tcx> {
+    fn interner(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        if let ty::Alias(ty::Projection, projection_ty) = ty.kind()
+            && let Some(ty::ImplTraitInTraitData::Trait { fn_def_id, .. })
+                = self.tcx.opt_rpitit_info(projection_ty.def_id)
+            && fn_def_id == self.fn_def_id
+        {
+            self.tcx.type_of(projection_ty.def_id).instantiate(self.tcx, projection_ty.args)
+        } else {
+            ty
+        }
+    }
+}