Auto merge of #124822 - matthiaskrgr:rollup-h7fc52t, r=matthiaskrgr

Rollup of 3 pull requests

Successful merges:

 - #124759 (Record impl args in the proof tree in new solver)
 - #124809 (borrowck: prepopulate opaque storage more eagerly)
 - #124815 (Update books)

r? `@ghost`
`@rustbot` modify labels: rollup

29 files changed, 226 insertions, 230 deletions

diff --git a/compiler/rustc_borrowck/src/consumers.rs b/compiler/rustc_borrowck/src/consumers.rs
index 64726eacca7..b9fa46ea883 100644
--- a/compiler/rustc_borrowck/src/consumers.rs
+++ b/compiler/rustc_borrowck/src/consumers.rs
@@ -2,7 +2,6 @@
 
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::{IndexSlice, IndexVec};
-use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::{Body, Promoted};
 use rustc_middle::ty::TyCtxt;
 use std::rc::Rc;
@@ -105,8 +104,7 @@ pub fn get_body_with_borrowck_facts(
     options: ConsumerOptions,
 ) -> BodyWithBorrowckFacts<'_> {
     let (input_body, promoted) = tcx.mir_promoted(def);
-    let infcx = tcx.infer_ctxt().with_opaque_type_inference(def).build();
     let input_body: &Body<'_> = &input_body.borrow();
     let promoted: &IndexSlice<_, _> = &promoted.borrow();
-    *super::do_mir_borrowck(&infcx, input_body, promoted, Some(options)).1.unwrap()
+    *super::do_mir_borrowck(tcx, input_body, promoted, Some(options)).1.unwrap()
 }
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 47c83e0bb2b..abe57e26af4 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -23,9 +23,8 @@ use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::bit_set::{BitSet, ChunkedBitSet};
 use rustc_index::{IndexSlice, IndexVec};
-use rustc_infer::infer::{
-    InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin, TyCtxtInferExt,
-};
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin};
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::*;
 use rustc_middle::query::Providers;
@@ -123,9 +122,8 @@ fn mir_borrowck(tcx: TyCtxt<'_>, def: LocalDefId) -> &BorrowCheckResult<'_> {
         return tcx.arena.alloc(result);
     }
 
-    let infcx = tcx.infer_ctxt().with_opaque_type_inference(def).build();
     let promoted: &IndexSlice<_, _> = &promoted.borrow();
-    let opt_closure_req = do_mir_borrowck(&infcx, input_body, promoted, None).0;
+    let opt_closure_req = do_mir_borrowck(tcx, input_body, promoted, None).0;
     debug!("mir_borrowck done");
 
     tcx.arena.alloc(opt_closure_req)
@@ -136,18 +134,15 @@ fn mir_borrowck(tcx: TyCtxt<'_>, def: LocalDefId) -> &BorrowCheckResult<'_> {
 /// Use `consumer_options: None` for the default behavior of returning
 /// [`BorrowCheckResult`] only. Otherwise, return [`BodyWithBorrowckFacts`] according
 /// to the given [`ConsumerOptions`].
-#[instrument(skip(infcx, input_body, input_promoted), fields(id=?input_body.source.def_id()), level = "debug")]
+#[instrument(skip(tcx, input_body, input_promoted), fields(id=?input_body.source.def_id()), level = "debug")]
 fn do_mir_borrowck<'tcx>(
-    infcx: &InferCtxt<'tcx>,
+    tcx: TyCtxt<'tcx>,
     input_body: &Body<'tcx>,
     input_promoted: &IndexSlice<Promoted, Body<'tcx>>,
     consumer_options: Option<ConsumerOptions>,
 ) -> (BorrowCheckResult<'tcx>, Option<Box<BodyWithBorrowckFacts<'tcx>>>) {
     let def = input_body.source.def_id().expect_local();
-    debug!(?def);
-
-    let tcx = infcx.tcx;
-    let infcx = BorrowckInferCtxt::new(infcx);
+    let infcx = BorrowckInferCtxt::new(tcx, def);
     let param_env = tcx.param_env(def);
 
     let mut local_names = IndexVec::from_elem(None, &input_body.local_decls);
@@ -187,6 +182,12 @@ fn do_mir_borrowck<'tcx>(
         nll::replace_regions_in_mir(&infcx, param_env, &mut body_owned, &mut promoted);
     let body = &body_owned; // no further changes
 
+    // FIXME(-Znext-solver): A bit dubious that we're only registering
+    // predefined opaques in the typeck root.
+    if infcx.next_trait_solver() && !infcx.tcx.is_typeck_child(body.source.def_id()) {
+        infcx.register_predefined_opaques_for_next_solver(def);
+    }
+
     let location_table = LocationTable::new(body);
 
     let move_data = MoveData::gather_moves(body, tcx, param_env, |_| true);
@@ -440,13 +441,14 @@ fn do_mir_borrowck<'tcx>(
     (result, body_with_facts)
 }
 
-pub struct BorrowckInferCtxt<'cx, 'tcx> {
-    pub(crate) infcx: &'cx InferCtxt<'tcx>,
+pub struct BorrowckInferCtxt<'tcx> {
+    pub(crate) infcx: InferCtxt<'tcx>,
     pub(crate) reg_var_to_origin: RefCell<FxIndexMap<ty::RegionVid, RegionCtxt>>,
 }
 
-impl<'cx, 'tcx> BorrowckInferCtxt<'cx, 'tcx> {
-    pub(crate) fn new(infcx: &'cx InferCtxt<'tcx>) -> Self {
+impl<'tcx> BorrowckInferCtxt<'tcx> {
+    pub(crate) fn new(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
+        let infcx = tcx.infer_ctxt().with_opaque_type_inference(def_id).build();
         BorrowckInferCtxt { infcx, reg_var_to_origin: RefCell::new(Default::default()) }
     }
 
@@ -492,18 +494,40 @@ impl<'cx, 'tcx> BorrowckInferCtxt<'cx, 'tcx> {
 
         next_region
     }
+
+    /// With the new solver we prepopulate the opaque type storage during
+    /// MIR borrowck with the hidden types from HIR typeck. This is necessary
+    /// to avoid ambiguities as earlier goals can rely on the hidden type
+    /// of an opaque which is only constrained by a later goal.
+    fn register_predefined_opaques_for_next_solver(&self, def_id: LocalDefId) {
+        let tcx = self.tcx;
+        // OK to use the identity arguments for each opaque type key, since
+        // we remap opaques from HIR typeck back to their definition params.
+        for data in tcx.typeck(def_id).concrete_opaque_types.iter().map(|(k, v)| (*k, *v)) {
+            // HIR typeck did not infer the regions of the opaque, so we instantiate
+            // them with fresh inference variables.
+            let (key, hidden_ty) = tcx.fold_regions(data, |_, _| {
+                self.next_nll_region_var_in_universe(
+                    NllRegionVariableOrigin::Existential { from_forall: false },
+                    ty::UniverseIndex::ROOT,
+                )
+            });
+
+            self.inject_new_hidden_type_unchecked(key, hidden_ty);
+        }
+    }
 }
 
-impl<'cx, 'tcx> Deref for BorrowckInferCtxt<'cx, 'tcx> {
+impl<'tcx> Deref for BorrowckInferCtxt<'tcx> {
     type Target = InferCtxt<'tcx>;
 
-    fn deref(&self) -> &'cx Self::Target {
-        self.infcx
+    fn deref(&self) -> &Self::Target {
+        &self.infcx
     }
 }
 
 struct MirBorrowckCtxt<'cx, 'tcx> {
-    infcx: &'cx BorrowckInferCtxt<'cx, 'tcx>,
+    infcx: &'cx BorrowckInferCtxt<'tcx>,
     param_env: ParamEnv<'tcx>,
     body: &'cx Body<'tcx>,
     move_data: &'cx MoveData<'tcx>,
diff --git a/compiler/rustc_borrowck/src/nll.rs b/compiler/rustc_borrowck/src/nll.rs
index 4aa32a61f7c..49f50babdcb 100644
--- a/compiler/rustc_borrowck/src/nll.rs
+++ b/compiler/rustc_borrowck/src/nll.rs
@@ -51,7 +51,7 @@ pub(crate) struct NllOutput<'tcx> {
 /// `compute_regions`.
 #[instrument(skip(infcx, param_env, body, promoted), level = "debug")]
 pub(crate) fn replace_regions_in_mir<'tcx>(
-    infcx: &BorrowckInferCtxt<'_, 'tcx>,
+    infcx: &BorrowckInferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     body: &mut Body<'tcx>,
     promoted: &mut IndexSlice<Promoted, Body<'tcx>>,
@@ -75,7 +75,7 @@ pub(crate) fn replace_regions_in_mir<'tcx>(
 ///
 /// This may result in errors being reported.
 pub(crate) fn compute_regions<'cx, 'tcx>(
-    infcx: &BorrowckInferCtxt<'_, 'tcx>,
+    infcx: &BorrowckInferCtxt<'tcx>,
     universal_regions: UniversalRegions<'tcx>,
     body: &Body<'tcx>,
     promoted: &IndexSlice<Promoted, Body<'tcx>>,
@@ -202,7 +202,7 @@ pub(crate) fn compute_regions<'cx, 'tcx>(
 }
 
 pub(super) fn dump_mir_results<'tcx>(
-    infcx: &BorrowckInferCtxt<'_, 'tcx>,
+    infcx: &BorrowckInferCtxt<'tcx>,
     body: &Body<'tcx>,
     regioncx: &RegionInferenceContext<'tcx>,
     closure_region_requirements: &Option<ClosureRegionRequirements<'tcx>>,
@@ -254,7 +254,7 @@ pub(super) fn dump_mir_results<'tcx>(
 #[allow(rustc::diagnostic_outside_of_impl)]
 #[allow(rustc::untranslatable_diagnostic)]
 pub(super) fn dump_annotation<'tcx>(
-    infcx: &BorrowckInferCtxt<'_, 'tcx>,
+    infcx: &BorrowckInferCtxt<'tcx>,
     body: &Body<'tcx>,
     regioncx: &RegionInferenceContext<'tcx>,
     closure_region_requirements: &Option<ClosureRegionRequirements<'tcx>>,
diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 78465ad7975..54574446b55 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -250,10 +250,7 @@ pub enum ExtraConstraintInfo {
 }
 
 #[instrument(skip(infcx, sccs), level = "debug")]
-fn sccs_info<'cx, 'tcx>(
-    infcx: &'cx BorrowckInferCtxt<'cx, 'tcx>,
-    sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>,
-) {
+fn sccs_info<'tcx>(infcx: &BorrowckInferCtxt<'tcx>, sccs: Rc<Sccs<RegionVid, ConstraintSccIndex>>) {
     use crate::renumber::RegionCtxt;
 
     let var_to_origin = infcx.reg_var_to_origin.borrow();
@@ -322,8 +319,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     ///
     /// The `outlives_constraints` and `type_tests` are an initial set
     /// of constraints produced by the MIR type check.
-    pub(crate) fn new<'cx>(
-        _infcx: &BorrowckInferCtxt<'cx, 'tcx>,
+    pub(crate) fn new(
+        _infcx: &BorrowckInferCtxt<'tcx>,
         var_infos: VarInfos,
         universal_regions: Rc<UniversalRegions<'tcx>>,
         placeholder_indices: Rc<PlaceholderIndices>,
diff --git a/compiler/rustc_borrowck/src/renumber.rs b/compiler/rustc_borrowck/src/renumber.rs
index dca8df32800..f5757bcaa1d 100644
--- a/compiler/rustc_borrowck/src/renumber.rs
+++ b/compiler/rustc_borrowck/src/renumber.rs
@@ -11,7 +11,7 @@ use rustc_span::Symbol;
 /// inference variables, returning the number of variables created.
 #[instrument(skip(infcx, body, promoted), level = "debug")]
 pub fn renumber_mir<'tcx>(
-    infcx: &BorrowckInferCtxt<'_, 'tcx>,
+    infcx: &BorrowckInferCtxt<'tcx>,
     body: &mut Body<'tcx>,
     promoted: &mut IndexSlice<Promoted, Body<'tcx>>,
 ) {
@@ -57,7 +57,7 @@ impl RegionCtxt {
 }
 
 struct RegionRenumberer<'a, 'tcx> {
-    infcx: &'a BorrowckInferCtxt<'a, 'tcx>,
+    infcx: &'a BorrowckInferCtxt<'tcx>,
 }
 
 impl<'a, 'tcx> RegionRenumberer<'a, 'tcx> {
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index b67c5d85818..13acc672def 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -24,7 +24,6 @@ use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::traits::query::NoSolution;
-use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::adjustment::PointerCoercion;
 use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::visit::TypeVisitableExt;
@@ -122,7 +121,7 @@ mod relate_tys;
 /// - `move_data` -- move-data constructed when performing the maybe-init dataflow analysis
 /// - `elements` -- MIR region map
 pub(crate) fn type_check<'mir, 'tcx>(
-    infcx: &BorrowckInferCtxt<'_, 'tcx>,
+    infcx: &BorrowckInferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     body: &Body<'tcx>,
     promoted: &IndexSlice<Promoted, Body<'tcx>>,
@@ -865,7 +864,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
 /// way, it accrues region constraints -- these can later be used by
 /// NLL region checking.
 struct TypeChecker<'a, 'tcx> {
-    infcx: &'a BorrowckInferCtxt<'a, 'tcx>,
+    infcx: &'a BorrowckInferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     last_span: Span,
     body: &'a Body<'tcx>,
@@ -1020,7 +1019,7 @@ impl Locations {
 
 impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
     fn new(
-        infcx: &'a BorrowckInferCtxt<'a, 'tcx>,
+        infcx: &'a BorrowckInferCtxt<'tcx>,
         body: &'a Body<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         region_bound_pairs: &'a RegionBoundPairs<'tcx>,
@@ -1028,7 +1027,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         implicit_region_bound: ty::Region<'tcx>,
         borrowck_context: &'a mut BorrowCheckContext<'a, 'tcx>,
     ) -> Self {
-        let mut checker = Self {
+        Self {
             infcx,
             last_span: body.span,
             body,
@@ -1039,74 +1038,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             implicit_region_bound,
             borrowck_context,
             reported_errors: Default::default(),
-        };
-
-        // FIXME(-Znext-solver): A bit dubious that we're only registering
-        // predefined opaques in the typeck root.
-        if infcx.next_trait_solver() && !infcx.tcx.is_typeck_child(body.source.def_id()) {
-            checker.register_predefined_opaques_for_next_solver();
-        }
-
-        checker
-    }
-
-    pub(super) fn register_predefined_opaques_for_next_solver(&mut self) {
-        // OK to use the identity arguments for each opaque type key, since
-        // we remap opaques from HIR typeck back to their definition params.
-        let opaques: Vec<_> = self
-            .infcx
-            .tcx
-            .typeck(self.body.source.def_id().expect_local())
-            .concrete_opaque_types
-            .iter()
-            .map(|(k, v)| (*k, *v))
-            .collect();
-
-        let renumbered_opaques = self.infcx.tcx.fold_regions(opaques, |_, _| {
-            self.infcx.next_nll_region_var_in_universe(
-                NllRegionVariableOrigin::Existential { from_forall: false },
-                ty::UniverseIndex::ROOT,
-            )
-        });
-
-        let param_env = self.param_env;
-        let result = self.fully_perform_op(
-            Locations::All(self.body.span),
-            ConstraintCategory::OpaqueType,
-            CustomTypeOp::new(
-                |ocx| {
-                    let mut obligations = Vec::new();
-                    for (opaque_type_key, hidden_ty) in renumbered_opaques {
-                        let cause = ObligationCause::dummy();
-                        ocx.infcx.insert_hidden_type(
-                            opaque_type_key,
-                            &cause,
-                            param_env,
-                            hidden_ty.ty,
-                            &mut obligations,
-                        )?;
-
-                        ocx.infcx.add_item_bounds_for_hidden_type(
-                            opaque_type_key.def_id.to_def_id(),
-                            opaque_type_key.args,
-                            cause,
-                            param_env,
-                            hidden_ty.ty,
-                            &mut obligations,
-                        );
-                    }
-
-                    ocx.register_obligations(obligations);
-                    Ok(())
-                },
-                "register pre-defined opaques",
-            ),
-        );
-
-        if result.is_err() {
-            self.infcx
-                .dcx()
-                .span_bug(self.body.span, "failed re-defining predefined opaques in mir typeck");
         }
     }
 
diff --git a/compiler/rustc_borrowck/src/universal_regions.rs b/compiler/rustc_borrowck/src/universal_regions.rs
index 070238fc378..f8123535e2d 100644
--- a/compiler/rustc_borrowck/src/universal_regions.rs
+++ b/compiler/rustc_borrowck/src/universal_regions.rs
@@ -240,7 +240,7 @@ impl<'tcx> UniversalRegions<'tcx> {
     /// signature. This will also compute the relationships that are
     /// known between those regions.
     pub fn new(
-        infcx: &BorrowckInferCtxt<'_, 'tcx>,
+        infcx: &BorrowckInferCtxt<'tcx>,
         mir_def: LocalDefId,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Self {
@@ -411,7 +411,7 @@ impl<'tcx> UniversalRegions<'tcx> {
 }
 
 struct UniversalRegionsBuilder<'cx, 'tcx> {
-    infcx: &'cx BorrowckInferCtxt<'cx, 'tcx>,
+    infcx: &'cx BorrowckInferCtxt<'tcx>,
     mir_def: LocalDefId,
     param_env: ty::ParamEnv<'tcx>,
 }
@@ -796,7 +796,7 @@ impl<'cx, 'tcx> UniversalRegionsBuilder<'cx, 'tcx> {
 }
 
 #[extension(trait InferCtxtExt<'tcx>)]
-impl<'cx, 'tcx> BorrowckInferCtxt<'cx, 'tcx> {
+impl<'tcx> BorrowckInferCtxt<'tcx> {
     #[instrument(skip(self), level = "debug")]
     fn replace_free_regions_with_nll_infer_vars<T>(
         &self,
diff --git a/compiler/rustc_infer/src/infer/opaque_types/mod.rs b/compiler/rustc_infer/src/infer/opaque_types/mod.rs
index 94a546f87ee..b7ee860cf07 100644
--- a/compiler/rustc_infer/src/infer/opaque_types/mod.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types/mod.rs
@@ -485,6 +485,19 @@ impl<'tcx> InferCtxt<'tcx> {
         Ok(InferOk { value: (), obligations })
     }
 
+    /// Insert a hidden type into the opaque type storage, making sure
+    /// it hasn't previously been defined. This does not emit any
+    /// constraints and it's the responsibility of the caller to make
+    /// sure that the item bounds of the opaque are checked.
+    pub fn inject_new_hidden_type_unchecked(
+        &self,
+        opaque_type_key: OpaqueTypeKey<'tcx>,
+        hidden_ty: OpaqueHiddenType<'tcx>,
+    ) {
+        let prev = self.inner.borrow_mut().opaque_types().register(opaque_type_key, hidden_ty);
+        assert_eq!(prev, None);
+    }
+
     /// Insert a hidden type into the opaque type storage, equating it
     /// with any previous entries if necessary.
     ///
diff --git a/compiler/rustc_middle/src/traits/solve/inspect.rs b/compiler/rustc_middle/src/traits/solve/inspect.rs
index cddf9d5f874..2ddcb8aab25 100644
--- a/compiler/rustc_middle/src/traits/solve/inspect.rs
+++ b/compiler/rustc_middle/src/traits/solve/inspect.rs
@@ -123,6 +123,8 @@ pub enum ProbeStep<'tcx> {
     /// used whenever there are multiple candidates to prove the
     /// current goalby .
     NestedProbe(Probe<'tcx>),
+    /// A trait goal was satisfied by an impl candidate.
+    RecordImplArgs { impl_args: CanonicalState<'tcx, ty::GenericArgsRef<'tcx>> },
     /// A call to `EvalCtxt::evaluate_added_goals_make_canonical_response` with
     /// `Certainty` was made. This is the certainty passed in, so it's not unified
     /// with the certainty of the `try_evaluate_added_goals` that is done within;
diff --git a/compiler/rustc_middle/src/traits/solve/inspect/format.rs b/compiler/rustc_middle/src/traits/solve/inspect/format.rs
index 11aa0e10931..e652f0586c4 100644
--- a/compiler/rustc_middle/src/traits/solve/inspect/format.rs
+++ b/compiler/rustc_middle/src/traits/solve/inspect/format.rs
@@ -136,6 +136,9 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
                     ProbeStep::MakeCanonicalResponse { shallow_certainty } => {
                         writeln!(this.f, "EVALUATE GOALS AND MAKE RESPONSE: {shallow_certainty:?}")?
                     }
+                    ProbeStep::RecordImplArgs { impl_args } => {
+                        writeln!(this.f, "RECORDED IMPL ARGS: {impl_args:?}")?
+                    }
                 }
             }
             Ok(())
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs
index d6bf2b596ef..5f73432750d 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs
@@ -236,7 +236,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             normalization_nested_goals,
         } = external_constraints.deref();
         self.register_region_constraints(region_constraints);
-        self.register_new_opaque_types(param_env, opaque_types);
+        self.register_new_opaque_types(opaque_types);
         (normalization_nested_goals.clone(), certainty)
     }
 
@@ -368,13 +368,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         assert!(region_constraints.member_constraints.is_empty());
     }
 
-    fn register_new_opaque_types(
-        &mut self,
-        param_env: ty::ParamEnv<'tcx>,
-        opaque_types: &[(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)],
-    ) {
+    fn register_new_opaque_types(&mut self, opaque_types: &[(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)]) {
         for &(key, ty) in opaque_types {
-            self.insert_hidden_type(key, param_env, ty).unwrap();
+            let hidden_ty = ty::OpaqueHiddenType { ty, span: DUMMY_SP };
+            self.infcx.inject_new_hidden_type_unchecked(key, hidden_ty);
         }
     }
 }
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
index 773babde0d7..55ade5e3e2f 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
@@ -248,8 +248,8 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         };
 
         for &(key, ty) in &input.predefined_opaques_in_body.opaque_types {
-            ecx.insert_hidden_type(key, input.goal.param_env, ty)
-                .expect("failed to prepopulate opaque types");
+            let hidden_ty = ty::OpaqueHiddenType { ty, span: DUMMY_SP };
+            ecx.infcx.inject_new_hidden_type_unchecked(key, hidden_ty);
         }
 
         if !ecx.nested_goals.is_empty() {
@@ -587,6 +587,11 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
 
         Ok(unchanged_certainty)
     }
+
+    /// Record impl args in the proof tree for later access by `InspectCandidate`.
+    pub(crate) fn record_impl_args(&mut self, impl_args: ty::GenericArgsRef<'tcx>) {
+        self.inspect.record_impl_args(self.infcx, self.max_input_universe, impl_args)
+    }
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
index 4d474b8e707..7efc951135b 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
@@ -1,12 +1,11 @@
 use std::ops::ControlFlow;
 
-use rustc_hir::def_id::DefId;
-use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, InferOk};
+use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::solve::inspect::ProbeKind;
 use rustc_infer::traits::solve::{CandidateSource, Certainty, Goal};
 use rustc_infer::traits::{
     BuiltinImplSource, ImplSource, ImplSourceUserDefinedData, Obligation, ObligationCause,
-    PolyTraitObligation, PredicateObligation, Selection, SelectionError, SelectionResult,
+    PolyTraitObligation, Selection, SelectionError, SelectionResult,
 };
 use rustc_macros::extension;
 use rustc_span::Span;
@@ -141,32 +140,32 @@ fn to_selection<'tcx>(
         return None;
     }
 
-    let make_nested = || {
-        cand.instantiate_nested_goals(span)
-            .into_iter()
-            .map(|nested| {
-                Obligation::new(
-                    nested.infcx().tcx,
-                    ObligationCause::dummy_with_span(span),
-                    nested.goal().param_env,
-                    nested.goal().predicate,
-                )
-            })
-            .collect()
-    };
+    let (nested, impl_args) = cand.instantiate_nested_goals_and_opt_impl_args(span);
+    let nested = nested
+        .into_iter()
+        .map(|nested| {
+            Obligation::new(
+                nested.infcx().tcx,
+                ObligationCause::dummy_with_span(span),
+                nested.goal().param_env,
+                nested.goal().predicate,
+            )
+        })
+        .collect();
 
     Some(match cand.kind() {
         ProbeKind::TraitCandidate { source, result: _ } => match source {
             CandidateSource::Impl(impl_def_id) => {
                 // FIXME: Remove this in favor of storing this in the tree
                 // For impl candidates, we do the rematch manually to compute the args.
-                ImplSource::UserDefined(rematch_impl(cand.goal(), impl_def_id, span))
-            }
-            CandidateSource::BuiltinImpl(builtin) => ImplSource::Builtin(builtin, make_nested()),
-            CandidateSource::ParamEnv(_) => ImplSource::Param(make_nested()),
-            CandidateSource::AliasBound => {
-                ImplSource::Builtin(BuiltinImplSource::Misc, make_nested())
+                ImplSource::UserDefined(ImplSourceUserDefinedData {
+                    impl_def_id,
+                    args: impl_args.expect("expected recorded impl args for impl candidate"),
+                    nested,
+                })
             }
+            CandidateSource::BuiltinImpl(builtin) => ImplSource::Builtin(builtin, nested),
+            CandidateSource::ParamEnv(_) | CandidateSource::AliasBound => ImplSource::Param(nested),
             CandidateSource::CoherenceUnknowable => {
                 span_bug!(span, "didn't expect to select an unknowable candidate")
             }
@@ -181,40 +180,3 @@ fn to_selection<'tcx>(
         }
     })
 }
-
-fn rematch_impl<'tcx>(
-    goal: &inspect::InspectGoal<'_, 'tcx>,
-    impl_def_id: DefId,
-    span: Span,
-) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> {
-    let infcx = goal.infcx();
-    let goal_trait_ref = infcx
-        .enter_forall_and_leak_universe(goal.goal().predicate.to_opt_poly_trait_pred().unwrap())
-        .trait_ref;
-
-    let args = infcx.fresh_args_for_item(span, impl_def_id);
-    let impl_trait_ref =
-        infcx.tcx.impl_trait_ref(impl_def_id).unwrap().instantiate(infcx.tcx, args);
-
-    let InferOk { value: (), obligations: mut nested } = infcx
-        .at(&ObligationCause::dummy_with_span(span), goal.goal().param_env)
-        .eq(DefineOpaqueTypes::Yes, goal_trait_ref, impl_trait_ref)
-        .expect("rematching impl failed");
-
-    // FIXME(-Znext-solver=coinductive): We need to add supertraits here eventually.
-
-    nested.extend(
-        infcx.tcx.predicates_of(impl_def_id).instantiate(infcx.tcx, args).into_iter().map(
-            |(clause, _)| {
-                Obligation::new(
-                    infcx.tcx,
-                    ObligationCause::dummy_with_span(span),
-                    goal.goal().param_env,
-                    clause,
-                )
-            },
-        ),
-    );
-
-    ImplSourceUserDefinedData { impl_def_id, nested, args }
-}
diff --git a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
index 4f79f1b2aaf..fa4323a3a94 100644
--- a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
+++ b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
@@ -93,6 +93,7 @@ pub struct InspectCandidate<'a, 'tcx> {
     kind: inspect::ProbeKind<'tcx>,
     nested_goals: Vec<(GoalSource, inspect::CanonicalState<'tcx, Goal<'tcx, ty::Predicate<'tcx>>>)>,
     final_state: inspect::CanonicalState<'tcx, ()>,
+    impl_args: Option<inspect::CanonicalState<'tcx, ty::GenericArgsRef<'tcx>>>,
     result: QueryResult<'tcx>,
     shallow_certainty: Certainty,
 }
@@ -135,7 +136,20 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
 
     /// Instantiate the nested goals for the candidate without rolling back their
     /// inference constraints. This function modifies the state of the `infcx`.
+    ///
+    /// See [`Self::instantiate_nested_goals_and_opt_impl_args`] if you need the impl args too.
     pub fn instantiate_nested_goals(&self, span: Span) -> Vec<InspectGoal<'a, 'tcx>> {
+        self.instantiate_nested_goals_and_opt_impl_args(span).0
+    }
+
+    /// Instantiate the nested goals for the candidate without rolling back their
+    /// inference constraints, and optionally the args of an impl if this candidate
+    /// came from a `CandidateSource::Impl`. This function modifies the state of the
+    /// `infcx`.
+    pub fn instantiate_nested_goals_and_opt_impl_args(
+        &self,
+        span: Span,
+    ) -> (Vec<InspectGoal<'a, 'tcx>>, Option<ty::GenericArgsRef<'tcx>>) {
         let infcx = self.goal.infcx;
         let param_env = self.goal.goal.param_env;
         let mut orig_values = self.goal.orig_values.to_vec();
@@ -164,6 +178,17 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
             self.final_state,
         );
 
+        let impl_args = self.impl_args.map(|impl_args| {
+            canonical::instantiate_canonical_state(
+                infcx,
+                span,
+                param_env,
+                &mut orig_values,
+                impl_args,
+            )
+            .fold_with(&mut EagerResolver::new(infcx))
+        });
+
         if let Some(term_hack) = self.goal.normalizes_to_term_hack {
             // FIXME: We ignore the expected term of `NormalizesTo` goals
             // when computing the result of its candidates. This is
@@ -171,7 +196,7 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
             let _ = term_hack.constrain(infcx, span, param_env);
         }
 
-        instantiated_goals
+        let goals = instantiated_goals
             .into_iter()
             .map(|(source, goal)| match goal.predicate.kind().no_bound_vars() {
                 Some(ty::PredicateKind::NormalizesTo(ty::NormalizesTo { alias, term })) => {
@@ -208,7 +233,9 @@ impl<'a, 'tcx> InspectCandidate<'a, 'tcx> {
                     source,
                 ),
             })
-            .collect()
+            .collect();
+
+        (goals, impl_args)
     }
 
     /// Visit all nested goals of this candidate, rolling back
@@ -245,9 +272,10 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         probe: &inspect::Probe<'tcx>,
     ) {
         let mut shallow_certainty = None;
+        let mut impl_args = None;
         for step in &probe.steps {
-            match step {
-                &inspect::ProbeStep::AddGoal(source, goal) => nested_goals.push((source, goal)),
+            match *step {
+                inspect::ProbeStep::AddGoal(source, goal) => nested_goals.push((source, goal)),
                 inspect::ProbeStep::NestedProbe(ref probe) => {
                     // Nested probes have to prove goals added in their parent
                     // but do not leak them, so we truncate the added goals
@@ -257,7 +285,10 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
                     nested_goals.truncate(num_goals);
                 }
                 inspect::ProbeStep::MakeCanonicalResponse { shallow_certainty: c } => {
-                    assert_eq!(shallow_certainty.replace(*c), None);
+                    assert_eq!(shallow_certainty.replace(c), None);
+                }
+                inspect::ProbeStep::RecordImplArgs { impl_args: i } => {
+                    assert_eq!(impl_args.replace(i), None);
                 }
                 inspect::ProbeStep::EvaluateGoals(_) => (),
             }
@@ -284,6 +315,7 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
                         final_state: probe.final_state,
                         result,
                         shallow_certainty,
+                        impl_args,
                     });
                 }
             }
diff --git a/compiler/rustc_trait_selection/src/solve/inspect/build.rs b/compiler/rustc_trait_selection/src/solve/inspect/build.rs
index 466d0d80060..79d8901a260 100644
--- a/compiler/rustc_trait_selection/src/solve/inspect/build.rs
+++ b/compiler/rustc_trait_selection/src/solve/inspect/build.rs
@@ -242,6 +242,7 @@ enum WipProbeStep<'tcx> {
     EvaluateGoals(WipAddedGoalsEvaluation<'tcx>),
     NestedProbe(WipProbe<'tcx>),
     MakeCanonicalResponse { shallow_certainty: Certainty },
+    RecordImplArgs { impl_args: inspect::CanonicalState<'tcx, ty::GenericArgsRef<'tcx>> },
 }
 
 impl<'tcx> WipProbeStep<'tcx> {
@@ -250,6 +251,9 @@ impl<'tcx> WipProbeStep<'tcx> {
             WipProbeStep::AddGoal(source, goal) => inspect::ProbeStep::AddGoal(source, goal),
             WipProbeStep::EvaluateGoals(eval) => inspect::ProbeStep::EvaluateGoals(eval.finalize()),
             WipProbeStep::NestedProbe(probe) => inspect::ProbeStep::NestedProbe(probe.finalize()),
+            WipProbeStep::RecordImplArgs { impl_args } => {
+                inspect::ProbeStep::RecordImplArgs { impl_args }
+            }
             WipProbeStep::MakeCanonicalResponse { shallow_certainty } => {
                 inspect::ProbeStep::MakeCanonicalResponse { shallow_certainty }
             }
@@ -534,6 +538,30 @@ impl<'tcx> ProofTreeBuilder<'tcx> {
         }
     }
 
+    pub(crate) fn record_impl_args(
+        &mut self,
+        infcx: &InferCtxt<'tcx>,
+        max_input_universe: ty::UniverseIndex,
+        impl_args: ty::GenericArgsRef<'tcx>,
+    ) {
+        match self.as_mut() {
+            Some(DebugSolver::GoalEvaluationStep(state)) => {
+                let impl_args = canonical::make_canonical_state(
+                    infcx,
+                    &state.var_values,
+                    max_input_universe,
+                    impl_args,
+                );
+                state
+                    .current_evaluation_scope()
+                    .steps
+                    .push(WipProbeStep::RecordImplArgs { impl_args });
+            }
+            None => {}
+            _ => bug!(),
+        }
+    }
+
     pub fn make_canonical_response(&mut self, shallow_certainty: Certainty) {
         match self.as_mut() {
             Some(DebugSolver::GoalEvaluationStep(state)) => {
@@ -543,7 +571,7 @@ impl<'tcx> ProofTreeBuilder<'tcx> {
                     .push(WipProbeStep::MakeCanonicalResponse { shallow_certainty });
             }
             None => {}
-            _ => {}
+            _ => bug!(),
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index d2b893d6383..0fde9dd4cd6 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -75,6 +75,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
 
         ecx.probe_trait_candidate(CandidateSource::Impl(impl_def_id)).enter(|ecx| {
             let impl_args = ecx.fresh_args_for_item(impl_def_id);
+            ecx.record_impl_args(impl_args);
             let impl_trait_ref = impl_trait_header.trait_ref.instantiate(tcx, impl_args);
 
             ecx.eq(goal.param_env, goal.predicate.trait_ref, impl_trait_ref)?;
diff --git a/src/doc/book b/src/doc/book
-Subproject d207d894cc5e1d496ab99beeacd1a420e5d4d23
+Subproject bebcf527e67755a989a1739b7cfaa8f0e6b3004
diff --git a/src/doc/embedded-book b/src/doc/embedded-book
-Subproject aa7d4b0b4653ddb47cb1de2036d090ec2ba9dab
+Subproject 17842ebb050f62e40a4618edeb8e8ee86e75870
diff --git a/src/doc/reference b/src/doc/reference
-Subproject 5854fcc286557ad3ab34d325073d11d8118096b
+Subproject 51817951d0d213a0011f82b62aae02c3b3f2472
diff --git a/src/doc/rust-by-example b/src/doc/rust-by-example
-Subproject 60d34b5fd33db1346f9aabfc0c9d0bda6c8e42b
+Subproject 229ad13b64d919b12e548d560f06d88963b25cd
diff --git a/src/doc/rustc-dev-guide b/src/doc/rustc-dev-guide
-Subproject 07425fed36b00e60341c5e29e28d37d40cbd445
+Subproject 2d1947ff34d50ca46dfe242ad75531a4c429bb5
diff --git a/tests/ui/impl-trait/dyn-trait-elided-two-inputs-ref-assoc.rs b/tests/ui/impl-trait/dyn-trait-elided-two-inputs-ref-assoc.rs
index 2dc19b9ad68..e9706b656f2 100644
--- a/tests/ui/impl-trait/dyn-trait-elided-two-inputs-ref-assoc.rs
+++ b/tests/ui/impl-trait/dyn-trait-elided-two-inputs-ref-assoc.rs
@@ -2,6 +2,9 @@
 // when there are multiple inputs.  The `dyn Bar` should default to `+
 // 'static`. This used to erroneously generate an error (cc #62517).
 //
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
 //@ check-pass
 
 trait Foo {
diff --git a/tests/ui/impl-trait/in-trait/placeholder-implied-bounds.rs b/tests/ui/impl-trait/in-trait/placeholder-implied-bounds.rs
index f7546a05bfd..df03150e29a 100644
--- a/tests/ui/impl-trait/in-trait/placeholder-implied-bounds.rs
+++ b/tests/ui/impl-trait/in-trait/placeholder-implied-bounds.rs
@@ -1,3 +1,6 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
 //@ check-pass
 
 pub fn main() {}
diff --git a/tests/ui/impl-trait/issues/issue-105826.rs b/tests/ui/impl-trait/issues/issue-105826.rs
index e3488140dcc..33c5ed5fdeb 100644
--- a/tests/ui/impl-trait/issues/issue-105826.rs
+++ b/tests/ui/impl-trait/issues/issue-105826.rs
@@ -1,3 +1,6 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
 //@ check-pass
 
 use std::io::Write;
diff --git a/tests/ui/traits/next-solver/select-alias-bound-as-param.rs b/tests/ui/traits/next-solver/select-alias-bound-as-param.rs
new file mode 100644
index 00000000000..fd40ef1f872
--- /dev/null
+++ b/tests/ui/traits/next-solver/select-alias-bound-as-param.rs
@@ -0,0 +1,13 @@
+//@ check-pass
+//@ compile-flags: -Znext-solver
+
+pub(crate) fn y() -> impl FnMut() {
+    || {}
+}
+
+pub(crate) fn x(a: (), b: ()) {
+    let x = ();
+    y()()
+}
+
+fn main() {}
diff --git a/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.next.stderr b/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.next.stderr
deleted file mode 100644
index 3c2bc0b9190..00000000000
--- a/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.next.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: internal compiler error: error performing operation: query type op
-  --> $DIR/illegal-upcast-from-impl-opaque.rs:25:1
-   |
-LL | fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: 
-  --> $DIR/illegal-upcast-from-impl-opaque.rs:25:1
-   |
-LL | fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-query stack during panic:
-end of query stack
diff --git a/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.rs b/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.rs
deleted file mode 100644
index f344474054a..00000000000
--- a/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.rs
+++ /dev/null
@@ -1,29 +0,0 @@
-//@ revisions: current next
-//@[next] compile-flags: -Znext-solver
-//@[next] failure-status: 101
-//@[next] known-bug: unknown
-//@[next] normalize-stderr-test "note: .*\n\n" -> ""
-//@[next] normalize-stderr-test "thread 'rustc' panicked.*\n.*\n" -> ""
-//@[next] normalize-stderr-test "(error: internal compiler error: [^:]+):\d+:\d+: " -> "$1:LL:CC: "
-//@[next] normalize-stderr-test "delayed at .*" -> ""
-//@[next] rustc-env:RUST_BACKTRACE=0
-
-#![feature(trait_upcasting, type_alias_impl_trait)]
-
-trait Super {
-    type Assoc;
-}
-
-trait Sub: Super {}
-
-impl<T: ?Sized> Super for T {
-    type Assoc = i32;
-}
-
-type Foo = impl Sized;
-
-fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-    x //[current]~ mismatched types
-}
-
-fn main() {}
diff --git a/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.current.stderr b/tests/ui/traits/trait-upcasting/upcast-defining-opaque.current.stderr
index c54a1c42bad..a259abb28ae 100644
--- a/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl-opaque.current.stderr
+++ b/tests/ui/traits/trait-upcasting/upcast-defining-opaque.current.stderr
@@ -1,11 +1,11 @@
 error[E0308]: mismatched types
-  --> $DIR/illegal-upcast-from-impl-opaque.rs:26:5
+  --> $DIR/upcast-defining-opaque.rs:21:5
    |
 LL | type Foo = impl Sized;
    |            ---------- the found opaque type
 LL |
-LL | fn illegal(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
-   |                                         ----------------------- expected `&dyn Super<Assoc = i32>` because of return type
+LL | fn upcast(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
+   |                                        ----------------------- expected `&dyn Super<Assoc = i32>` because of return type
 LL |     x
    |     ^ expected trait `Super`, found trait `Sub`
    |
diff --git a/tests/ui/traits/trait-upcasting/upcast-defining-opaque.rs b/tests/ui/traits/trait-upcasting/upcast-defining-opaque.rs
new file mode 100644
index 00000000000..cb1501a94a2
--- /dev/null
+++ b/tests/ui/traits/trait-upcasting/upcast-defining-opaque.rs
@@ -0,0 +1,24 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] check-pass
+
+#![feature(trait_upcasting, type_alias_impl_trait)]
+
+trait Super {
+    type Assoc;
+}
+
+trait Sub: Super {}
+
+impl<T: ?Sized> Super for T {
+    type Assoc = i32;
+}
+
+type Foo = impl Sized;
+
+fn upcast(x: &dyn Sub<Assoc = Foo>) -> &dyn Super<Assoc = i32> {
+    x //[current]~ mismatched types
+}
+
+fn main() {}