diff --git a/Cargo.lock b/Cargo.lock
index 3d0e294217912..684dfbca6f153 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3199,14 +3199,12 @@ dependencies = [
 
 [[package]]
 name = "rustc-rayon-core"
-version = "0.5.0"
+version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "67668daaf00e359c126f6dcb40d652d89b458a008c8afa727a42a2d20fca0b7f"
+checksum = "2f42932dcd3bcbe484b38a3ccf79b7906fac41c02d408b5b1bac26da3416efdb"
 dependencies = [
- "crossbeam-channel",
  "crossbeam-deque",
  "crossbeam-utils",
- "num_cpus",
 ]
 
 [[package]]
diff --git a/compiler/rustc_hir_typeck/src/expr_use_visitor.rs b/compiler/rustc_hir_typeck/src/expr_use_visitor.rs
index 27df8f0e98a13..0511f4e25ad3d 100644
--- a/compiler/rustc_hir_typeck/src/expr_use_visitor.rs
+++ b/compiler/rustc_hir_typeck/src/expr_use_visitor.rs
@@ -1502,16 +1502,21 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
         let mut projections = base_place.place.projections;
 
         let node_ty = self.cx.typeck_results().node_type(node);
-        // Opaque types can't have field projections, but we can instead convert
-        // the current place in-place (heh) to the hidden type, and then apply all
-        // follow up projections on that.
-        if node_ty != place_ty
-            && self
-                .cx
-                .try_structurally_resolve_type(self.cx.tcx().hir_span(base_place.hir_id), place_ty)
-                .is_impl_trait()
-        {
-            projections.push(Projection { kind: ProjectionKind::OpaqueCast, ty: node_ty });
+        if !self.cx.tcx().next_trait_solver_globally() {
+            // Opaque types can't have field projections, but we can instead convert
+            // the current place in-place (heh) to the hidden type, and then apply all
+            // follow up projections on that.
+            if node_ty != place_ty
+                && self
+                    .cx
+                    .try_structurally_resolve_type(
+                        self.cx.tcx().hir_span(base_place.hir_id),
+                        place_ty,
+                    )
+                    .is_impl_trait()
+            {
+                projections.push(Projection { kind: ProjectionKind::OpaqueCast, ty: node_ty });
+            }
         }
         projections.push(Projection { kind, ty });
         PlaceWithHirId::new(node, base_place.place.base_ty, base_place.place.base, projections)
diff --git a/compiler/rustc_middle/src/hir/place.rs b/compiler/rustc_middle/src/hir/place.rs
index 60ce8544aa0ed..c3d10615cf10c 100644
--- a/compiler/rustc_middle/src/hir/place.rs
+++ b/compiler/rustc_middle/src/hir/place.rs
@@ -40,6 +40,8 @@ pub enum ProjectionKind {
 
     /// A conversion from an opaque type to its hidden type so we can
     /// do further projections on it.
+    ///
+    /// This is unused if `-Znext-solver` is enabled.
     OpaqueCast,
 }
 
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index c7561f8afef9a..304b3caa6e19f 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -1242,6 +1242,8 @@ pub enum ProjectionElem<V, T> {
 
     /// Like an explicit cast from an opaque type to a concrete type, but without
     /// requiring an intermediate variable.
+    ///
+    /// This is unused with `-Znext-solver`.
     OpaqueCast(T),
 
     /// A transmute from an unsafe binder to the type that it wraps. This is a projection
diff --git a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
index 9670c1716f585..d66b38c5b005c 100644
--- a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
@@ -101,18 +101,21 @@ impl<'tcx> MatchPairTree<'tcx> {
             place_builder = resolved;
         }
 
-        // Only add the OpaqueCast projection if the given place is an opaque type and the
-        // expected type from the pattern is not.
-        let may_need_cast = match place_builder.base() {
-            PlaceBase::Local(local) => {
-                let ty =
-                    Place::ty_from(local, place_builder.projection(), &cx.local_decls, cx.tcx).ty;
-                ty != pattern.ty && ty.has_opaque_types()
+        if !cx.tcx.next_trait_solver_globally() {
+            // Only add the OpaqueCast projection if the given place is an opaque type and the
+            // expected type from the pattern is not.
+            let may_need_cast = match place_builder.base() {
+                PlaceBase::Local(local) => {
+                    let ty =
+                        Place::ty_from(local, place_builder.projection(), &cx.local_decls, cx.tcx)
+                            .ty;
+                    ty != pattern.ty && ty.has_opaque_types()
+                }
+                _ => true,
+            };
+            if may_need_cast {
+                place_builder = place_builder.project(ProjectionElem::OpaqueCast(pattern.ty));
             }
-            _ => true,
-        };
-        if may_need_cast {
-            place_builder = place_builder.project(ProjectionElem::OpaqueCast(pattern.ty));
         }
 
         let place = place_builder.try_to_place(cx);
diff --git a/compiler/rustc_mir_transform/src/post_analysis_normalize.rs b/compiler/rustc_mir_transform/src/post_analysis_normalize.rs
index 76c2f082c0bfc..5599dee4ccad3 100644
--- a/compiler/rustc_mir_transform/src/post_analysis_normalize.rs
+++ b/compiler/rustc_mir_transform/src/post_analysis_normalize.rs
@@ -39,20 +39,22 @@ impl<'tcx> MutVisitor<'tcx> for PostAnalysisNormalizeVisitor<'tcx> {
         _context: PlaceContext,
         _location: Location,
     ) {
-        // Performance optimization: don't reintern if there is no `OpaqueCast` to remove.
-        if place.projection.iter().all(|elem| !matches!(elem, ProjectionElem::OpaqueCast(_))) {
-            return;
+        if !self.tcx.next_trait_solver_globally() {
+            // `OpaqueCast` projections are only needed if there are opaque types on which projections
+            // are performed. After the `PostAnalysisNormalize` pass, all opaque types are replaced with their
+            // hidden types, so we don't need these projections anymore.
+            //
+            // Performance optimization: don't reintern if there is no `OpaqueCast` to remove.
+            if place.projection.iter().any(|elem| matches!(elem, ProjectionElem::OpaqueCast(_))) {
+                place.projection = self.tcx.mk_place_elems(
+                    &place
+                        .projection
+                        .into_iter()
+                        .filter(|elem| !matches!(elem, ProjectionElem::OpaqueCast(_)))
+                        .collect::<Vec<_>>(),
+                );
+            };
         }
-        // `OpaqueCast` projections are only needed if there are opaque types on which projections
-        // are performed. After the `PostAnalysisNormalize` pass, all opaque types are replaced with their
-        // hidden types, so we don't need these projections anymore.
-        place.projection = self.tcx.mk_place_elems(
-            &place
-                .projection
-                .into_iter()
-                .filter(|elem| !matches!(elem, ProjectionElem::OpaqueCast(_)))
-                .collect::<Vec<_>>(),
-        );
         self.super_place(place, _context, _location);
     }
 
diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
index ee000b1174866..83b2465d05aa7 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -92,16 +92,20 @@ where
             let ty::Dynamic(bounds, _, _) = goal.predicate.self_ty().kind() else {
                 panic!("expected object type in `probe_and_consider_object_bound_candidate`");
             };
-            ecx.add_goals(
-                GoalSource::ImplWhereBound,
-                structural_traits::predicates_for_object_candidate(
-                    ecx,
-                    goal.param_env,
-                    goal.predicate.trait_ref(cx),
-                    bounds,
-                ),
-            );
-            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            match structural_traits::predicates_for_object_candidate(
+                ecx,
+                goal.param_env,
+                goal.predicate.trait_ref(cx),
+                bounds,
+            ) {
+                Ok(requirements) => {
+                    ecx.add_goals(GoalSource::ImplWhereBound, requirements);
+                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                }
+                Err(_) => {
+                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+                }
+            }
         })
     }
 
diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
index c2fb592c3f3aa..1526049719ea0 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
@@ -5,9 +5,10 @@ use derive_where::derive_where;
 use rustc_type_ir::data_structures::HashMap;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
+use rustc_type_ir::solve::inspect::ProbeKind;
 use rustc_type_ir::{
-    self as ty, Interner, Movability, Mutability, TypeFoldable, TypeFolder, TypeSuperFoldable,
-    Upcast as _, elaborate,
+    self as ty, FallibleTypeFolder, Interner, Movability, Mutability, TypeFoldable,
+    TypeSuperFoldable, Upcast as _, elaborate,
 };
 use rustc_type_ir_macros::{TypeFoldable_Generic, TypeVisitable_Generic};
 use tracing::instrument;
@@ -822,22 +823,16 @@ pub(in crate::solve) fn const_conditions_for_destruct<I: Interner>(
 /// impl Baz for dyn Foo<Item = Ty> {}
 /// ```
 ///
-/// However, in order to make such impls well-formed, we need to do an
+/// However, in order to make such impls non-cyclical, we need to do an
 /// additional step of eagerly folding the associated types in the where
 /// clauses of the impl. In this example, that means replacing
 /// `<Self as Foo>::Bar` with `Ty` in the first impl.
-///
-// FIXME: This is only necessary as `<Self as Trait>::Assoc: ItemBound`
-// bounds in impls are trivially proven using the item bound candidates.
-// This is unsound in general and once that is fixed, we don't need to
-// normalize eagerly here. See https://github.com/lcnr/solver-woes/issues/9
-// for more details.
 pub(in crate::solve) fn predicates_for_object_candidate<D, I>(
-    ecx: &EvalCtxt<'_, D>,
+    ecx: &mut EvalCtxt<'_, D>,
     param_env: I::ParamEnv,
     trait_ref: ty::TraitRef<I>,
     object_bounds: I::BoundExistentialPredicates,
-) -> Vec<Goal<I, I::Predicate>>
+) -> Result<Vec<Goal<I, I::Predicate>>, Ambiguous>
 where
     D: SolverDelegate<Interner = I>,
     I: Interner,
@@ -871,72 +866,130 @@ where
             .extend(cx.item_bounds(associated_type_def_id).iter_instantiated(cx, trait_ref.args));
     }
 
-    let mut replace_projection_with = HashMap::default();
+    let mut replace_projection_with: HashMap<_, Vec<_>> = HashMap::default();
     for bound in object_bounds.iter() {
         if let ty::ExistentialPredicate::Projection(proj) = bound.skip_binder() {
+            // FIXME: We *probably* should replace this with a dummy placeholder,
+            // b/c don't want to replace literal instances of this dyn type that
+            // show up in the bounds, but just ones that come from substituting
+            // `Self` with the dyn type.
             let proj = proj.with_self_ty(cx, trait_ref.self_ty());
-            let old_ty = replace_projection_with.insert(proj.def_id(), bound.rebind(proj));
-            assert_eq!(
-                old_ty,
-                None,
-                "{:?} has two generic parameters: {:?} and {:?}",
-                proj.projection_term,
-                proj.term,
-                old_ty.unwrap()
-            );
+            replace_projection_with.entry(proj.def_id()).or_default().push(bound.rebind(proj));
         }
     }
 
-    let mut folder =
-        ReplaceProjectionWith { ecx, param_env, mapping: replace_projection_with, nested: vec![] };
-    let folded_requirements = requirements.fold_with(&mut folder);
+    let mut folder = ReplaceProjectionWith {
+        ecx,
+        param_env,
+        self_ty: trait_ref.self_ty(),
+        mapping: &replace_projection_with,
+        nested: vec![],
+    };
 
-    folder
+    let requirements = requirements.try_fold_with(&mut folder)?;
+    Ok(folder
         .nested
         .into_iter()
-        .chain(folded_requirements.into_iter().map(|clause| Goal::new(cx, param_env, clause)))
-        .collect()
+        .chain(requirements.into_iter().map(|clause| Goal::new(cx, param_env, clause)))
+        .collect())
 }
 
-struct ReplaceProjectionWith<'a, D: SolverDelegate<Interner = I>, I: Interner> {
-    ecx: &'a EvalCtxt<'a, D>,
+struct ReplaceProjectionWith<'a, 'b, I: Interner, D: SolverDelegate<Interner = I>> {
+    ecx: &'a mut EvalCtxt<'b, D>,
     param_env: I::ParamEnv,
-    mapping: HashMap<I::DefId, ty::Binder<I, ty::ProjectionPredicate<I>>>,
+    self_ty: I::Ty,
+    mapping: &'a HashMap<I::DefId, Vec<ty::Binder<I, ty::ProjectionPredicate<I>>>>,
     nested: Vec<Goal<I, I::Predicate>>,
 }
 
-impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I>
-    for ReplaceProjectionWith<'_, D, I>
+impl<D, I> ReplaceProjectionWith<'_, '_, I, D>
+where
+    D: SolverDelegate<Interner = I>,
+    I: Interner,
 {
+    fn projection_may_match(
+        &mut self,
+        source_projection: ty::Binder<I, ty::ProjectionPredicate<I>>,
+        target_projection: ty::AliasTerm<I>,
+    ) -> bool {
+        source_projection.item_def_id() == target_projection.def_id
+            && self
+                .ecx
+                .probe(|_| ProbeKind::ProjectionCompatibility)
+                .enter(|ecx| -> Result<_, NoSolution> {
+                    let source_projection = ecx.instantiate_binder_with_infer(source_projection);
+                    ecx.eq(self.param_env, source_projection.projection_term, target_projection)?;
+                    ecx.try_evaluate_added_goals()
+                })
+                .is_ok()
+    }
+
+    /// Try to replace an alias with the term present in the projection bounds of the self type.
+    /// Returns `Ok<None>` if this alias is not eligible to be replaced, or bail with
+    /// `Err(Ambiguous)` if it's uncertain which projection bound to replace the term with due
+    /// to multiple bounds applying.
+    fn try_eagerly_replace_alias(
+        &mut self,
+        alias_term: ty::AliasTerm<I>,
+    ) -> Result<Option<I::Term>, Ambiguous> {
+        if alias_term.self_ty() != self.self_ty {
+            return Ok(None);
+        }
+
+        let Some(replacements) = self.mapping.get(&alias_term.def_id) else {
+            return Ok(None);
+        };
+
+        // This is quite similar to the `projection_may_match` we use in unsizing,
+        // but here we want to unify a projection predicate against an alias term
+        // so we can replace it with the the projection predicate's term.
+        let mut matching_projections = replacements
+            .iter()
+            .filter(|source_projection| self.projection_may_match(**source_projection, alias_term));
+        let Some(replacement) = matching_projections.next() else {
+            // This shouldn't happen.
+            panic!("could not replace {alias_term:?} with term from from {:?}", self.self_ty);
+        };
+        // FIXME: This *may* have issues with duplicated projections.
+        if matching_projections.next().is_some() {
+            // If there's more than one projection that we can unify here, then we
+            // need to stall until inference constrains things so that there's only
+            // one choice.
+            return Err(Ambiguous);
+        }
+
+        let replacement = self.ecx.instantiate_binder_with_infer(*replacement);
+        self.nested.extend(
+            self.ecx
+                .eq_and_get_goals(self.param_env, alias_term, replacement.projection_term)
+                .expect("expected to be able to unify goal projection with dyn's projection"),
+        );
+
+        Ok(Some(replacement.term))
+    }
+}
+
+/// Marker for bailing with ambiguity.
+pub(crate) struct Ambiguous;
+
+impl<D, I> FallibleTypeFolder<I> for ReplaceProjectionWith<'_, '_, I, D>
+where
+    D: SolverDelegate<Interner = I>,
+    I: Interner,
+{
+    type Error = Ambiguous;
+
     fn cx(&self) -> I {
         self.ecx.cx()
     }
 
-    fn fold_ty(&mut self, ty: I::Ty) -> I::Ty {
+    fn try_fold_ty(&mut self, ty: I::Ty) -> Result<I::Ty, Ambiguous> {
         if let ty::Alias(ty::Projection, alias_ty) = ty.kind() {
-            if let Some(replacement) = self.mapping.get(&alias_ty.def_id) {
-                // We may have a case where our object type's projection bound is higher-ranked,
-                // but the where clauses we instantiated are not. We can solve this by instantiating
-                // the binder at the usage site.
-                let proj = self.ecx.instantiate_binder_with_infer(*replacement);
-                // FIXME: Technically this equate could be fallible...
-                self.nested.extend(
-                    self.ecx
-                        .eq_and_get_goals(
-                            self.param_env,
-                            alias_ty,
-                            proj.projection_term.expect_ty(self.ecx.cx()),
-                        )
-                        .expect(
-                            "expected to be able to unify goal projection with dyn's projection",
-                        ),
-                );
-                proj.term.expect_ty()
-            } else {
-                ty.super_fold_with(self)
+            if let Some(term) = self.try_eagerly_replace_alias(alias_ty.into())? {
+                return Ok(term.expect_ty());
             }
-        } else {
-            ty.super_fold_with(self)
         }
+
+        ty.try_super_fold_with(self)
     }
 }
diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
index 9262da2906d08..409af8568d7a3 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -944,7 +944,7 @@ where
              target_projection: ty::Binder<I, ty::ExistentialProjection<I>>| {
                 source_projection.item_def_id() == target_projection.item_def_id()
                     && ecx
-                        .probe(|_| ProbeKind::UpcastProjectionCompatibility)
+                        .probe(|_| ProbeKind::ProjectionCompatibility)
                         .enter(|ecx| -> Result<_, NoSolution> {
                             ecx.enter_forall(target_projection, |ecx, target_projection| {
                                 let source_projection =
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 202378560eed7..ff7ea5bd71896 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -2065,7 +2065,8 @@ fn collect_print_requests(
             check_print_request_stability(early_dcx, unstable_opts, (print_name, *print_kind));
             *print_kind
         } else {
-            emit_unknown_print_request_help(early_dcx, req)
+            let is_nightly = nightly_options::match_is_nightly_build(matches);
+            emit_unknown_print_request_help(early_dcx, req, is_nightly)
         };
 
         let out = out.unwrap_or(OutFileName::Stdout);
@@ -2089,25 +2090,37 @@ fn check_print_request_stability(
     unstable_opts: &UnstableOptions,
     (print_name, print_kind): (&str, PrintKind),
 ) {
+    if !is_print_request_stable(print_kind) && !unstable_opts.unstable_options {
+        early_dcx.early_fatal(format!(
+            "the `-Z unstable-options` flag must also be passed to enable the `{print_name}` \
+                print option"
+        ));
+    }
+}
+
+fn is_print_request_stable(print_kind: PrintKind) -> bool {
     match print_kind {
         PrintKind::AllTargetSpecsJson
         | PrintKind::CheckCfg
         | PrintKind::CrateRootLintLevels
         | PrintKind::SupportedCrateTypes
-        | PrintKind::TargetSpecJson
-            if !unstable_opts.unstable_options =>
-        {
-            early_dcx.early_fatal(format!(
-                "the `-Z unstable-options` flag must also be passed to enable the `{print_name}` \
-                print option"
-            ));
-        }
-        _ => {}
+        | PrintKind::TargetSpecJson => false,
+        _ => true,
     }
 }
 
-fn emit_unknown_print_request_help(early_dcx: &EarlyDiagCtxt, req: &str) -> ! {
-    let prints = PRINT_KINDS.iter().map(|(name, _)| format!("`{name}`")).collect::<Vec<_>>();
+fn emit_unknown_print_request_help(early_dcx: &EarlyDiagCtxt, req: &str, is_nightly: bool) -> ! {
+    let prints = PRINT_KINDS
+        .iter()
+        .filter_map(|(name, kind)| {
+            // If we're not on nightly, we don't want to print unstable options
+            if !is_nightly && !is_print_request_stable(*kind) {
+                None
+            } else {
+                Some(format!("`{name}`"))
+            }
+        })
+        .collect::<Vec<_>>();
     let prints = prints.join(", ");
 
     let mut diag = early_dcx.early_struct_fatal(format!("unknown print request: `{req}`"));
diff --git a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
index 48a05ad29fbd9..24b87000e32bc 100644
--- a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
+++ b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
@@ -292,7 +292,7 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
                 inspect::ProbeStep::NestedProbe(ref probe) => {
                     match probe.kind {
                         // These never assemble candidates for the goal we're trying to solve.
-                        inspect::ProbeKind::UpcastProjectionCompatibility
+                        inspect::ProbeKind::ProjectionCompatibility
                         | inspect::ProbeKind::ShadowedEnvProbing => continue,
 
                         inspect::ProbeKind::NormalizedSelfTyAssembly
@@ -314,8 +314,10 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         }
 
         match probe.kind {
-            inspect::ProbeKind::UpcastProjectionCompatibility
-            | inspect::ProbeKind::ShadowedEnvProbing => bug!(),
+            inspect::ProbeKind::ProjectionCompatibility
+            | inspect::ProbeKind::ShadowedEnvProbing => {
+                bug!()
+            }
 
             inspect::ProbeKind::NormalizedSelfTyAssembly | inspect::ProbeKind::UnsizeAssembly => {}
 
diff --git a/compiler/rustc_trait_selection/src/solve/select.rs b/compiler/rustc_trait_selection/src/solve/select.rs
index 4437fc5b0295f..4fdaf740287ba 100644
--- a/compiler/rustc_trait_selection/src/solve/select.rs
+++ b/compiler/rustc_trait_selection/src/solve/select.rs
@@ -177,7 +177,7 @@ fn to_selection<'tcx>(
         },
         ProbeKind::NormalizedSelfTyAssembly
         | ProbeKind::UnsizeAssembly
-        | ProbeKind::UpcastProjectionCompatibility
+        | ProbeKind::ProjectionCompatibility
         | ProbeKind::OpaqueTypeStorageLookup { result: _ }
         | ProbeKind::Root { result: _ }
         | ProbeKind::ShadowedEnvProbing
diff --git a/compiler/rustc_type_ir/src/solve/inspect.rs b/compiler/rustc_type_ir/src/solve/inspect.rs
index 18fb71dd290e7..b10641b287d4a 100644
--- a/compiler/rustc_type_ir/src/solve/inspect.rs
+++ b/compiler/rustc_type_ir/src/solve/inspect.rs
@@ -118,10 +118,12 @@ pub enum ProbeKind<I: Interner> {
     /// Used in the probe that wraps normalizing the non-self type for the unsize
     /// trait, which is also structurally matched on.
     UnsizeAssembly,
-    /// During upcasting from some source object to target object type, used to
-    /// do a probe to find out what projection type(s) may be used to prove that
-    /// the source type upholds all of the target type's object bounds.
-    UpcastProjectionCompatibility,
+    /// Used to do a probe to find out what projection type(s) match a given
+    /// alias bound or projection predicate. For trait upcasting, this is used
+    /// to prove that the source type upholds all of the target type's object
+    /// bounds. For object type bounds, this is used when eagerly replacing
+    /// supertrait aliases.
+    ProjectionCompatibility,
     /// Looking for param-env candidates that satisfy the trait ref for a projection.
     ShadowedEnvProbing,
     /// Try to unify an opaque type with an existing key in the storage.
diff --git a/src/bootstrap/download-ci-llvm-stamp b/src/bootstrap/download-ci-llvm-stamp
index e157ff233bbf7..b70d452b427c8 100644
--- a/src/bootstrap/download-ci-llvm-stamp
+++ b/src/bootstrap/download-ci-llvm-stamp
@@ -1,4 +1,4 @@
 Change this file to make users of the `download-ci-llvm` configuration download
 a new version of LLVM from CI, even if the LLVM submodule hasn’t changed.
 
-Last change is for: https://github.com/rust-lang/rust/pull/138784
+Last change is for: https://github.com/rust-lang/rust/pull/139931
diff --git a/src/bootstrap/src/core/build_steps/llvm.rs b/src/bootstrap/src/core/build_steps/llvm.rs
index 69a8bd59f16ca..6e84b83d17dcc 100644
--- a/src/bootstrap/src/core/build_steps/llvm.rs
+++ b/src/bootstrap/src/core/build_steps/llvm.rs
@@ -370,8 +370,8 @@ impl Step for Llvm {
             cfg.define("LLVM_PROFDATA_FILE", path);
         }
 
-        // Libraries for ELF section compression.
-        if !target.is_windows() {
+        // Libraries for ELF section compression and profraw files merging.
+        if !target.is_msvc() {
             cfg.define("LLVM_ENABLE_ZLIB", "ON");
         } else {
             cfg.define("LLVM_ENABLE_ZLIB", "OFF");
diff --git a/src/build_helper/src/fs/mod.rs b/src/build_helper/src/fs/mod.rs
index 02029846fd147..123df76e6a2e9 100644
--- a/src/build_helper/src/fs/mod.rs
+++ b/src/build_helper/src/fs/mod.rs
@@ -22,21 +22,27 @@ where
 /// A wrapper around [`std::fs::remove_dir_all`] that can also be used on *non-directory entries*,
 /// including files and symbolic links.
 ///
-/// - This will produce an error if the target path is not found.
+/// - This will not produce an error if the target path is not found.
 /// - Like [`std::fs::remove_dir_all`], this helper does not traverse symbolic links, will remove
 ///   symbolic link itself.
 /// - This helper is **not** robust against races on the underlying filesystem, behavior is
 ///   unspecified if this helper is called concurrently.
 /// - This helper is not robust against TOCTOU problems.
 ///
-/// FIXME: this implementation is insufficiently robust to replace bootstrap's clean `rm_rf`
-/// implementation:
-///
-/// - This implementation currently does not perform retries.
+/// FIXME: Audit whether this implementation is robust enough to replace bootstrap's clean `rm_rf`.
 #[track_caller]
 pub fn recursive_remove<P: AsRef<Path>>(path: P) -> io::Result<()> {
     let path = path.as_ref();
-    let metadata = fs::symlink_metadata(path)?;
+
+    // If the path doesn't exist, we treat it as a successful no-op.
+    // From the caller's perspective, the goal is simply "ensure this file/dir is gone" —
+    // if it's already not there, that's a success, not an error.
+    let metadata = match fs::symlink_metadata(path) {
+        Ok(m) => m,
+        Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(()),
+        Err(e) => return Err(e),
+    };
+
     #[cfg(windows)]
     let is_dir_like = |meta: &fs::Metadata| {
         use std::os::windows::fs::FileTypeExt;
@@ -45,11 +51,35 @@ pub fn recursive_remove<P: AsRef<Path>>(path: P) -> io::Result<()> {
     #[cfg(not(windows))]
     let is_dir_like = fs::Metadata::is_dir;
 
-    if is_dir_like(&metadata) {
-        fs::remove_dir_all(path)
-    } else {
-        try_remove_op_set_perms(fs::remove_file, path, metadata)
+    const MAX_RETRIES: usize = 5;
+    const RETRY_DELAY_MS: u64 = 100;
+
+    let try_remove = || {
+        if is_dir_like(&metadata) {
+            fs::remove_dir_all(path)
+        } else {
+            try_remove_op_set_perms(fs::remove_file, path, metadata.clone())
+        }
+    };
+
+    // Retry deletion a few times to handle transient filesystem errors.
+    // This is unusual for local file operations, but it's a mitigation
+    // against unlikely events where malware scanners may be holding a
+    // file beyond our control, to give the malware scanners some opportunity
+    // to release their hold.
+    for attempt in 0..MAX_RETRIES {
+        match try_remove() {
+            Ok(()) => return Ok(()),
+            Err(e) if e.kind() == io::ErrorKind::NotFound => return Ok(()),
+            Err(_) if attempt < MAX_RETRIES - 1 => {
+                std::thread::sleep(std::time::Duration::from_millis(RETRY_DELAY_MS));
+                continue;
+            }
+            Err(e) => return Err(e),
+        }
     }
+
+    Ok(())
 }
 
 fn try_remove_op_set_perms<'p, Op>(mut op: Op, path: &'p Path, metadata: Metadata) -> io::Result<()>
@@ -67,3 +97,9 @@ where
         Err(e) => Err(e),
     }
 }
+
+pub fn remove_and_create_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
+    let path = path.as_ref();
+    recursive_remove(path)?;
+    fs::create_dir_all(path)
+}
diff --git a/src/build_helper/src/fs/tests.rs b/src/build_helper/src/fs/tests.rs
index 1e694393127cb..7ce1d8928d1cb 100644
--- a/src/build_helper/src/fs/tests.rs
+++ b/src/build_helper/src/fs/tests.rs
@@ -14,7 +14,7 @@ mod recursive_remove_tests {
         let tmpdir = env::temp_dir();
         let path = tmpdir.join("__INTERNAL_BOOTSTRAP_nonexistent_path");
         assert!(fs::symlink_metadata(&path).is_err_and(|e| e.kind() == io::ErrorKind::NotFound));
-        assert!(recursive_remove(&path).is_err_and(|e| e.kind() == io::ErrorKind::NotFound));
+        assert!(recursive_remove(&path).is_ok());
     }
 
     #[test]
diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index 2f6870e3c368f..55a116a018a8a 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -67,9 +67,13 @@ pub(crate) fn try_inline(
             record_extern_fqn(cx, did, ItemType::Trait);
             cx.with_param_env(did, |cx| {
                 build_impls(cx, did, attrs_without_docs, &mut ret);
-                clean::TraitItem(Box::new(build_external_trait(cx, did)))
+                clean::TraitItem(Box::new(build_trait(cx, did)))
             })
         }
+        Res::Def(DefKind::TraitAlias, did) => {
+            record_extern_fqn(cx, did, ItemType::TraitAlias);
+            cx.with_param_env(did, |cx| clean::TraitAliasItem(build_trait_alias(cx, did)))
+        }
         Res::Def(DefKind::Fn, did) => {
             record_extern_fqn(cx, did, ItemType::Function);
             cx.with_param_env(did, |cx| {
@@ -251,7 +255,7 @@ pub(crate) fn record_extern_fqn(cx: &mut DocContext<'_>, did: DefId, kind: ItemT
     }
 }
 
-pub(crate) fn build_external_trait(cx: &mut DocContext<'_>, did: DefId) -> clean::Trait {
+pub(crate) fn build_trait(cx: &mut DocContext<'_>, did: DefId) -> clean::Trait {
     let trait_items = cx
         .tcx
         .associated_items(did)
@@ -263,11 +267,18 @@ pub(crate) fn build_external_trait(cx: &mut DocContext<'_>, did: DefId) -> clean
     let predicates = cx.tcx.predicates_of(did);
     let generics = clean_ty_generics(cx, cx.tcx.generics_of(did), predicates);
     let generics = filter_non_trait_generics(did, generics);
-    let (generics, supertrait_bounds) = separate_supertrait_bounds(generics);
+    let (generics, supertrait_bounds) = separate_self_bounds(generics);
     clean::Trait { def_id: did, generics, items: trait_items, bounds: supertrait_bounds }
 }
 
-pub(crate) fn build_function(cx: &mut DocContext<'_>, def_id: DefId) -> Box<clean::Function> {
+fn build_trait_alias(cx: &mut DocContext<'_>, did: DefId) -> clean::TraitAlias {
+    let predicates = cx.tcx.predicates_of(did);
+    let generics = clean_ty_generics(cx, cx.tcx.generics_of(did), predicates);
+    let (generics, bounds) = separate_self_bounds(generics);
+    clean::TraitAlias { generics, bounds }
+}
+
+pub(super) fn build_function(cx: &mut DocContext<'_>, def_id: DefId) -> Box<clean::Function> {
     let sig = cx.tcx.fn_sig(def_id).instantiate_identity();
     // The generics need to be cleaned before the signature.
     let mut generics =
@@ -788,12 +799,7 @@ fn filter_non_trait_generics(trait_did: DefId, mut g: clean::Generics) -> clean:
     g
 }
 
-/// Supertrait bounds for a trait are also listed in the generics coming from
-/// the metadata for a crate, so we want to separate those out and create a new
-/// list of explicit supertrait bounds to render nicely.
-fn separate_supertrait_bounds(
-    mut g: clean::Generics,
-) -> (clean::Generics, Vec<clean::GenericBound>) {
+fn separate_self_bounds(mut g: clean::Generics) -> (clean::Generics, Vec<clean::GenericBound>) {
     let mut ty_bounds = Vec::new();
     g.where_predicates.retain(|pred| match *pred {
         clean::WherePredicate::BoundPredicate { ty: clean::SelfTy, ref bounds, .. } => {
@@ -806,22 +812,17 @@ fn separate_supertrait_bounds(
 }
 
 pub(crate) fn record_extern_trait(cx: &mut DocContext<'_>, did: DefId) {
-    if did.is_local() {
-        return;
-    }
-
+    if did.is_local()
+        || cx.external_traits.contains_key(&did)
+        || cx.active_extern_traits.contains(&did)
     {
-        if cx.external_traits.contains_key(&did) || cx.active_extern_traits.contains(&did) {
-            return;
-        }
+        return;
     }
 
-    {
-        cx.active_extern_traits.insert(did);
-    }
+    cx.active_extern_traits.insert(did);
 
     debug!("record_extern_trait: {did:?}");
-    let trait_ = build_external_trait(cx, did);
+    let trait_ = build_trait(cx, did);
 
     cx.external_traits.insert(did, trait_);
     cx.active_extern_traits.remove(&did);
diff --git a/src/librustdoc/core.rs b/src/librustdoc/core.rs
index c4dea79370d1f..41688b41c6e7f 100644
--- a/src/librustdoc/core.rs
+++ b/src/librustdoc/core.rs
@@ -27,7 +27,7 @@ use rustc_span::source_map;
 use rustc_span::symbol::sym;
 use tracing::{debug, info};
 
-use crate::clean::inline::build_external_trait;
+use crate::clean::inline::build_trait;
 use crate::clean::{self, ItemId};
 use crate::config::{Options as RustdocOptions, OutputFormat, RenderOptions};
 use crate::formats::cache::Cache;
@@ -385,7 +385,7 @@ pub(crate) fn run_global_ctxt(
     //
     // Note that in case of `#![no_core]`, the trait is not available.
     if let Some(sized_trait_did) = ctxt.tcx.lang_items().sized_trait() {
-        let sized_trait = build_external_trait(&mut ctxt, sized_trait_did);
+        let sized_trait = build_trait(&mut ctxt, sized_trait_did);
         ctxt.external_traits.insert(sized_trait_did, sized_trait);
     }
 
diff --git a/src/librustdoc/html/render/print_item.rs b/src/librustdoc/html/render/print_item.rs
index 45cb0adecf3bf..39a631b637bd4 100644
--- a/src/librustdoc/html/render/print_item.rs
+++ b/src/librustdoc/html/render/print_item.rs
@@ -1232,12 +1232,13 @@ fn item_trait_alias(
         wrap_item(w, |w| {
             write!(
                 w,
-                "{attrs}trait {name}{generics}{where_b} = {bounds};",
+                "{attrs}trait {name}{generics} = {bounds}{where_clause};",
                 attrs = render_attributes_in_pre(it, "", cx),
                 name = it.name.unwrap(),
                 generics = t.generics.print(cx),
-                where_b = print_where_clause(&t.generics, cx, 0, Ending::Newline).maybe_display(),
                 bounds = bounds(&t.bounds, true, cx),
+                where_clause =
+                    print_where_clause(&t.generics, cx, 0, Ending::NoNewline).maybe_display(),
             )
         })?;
 
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index eb298060b2ece..d11f5c1a3a6f4 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -9,6 +9,7 @@ use std::process::{Child, Command, ExitStatus, Output, Stdio};
 use std::sync::Arc;
 use std::{env, iter, str};
 
+use build_helper::fs::remove_and_create_dir_all;
 use camino::{Utf8Path, Utf8PathBuf};
 use colored::Colorize;
 use regex::{Captures, Regex};
@@ -207,12 +208,6 @@ pub fn compute_stamp_hash(config: &Config) -> String {
     format!("{:x}", hash.finish())
 }
 
-fn remove_and_create_dir_all(path: &Utf8Path) {
-    let path = path.as_std_path();
-    let _ = fs::remove_dir_all(path);
-    fs::create_dir_all(path).unwrap();
-}
-
 #[derive(Copy, Clone, Debug)]
 struct TestCx<'test> {
     config: &'test Config,
@@ -523,7 +518,9 @@ impl<'test> TestCx<'test> {
         let mut rustc = Command::new(&self.config.rustc_path);
 
         let out_dir = self.output_base_name().with_extension("pretty-out");
-        remove_and_create_dir_all(&out_dir);
+        remove_and_create_dir_all(&out_dir).unwrap_or_else(|e| {
+            panic!("failed to remove and recreate output directory `{out_dir}`: {e}")
+        });
 
         let target = if self.props.force_host { &*self.config.host } else { &*self.config.target };
 
@@ -1098,13 +1095,19 @@ impl<'test> TestCx<'test> {
         let aux_dir = self.aux_output_dir_name();
 
         if !self.props.aux.builds.is_empty() {
-            remove_and_create_dir_all(&aux_dir);
+            remove_and_create_dir_all(&aux_dir).unwrap_or_else(|e| {
+                panic!("failed to remove and recreate output directory `{aux_dir}`: {e}")
+            });
         }
 
         if !self.props.aux.bins.is_empty() {
             let aux_bin_dir = self.aux_bin_output_dir_name();
-            remove_and_create_dir_all(&aux_dir);
-            remove_and_create_dir_all(&aux_bin_dir);
+            remove_and_create_dir_all(&aux_dir).unwrap_or_else(|e| {
+                panic!("failed to remove and recreate output directory `{aux_dir}`: {e}")
+            });
+            remove_and_create_dir_all(&aux_bin_dir).unwrap_or_else(|e| {
+                panic!("failed to remove and recreate output directory `{aux_bin_dir}`: {e}")
+            });
         }
 
         aux_dir
@@ -1509,7 +1512,9 @@ impl<'test> TestCx<'test> {
 
         let set_mir_dump_dir = |rustc: &mut Command| {
             let mir_dump_dir = self.get_mir_dump_dir();
-            remove_and_create_dir_all(&mir_dump_dir);
+            remove_and_create_dir_all(&mir_dump_dir).unwrap_or_else(|e| {
+                panic!("failed to remove and recreate output directory `{mir_dump_dir}`: {e}")
+            });
             let mut dir_opt = "-Zdump-mir-dir=".to_string();
             dir_opt.push_str(mir_dump_dir.as_str());
             debug!("dir_opt: {:?}", dir_opt);
@@ -1969,7 +1974,9 @@ impl<'test> TestCx<'test> {
         let suffix =
             self.safe_revision().map_or("nightly".into(), |path| path.to_owned() + "-nightly");
         let compare_dir = output_base_dir(self.config, self.testpaths, Some(&suffix));
-        remove_and_create_dir_all(&compare_dir);
+        remove_and_create_dir_all(&compare_dir).unwrap_or_else(|e| {
+            panic!("failed to remove and recreate output directory `{compare_dir}`: {e}")
+        });
 
         // We need to create a new struct for the lifetimes on `config` to work.
         let new_rustdoc = TestCx {
diff --git a/src/tools/compiletest/src/runtest/rustdoc.rs b/src/tools/compiletest/src/runtest/rustdoc.rs
index 2583ae96a6788..637ea833357a2 100644
--- a/src/tools/compiletest/src/runtest/rustdoc.rs
+++ b/src/tools/compiletest/src/runtest/rustdoc.rs
@@ -7,7 +7,9 @@ impl TestCx<'_> {
         assert!(self.revision.is_none(), "revisions not relevant here");
 
         let out_dir = self.output_base_dir();
-        remove_and_create_dir_all(&out_dir);
+        remove_and_create_dir_all(&out_dir).unwrap_or_else(|e| {
+            panic!("failed to remove and recreate output directory `{out_dir}`: {e}")
+        });
 
         let proc_res = self.document(&out_dir, &self.testpaths);
         if !proc_res.status.success() {
diff --git a/src/tools/compiletest/src/runtest/rustdoc_json.rs b/src/tools/compiletest/src/runtest/rustdoc_json.rs
index bf7eb2e109a46..9f88faca89268 100644
--- a/src/tools/compiletest/src/runtest/rustdoc_json.rs
+++ b/src/tools/compiletest/src/runtest/rustdoc_json.rs
@@ -9,7 +9,9 @@ impl TestCx<'_> {
         assert!(self.revision.is_none(), "revisions not relevant here");
 
         let out_dir = self.output_base_dir();
-        remove_and_create_dir_all(&out_dir);
+        remove_and_create_dir_all(&out_dir).unwrap_or_else(|e| {
+            panic!("failed to remove and recreate output directory `{out_dir}`: {e}")
+        });
 
         let proc_res = self.document(&out_dir, &self.testpaths);
         if !proc_res.status.success() {
diff --git a/src/tools/opt-dist/src/environment.rs b/src/tools/opt-dist/src/environment.rs
index 90d0ca717b25c..9342d164be411 100644
--- a/src/tools/opt-dist/src/environment.rs
+++ b/src/tools/opt-dist/src/environment.rs
@@ -25,6 +25,7 @@ pub struct Environment {
     prebuilt_rustc_perf: Option<Utf8PathBuf>,
     use_bolt: bool,
     shared_llvm: bool,
+    run_tests: bool,
 }
 
 impl Environment {
@@ -101,6 +102,10 @@ impl Environment {
     pub fn benchmark_cargo_config(&self) -> &[String] {
         &self.benchmark_cargo_config
     }
+
+    pub fn run_tests(&self) -> bool {
+        self.run_tests
+    }
 }
 
 /// What is the extension of binary executables on this platform?
diff --git a/src/tools/opt-dist/src/main.rs b/src/tools/opt-dist/src/main.rs
index ff4ddbaea49b7..d5e6640fb430a 100644
--- a/src/tools/opt-dist/src/main.rs
+++ b/src/tools/opt-dist/src/main.rs
@@ -94,6 +94,10 @@ enum EnvironmentCmd {
         /// Arguments passed to `rustc-perf --cargo-config <value>` when running benchmarks.
         #[arg(long)]
         benchmark_cargo_config: Vec<String>,
+
+        /// Perform tests after final build if it's not a try build
+        #[arg(long)]
+        run_tests: bool,
     },
     /// Perform an optimized build on Linux CI, from inside Docker.
     LinuxCi {
@@ -125,6 +129,7 @@ fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)>
             skipped_tests,
             benchmark_cargo_config,
             shared,
+            run_tests,
         } => {
             let env = EnvironmentBuilder::default()
                 .host_tuple(target_triple)
@@ -138,6 +143,7 @@ fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)>
                 .use_bolt(use_bolt)
                 .skipped_tests(skipped_tests)
                 .benchmark_cargo_config(benchmark_cargo_config)
+                .run_tests(run_tests)
                 .build()?;
 
             (env, shared.build_args)
@@ -160,6 +166,7 @@ fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)>
                 // FIXME: Enable bolt for aarch64 once it's fixed upstream. Broken as of December 2024.
                 .use_bolt(!is_aarch64)
                 .skipped_tests(vec![])
+                .run_tests(true)
                 .build()?;
 
             (env, shared.build_args)
@@ -179,6 +186,7 @@ fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)>
                 .shared_llvm(false)
                 .use_bolt(false)
                 .skipped_tests(vec![])
+                .run_tests(true)
                 .build()?;
 
             (env, shared.build_args)
@@ -344,7 +352,7 @@ fn execute_pipeline(
     // possible regressions.
     // The tests are not executed for try builds, which can be in various broken states, so we don't
     // want to gatekeep them with tests.
-    if !is_try_build() {
+    if !is_try_build() && env.run_tests() {
         timer.section("Run tests", |_| run_tests(env))?;
     }
 
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index fd8e4eacc0fa7..dd7f9c6b146a7 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -260,7 +260,6 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "constant_time_eq",
     "cpufeatures",
     "crc32fast",
-    "crossbeam-channel",
     "crossbeam-deque",
     "crossbeam-epoch",
     "crossbeam-utils",
@@ -295,7 +294,6 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "gimli",
     "gsgdt",
     "hashbrown",
-    "hermit-abi",
     "icu_list",
     "icu_list_data",
     "icu_locid",
@@ -329,7 +327,6 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "miniz_oxide",
     "nix",
     "nu-ansi-term",
-    "num_cpus",
     "object",
     "odht",
     "once_cell",
diff --git a/tests/run-make/print-request-help-stable-unstable/help-diff.diff b/tests/run-make/print-request-help-stable-unstable/help-diff.diff
new file mode 100644
index 0000000000000..07eafca327108
--- /dev/null
+++ b/tests/run-make/print-request-help-stable-unstable/help-diff.diff
@@ -0,0 +1,7 @@
+@@ -1,5 +1,5 @@
+ error: unknown print request: `xxx`
+   |
+-  = help: valid print requests are: `calling-conventions`, `cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `host-tuple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `tls-models`
++  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `check-cfg`, `code-models`, `crate-name`, `crate-root-lint-levels`, `deployment-target`, `file-names`, `host-tuple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `supported-crate-types`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
+   = help: for more information, see the rustc book: https://doc.rust-lang.org/rustc/command-line-arguments.html#--print-print-compiler-information
+ 
diff --git a/tests/run-make/print-request-help-stable-unstable/rmake.rs b/tests/run-make/print-request-help-stable-unstable/rmake.rs
new file mode 100644
index 0000000000000..a59963da5c497
--- /dev/null
+++ b/tests/run-make/print-request-help-stable-unstable/rmake.rs
@@ -0,0 +1,33 @@
+//! Check that unstable print requests are omitted from help if compiler is in stable channel.
+//!
+//! Issue: <https://github.com/rust-lang/rust/issues/138698>
+use run_make_support::{diff, rustc, similar};
+
+fn main() {
+    let stable_invalid_print_request_help = rustc()
+        .env("RUSTC_BOOTSTRAP", "-1")
+        .cfg("force_stable")
+        .print("xxx")
+        .run_fail()
+        .stderr_utf8();
+    assert!(!stable_invalid_print_request_help.contains("all-target-specs-json"));
+    diff()
+        .expected_file("stable-invalid-print-request-help.err")
+        .actual_text("stable_invalid_print_request_help", &stable_invalid_print_request_help)
+        .run();
+
+    let unstable_invalid_print_request_help = rustc().print("xxx").run_fail().stderr_utf8();
+    assert!(unstable_invalid_print_request_help.contains("all-target-specs-json"));
+    diff()
+        .expected_file("unstable-invalid-print-request-help.err")
+        .actual_text("unstable_invalid_print_request_help", &unstable_invalid_print_request_help)
+        .run();
+
+    let help_diff = similar::TextDiff::from_lines(
+        &stable_invalid_print_request_help,
+        &unstable_invalid_print_request_help,
+    )
+    .unified_diff()
+    .to_string();
+    diff().expected_file("help-diff.diff").actual_text("help_diff", help_diff).run();
+}
diff --git a/tests/run-make/print-request-help-stable-unstable/stable-invalid-print-request-help.err b/tests/run-make/print-request-help-stable-unstable/stable-invalid-print-request-help.err
new file mode 100644
index 0000000000000..019a578dad3e0
--- /dev/null
+++ b/tests/run-make/print-request-help-stable-unstable/stable-invalid-print-request-help.err
@@ -0,0 +1,5 @@
+error: unknown print request: `xxx`
+  |
+  = help: valid print requests are: `calling-conventions`, `cfg`, `code-models`, `crate-name`, `deployment-target`, `file-names`, `host-tuple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `tls-models`
+  = help: for more information, see the rustc book: https://doc.rust-lang.org/rustc/command-line-arguments.html#--print-print-compiler-information
+
diff --git a/tests/run-make/print-request-help-stable-unstable/unstable-invalid-print-request-help.err b/tests/run-make/print-request-help-stable-unstable/unstable-invalid-print-request-help.err
new file mode 100644
index 0000000000000..50ef340e3dd02
--- /dev/null
+++ b/tests/run-make/print-request-help-stable-unstable/unstable-invalid-print-request-help.err
@@ -0,0 +1,5 @@
+error: unknown print request: `xxx`
+  |
+  = help: valid print requests are: `all-target-specs-json`, `calling-conventions`, `cfg`, `check-cfg`, `code-models`, `crate-name`, `crate-root-lint-levels`, `deployment-target`, `file-names`, `host-tuple`, `link-args`, `native-static-libs`, `relocation-models`, `split-debuginfo`, `stack-protector-strategies`, `supported-crate-types`, `sysroot`, `target-cpus`, `target-features`, `target-libdir`, `target-list`, `target-spec-json`, `tls-models`
+  = help: for more information, see the rustc book: https://doc.rust-lang.org/rustc/command-line-arguments.html#--print-print-compiler-information
+
diff --git a/tests/rustdoc/auxiliary/ext-trait-aliases.rs b/tests/rustdoc/auxiliary/ext-trait-aliases.rs
new file mode 100644
index 0000000000000..8454c04063c12
--- /dev/null
+++ b/tests/rustdoc/auxiliary/ext-trait-aliases.rs
@@ -0,0 +1,13 @@
+#![feature(trait_alias)]
+
+pub trait ExtAlias0 = Copy + Iterator<Item = u8>;
+
+pub trait ExtAlias1<'a, T: 'a + Clone, const N: usize> = From<[&'a T; N]>;
+
+pub trait ExtAlias2<T> = where T: From<String>, String: Into<T>;
+
+pub trait ExtAlias3 = Sized;
+
+pub trait ExtAlias4 = where Self: Sized;
+
+pub trait ExtAlias5 = ;
diff --git a/tests/rustdoc/auxiliary/trait-alias-mention.rs b/tests/rustdoc/auxiliary/trait-alias-mention.rs
deleted file mode 100644
index 6df06c87a09d5..0000000000000
--- a/tests/rustdoc/auxiliary/trait-alias-mention.rs
+++ /dev/null
@@ -1,3 +0,0 @@
-#![feature(trait_alias)]
-
-pub trait SomeAlias = std::fmt::Debug + std::marker::Copy;
diff --git a/tests/rustdoc/trait-alias-mention.rs b/tests/rustdoc/trait-alias-mention.rs
deleted file mode 100644
index b6ef926e644e5..0000000000000
--- a/tests/rustdoc/trait-alias-mention.rs
+++ /dev/null
@@ -1,10 +0,0 @@
-//@ aux-build:trait-alias-mention.rs
-//@ build-aux-docs
-
-#![crate_name = "foo"]
-
-extern crate trait_alias_mention;
-
-//@ has foo/fn.mention_alias_in_bounds.html '//a[@href="../trait_alias_mention/traitalias.SomeAlias.html"]' 'SomeAlias'
-pub fn mention_alias_in_bounds<T: trait_alias_mention::SomeAlias>() {
-}
diff --git a/tests/rustdoc/trait-aliases.rs b/tests/rustdoc/trait-aliases.rs
new file mode 100644
index 0000000000000..1be93f72042c2
--- /dev/null
+++ b/tests/rustdoc/trait-aliases.rs
@@ -0,0 +1,82 @@
+// Basic testing for trait aliases.
+#![feature(trait_alias)]
+#![crate_name = "it"]
+
+// Check the "local case" (HIR cleaning) //
+
+//@ has it/all.html '//a[@href="traitalias.Alias0.html"]' 'Alias0'
+//@ has it/index.html '//h2[@id="trait-aliases"]' 'Trait Aliases'
+//@ has it/index.html '//a[@class="traitalias"]' 'Alias0'
+//@ has it/traitalias.Alias0.html
+//@ has - '//*[@class="rust item-decl"]//code' 'trait Alias0 = Copy + Iterator<Item = u8>;'
+pub trait Alias0 = Copy + Iterator<Item = u8>;
+
+//@ has it/traitalias.Alias1.html
+//@ has - '//pre[@class="rust item-decl"]' \
+//        "trait Alias1<'a, T: 'a + Clone, const N: usize> = From<[&'a T; N]>;"
+pub trait Alias1<'a, T: 'a + Clone, const N: usize> = From<[&'a T; N]>;
+
+//@ has it/traitalias.Alias2.html
+//@ has - '//pre[@class="rust item-decl"]' \
+//        'trait Alias2<T> = where T: From<String>, String: Into<T>;'
+pub trait Alias2<T> = where T: From<String>, String: Into<T>;
+
+//@ has it/traitalias.Alias3.html
+//@ has - '//pre[@class="rust item-decl"]' 'trait Alias3 = ;'
+pub trait Alias3 =;
+
+//@ has it/traitalias.Alias4.html
+//@ has - '//pre[@class="rust item-decl"]' 'trait Alias4 = ;'
+pub trait Alias4 = where;
+
+//@ has it/fn.usage0.html
+//@ has - '//pre[@class="rust item-decl"]' "pub fn usage0(_: impl Alias0)"
+//@ has - '//a[@href="traitalias.Alias0.html"]' 'Alias0'
+pub fn usage0(_: impl Alias0) {}
+
+// FIXME: One can only "disambiguate" intra-doc links to trait aliases with `type@` but not with
+// `trait@` (fails to resolve) or `traitalias@` (doesn't exist). We should make at least one of
+// the latter two work, right?
+
+//@ has it/link0/index.html
+//@ has - '//a/@href' 'traitalias.Alias0.html'
+//@ has - '//a/@href' 'traitalias.Alias1.html'
+/// [Alias0], [type@Alias1]
+pub mod link0 {}
+
+// Check the "extern case" (middle cleaning) //
+
+//@ aux-build: ext-trait-aliases.rs
+extern crate ext_trait_aliases as ext;
+
+//@ has it/traitalias.ExtAlias0.html
+//@ has - '//pre[@class="rust item-decl"]' 'trait ExtAlias0 = Copy + Iterator<Item = u8>;'
+pub use ext::ExtAlias0;
+
+//@ has it/traitalias.ExtAlias1.html
+//@ has - '//pre[@class="rust item-decl"]' \
+//        "trait ExtAlias1<'a, T, const N: usize> = From<[&'a T; N]> where T: 'a + Clone;"
+pub use ext::ExtAlias1;
+
+//@ has it/traitalias.ExtAlias2.html
+//@ has - '//pre[@class="rust item-decl"]' \
+//        'trait ExtAlias2<T> = where T: From<String>, String: Into<T>;'
+pub use ext::ExtAlias2;
+
+//@ has it/traitalias.ExtAlias3.html
+//@ has - '//pre[@class="rust item-decl"]' 'trait ExtAlias3 = Sized;'
+pub use ext::ExtAlias3;
+
+// NOTE: Middle cleaning can't discern `= Sized` and `= where Self: Sized` and that's okay.
+//@ has it/traitalias.ExtAlias4.html
+//@ has - '//pre[@class="rust item-decl"]' 'trait ExtAlias4 = Sized;'
+pub use ext::ExtAlias4;
+
+//@ has it/traitalias.ExtAlias5.html
+//@ has - '//pre[@class="rust item-decl"]' 'trait ExtAlias5 = ;'
+pub use ext::ExtAlias5;
+
+//@ has it/fn.usage1.html
+//@ has - '//pre[@class="rust item-decl"]' "pub fn usage1(_: impl ExtAlias0)"
+//@ has - '//a[@href="traitalias.ExtAlias0.html"]' 'ExtAlias0'
+pub fn usage1(_: impl ExtAlias0) {}
diff --git a/tests/rustdoc/trait_alias.rs b/tests/rustdoc/trait_alias.rs
deleted file mode 100644
index bfdb9d40e2d2e..0000000000000
--- a/tests/rustdoc/trait_alias.rs
+++ /dev/null
@@ -1,26 +0,0 @@
-#![feature(trait_alias)]
-
-#![crate_name = "foo"]
-
-use std::fmt::Debug;
-
-//@ has foo/all.html '//a[@href="traitalias.CopyAlias.html"]' 'CopyAlias'
-//@ has foo/all.html '//a[@href="traitalias.Alias2.html"]' 'Alias2'
-//@ has foo/all.html '//a[@href="traitalias.Foo.html"]' 'Foo'
-
-//@ has foo/index.html '//h2[@id="trait-aliases"]' 'Trait Aliases'
-//@ has foo/index.html '//a[@class="traitalias"]' 'CopyAlias'
-//@ has foo/index.html '//a[@class="traitalias"]' 'Alias2'
-//@ has foo/index.html '//a[@class="traitalias"]' 'Foo'
-
-//@ has foo/traitalias.CopyAlias.html
-//@ has - '//section[@id="main-content"]/pre[@class="rust item-decl"]' 'trait CopyAlias = Copy;'
-pub trait CopyAlias = Copy;
-//@ has foo/traitalias.Alias2.html
-//@ has - '//section[@id="main-content"]/pre[@class="rust item-decl"]' 'trait Alias2 = Copy + Debug;'
-pub trait Alias2 = Copy + Debug;
-//@ has foo/traitalias.Foo.html
-//@ has - '//section[@id="main-content"]/pre[@class="rust item-decl"]' 'trait Foo<T> = Into<T> + Debug;'
-pub trait Foo<T> = Into<T> + Debug;
-//@ has foo/fn.bar.html '//a[@href="traitalias.Alias2.html"]' 'Alias2'
-pub fn bar<T>() where T: Alias2 {}
diff --git a/tests/ui/borrowck/static-trait-bound-lost.rs b/tests/ui/borrowck/static-trait-bound-lost.rs
new file mode 100644
index 0000000000000..0288acea0f604
--- /dev/null
+++ b/tests/ui/borrowck/static-trait-bound-lost.rs
@@ -0,0 +1,54 @@
+// This test is a reduced version of a bug introduced during work on type-tests for Polonius.
+// The underlying problem is that the 'static bound is lost for a type parameter that is
+// threaded deeply enough, causing an error.
+// The bug was first observed in exr-1.4.1/src/image/read/mod.rs:124:5 during perf test.
+
+//@ check-pass
+
+use std::marker::PhantomData;
+
+struct ReadAllLayers<ReadChannels> {
+    px: PhantomData<ReadChannels>,
+}
+
+trait ReadLayers<'s> {}
+
+impl<'s, C> ReadLayers<'s> for ReadAllLayers<C> where C: ReadChannels<'s> {}
+
+fn make_builder<A, Set, Pixels>(
+    _: Set,
+) -> ReadAllLayers<CollectPixels<A, Pixels, Set>>
+where
+    Set: Fn(&mut Pixels),
+{
+    todo!()
+}
+
+struct CollectPixels<Pixel, PixelStorage, SetPixel> {
+    px: PhantomData<(SetPixel, Pixel, PixelStorage)>,
+}
+
+impl<'s, PixelStorage, SetPixel: 's> ReadChannels<'s>
+    for CollectPixels<usize, PixelStorage, SetPixel>
+where
+    SetPixel: Fn(&mut PixelStorage),
+{
+}
+
+trait ReadChannels<'s> {}
+
+fn from_file<L>(_: L)
+where
+    for<'s> L: ReadLayers<'s>,
+{
+}
+
+pub fn read_all_rgba_layers_from_file<Set: 'static, Pixels: 'static>(
+    set_pixel: Set,
+) where
+    Set: Fn(&mut Pixels),
+{
+    from_file(make_builder(set_pixel)); // Error triggered.
+}
+
+pub fn main() {}
diff --git a/tests/ui/nll/relate_tys/placeholder-outlives-existential.rs b/tests/ui/nll/relate_tys/placeholder-outlives-existential.rs
new file mode 100644
index 0000000000000..3f22e9a313af6
--- /dev/null
+++ b/tests/ui/nll/relate_tys/placeholder-outlives-existential.rs
@@ -0,0 +1,31 @@
+// Test that we correctly handle some cases of placeholder leaks.
+//
+//@ compile-flags:-Zno-leak-check
+
+
+struct Co<'a>(&'a ());
+struct Inv<'a>(*mut &'a ());
+struct Contra<'a>(fn(&'a ()));
+
+// `exists<'e> forall<'p> 'p: 'e` -> ERROR
+fn p_outlives_e(
+    x: for<'e> fn(for<'p> fn(fn(fn(Contra<'e>, Co<'p>)))),
+) -> fn(fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>)))) {
+    x //~ ERROR mismatched types [E0308]
+}
+
+// `exists<'e> forall<'p> 'e: 'p` -> Ok, 'e: 'static
+fn e_outlives_p_static(
+    x: for<'e> fn(Inv<'e>, for<'p> fn(fn(fn(Contra<'p>, Co<'e>)))),
+) -> fn(Inv<'static>, fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>)))) {
+    x
+}
+
+// `exists<'e> forall<'p> 'e: 'p` -> Ok, 'e: 'static -> ERROR
+fn e_outlives_p_static_err<'not_static>(
+    x: for<'e> fn(Inv<'e>, for<'p> fn(fn(fn(Contra<'p>, Co<'e>)))),
+) -> fn(Inv<'not_static>, fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>)))) {
+    x //~ ERROR lifetime may not live long enough
+}
+
+fn main() {}
diff --git a/tests/ui/nll/relate_tys/placeholder-outlives-existential.stderr b/tests/ui/nll/relate_tys/placeholder-outlives-existential.stderr
new file mode 100644
index 0000000000000..a9ca07c33943a
--- /dev/null
+++ b/tests/ui/nll/relate_tys/placeholder-outlives-existential.stderr
@@ -0,0 +1,25 @@
+error[E0308]: mismatched types
+  --> $DIR/placeholder-outlives-existential.rs:15:5
+   |
+LL |     x
+   |     ^ one type is more general than the other
+   |
+   = note: expected fn pointer `fn(fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>))))`
+              found fn pointer `for<'e> fn(for<'e, 'p> fn(for<'e, 'p> fn(for<'e, 'p> fn(Contra<'e>, Co<'p>))))`
+
+error: lifetime may not live long enough
+  --> $DIR/placeholder-outlives-existential.rs:29:5
+   |
+LL | fn e_outlives_p_static_err<'not_static>(
+   |                            ----------- lifetime `'not_static` defined here
+...
+LL |     x
+   |     ^ returning this value requires that `'not_static` must outlive `'static`
+   |
+   = note: requirement occurs because of the type `Inv<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Inv<'a>` is invariant over the parameter `'a`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/traits/next-solver/supertrait-alias-1.rs b/tests/ui/traits/next-solver/supertrait-alias-1.rs
new file mode 100644
index 0000000000000..579a44677c2ee
--- /dev/null
+++ b/tests/ui/traits/next-solver/supertrait-alias-1.rs
@@ -0,0 +1,22 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/171>.
+// Tests that we don't try to replace `<V as Super>::Output` when replacing projections in the
+// required bounds for `dyn Trait`, b/c `V` is not relevant to the dyn type, which we were
+// previously encountering b/c we were walking into the existential projection bounds of the dyn
+// type itself.
+
+pub trait Trait: Super {}
+
+pub trait Super {
+    type Output;
+}
+
+fn bound<T: Trait + ?Sized>() {}
+
+fn visit_simd_operator<V: Super + ?Sized>() {
+    bound::<dyn Trait<Output = <V as Super>::Output>>();
+}
+
+fn main() {}
diff --git a/tests/ui/traits/next-solver/supertrait-alias-2.rs b/tests/ui/traits/next-solver/supertrait-alias-2.rs
new file mode 100644
index 0000000000000..a0f3e038dca63
--- /dev/null
+++ b/tests/ui/traits/next-solver/supertrait-alias-2.rs
@@ -0,0 +1,25 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/171>.
+// Tests that we don't try to replace `<T as Other>::Assoc` when replacing projections in the
+// required bounds for `dyn Foo`, b/c `T` is not relevant to the dyn type, which we were
+// encountering when walking through the elaborated supertraits of `dyn Foo`.
+
+trait Other<X> {}
+
+trait Foo<T: Foo<T>>: Other<<T as Foo<T>>::Assoc> {
+    type Assoc;
+}
+
+impl<T> Foo<T> for T {
+    type Assoc = ();
+}
+
+impl<T: ?Sized> Other<()> for T {}
+
+fn is_foo<T: Foo<()> + ?Sized>() {}
+
+fn main() {
+    is_foo::<dyn Foo<(), Assoc = ()>>();
+}
diff --git a/tests/ui/traits/next-solver/supertrait-alias-3.rs b/tests/ui/traits/next-solver/supertrait-alias-3.rs
new file mode 100644
index 0000000000000..78182bbc41568
--- /dev/null
+++ b/tests/ui/traits/next-solver/supertrait-alias-3.rs
@@ -0,0 +1,32 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/171>.
+// Exercises a case where structural equality is insufficient when replacing projections in a dyn's
+// bounds. In this case, the bound will contain `<Self as Super<<i32 as Mirror>:Assoc>::Assoc`, but
+// the existential projections from the dyn will have `<Self as Super<i32>>::Assoc` because as an
+// optimization we eagerly normalize aliases in goals.
+
+trait Other<T> {}
+impl<T> Other<T> for T {}
+
+trait Super<T> {
+    type Assoc;
+}
+
+trait Mirror {
+    type Assoc;
+}
+impl<T> Mirror for T {
+    type Assoc = T;
+}
+
+trait Foo<A, B>: Super<<A as Mirror>::Assoc, Assoc = A> {
+    type FooAssoc: Other<<Self as Super<<A as Mirror>::Assoc>>::Assoc>;
+}
+
+fn is_foo<F: Foo<T, U> + ?Sized, T, U>() {}
+
+fn main() {
+    is_foo::<dyn Foo<i32, u32, FooAssoc = i32>, _, _>();
+}
diff --git a/tests/ui/traits/next-solver/supertrait-alias-4.rs b/tests/ui/traits/next-solver/supertrait-alias-4.rs
new file mode 100644
index 0000000000000..919a768fcf281
--- /dev/null
+++ b/tests/ui/traits/next-solver/supertrait-alias-4.rs
@@ -0,0 +1,24 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Exercises the ambiguity that comes from replacing the associated types within the bounds
+// that are required for a `impl Trait for dyn Trait` built-in object impl to hold.
+
+trait Sup<T> {
+    type Assoc;
+}
+
+trait Foo<A, B>: Sup<A, Assoc = A> + Sup<B, Assoc = B> {
+    type Other: Bar<<Self as Sup<A>>::Assoc>;
+}
+
+trait Bar<T> {}
+impl Bar<i32> for () {}
+
+fn foo<A, B>(x: &(impl Foo<A, B> + ?Sized)) {}
+
+fn main() {
+    let x: &dyn Foo<_, _, Other = ()> = todo!();
+    foo(x);
+    let y: &dyn Foo<i32, u32, Other = ()> = x;
+}
diff --git a/tests/ui/type-alias-impl-trait/cross_inference_pattern_bug_no_type.rs b/tests/ui/type-alias-impl-trait/cross_inference_pattern_bug_no_type.rs
index 736a9dfb49033..be8b7fa6a9352 100644
--- a/tests/ui/type-alias-impl-trait/cross_inference_pattern_bug_no_type.rs
+++ b/tests/ui/type-alias-impl-trait/cross_inference_pattern_bug_no_type.rs
@@ -1,6 +1,5 @@
 //@ compile-flags: --crate-type=lib
 //@ edition: 2021
-//@ rustc-env:RUST_BACKTRACE=0
 //@ check-pass
 
 // tracked in https://github.com/rust-lang/rust/issues/96572
diff --git a/tests/ui/type-alias-impl-trait/destructure_tait-ice-113594.rs b/tests/ui/type-alias-impl-trait/destructure_tait-ice-113594.rs
index a3b1aba70411f..a42ea083d740c 100644
--- a/tests/ui/type-alias-impl-trait/destructure_tait-ice-113594.rs
+++ b/tests/ui/type-alias-impl-trait/destructure_tait-ice-113594.rs
@@ -1,3 +1,5 @@
+//@ revisions: current next
+//@ [next] compile-flags: -Znext-solver
 //@ build-pass
 //@ edition: 2021
 
diff --git a/tests/ui/type-alias-impl-trait/tait-normalize.rs b/tests/ui/type-alias-impl-trait/tait-normalize.rs
index 38e09b6087b64..a34d167bcc391 100644
--- a/tests/ui/type-alias-impl-trait/tait-normalize.rs
+++ b/tests/ui/type-alias-impl-trait/tait-normalize.rs
@@ -1,3 +1,5 @@
+//@ revisions: current next
+//@ [next] compile-flags: -Znext-solver
 //@ check-pass
 
 #![feature(type_alias_impl_trait)]