Adding upstream version 1.66.0+dfsg1.upstream/1.66.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

34 files changed, 2733 insertions, 1903 deletions

diff --git a/compiler/rustc_trait_selection/Cargo.toml b/compiler/rustc_trait_selection/Cargo.toml
index 566f236f2..67613e1a4 100644
--- a/compiler/rustc_trait_selection/Cargo.toml
+++ b/compiler/rustc_trait_selection/Cargo.toml
@@ -4,7 +4,6 @@ version = "0.0.0"
 edition = "2021"
 
 [lib]
-doctest = false
 
 [dependencies]
 rustc_parse_format = { path = "../rustc_parse_format" }
diff --git a/compiler/rustc_trait_selection/src/autoderef.rs b/compiler/rustc_trait_selection/src/autoderef.rs
index 8b7e8984a..61cfeec4b 100644
--- a/compiler/rustc_trait_selection/src/autoderef.rs
+++ b/compiler/rustc_trait_selection/src/autoderef.rs
@@ -1,6 +1,6 @@
+use crate::errors::AutoDerefReachedRecursionLimit;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{self, TraitEngine};
-use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_infer::infer::InferCtxt;
 use rustc_middle::ty::{self, TraitRef, Ty, TyCtxt};
@@ -25,7 +25,7 @@ struct AutoderefSnapshot<'tcx> {
 
 pub struct Autoderef<'a, 'tcx> {
     // Meta infos:
-    infcx: &'a InferCtxt<'a, 'tcx>,
+    infcx: &'a InferCtxt<'tcx>,
     span: Span,
     overloaded_span: Span,
     body_id: hir::HirId,
@@ -94,7 +94,7 @@ impl<'a, 'tcx> Iterator for Autoderef<'a, 'tcx> {
 
 impl<'a, 'tcx> Autoderef<'a, 'tcx> {
     pub fn new(
-        infcx: &'a InferCtxt<'a, 'tcx>,
+        infcx: &'a InferCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         body_id: hir::HirId,
         span: Span,
@@ -222,19 +222,10 @@ pub fn report_autoderef_recursion_limit_error<'tcx>(tcx: TyCtxt<'tcx>, span: Spa
         Limit(0) => Limit(2),
         limit => limit * 2,
     };
-    struct_span_err!(
-        tcx.sess,
+    tcx.sess.emit_err(AutoDerefReachedRecursionLimit {
         span,
-        E0055,
-        "reached the recursion limit while auto-dereferencing `{:?}`",
-        ty
-    )
-    .span_label(span, "deref recursion limit reached")
-    .help(&format!(
-        "consider increasing the recursion limit by adding a \
-             `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
+        ty,
         suggested_limit,
-        tcx.crate_name(LOCAL_CRATE),
-    ))
-    .emit();
+        crate_name: tcx.crate_name(LOCAL_CRATE),
+    });
 }
diff --git a/compiler/rustc_trait_selection/src/errors.rs b/compiler/rustc_trait_selection/src/errors.rs
new file mode 100644
index 000000000..7f8705824
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/errors.rs
@@ -0,0 +1,102 @@
+use rustc_errors::{fluent, ErrorGuaranteed, Handler, IntoDiagnostic};
+use rustc_macros::Diagnostic;
+use rustc_middle::ty::{self, PolyTraitRef, Ty};
+use rustc_session::Limit;
+use rustc_span::{Span, Symbol};
+
+#[derive(Diagnostic)]
+#[diag(trait_selection_dump_vtable_entries)]
+pub struct DumpVTableEntries<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub trait_ref: PolyTraitRef<'a>,
+    pub entries: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(trait_selection_unable_to_construct_constant_value)]
+pub struct UnableToConstructConstantValue<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub unevaluated: ty::UnevaluatedConst<'a>,
+}
+
+#[derive(Diagnostic)]
+#[help]
+#[diag(trait_selection_auto_deref_reached_recursion_limit, code = "E0055")]
+pub struct AutoDerefReachedRecursionLimit<'a> {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub ty: Ty<'a>,
+    pub suggested_limit: Limit,
+    pub crate_name: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(trait_selection_empty_on_clause_in_rustc_on_unimplemented, code = "E0232")]
+pub struct EmptyOnClauseInOnUnimplemented {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(trait_selection_invalid_on_clause_in_rustc_on_unimplemented, code = "E0232")]
+pub struct InvalidOnClauseInOnUnimplemented {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(trait_selection_no_value_in_rustc_on_unimplemented, code = "E0232")]
+#[note]
+pub struct NoValueInOnUnimplemented {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+pub struct NegativePositiveConflict<'a> {
+    pub impl_span: Span,
+    pub trait_desc: &'a str,
+    pub self_desc: &'a Option<String>,
+    pub negative_impl_span: Result<Span, Symbol>,
+    pub positive_impl_span: Result<Span, Symbol>,
+}
+
+impl IntoDiagnostic<'_> for NegativePositiveConflict<'_> {
+    fn into_diagnostic(
+        self,
+        handler: &Handler,
+    ) -> rustc_errors::DiagnosticBuilder<'_, ErrorGuaranteed> {
+        let mut diag = handler.struct_err(fluent::trait_selection_negative_positive_conflict);
+        diag.set_arg("trait_desc", self.trait_desc);
+        diag.set_arg(
+            "self_desc",
+            self.self_desc.clone().map_or_else(|| String::from("none"), |ty| ty),
+        );
+        diag.set_span(self.impl_span);
+        diag.code(rustc_errors::error_code!(E0751));
+        match self.negative_impl_span {
+            Ok(span) => {
+                diag.span_label(span, fluent::negative_implementation_here);
+            }
+            Err(cname) => {
+                diag.note(fluent::negative_implementation_in_crate);
+                diag.set_arg("negative_impl_cname", cname.to_string());
+            }
+        }
+        match self.positive_impl_span {
+            Ok(span) => {
+                diag.span_label(span, fluent::positive_implementation_here);
+            }
+            Err(cname) => {
+                diag.note(fluent::positive_implementation_in_crate);
+                diag.set_arg("positive_impl_cname", cname.to_string());
+            }
+        }
+        diag
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/infer.rs b/compiler/rustc_trait_selection/src/infer.rs
index 9d30374f8..a335f8e06 100644
--- a/compiler/rustc_trait_selection/src/infer.rs
+++ b/compiler/rustc_trait_selection/src/infer.rs
@@ -24,6 +24,13 @@ pub trait InferCtxtExt<'tcx> {
         span: Span,
     ) -> bool;
 
+    fn type_is_sized_modulo_regions(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+        span: Span,
+    ) -> bool;
+
     fn partially_normalize_associated_types_in<T>(
         &self,
         cause: ObligationCause<'tcx>,
@@ -52,7 +59,7 @@ pub trait InferCtxtExt<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
     ) -> traits::EvaluationResult;
 }
-impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
+impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
     fn type_is_copy_modulo_regions(
         &self,
         param_env: ty::ParamEnv<'tcx>,
@@ -62,7 +69,7 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         let ty = self.resolve_vars_if_possible(ty);
 
         if !(param_env, ty).needs_infer() {
-            return ty.is_copy_modulo_regions(self.tcx.at(span), param_env);
+            return ty.is_copy_modulo_regions(self.tcx, param_env);
         }
 
         let copy_def_id = self.tcx.require_lang_item(LangItem::Copy, None);
@@ -74,6 +81,16 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, copy_def_id, span)
     }
 
+    fn type_is_sized_modulo_regions(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+        span: Span,
+    ) -> bool {
+        let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
+        traits::type_known_to_meet_bound_modulo_regions(self, param_env, ty, lang_item, span)
+    }
+
     /// Normalizes associated types in `value`, potentially returning
     /// new obligations that must further be processed.
     fn partially_normalize_associated_types_in<T>(
@@ -125,7 +142,7 @@ pub trait InferCtxtBuilderExt<'tcx> {
     fn enter_canonical_trait_query<K, R>(
         &mut self,
         canonical_key: &Canonical<'tcx, K>,
-        operation: impl FnOnce(&InferCtxt<'_, 'tcx>, &mut dyn TraitEngine<'tcx>, K) -> Fallible<R>,
+        operation: impl FnOnce(&InferCtxt<'tcx>, &mut dyn TraitEngine<'tcx>, K) -> Fallible<R>,
     ) -> Fallible<CanonicalizedQueryResponse<'tcx, R>>
     where
         K: TypeFoldable<'tcx>,
@@ -153,25 +170,17 @@ impl<'tcx> InferCtxtBuilderExt<'tcx> for InferCtxtBuilder<'tcx> {
     fn enter_canonical_trait_query<K, R>(
         &mut self,
         canonical_key: &Canonical<'tcx, K>,
-        operation: impl FnOnce(&InferCtxt<'_, 'tcx>, &mut dyn TraitEngine<'tcx>, K) -> Fallible<R>,
+        operation: impl FnOnce(&InferCtxt<'tcx>, &mut dyn TraitEngine<'tcx>, K) -> Fallible<R>,
     ) -> Fallible<CanonicalizedQueryResponse<'tcx, R>>
     where
         K: TypeFoldable<'tcx>,
         R: Debug + TypeFoldable<'tcx>,
         Canonical<'tcx, QueryResponse<'tcx, R>>: ArenaAllocatable<'tcx>,
     {
-        self.enter_with_canonical(
-            DUMMY_SP,
-            canonical_key,
-            |ref infcx, key, canonical_inference_vars| {
-                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
-                let value = operation(infcx, &mut *fulfill_cx, key)?;
-                infcx.make_canonicalized_query_response(
-                    canonical_inference_vars,
-                    value,
-                    &mut *fulfill_cx,
-                )
-            },
-        )
+        let (ref infcx, key, canonical_inference_vars) =
+            self.build_with_canonical(DUMMY_SP, canonical_key);
+        let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
+        let value = operation(infcx, &mut *fulfill_cx, key)?;
+        infcx.make_canonicalized_query_response(canonical_inference_vars, value, &mut *fulfill_cx)
     }
 }
diff --git a/compiler/rustc_trait_selection/src/lib.rs b/compiler/rustc_trait_selection/src/lib.rs
index 282ee632c..5d52aa075 100644
--- a/compiler/rustc_trait_selection/src/lib.rs
+++ b/compiler/rustc_trait_selection/src/lib.rs
@@ -16,11 +16,10 @@
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]
 #![feature(hash_drain_filter)]
-#![feature(label_break_value)]
 #![feature(let_chains)]
-#![feature(let_else)]
 #![feature(if_let_guard)]
 #![feature(never_type)]
+#![feature(type_alias_impl_trait)]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]
@@ -36,5 +35,6 @@ extern crate rustc_middle;
 extern crate smallvec;
 
 pub mod autoderef;
+pub mod errors;
 pub mod infer;
 pub mod traits;
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index 294c81d0b..ed34ab95a 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -3,13 +3,14 @@
 
 use super::*;
 
+use crate::errors::UnableToConstructConstantValue;
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::InferCtxt;
 use crate::traits::project::ProjectAndUnifyResult;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::visit::TypeVisitable;
-use rustc_middle::ty::{Region, RegionVid, Term};
+use rustc_middle::ty::{PolyTraitRef, Region, RegionVid};
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 
@@ -65,13 +66,13 @@ impl<'tcx> AutoTraitFinder<'tcx> {
     /// struct Foo<T> { data: Box<T> }
     /// ```
     ///
-    /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
-    /// The analysis attempts to account for custom impls as well as other complex cases. This
-    /// result is intended for use by rustdoc and other such consumers.
+    /// then this might return that `Foo<T>: Send` if `T: Send` (encoded in the AutoTraitResult
+    /// type). The analysis attempts to account for custom impls as well as other complex cases.
+    /// This result is intended for use by rustdoc and other such consumers.
     ///
     /// (Note that due to the coinductive nature of Send, the full and correct result is actually
     /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
-    /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
+    /// types are all Send. So, in our example, we might have that `Foo<T>: Send` if `Box<T>: Send`.
     /// But this is often not the best way to present to the user.)
     ///
     /// Warning: The API should be considered highly unstable, and it may be refactored or removed
@@ -89,145 +90,105 @@ impl<'tcx> AutoTraitFinder<'tcx> {
 
         let trait_pred = ty::Binder::dummy(trait_ref);
 
-        let bail_out = tcx.infer_ctxt().enter(|infcx| {
-            let mut selcx = SelectionContext::new(&infcx);
-            let result = selcx.select(&Obligation::new(
-                ObligationCause::dummy(),
-                orig_env,
-                trait_pred.to_poly_trait_predicate(),
-            ));
-
-            match result {
-                Ok(Some(ImplSource::UserDefined(_))) => {
-                    debug!(
-                        "find_auto_trait_generics({:?}): \
-                         manual impl found, bailing out",
-                        trait_ref
-                    );
-                    return true;
-                }
-                _ => {}
-            }
-
-            let result = selcx.select(&Obligation::new(
-                ObligationCause::dummy(),
-                orig_env,
-                trait_pred.to_poly_trait_predicate_negative_polarity(),
-            ));
-
-            match result {
-                Ok(Some(ImplSource::UserDefined(_))) => {
-                    debug!(
-                        "find_auto_trait_generics({:?}): \
-                         manual impl found, bailing out",
-                        trait_ref
-                    );
-                    true
-                }
-                _ => false,
+        let infcx = tcx.infer_ctxt().build();
+        let mut selcx = SelectionContext::new(&infcx);
+        for f in [
+            PolyTraitRef::to_poly_trait_predicate,
+            PolyTraitRef::to_poly_trait_predicate_negative_polarity,
+        ] {
+            let result =
+                selcx.select(&Obligation::new(ObligationCause::dummy(), orig_env, f(&trait_pred)));
+            if let Ok(Some(ImplSource::UserDefined(_))) = result {
+                debug!(
+                    "find_auto_trait_generics({:?}): \
+                 manual impl found, bailing out",
+                    trait_ref
+                );
+                // If an explicit impl exists, it always takes priority over an auto impl
+                return AutoTraitResult::ExplicitImpl;
             }
-        });
-
-        // If an explicit impl exists, it always takes priority over an auto impl
-        if bail_out {
-            return AutoTraitResult::ExplicitImpl;
         }
 
-        tcx.infer_ctxt().enter(|infcx| {
-            let mut fresh_preds = FxHashSet::default();
+        let infcx = tcx.infer_ctxt().build();
+        let mut fresh_preds = FxHashSet::default();
+
+        // Due to the way projections are handled by SelectionContext, we need to run
+        // evaluate_predicates twice: once on the original param env, and once on the result of
+        // the first evaluate_predicates call.
+        //
+        // The problem is this: most of rustc, including SelectionContext and traits::project,
+        // are designed to work with a concrete usage of a type (e.g., Vec<u8>
+        // fn<T>() { Vec<T> }. This information will generally never change - given
+        // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
+        // If we're unable to prove that 'T' implements a particular trait, we're done -
+        // there's nothing left to do but error out.
+        //
+        // However, synthesizing an auto trait impl works differently. Here, we start out with
+        // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
+        // with - and progressively discover the conditions we need to fulfill for it to
+        // implement a certain auto trait. This ends up breaking two assumptions made by trait
+        // selection and projection:
+        //
+        // * We can always cache the result of a particular trait selection for the lifetime of
+        // an InfCtxt
+        // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
+        // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
+        //
+        // We fix the first assumption by manually clearing out all of the InferCtxt's caches
+        // in between calls to SelectionContext.select. This allows us to keep all of the
+        // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
+        // them between calls.
+        //
+        // We fix the second assumption by reprocessing the result of our first call to
+        // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
+        // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
+        // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
+        // SelectionContext to return it back to us.
+
+        let Some((new_env, user_env)) = self.evaluate_predicates(
+            &infcx,
+            trait_did,
+            ty,
+            orig_env,
+            orig_env,
+            &mut fresh_preds,
+            false,
+        ) else {
+            return AutoTraitResult::NegativeImpl;
+        };
+
+        let (full_env, full_user_env) = self
+            .evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds, true)
+            .unwrap_or_else(|| {
+                panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
+            });
 
-            // Due to the way projections are handled by SelectionContext, we need to run
-            // evaluate_predicates twice: once on the original param env, and once on the result of
-            // the first evaluate_predicates call.
-            //
-            // The problem is this: most of rustc, including SelectionContext and traits::project,
-            // are designed to work with a concrete usage of a type (e.g., Vec<u8>
-            // fn<T>() { Vec<T> }. This information will generally never change - given
-            // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
-            // If we're unable to prove that 'T' implements a particular trait, we're done -
-            // there's nothing left to do but error out.
-            //
-            // However, synthesizing an auto trait impl works differently. Here, we start out with
-            // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
-            // with - and progressively discover the conditions we need to fulfill for it to
-            // implement a certain auto trait. This ends up breaking two assumptions made by trait
-            // selection and projection:
-            //
-            // * We can always cache the result of a particular trait selection for the lifetime of
-            // an InfCtxt
-            // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
-            // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
-            //
-            // We fix the first assumption by manually clearing out all of the InferCtxt's caches
-            // in between calls to SelectionContext.select. This allows us to keep all of the
-            // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
-            // them between calls.
-            //
-            // We fix the second assumption by reprocessing the result of our first call to
-            // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
-            // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
-            // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
-            // SelectionContext to return it back to us.
-
-            let Some((new_env, user_env)) = self.evaluate_predicates(
-                &infcx,
-                trait_did,
-                ty,
-                orig_env,
-                orig_env,
-                &mut fresh_preds,
-                false,
-            ) else {
-                return AutoTraitResult::NegativeImpl;
-            };
-
-            let (full_env, full_user_env) = self
-                .evaluate_predicates(
-                    &infcx,
-                    trait_did,
-                    ty,
-                    new_env,
-                    user_env,
-                    &mut fresh_preds,
-                    true,
-                )
-                .unwrap_or_else(|| {
-                    panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
-                });
-
-            debug!(
-                "find_auto_trait_generics({:?}): fulfilling \
-                 with {:?}",
-                trait_ref, full_env
-            );
-            infcx.clear_caches();
-
-            // At this point, we already have all of the bounds we need. FulfillmentContext is used
-            // to store all of the necessary region/lifetime bounds in the InferContext, as well as
-            // an additional sanity check.
-            let mut fulfill = <dyn TraitEngine<'tcx>>::new(tcx);
-            fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
-            let errors = fulfill.select_all_or_error(&infcx);
-
-            if !errors.is_empty() {
-                panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
-            }
+        debug!(
+            "find_auto_trait_generics({:?}): fulfilling \
+             with {:?}",
+            trait_ref, full_env
+        );
+        infcx.clear_caches();
+
+        // At this point, we already have all of the bounds we need. FulfillmentContext is used
+        // to store all of the necessary region/lifetime bounds in the InferContext, as well as
+        // an additional sanity check.
+        let errors =
+            super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
+        if !errors.is_empty() {
+            panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
+        }
 
-            infcx.process_registered_region_obligations(&Default::default(), full_env);
+        infcx.process_registered_region_obligations(&Default::default(), full_env);
 
-            let region_data = infcx
-                .inner
-                .borrow_mut()
-                .unwrap_region_constraints()
-                .region_constraint_data()
-                .clone();
+        let region_data =
+            infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone();
 
-            let vid_to_region = self.map_vid_to_region(&region_data);
+        let vid_to_region = self.map_vid_to_region(&region_data);
 
-            let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
+        let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
 
-            AutoTraitResult::PositiveImpl(auto_trait_callback(info))
-        })
+        AutoTraitResult::PositiveImpl(auto_trait_callback(info))
     }
 }
 
@@ -273,7 +234,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
     /// user.
     fn evaluate_predicates(
         &self,
-        infcx: &InferCtxt<'_, 'tcx>,
+        infcx: &InferCtxt<'tcx>,
         trait_did: DefId,
         ty: Ty<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
@@ -343,7 +304,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                         }
                     }
 
-                    let obligations = impl_source.clone().nested_obligations().into_iter();
+                    let obligations = impl_source.borrow_nested_obligations().iter().cloned();
 
                     if !self.evaluate_nested_obligations(
                         ty,
@@ -613,7 +574,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
     }
 
     fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
-        if let Term::Ty(ty) = p.term().skip_binder() {
+        if let Some(ty) = p.term().skip_binder().ty() {
             matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
         } else {
             false
@@ -832,8 +793,11 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                                 Ok(None) => {
                                     let tcx = self.tcx;
                                     let def_id = unevaluated.def.did;
-                                    let reported = tcx.sess.struct_span_err(tcx.def_span(def_id), &format!("unable to construct a constant value for the unevaluated constant {:?}", unevaluated)).emit();
-
+                                    let reported =
+                                        tcx.sess.emit_err(UnableToConstructConstantValue {
+                                            span: tcx.def_span(def_id),
+                                            unevaluated: unevaluated,
+                                        });
                                     Err(ErrorHandled::Reported(reported))
                                 }
                                 Err(err) => Err(err),
@@ -875,7 +839,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
 
     pub fn clean_pred(
         &self,
-        infcx: &InferCtxt<'_, 'tcx>,
+        infcx: &InferCtxt<'tcx>,
         p: ty::Predicate<'tcx>,
     ) -> ty::Predicate<'tcx> {
         infcx.freshen(p)
diff --git a/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs b/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
index 9ef7ac9a8..81e1d6449 100644
--- a/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
@@ -28,7 +28,7 @@ impl FulfillmentContext<'_> {
 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
     fn normalize_projection_type(
         &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
+        infcx: &InferCtxt<'tcx>,
         _param_env: ty::ParamEnv<'tcx>,
         projection_ty: ty::ProjectionTy<'tcx>,
         _cause: ObligationCause<'tcx>,
@@ -38,7 +38,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 
     fn register_predicate_obligation(
         &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
+        infcx: &InferCtxt<'tcx>,
         obligation: PredicateObligation<'tcx>,
     ) {
         assert!(!infcx.is_in_snapshot());
@@ -49,7 +49,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.obligations.insert(obligation);
     }
 
-    fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
         {
             let errors = self.select_where_possible(infcx);
 
@@ -71,10 +71,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
             .collect()
     }
 
-    fn select_where_possible(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-    ) -> Vec<FulfillmentError<'tcx>> {
+    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
         assert!(!infcx.is_in_snapshot());
 
         let mut errors = Vec::new();
diff --git a/compiler/rustc_trait_selection/src/traits/codegen.rs b/compiler/rustc_trait_selection/src/traits/codegen.rs
index c0700748c..8a62bf015 100644
--- a/compiler/rustc_trait_selection/src/traits/codegen.rs
+++ b/compiler/rustc_trait_selection/src/traits/codegen.rs
@@ -4,10 +4,12 @@
 // general routines.
 
 use crate::infer::{DefiningAnchor, TyCtxtInferExt};
+use crate::traits::error_reporting::TypeErrCtxtExt;
 use crate::traits::{
     ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngine, TraitEngineExt,
     Unimplemented,
 };
+use rustc_infer::traits::FulfillmentErrorCode;
 use rustc_middle::traits::CodegenObligationError;
 use rustc_middle::ty::{self, TyCtxt};
 
@@ -18,8 +20,7 @@ use rustc_middle::ty::{self, TyCtxt};
 /// obligations *could be* resolved if we wanted to.
 ///
 /// This also expects that `trait_ref` is fully normalized.
-#[instrument(level = "debug", skip(tcx))]
-pub fn codegen_fulfill_obligation<'tcx>(
+pub fn codegen_select_candidate<'tcx>(
     tcx: TyCtxt<'tcx>,
     (param_env, trait_ref): (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>),
 ) -> Result<&'tcx ImplSource<'tcx, ()>, CodegenObligationError> {
@@ -28,53 +29,61 @@ pub fn codegen_fulfill_obligation<'tcx>(
 
     // Do the initial selection for the obligation. This yields the
     // shallow result we are looking for -- that is, what specific impl.
-    let mut infcx_builder =
-        tcx.infer_ctxt().ignoring_regions().with_opaque_type_inference(DefiningAnchor::Bubble);
-    infcx_builder.enter(|infcx| {
-        //~^ HACK `Bubble` is required for
-        // this test to pass: type-alias-impl-trait/assoc-projection-ice.rs
-        let mut selcx = SelectionContext::new(&infcx);
+    let infcx = tcx
+        .infer_ctxt()
+        .ignoring_regions()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .build();
+    //~^ HACK `Bubble` is required for
+    // this test to pass: type-alias-impl-trait/assoc-projection-ice.rs
+    let mut selcx = SelectionContext::new(&infcx);
 
-        let obligation_cause = ObligationCause::dummy();
-        let obligation =
-            Obligation::new(obligation_cause, param_env, trait_ref.to_poly_trait_predicate());
+    let obligation_cause = ObligationCause::dummy();
+    let obligation =
+        Obligation::new(obligation_cause, param_env, trait_ref.to_poly_trait_predicate());
 
-        let selection = match selcx.select(&obligation) {
-            Ok(Some(selection)) => selection,
-            Ok(None) => return Err(CodegenObligationError::Ambiguity),
-            Err(Unimplemented) => return Err(CodegenObligationError::Unimplemented),
-            Err(e) => {
-                bug!("Encountered error `{:?}` selecting `{:?}` during codegen", e, trait_ref)
-            }
-        };
+    let selection = match selcx.select(&obligation) {
+        Ok(Some(selection)) => selection,
+        Ok(None) => return Err(CodegenObligationError::Ambiguity),
+        Err(Unimplemented) => return Err(CodegenObligationError::Unimplemented),
+        Err(e) => {
+            bug!("Encountered error `{:?}` selecting `{:?}` during codegen", e, trait_ref)
+        }
+    };
 
-        debug!(?selection);
+    debug!(?selection);
 
-        // Currently, we use a fulfillment context to completely resolve
-        // all nested obligations. This is because they can inform the
-        // inference of the impl's type parameters.
-        let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx);
-        let impl_source = selection.map(|predicate| {
-            fulfill_cx.register_predicate_obligation(&infcx, predicate);
-        });
+    // Currently, we use a fulfillment context to completely resolve
+    // all nested obligations. This is because they can inform the
+    // inference of the impl's type parameters.
+    let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx);
+    let impl_source = selection.map(|predicate| {
+        fulfill_cx.register_predicate_obligation(&infcx, predicate);
+    });
 
-        // In principle, we only need to do this so long as `impl_source`
-        // contains unbound type parameters. It could be a slight
-        // optimization to stop iterating early.
-        let errors = fulfill_cx.select_all_or_error(&infcx);
-        if !errors.is_empty() {
-            return Err(CodegenObligationError::FulfillmentError);
+    // In principle, we only need to do this so long as `impl_source`
+    // contains unbound type parameters. It could be a slight
+    // optimization to stop iterating early.
+    let errors = fulfill_cx.select_all_or_error(&infcx);
+    if !errors.is_empty() {
+        // `rustc_monomorphize::collector` assumes there are no type errors.
+        // Cycle errors are the only post-monomorphization errors possible; emit them now so
+        // `rustc_ty_utils::resolve_associated_item` doesn't return `None` post-monomorphization.
+        for err in errors {
+            if let FulfillmentErrorCode::CodeCycle(cycle) = err.code {
+                infcx.err_ctxt().report_overflow_error_cycle(&cycle);
+            }
         }
+        return Err(CodegenObligationError::FulfillmentError);
+    }
 
-        let impl_source = infcx.resolve_vars_if_possible(impl_source);
-        let impl_source = infcx.tcx.erase_regions(impl_source);
+    let impl_source = infcx.resolve_vars_if_possible(impl_source);
+    let impl_source = infcx.tcx.erase_regions(impl_source);
 
-        // Opaque types may have gotten their hidden types constrained, but we can ignore them safely
-        // as they will get constrained elsewhere, too.
-        // (ouz-a) This is required for `type-alias-impl-trait/assoc-projection-ice.rs` to pass
-        let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
+    // Opaque types may have gotten their hidden types constrained, but we can ignore them safely
+    // as they will get constrained elsewhere, too.
+    // (ouz-a) This is required for `type-alias-impl-trait/assoc-projection-ice.rs` to pass
+    let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
 
-        debug!("Cache miss: {trait_ref:?} => {impl_source:?}");
-        Ok(&*tcx.arena.alloc(impl_source))
-    })
+    Ok(&*tcx.arena.alloc(impl_source))
 }
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 1c8cdf4ca..8aab75490 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -6,21 +6,22 @@
 
 use crate::infer::outlives::env::OutlivesEnvironment;
 use crate::infer::{CombinedSnapshot, InferOk};
+use crate::traits::outlives_bounds::InferCtxtExt as _;
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::util::impl_subject_and_oblig;
 use crate::traits::SkipLeakCheck;
 use crate::traits::{
-    self, FulfillmentContext, Normalized, Obligation, ObligationCause, PredicateObligation,
-    PredicateObligations, SelectionContext, TraitEngineExt,
+    self, Normalized, Obligation, ObligationCause, ObligationCtxt, PredicateObligation,
+    PredicateObligations, SelectionContext,
 };
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::Diagnostic;
-use rustc_hir::def_id::{DefId, LOCAL_CRATE};
+use rustc_hir::def_id::{DefId, CRATE_DEF_ID, LOCAL_CRATE};
+use rustc_hir::CRATE_HIR_ID;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
-use rustc_infer::traits::{util, TraitEngine};
+use rustc_infer::traits::util;
 use rustc_middle::traits::specialization_graph::OverlapMode;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
-use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::visit::TypeVisitable;
 use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitor};
 use rustc_span::symbol::sym;
@@ -59,23 +60,17 @@ pub fn add_placeholder_note(err: &mut Diagnostic) {
     );
 }
 
-/// If there are types that satisfy both impls, invokes `on_overlap`
+/// If there are types that satisfy both impls, returns `Some`
 /// with a suitably-freshened `ImplHeader` with those types
-/// substituted. Otherwise, invokes `no_overlap`.
-#[instrument(skip(tcx, skip_leak_check, on_overlap, no_overlap), level = "debug")]
-pub fn overlapping_impls<F1, F2, R>(
+/// substituted. Otherwise, returns `None`.
+#[instrument(skip(tcx, skip_leak_check), level = "debug")]
+pub fn overlapping_impls(
     tcx: TyCtxt<'_>,
     impl1_def_id: DefId,
     impl2_def_id: DefId,
     skip_leak_check: SkipLeakCheck,
     overlap_mode: OverlapMode,
-    on_overlap: F1,
-    no_overlap: F2,
-) -> R
-where
-    F1: FnOnce(OverlapResult<'_>) -> R,
-    F2: FnOnce() -> R,
-{
+) -> Option<OverlapResult<'_>> {
     // Before doing expensive operations like entering an inference context, do
     // a quick check via fast_reject to tell if the impl headers could possibly
     // unify.
@@ -96,28 +91,24 @@ where
     if !may_overlap {
         // Some types involved are definitely different, so the impls couldn't possibly overlap.
         debug!("overlapping_impls: fast_reject early-exit");
-        return no_overlap();
+        return None;
     }
 
-    let overlaps = tcx.infer_ctxt().enter(|infcx| {
-        let selcx = &mut SelectionContext::intercrate(&infcx);
-        overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some()
-    });
-
+    let infcx = tcx.infer_ctxt().build();
+    let selcx = &mut SelectionContext::intercrate(&infcx);
+    let overlaps =
+        overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some();
     if !overlaps {
-        return no_overlap();
+        return None;
     }
 
     // In the case where we detect an error, run the check again, but
     // this time tracking intercrate ambiguity causes for better
     // diagnostics. (These take time and can lead to false errors.)
-    tcx.infer_ctxt().enter(|infcx| {
-        let selcx = &mut SelectionContext::intercrate(&infcx);
-        selcx.enable_tracking_intercrate_ambiguity_causes();
-        on_overlap(
-            overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap(),
-        )
-    })
+    let infcx = tcx.infer_ctxt().build();
+    let selcx = &mut SelectionContext::intercrate(&infcx);
+    selcx.enable_tracking_intercrate_ambiguity_causes();
+    Some(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap())
 }
 
 fn with_fresh_ty_vars<'cx, 'tcx>(
@@ -166,7 +157,7 @@ fn overlap_within_probe<'cx, 'tcx>(
     impl1_def_id: DefId,
     impl2_def_id: DefId,
     overlap_mode: OverlapMode,
-    snapshot: &CombinedSnapshot<'_, 'tcx>,
+    snapshot: &CombinedSnapshot<'tcx>,
 ) -> Option<OverlapResult<'tcx>> {
     let infcx = selcx.infcx();
 
@@ -196,7 +187,7 @@ fn overlap_within_probe<'cx, 'tcx>(
         }
     }
 
-    // We disable the leak when when creating the `snapshot` by using
+    // We disable the leak when creating the `snapshot` by using
     // `infcx.probe_maybe_disable_leak_check`.
     if infcx.leak_check(true, snapshot).is_err() {
         debug!("overlap: leak check failed");
@@ -297,36 +288,41 @@ fn negative_impl<'cx, 'tcx>(
     let tcx = selcx.infcx().tcx;
 
     // Create an infcx, taking the predicates of impl1 as assumptions:
-    tcx.infer_ctxt().enter(|infcx| {
-        // create a parameter environment corresponding to a (placeholder) instantiation of impl1
-        let impl_env = tcx.param_env(impl1_def_id);
-        let subject1 = match traits::fully_normalize(
-            &infcx,
-            FulfillmentContext::new(),
-            ObligationCause::dummy(),
-            impl_env,
-            tcx.impl_subject(impl1_def_id),
-        ) {
-            Ok(s) => s,
-            Err(err) => bug!("failed to fully normalize {:?}: {:?}", impl1_def_id, err),
-        };
+    let infcx = tcx.infer_ctxt().build();
+    // create a parameter environment corresponding to a (placeholder) instantiation of impl1
+    let impl_env = tcx.param_env(impl1_def_id);
+    let subject1 = match traits::fully_normalize(
+        &infcx,
+        ObligationCause::dummy(),
+        impl_env,
+        tcx.impl_subject(impl1_def_id),
+    ) {
+        Ok(s) => s,
+        Err(err) => {
+            tcx.sess.delay_span_bug(
+                tcx.def_span(impl1_def_id),
+                format!("failed to fully normalize {:?}: {:?}", impl1_def_id, err),
+            );
+            return false;
+        }
+    };
 
-        // Attempt to prove that impl2 applies, given all of the above.
-        let selcx = &mut SelectionContext::new(&infcx);
-        let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
-        let (subject2, obligations) =
-            impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
+    // Attempt to prove that impl2 applies, given all of the above.
+    let selcx = &mut SelectionContext::new(&infcx);
+    let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
+    let (subject2, obligations) =
+        impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
 
-        !equate(&infcx, impl_env, subject1, subject2, obligations)
-    })
+    !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
 }
 
-fn equate<'cx, 'tcx>(
-    infcx: &InferCtxt<'cx, 'tcx>,
+fn equate<'tcx>(
+    infcx: &InferCtxt<'tcx>,
     impl_env: ty::ParamEnv<'tcx>,
     subject1: ImplSubject<'tcx>,
     subject2: ImplSubject<'tcx>,
     obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
+    body_def_id: DefId,
 ) -> bool {
     // do the impls unify? If not, not disjoint.
     let Ok(InferOk { obligations: more_obligations, .. }) =
@@ -340,7 +336,7 @@ fn equate<'cx, 'tcx>(
     let opt_failing_obligation = obligations
         .into_iter()
         .chain(more_obligations)
-        .find(|o| negative_impl_exists(selcx, impl_env, o));
+        .find(|o| negative_impl_exists(selcx, o, body_def_id));
 
     if let Some(failing_obligation) = opt_failing_obligation {
         debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
@@ -354,18 +350,16 @@ fn equate<'cx, 'tcx>(
 #[instrument(level = "debug", skip(selcx))]
 fn negative_impl_exists<'cx, 'tcx>(
     selcx: &SelectionContext<'cx, 'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
     o: &PredicateObligation<'tcx>,
+    body_def_id: DefId,
 ) -> bool {
-    let infcx = &selcx.infcx().fork();
-
-    if resolve_negative_obligation(infcx, param_env, o) {
+    if resolve_negative_obligation(selcx.infcx().fork(), o, body_def_id) {
         return true;
     }
 
     // Try to prove a negative obligation exists for super predicates
-    for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
-        if resolve_negative_obligation(infcx, param_env, &o) {
+    for o in util::elaborate_predicates(selcx.tcx(), iter::once(o.predicate)) {
+        if resolve_negative_obligation(selcx.infcx().fork(), &o, body_def_id) {
             return true;
         }
     }
@@ -374,10 +368,10 @@ fn negative_impl_exists<'cx, 'tcx>(
 }
 
 #[instrument(level = "debug", skip(infcx))]
-fn resolve_negative_obligation<'cx, 'tcx>(
-    infcx: &InferCtxt<'cx, 'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
+fn resolve_negative_obligation<'tcx>(
+    infcx: InferCtxt<'tcx>,
     o: &PredicateObligation<'tcx>,
+    body_def_id: DefId,
 ) -> bool {
     let tcx = infcx.tcx;
 
@@ -385,17 +379,25 @@ fn resolve_negative_obligation<'cx, 'tcx>(
         return false;
     };
 
-    let mut fulfillment_cx = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
-    fulfillment_cx.register_predicate_obligation(infcx, o);
-
-    let errors = fulfillment_cx.select_all_or_error(infcx);
-
-    if !errors.is_empty() {
+    let param_env = o.param_env;
+    if !super::fully_solve_obligation(&infcx, o).is_empty() {
         return false;
     }
 
-    // FIXME -- also add "assumed to be well formed" types into the `outlives_env`
-    let outlives_env = OutlivesEnvironment::new(param_env);
+    let (body_id, body_def_id) = if let Some(body_def_id) = body_def_id.as_local() {
+        (tcx.hir().local_def_id_to_hir_id(body_def_id), body_def_id)
+    } else {
+        (CRATE_HIR_ID, CRATE_DEF_ID)
+    };
+
+    let ocx = ObligationCtxt::new(&infcx);
+    let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
+    let outlives_env = OutlivesEnvironment::with_bounds(
+        param_env,
+        Some(&infcx),
+        infcx.implied_bounds_tys(param_env, body_id, wf_tys),
+    );
+
     infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
 
     infcx.resolve_regions(&outlives_env).is_empty()
@@ -404,12 +406,12 @@ fn resolve_negative_obligation<'cx, 'tcx>(
 pub fn trait_ref_is_knowable<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
-) -> Option<Conflict> {
+) -> Result<(), Conflict> {
     debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
         // A downstream or cousin crate is allowed to implement some
         // substitution of this trait-ref.
-        return Some(Conflict::Downstream);
+        return Err(Conflict::Downstream);
     }
 
     if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
@@ -418,7 +420,7 @@ pub fn trait_ref_is_knowable<'tcx>(
         // allowed to implement a substitution of this trait ref, which
         // means impls could only come from dependencies of this crate,
         // which we already know about.
-        return None;
+        return Ok(());
     }
 
     // This is a remote non-fundamental trait, so if another crate
@@ -431,10 +433,10 @@ pub fn trait_ref_is_knowable<'tcx>(
     // we are an owner.
     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
         debug!("trait_ref_is_knowable: orphan check passed");
-        None
+        Ok(())
     } else {
         debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
-        Some(Conflict::Upstream)
+        Err(Conflict::Upstream)
     }
 }
 
@@ -740,7 +742,21 @@ impl<'tcx> TypeVisitor<'tcx> for OrphanChecker<'tcx> {
         result
     }
 
-    // FIXME: Constants should participate in orphan checking.
+    /// All possible values for a constant parameter already exist
+    /// in the crate defining the trait, so they are always non-local[^1].
+    ///
+    /// Because there's no way to have an impl where the first local
+    /// generic argument is a constant, we also don't have to fail
+    /// the orphan check when encountering a parameter or a generic constant.
+    ///
+    /// This means that we can completely ignore constants during the orphan check.
+    ///
+    /// See `src/test/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
+    ///
+    /// [^1]: This might not hold for function pointers or trait objects in the future.
+    /// As these should be quite rare as const arguments and especially rare as impl
+    /// parameters, allowing uncovered const parameters in impls seems more useful
+    /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
     fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         ControlFlow::CONTINUE
     }
diff --git a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
index 254bc4ab6..84038625f 100644
--- a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
+++ b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
@@ -9,14 +9,12 @@
 //! `thir_abstract_const` which can then be checked for structural equality with other
 //! generic constants mentioned in the `caller_bounds` of the current environment.
 use rustc_errors::ErrorGuaranteed;
-use rustc_hir::def::DefKind;
 use rustc_infer::infer::InferCtxt;
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::abstract_const::{
     walk_abstract_const, AbstractConst, FailureKind, Node, NotConstEvaluatable,
 };
 use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
-use rustc_session::lint;
 use rustc_span::Span;
 
 use std::iter;
@@ -101,7 +99,7 @@ impl<'tcx> ConstUnifyCtxt<'tcx> {
                         a_uv == b_uv
                     }
                     // FIXME(generic_const_exprs): We may want to either actually try
-                    // to evaluate `a_ct` and `b_ct` if they are are fully concrete or something like
+                    // to evaluate `a_ct` and `b_ct` if they are fully concrete or something like
                     // this, for now we just return false here.
                     _ => false,
                 }
@@ -138,7 +136,7 @@ impl<'tcx> ConstUnifyCtxt<'tcx> {
 #[instrument(skip(tcx), level = "debug")]
 pub fn try_unify_abstract_consts<'tcx>(
     tcx: TyCtxt<'tcx>,
-    (a, b): (ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>),
+    (a, b): (ty::UnevaluatedConst<'tcx>, ty::UnevaluatedConst<'tcx>),
     param_env: ty::ParamEnv<'tcx>,
 ) -> bool {
     (|| {
@@ -159,13 +157,22 @@ pub fn try_unify_abstract_consts<'tcx>(
 
 /// Check if a given constant can be evaluated.
 #[instrument(skip(infcx), level = "debug")]
-pub fn is_const_evaluatable<'cx, 'tcx>(
-    infcx: &InferCtxt<'cx, 'tcx>,
-    uv: ty::Unevaluated<'tcx, ()>,
+pub fn is_const_evaluatable<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    ct: ty::Const<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     span: Span,
 ) -> Result<(), NotConstEvaluatable> {
     let tcx = infcx.tcx;
+    let uv = match ct.kind() {
+        ty::ConstKind::Unevaluated(uv) => uv,
+        ty::ConstKind::Param(_)
+        | ty::ConstKind::Bound(_, _)
+        | ty::ConstKind::Placeholder(_)
+        | ty::ConstKind::Value(_)
+        | ty::ConstKind::Error(_) => return Ok(()),
+        ty::ConstKind::Infer(_) => return Err(NotConstEvaluatable::MentionsInfer),
+    };
 
     if tcx.features().generic_const_exprs {
         if let Some(ct) = AbstractConst::new(tcx, uv)? {
@@ -183,7 +190,7 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
                 FailureKind::Concrete => {}
             }
         }
-        let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
+        let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
         match concrete {
             Err(ErrorHandled::TooGeneric) => {
                 Err(NotConstEvaluatable::Error(infcx.tcx.sess.delay_span_bug(
@@ -210,7 +217,7 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
         // and hopefully soon change this to an error.
         //
         // See #74595 for more details about this.
-        let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
+        let concrete = infcx.const_eval_resolve(param_env, uv, Some(span));
 
         match concrete {
           // If we're evaluating a foreign constant, under a nightly compiler without generic
@@ -235,39 +242,25 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
                   .emit()
             }
 
-            Err(ErrorHandled::TooGeneric) => Err(if uv.has_infer_types_or_consts() {
-                NotConstEvaluatable::MentionsInfer
-                } else if uv.has_param_types_or_consts() {
-                NotConstEvaluatable::MentionsParam
-            } else {
-                let guar = infcx.tcx.sess.delay_span_bug(span, format!("Missing value for constant, but no error reported?"));
-                NotConstEvaluatable::Error(guar)
-            }),
+            Err(ErrorHandled::TooGeneric) => {
+                let err = if uv.has_non_region_infer() {
+                    NotConstEvaluatable::MentionsInfer
+                } else if uv.has_non_region_param() {
+                    NotConstEvaluatable::MentionsParam
+                } else {
+                    let guar = infcx.tcx.sess.delay_span_bug(span, format!("Missing value for constant, but no error reported?"));
+                    NotConstEvaluatable::Error(guar)
+                };
+
+                Err(err)
+            },
             Err(ErrorHandled::Linted) => {
                 let reported =
                     infcx.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
                 Err(NotConstEvaluatable::Error(reported))
             }
             Err(ErrorHandled::Reported(e)) => Err(NotConstEvaluatable::Error(e)),
-            Ok(_) => {
-              if uv.substs.has_param_types_or_consts() {
-                  assert!(matches!(infcx.tcx.def_kind(uv.def.did), DefKind::AnonConst));
-                  let mir_body = infcx.tcx.mir_for_ctfe_opt_const_arg(uv.def);
-
-                  if mir_body.is_polymorphic {
-                      let Some(local_def_id) = uv.def.did.as_local() else { return Ok(()) };
-                      tcx.struct_span_lint_hir(
-                          lint::builtin::CONST_EVALUATABLE_UNCHECKED,
-                          tcx.hir().local_def_id_to_hir_id(local_def_id),
-                          span,
-                          |err| {
-                              err.build("cannot use constants which depend on generic parameters in types").emit();
-                        })
-                  }
-              }
-
-              Ok(())
-            },
+            Ok(_) => Ok(()),
         }
     }
 }
@@ -281,7 +274,7 @@ fn satisfied_from_param_env<'tcx>(
     for pred in param_env.caller_bounds() {
         match pred.kind().skip_binder() {
             ty::PredicateKind::ConstEvaluatable(uv) => {
-                if let Some(b_ct) = AbstractConst::new(tcx, uv)? {
+                if let Some(b_ct) = AbstractConst::from_const(tcx, uv)? {
                     let const_unify_ctxt = ConstUnifyCtxt { tcx, param_env };
 
                     // Try to unify with each subtree in the AbstractConst to allow for
diff --git a/compiler/rustc_trait_selection/src/traits/engine.rs b/compiler/rustc_trait_selection/src/traits/engine.rs
index 6c177f638..e0c8deec9 100644
--- a/compiler/rustc_trait_selection/src/traits/engine.rs
+++ b/compiler/rustc_trait_selection/src/traits/engine.rs
@@ -3,7 +3,8 @@ use std::cell::RefCell;
 use super::TraitEngine;
 use super::{ChalkFulfillmentContext, FulfillmentContext};
 use crate::infer::InferCtxtExt;
-use rustc_hir::def_id::DefId;
+use rustc_data_structures::fx::FxHashSet;
+use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::infer::{InferCtxt, InferOk};
 use rustc_infer::traits::{
     FulfillmentError, Obligation, ObligationCause, PredicateObligation, TraitEngineExt as _,
@@ -12,9 +13,11 @@ use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::ToPredicate;
 use rustc_middle::ty::TypeFoldable;
 use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::Span;
 
 pub trait TraitEngineExt<'tcx> {
     fn new(tcx: TyCtxt<'tcx>) -> Box<Self>;
+    fn new_in_snapshot(tcx: TyCtxt<'tcx>) -> Box<Self>;
 }
 
 impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
@@ -25,20 +28,32 @@ impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
             Box::new(FulfillmentContext::new())
         }
     }
+
+    fn new_in_snapshot(tcx: TyCtxt<'tcx>) -> Box<Self> {
+        if tcx.sess.opts.unstable_opts.chalk {
+            Box::new(ChalkFulfillmentContext::new())
+        } else {
+            Box::new(FulfillmentContext::new_in_snapshot())
+        }
+    }
 }
 
 /// Used if you want to have pleasant experience when dealing
 /// with obligations outside of hir or mir typeck.
 pub struct ObligationCtxt<'a, 'tcx> {
-    pub infcx: &'a InferCtxt<'a, 'tcx>,
+    pub infcx: &'a InferCtxt<'tcx>,
     engine: RefCell<Box<dyn TraitEngine<'tcx>>>,
 }
 
 impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
-    pub fn new(infcx: &'a InferCtxt<'a, 'tcx>) -> Self {
+    pub fn new(infcx: &'a InferCtxt<'tcx>) -> Self {
         Self { infcx, engine: RefCell::new(<dyn TraitEngine<'_>>::new(infcx.tcx)) }
     }
 
+    pub fn new_in_snapshot(infcx: &'a InferCtxt<'tcx>) -> Self {
+        Self { infcx, engine: RefCell::new(<dyn TraitEngine<'_>>::new_in_snapshot(infcx.tcx)) }
+    }
+
     pub fn register_obligation(&self, obligation: PredicateObligation<'tcx>) {
         self.engine.borrow_mut().register_predicate_obligation(self.infcx, obligation);
     }
@@ -109,4 +124,34 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     pub fn select_all_or_error(&self) -> Vec<FulfillmentError<'tcx>> {
         self.engine.borrow_mut().select_all_or_error(self.infcx)
     }
+
+    pub fn assumed_wf_types(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        span: Span,
+        def_id: LocalDefId,
+    ) -> FxHashSet<Ty<'tcx>> {
+        let tcx = self.infcx.tcx;
+        let assumed_wf_types = tcx.assumed_wf_types(def_id);
+        let mut implied_bounds = FxHashSet::default();
+        let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+        let cause = ObligationCause::misc(span, hir_id);
+        for ty in assumed_wf_types {
+            // FIXME(@lcnr): rustc currently does not check wf for types
+            // pre-normalization, meaning that implied bounds are sometimes
+            // incorrect. See #100910 for more details.
+            //
+            // Not adding the unnormalized types here mostly fixes that, except
+            // that there are projections which are still ambiguous in the item definition
+            // but do normalize successfully when using the item, see #98543.
+            //
+            // Anyways, I will hopefully soon change implied bounds to make all of this
+            // sound and then uncomment this line again.
+
+            // implied_bounds.insert(ty);
+            let normalized = self.normalize(cause.clone(), param_env, ty);
+            implied_bounds.insert(normalized);
+        }
+        implied_bounds
+    }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index e442c5c91..1217d264a 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -2,10 +2,10 @@ pub mod on_unimplemented;
 pub mod suggestions;
 
 use super::{
-    EvaluationResult, FulfillmentContext, FulfillmentError, FulfillmentErrorCode,
-    MismatchedProjectionTypes, Obligation, ObligationCause, ObligationCauseCode,
-    OnUnimplementedDirective, OnUnimplementedNote, OutputTypeParameterMismatch, Overflow,
-    PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
+    FulfillmentContext, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
+    Obligation, ObligationCause, ObligationCauseCode, OnUnimplementedDirective,
+    OnUnimplementedNote, OutputTypeParameterMismatch, Overflow, PredicateObligation,
+    SelectionContext, SelectionError, TraitNotObjectSafe,
 };
 
 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
@@ -22,6 +22,8 @@ use rustc_hir::intravisit::Visitor;
 use rustc_hir::GenericParam;
 use rustc_hir::Item;
 use rustc_hir::Node;
+use rustc_infer::infer::error_reporting::TypeErrCtxt;
+use rustc_infer::infer::TypeTrace;
 use rustc_infer::traits::TraitEngine;
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
@@ -31,6 +33,8 @@ use rustc_middle::ty::{
     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
     TypeVisitable,
 };
+use rustc_session::Limit;
+use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::symbol::{kw, sym};
 use rustc_span::{ExpnKind, Span, DUMMY_SP};
 use std::fmt;
@@ -40,8 +44,8 @@ use std::ops::ControlFlow;
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use crate::traits::query::normalize::AtExt as _;
 use crate::traits::specialize::to_pretty_impl_header;
-use on_unimplemented::InferCtxtExt as _;
-use suggestions::InferCtxtExt as _;
+use on_unimplemented::TypeErrCtxtExt as _;
+use suggestions::TypeErrCtxtExt as _;
 
 pub use rustc_infer::traits::error_reporting::*;
 
@@ -62,6 +66,37 @@ pub struct ImplCandidate<'tcx> {
 }
 
 pub trait InferCtxtExt<'tcx> {
+    /// Given some node representing a fn-like thing in the HIR map,
+    /// returns a span and `ArgKind` information that describes the
+    /// arguments it expects. This can be supplied to
+    /// `report_arg_count_mismatch`.
+    fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
+
+    /// Reports an error when the number of arguments needed by a
+    /// trait match doesn't match the number that the expression
+    /// provides.
+    fn report_arg_count_mismatch(
+        &self,
+        span: Span,
+        found_span: Option<Span>,
+        expected_args: Vec<ArgKind>,
+        found_args: Vec<ArgKind>,
+        is_closure: bool,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
+
+    /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
+    /// in that order, and returns the generic type corresponding to the
+    /// argument of that trait (corresponding to the closure arguments).
+    fn type_implements_fn_trait(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        ty: ty::Binder<'tcx, Ty<'tcx>>,
+        constness: ty::BoundConstness,
+        polarity: ty::ImplPolarity,
+    ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
+}
+
+pub trait TypeErrCtxtExt<'tcx> {
     fn report_fulfillment_errors(
         &self,
         errors: &[FulfillmentError<'tcx>],
@@ -77,6 +112,8 @@ pub trait InferCtxtExt<'tcx> {
     where
         T: fmt::Display + TypeFoldable<'tcx>;
 
+    fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
+
     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 
     /// The `root_obligation` parameter should be the `root_obligation` field
@@ -89,12 +126,71 @@ pub trait InferCtxtExt<'tcx> {
         error: &SelectionError<'tcx>,
         fallback_has_occurred: bool,
     );
+}
 
+impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
     /// Given some node representing a fn-like thing in the HIR map,
     /// returns a span and `ArgKind` information that describes the
     /// arguments it expects. This can be supplied to
     /// `report_arg_count_mismatch`.
-    fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
+    fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
+        let sm = self.tcx.sess.source_map();
+        let hir = self.tcx.hir();
+        Some(match node {
+            Node::Expr(&hir::Expr {
+                kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, .. }),
+                ..
+            }) => (
+                fn_decl_span,
+                hir.body(body)
+                    .params
+                    .iter()
+                    .map(|arg| {
+                        if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
+                            *arg.pat
+                        {
+                            Some(ArgKind::Tuple(
+                                Some(span),
+                                args.iter()
+                                    .map(|pat| {
+                                        sm.span_to_snippet(pat.span)
+                                            .ok()
+                                            .map(|snippet| (snippet, "_".to_owned()))
+                                    })
+                                    .collect::<Option<Vec<_>>>()?,
+                            ))
+                        } else {
+                            let name = sm.span_to_snippet(arg.pat.span).ok()?;
+                            Some(ArgKind::Arg(name, "_".to_owned()))
+                        }
+                    })
+                    .collect::<Option<Vec<ArgKind>>>()?,
+            ),
+            Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
+            | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
+            | Node::TraitItem(&hir::TraitItem {
+                kind: hir::TraitItemKind::Fn(ref sig, _), ..
+            }) => (
+                sig.span,
+                sig.decl
+                    .inputs
+                    .iter()
+                    .map(|arg| match arg.kind {
+                        hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
+                            Some(arg.span),
+                            vec![("_".to_owned(), "_".to_owned()); tys.len()],
+                        ),
+                        _ => ArgKind::empty(),
+                    })
+                    .collect::<Vec<ArgKind>>(),
+            ),
+            Node::Ctor(ref variant_data) => {
+                let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
+                (span, vec![ArgKind::empty(); variant_data.fields().len()])
+            }
+            _ => panic!("non-FnLike node found: {:?}", node),
+        })
+    }
 
     /// Reports an error when the number of arguments needed by a
     /// trait match doesn't match the number that the expression
@@ -106,21 +202,175 @@ pub trait InferCtxtExt<'tcx> {
         expected_args: Vec<ArgKind>,
         found_args: Vec<ArgKind>,
         is_closure: bool,
-    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let kind = if is_closure { "closure" } else { "function" };
+
+        let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
+            let arg_length = arguments.len();
+            let distinct = matches!(other, &[ArgKind::Tuple(..)]);
+            match (arg_length, arguments.get(0)) {
+                (1, Some(&ArgKind::Tuple(_, ref fields))) => {
+                    format!("a single {}-tuple as argument", fields.len())
+                }
+                _ => format!(
+                    "{} {}argument{}",
+                    arg_length,
+                    if distinct && arg_length > 1 { "distinct " } else { "" },
+                    pluralize!(arg_length)
+                ),
+            }
+        };
+
+        let expected_str = args_str(&expected_args, &found_args);
+        let found_str = args_str(&found_args, &expected_args);
+
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            span,
+            E0593,
+            "{} is expected to take {}, but it takes {}",
+            kind,
+            expected_str,
+            found_str,
+        );
+
+        err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
+
+        if let Some(found_span) = found_span {
+            err.span_label(found_span, format!("takes {}", found_str));
+
+            // move |_| { ... }
+            // ^^^^^^^^-- def_span
+            //
+            // move |_| { ... }
+            // ^^^^^-- prefix
+            let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
+            // move |_| { ... }
+            //      ^^^-- pipe_span
+            let pipe_span =
+                if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
+
+            // Suggest to take and ignore the arguments with expected_args_length `_`s if
+            // found arguments is empty (assume the user just wants to ignore args in this case).
+            // For example, if `expected_args_length` is 2, suggest `|_, _|`.
+            if found_args.is_empty() && is_closure {
+                let underscores = vec!["_"; expected_args.len()].join(", ");
+                err.span_suggestion_verbose(
+                    pipe_span,
+                    &format!(
+                        "consider changing the closure to take and ignore the expected argument{}",
+                        pluralize!(expected_args.len())
+                    ),
+                    format!("|{}|", underscores),
+                    Applicability::MachineApplicable,
+                );
+            }
+
+            if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
+                if fields.len() == expected_args.len() {
+                    let sugg = fields
+                        .iter()
+                        .map(|(name, _)| name.to_owned())
+                        .collect::<Vec<String>>()
+                        .join(", ");
+                    err.span_suggestion_verbose(
+                        found_span,
+                        "change the closure to take multiple arguments instead of a single tuple",
+                        format!("|{}|", sugg),
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
+            if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
+                && fields.len() == found_args.len()
+                && is_closure
+            {
+                let sugg = format!(
+                    "|({}){}|",
+                    found_args
+                        .iter()
+                        .map(|arg| match arg {
+                            ArgKind::Arg(name, _) => name.to_owned(),
+                            _ => "_".to_owned(),
+                        })
+                        .collect::<Vec<String>>()
+                        .join(", "),
+                    // add type annotations if available
+                    if found_args.iter().any(|arg| match arg {
+                        ArgKind::Arg(_, ty) => ty != "_",
+                        _ => false,
+                    }) {
+                        format!(
+                            ": ({})",
+                            fields
+                                .iter()
+                                .map(|(_, ty)| ty.to_owned())
+                                .collect::<Vec<String>>()
+                                .join(", ")
+                        )
+                    } else {
+                        String::new()
+                    },
+                );
+                err.span_suggestion_verbose(
+                    found_span,
+                    "change the closure to accept a tuple instead of individual arguments",
+                    sugg,
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+
+        err
+    }
 
-    /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
-    /// in that order, and returns the generic type corresponding to the
-    /// argument of that trait (corresponding to the closure arguments).
     fn type_implements_fn_trait(
         &self,
         param_env: ty::ParamEnv<'tcx>,
         ty: ty::Binder<'tcx, Ty<'tcx>>,
         constness: ty::BoundConstness,
         polarity: ty::ImplPolarity,
-    ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
-}
+    ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
+        self.commit_if_ok(|_| {
+            for trait_def_id in [
+                self.tcx.lang_items().fn_trait(),
+                self.tcx.lang_items().fn_mut_trait(),
+                self.tcx.lang_items().fn_once_trait(),
+            ] {
+                let Some(trait_def_id) = trait_def_id else { continue };
+                // Make a fresh inference variable so we can determine what the substitutions
+                // of the trait are.
+                let var = self.next_ty_var(TypeVariableOrigin {
+                    span: DUMMY_SP,
+                    kind: TypeVariableOriginKind::MiscVariable,
+                });
+                let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
+                let obligation = Obligation::new(
+                    ObligationCause::dummy(),
+                    param_env,
+                    ty.rebind(ty::TraitPredicate {
+                        trait_ref: ty::TraitRef::new(trait_def_id, substs),
+                        constness,
+                        polarity,
+                    })
+                    .to_predicate(self.tcx),
+                );
+                let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
+                fulfill_cx.register_predicate_obligation(self, obligation);
+                if fulfill_cx.select_all_or_error(self).is_empty() {
+                    return Ok((
+                        ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
+                            .expect("expected to map DefId to ClosureKind"),
+                        ty.rebind(self.resolve_vars_if_possible(var)),
+                    ));
+                }
+            }
 
-impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
+            Err(())
+        })
+    }
+}
+impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     fn report_fulfillment_errors(
         &self,
         errors: &[FulfillmentError<'tcx>],
@@ -250,6 +500,19 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         bug!();
     }
 
+    fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
+        let suggested_limit = match self.tcx.recursion_limit() {
+            Limit(0) => Limit(2),
+            limit => limit * 2,
+        };
+        err.help(&format!(
+            "consider increasing the recursion limit by adding a \
+             `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
+            suggested_limit,
+            self.tcx.crate_name(LOCAL_CRATE),
+        ));
+    }
+
     /// Reports that a cycle was detected which led to overflow and halts
     /// compilation. This is equivalent to `report_overflow_error` except
     /// that we can give a more helpful error message (and, in particular,
@@ -348,7 +611,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             message,
                             label,
                             note,
-                            enclosing_scope,
+                            parent_label,
                             append_const_msg,
                         } = self.on_unimplemented_note(trait_ref, &obligation);
                         let have_alt_message = message.is_some() || label.is_some();
@@ -449,12 +712,27 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         {
                             "consider using `()`, or a `Result`".to_owned()
                         } else {
-                            format!(
-                                "{}the trait `{}` is not implemented for `{}`",
-                                pre_message,
-                                trait_predicate.print_modifiers_and_trait_path(),
-                                trait_ref.skip_binder().self_ty(),
-                            )
+                            let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
+                                ty::FnDef(_, _) => Some("fn item"),
+                                ty::Closure(_, _) => Some("closure"),
+                                _ => None,
+                            };
+
+                            match ty_desc {
+                                Some(desc) => format!(
+                                    "{}the trait `{}` is not implemented for {} `{}`",
+                                    pre_message,
+                                    trait_predicate.print_modifiers_and_trait_path(),
+                                    desc,
+                                    trait_ref.skip_binder().self_ty(),
+                                ),
+                                None => format!(
+                                    "{}the trait `{}` is not implemented for `{}`",
+                                    pre_message,
+                                    trait_predicate.print_modifiers_and_trait_path(),
+                                    trait_ref.skip_binder().self_ty(),
+                                ),
+                            }
                         };
 
                         if self.suggest_add_reference_to_arg(
@@ -482,10 +760,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         }
 
                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
-                           Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
+                            Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
                         }
 
+                        let mut unsatisfied_const = false;
                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
                             let non_const_predicate = trait_ref.without_const();
                             let non_const_obligation = Obligation {
@@ -495,6 +774,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                 recursion_depth: obligation.recursion_depth,
                             };
                             if self.predicate_may_hold(&non_const_obligation) {
+                                unsatisfied_const = true;
                                 err.span_note(
                                     span,
                                     &format!(
@@ -514,7 +794,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
                             err.note(s.as_str());
                         }
-                        if let Some(ref s) = enclosing_scope {
+                        if let Some(ref s) = parent_label {
                             let body = tcx
                                 .hir()
                                 .opt_local_def_id(obligation.cause.body_id)
@@ -523,11 +803,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                         hir_id: obligation.cause.body_id,
                                     })
                                 });
-
-                            let enclosing_scope_span =
-                                tcx.hir().span_with_body(tcx.hir().local_def_id_to_hir_id(body));
-
-                            err.span_label(enclosing_scope_span, s);
+                            err.span_label(tcx.def_span(body), s);
                         }
 
                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
@@ -594,11 +870,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         // Try to report a help message
                         if is_fn_trait
                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
-                                obligation.param_env,
-                                trait_ref.self_ty(),
-                                trait_predicate.skip_binder().constness,
-                                trait_predicate.skip_binder().polarity,
-                            )
+                            obligation.param_env,
+                            trait_ref.self_ty(),
+                            trait_predicate.skip_binder().constness,
+                            trait_predicate.skip_binder().polarity,
+                        )
                         {
                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
                             // suggestion to add trait bounds for the type, since we only typically implement
@@ -649,8 +925,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                     );
                                 }
                             }
-                        } else if !trait_ref.has_infer_types_or_consts()
-                            && self.predicate_can_apply(obligation.param_env, trait_ref)
+                        } else if !trait_ref.has_non_region_infer()
+                            && self.predicate_can_apply(obligation.param_env, trait_predicate)
                         {
                             // If a where-clause may be useful, remind the
                             // user that they can add it.
@@ -665,7 +941,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                 None,
                                 obligation.cause.body_id,
                             );
-                        } else if !suggested {
+                        } else if !suggested && !unsatisfied_const {
                             // Can't show anything else useful, try to find similar impls.
                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
                             if !self.report_similar_impl_candidates(
@@ -828,12 +1104,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
                         // Additional context information explaining why the closure only implements
                         // a particular trait.
-                        if let Some(typeck_results) = self.in_progress_typeck_results {
+                        if let Some(typeck_results) = &self.typeck_results {
                             let hir_id = self
                                 .tcx
                                 .hir()
                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
-                            let typeck_results = typeck_results.borrow();
                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
                                     err.span_label(
@@ -859,8 +1134,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             }
                         }
 
-                        err.emit();
-                        return;
+                        err
                     }
 
                     ty::PredicateKind::WellFormed(ty) => {
@@ -941,9 +1215,14 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 
+                let mut not_tupled = false;
+
                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
-                    _ => vec![ArgKind::empty()],
+                    _ => {
+                        not_tupled = true;
+                        vec![ArgKind::empty()]
+                    }
                 };
 
                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
@@ -951,15 +1230,34 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     ty::Tuple(ref tys) => {
                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
                     }
-                    _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
+                    _ => {
+                        not_tupled = true;
+                        vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
+                    }
                 };
 
-                if found.len() == expected.len() {
+                // If this is a `Fn` family trait and either the expected or found
+                // is not tupled, then fall back to just a regular mismatch error.
+                // This shouldn't be common unless manually implementing one of the
+                // traits manually, but don't make it more confusing when it does
+                // happen.
+                if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
+                    self.report_and_explain_type_error(
+                        TypeTrace::poly_trait_refs(
+                            &obligation.cause,
+                            true,
+                            expected_trait_ref,
+                            found_trait_ref,
+                        ),
+                        ty::error::TypeError::Mismatch,
+                    )
+                } else if found.len() == expected.len() {
                     self.report_closure_arg_mismatch(
                         span,
                         found_span,
                         found_trait_ref,
                         expected_trait_ref,
+                        obligation.cause.code(),
                     )
                 } else {
                     let (closure_span, found) = found_did
@@ -1008,7 +1306,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 }
 
                 match obligation.predicate.kind().skip_binder() {
-                    ty::PredicateKind::ConstEvaluatable(uv) => {
+                    ty::PredicateKind::ConstEvaluatable(ct) => {
+                        let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
+                            bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
+                        };
                         let mut err =
                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
                         let const_span = self.tcx.def_span(uv.def.did);
@@ -1054,250 +1355,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         err.emit();
     }
-
-    /// Given some node representing a fn-like thing in the HIR map,
-    /// returns a span and `ArgKind` information that describes the
-    /// arguments it expects. This can be supplied to
-    /// `report_arg_count_mismatch`.
-    fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
-        let sm = self.tcx.sess.source_map();
-        let hir = self.tcx.hir();
-        Some(match node {
-            Node::Expr(&hir::Expr {
-                kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, .. }),
-                ..
-            }) => (
-                fn_decl_span,
-                hir.body(body)
-                    .params
-                    .iter()
-                    .map(|arg| {
-                        if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
-                            *arg.pat
-                        {
-                            Some(ArgKind::Tuple(
-                                Some(span),
-                                args.iter()
-                                    .map(|pat| {
-                                        sm.span_to_snippet(pat.span)
-                                            .ok()
-                                            .map(|snippet| (snippet, "_".to_owned()))
-                                    })
-                                    .collect::<Option<Vec<_>>>()?,
-                            ))
-                        } else {
-                            let name = sm.span_to_snippet(arg.pat.span).ok()?;
-                            Some(ArgKind::Arg(name, "_".to_owned()))
-                        }
-                    })
-                    .collect::<Option<Vec<ArgKind>>>()?,
-            ),
-            Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
-            | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
-            | Node::TraitItem(&hir::TraitItem {
-                kind: hir::TraitItemKind::Fn(ref sig, _), ..
-            }) => (
-                sig.span,
-                sig.decl
-                    .inputs
-                    .iter()
-                    .map(|arg| match arg.kind {
-                        hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
-                            Some(arg.span),
-                            vec![("_".to_owned(), "_".to_owned()); tys.len()],
-                        ),
-                        _ => ArgKind::empty(),
-                    })
-                    .collect::<Vec<ArgKind>>(),
-            ),
-            Node::Ctor(ref variant_data) => {
-                let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
-                (span, vec![ArgKind::empty(); variant_data.fields().len()])
-            }
-            _ => panic!("non-FnLike node found: {:?}", node),
-        })
-    }
-
-    /// Reports an error when the number of arguments needed by a
-    /// trait match doesn't match the number that the expression
-    /// provides.
-    fn report_arg_count_mismatch(
-        &self,
-        span: Span,
-        found_span: Option<Span>,
-        expected_args: Vec<ArgKind>,
-        found_args: Vec<ArgKind>,
-        is_closure: bool,
-    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
-        let kind = if is_closure { "closure" } else { "function" };
-
-        let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
-            let arg_length = arguments.len();
-            let distinct = matches!(other, &[ArgKind::Tuple(..)]);
-            match (arg_length, arguments.get(0)) {
-                (1, Some(&ArgKind::Tuple(_, ref fields))) => {
-                    format!("a single {}-tuple as argument", fields.len())
-                }
-                _ => format!(
-                    "{} {}argument{}",
-                    arg_length,
-                    if distinct && arg_length > 1 { "distinct " } else { "" },
-                    pluralize!(arg_length)
-                ),
-            }
-        };
-
-        let expected_str = args_str(&expected_args, &found_args);
-        let found_str = args_str(&found_args, &expected_args);
-
-        let mut err = struct_span_err!(
-            self.tcx.sess,
-            span,
-            E0593,
-            "{} is expected to take {}, but it takes {}",
-            kind,
-            expected_str,
-            found_str,
-        );
-
-        err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
-
-        if let Some(found_span) = found_span {
-            err.span_label(found_span, format!("takes {}", found_str));
-
-            // move |_| { ... }
-            // ^^^^^^^^-- def_span
-            //
-            // move |_| { ... }
-            // ^^^^^-- prefix
-            let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
-            // move |_| { ... }
-            //      ^^^-- pipe_span
-            let pipe_span =
-                if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
-
-            // Suggest to take and ignore the arguments with expected_args_length `_`s if
-            // found arguments is empty (assume the user just wants to ignore args in this case).
-            // For example, if `expected_args_length` is 2, suggest `|_, _|`.
-            if found_args.is_empty() && is_closure {
-                let underscores = vec!["_"; expected_args.len()].join(", ");
-                err.span_suggestion_verbose(
-                    pipe_span,
-                    &format!(
-                        "consider changing the closure to take and ignore the expected argument{}",
-                        pluralize!(expected_args.len())
-                    ),
-                    format!("|{}|", underscores),
-                    Applicability::MachineApplicable,
-                );
-            }
-
-            if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
-                if fields.len() == expected_args.len() {
-                    let sugg = fields
-                        .iter()
-                        .map(|(name, _)| name.to_owned())
-                        .collect::<Vec<String>>()
-                        .join(", ");
-                    err.span_suggestion_verbose(
-                        found_span,
-                        "change the closure to take multiple arguments instead of a single tuple",
-                        format!("|{}|", sugg),
-                        Applicability::MachineApplicable,
-                    );
-                }
-            }
-            if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
-                && fields.len() == found_args.len()
-                && is_closure
-            {
-                let sugg = format!(
-                    "|({}){}|",
-                    found_args
-                        .iter()
-                        .map(|arg| match arg {
-                            ArgKind::Arg(name, _) => name.to_owned(),
-                            _ => "_".to_owned(),
-                        })
-                        .collect::<Vec<String>>()
-                        .join(", "),
-                    // add type annotations if available
-                    if found_args.iter().any(|arg| match arg {
-                        ArgKind::Arg(_, ty) => ty != "_",
-                        _ => false,
-                    }) {
-                        format!(
-                            ": ({})",
-                            fields
-                                .iter()
-                                .map(|(_, ty)| ty.to_owned())
-                                .collect::<Vec<String>>()
-                                .join(", ")
-                        )
-                    } else {
-                        String::new()
-                    },
-                );
-                err.span_suggestion_verbose(
-                    found_span,
-                    "change the closure to accept a tuple instead of individual arguments",
-                    sugg,
-                    Applicability::MachineApplicable,
-                );
-            }
-        }
-
-        err
-    }
-
-    fn type_implements_fn_trait(
-        &self,
-        param_env: ty::ParamEnv<'tcx>,
-        ty: ty::Binder<'tcx, Ty<'tcx>>,
-        constness: ty::BoundConstness,
-        polarity: ty::ImplPolarity,
-    ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
-        self.commit_if_ok(|_| {
-            for trait_def_id in [
-                self.tcx.lang_items().fn_trait(),
-                self.tcx.lang_items().fn_mut_trait(),
-                self.tcx.lang_items().fn_once_trait(),
-            ] {
-                let Some(trait_def_id) = trait_def_id else { continue };
-                // Make a fresh inference variable so we can determine what the substitutions
-                // of the trait are.
-                let var = self.next_ty_var(TypeVariableOrigin {
-                    span: DUMMY_SP,
-                    kind: TypeVariableOriginKind::MiscVariable,
-                });
-                let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
-                let obligation = Obligation::new(
-                    ObligationCause::dummy(),
-                    param_env,
-                    ty.rebind(ty::TraitPredicate {
-                        trait_ref: ty::TraitRef::new(trait_def_id, substs),
-                        constness,
-                        polarity,
-                    })
-                    .to_predicate(self.tcx),
-                );
-                let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
-                fulfill_cx.register_predicate_obligation(self, obligation);
-                if fulfill_cx.select_all_or_error(self).is_empty() {
-                    return Ok((
-                        ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
-                            .expect("expected to map DefId to ClosureKind"),
-                        ty.rebind(self.resolve_vars_if_possible(var)),
-                    ));
-                }
-            }
-
-            Err(())
-        })
-    }
 }
 
-trait InferCtxtPrivExt<'hir, 'tcx> {
+trait InferCtxtPrivExt<'tcx> {
     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
     // `error` occurring implies that `cond` occurs.
     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
@@ -1315,6 +1375,13 @@ trait InferCtxtPrivExt<'hir, 'tcx> {
         error: &MismatchedProjectionTypes<'tcx>,
     );
 
+    fn maybe_detailed_projection_msg(
+        &self,
+        pred: ty::ProjectionPredicate<'tcx>,
+        normalized_ty: ty::Term<'tcx>,
+        expected_ty: ty::Term<'tcx>,
+    ) -> Option<String>;
+
     fn fuzzy_match_tys(
         &self,
         a: Ty<'tcx>,
@@ -1371,7 +1438,7 @@ trait InferCtxtPrivExt<'hir, 'tcx> {
     fn predicate_can_apply(
         &self,
         param_env: ty::ParamEnv<'tcx>,
-        pred: ty::PolyTraitRef<'tcx>,
+        pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool;
 
     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
@@ -1389,13 +1456,13 @@ trait InferCtxtPrivExt<'hir, 'tcx> {
         predicate: ty::Predicate<'tcx>,
     );
 
-    fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'hir>);
+    fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
 
     fn maybe_indirection_for_unsized(
         &self,
         err: &mut Diagnostic,
-        item: &'hir Item<'hir>,
-        param: &'hir GenericParam<'hir>,
+        item: &'tcx Item<'tcx>,
+        param: &'tcx GenericParam<'tcx>,
     ) -> bool;
 
     fn is_recursive_obligation(
@@ -1405,7 +1472,7 @@ trait InferCtxtPrivExt<'hir, 'tcx> {
     ) -> bool;
 }
 
-impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
+impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
     // `error` occurring implies that `cond` occurs.
     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
@@ -1476,13 +1543,31 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 .emit();
             }
             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
-                self.report_mismatched_consts(
+                let mut diag = self.report_mismatched_consts(
                     &error.obligation.cause,
                     expected_found.expected,
                     expected_found.found,
                     err.clone(),
-                )
-                .emit();
+                );
+                let code = error.obligation.cause.code().peel_derives().peel_match_impls();
+                if let ObligationCauseCode::BindingObligation(..)
+                | ObligationCauseCode::ItemObligation(..)
+                | ObligationCauseCode::ExprBindingObligation(..)
+                | ObligationCauseCode::ExprItemObligation(..) = code
+                {
+                    self.note_obligation_cause_code(
+                        &mut diag,
+                        &error.obligation.predicate,
+                        error.obligation.param_env,
+                        code,
+                        &mut vec![],
+                        &mut Default::default(),
+                    );
+                }
+                diag.emit();
+            }
+            FulfillmentErrorCode::CodeCycle(ref cycle) => {
+                self.report_overflow_error_cycle(cycle);
             }
         }
     }
@@ -1500,8 +1585,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         }
 
         self.probe(|_| {
-            let err_buf;
-            let mut err = &error.err;
+            let mut err = error.err;
             let mut values = None;
 
             // try to find the mismatched types to report the error with.
@@ -1534,31 +1618,28 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                     obligation.cause.code().peel_derives(),
                     ObligationCauseCode::ItemObligation(_)
                         | ObligationCauseCode::BindingObligation(_, _)
+                        | ObligationCauseCode::ExprItemObligation(..)
+                        | ObligationCauseCode::ExprBindingObligation(..)
                         | ObligationCauseCode::ObjectCastObligation(..)
                         | ObligationCauseCode::OpaqueType
                 );
-                if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
+                if let Err(new_err) = self.at(&obligation.cause, obligation.param_env).eq_exp(
                     is_normalized_ty_expected,
                     normalized_ty,
                     data.term,
                 ) {
-                    values = Some(infer::ValuePairs::Terms(ExpectedFound::new(
-                        is_normalized_ty_expected,
-                        normalized_ty,
-                        data.term,
-                    )));
-                    err_buf = error;
-                    err = &err_buf;
+                    values = Some((data, is_normalized_ty_expected, normalized_ty, data.term));
+                    err = new_err;
                 }
             }
 
-            let mut diag = struct_span_err!(
-                self.tcx.sess,
-                obligation.cause.span,
-                E0271,
-                "type mismatch resolving `{}`",
-                predicate
-            );
+            let msg = values
+                .and_then(|(predicate, _, normalized_ty, expected_ty)| {
+                    self.maybe_detailed_projection_msg(predicate, normalized_ty, expected_ty)
+                })
+                .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
+            let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
+
             let secondary_span = match predicate.kind().skip_binder() {
                 ty::PredicateKind::Projection(proj) => self
                     .tcx
@@ -1584,7 +1665,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                                 ..
                             })
                             | hir::Node::ImplItem(hir::ImplItem {
-                                kind: hir::ImplItemKind::TyAlias(ty),
+                                kind: hir::ImplItemKind::Type(ty),
                                 ..
                             }),
                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
@@ -1596,7 +1677,13 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 &mut diag,
                 &obligation.cause,
                 secondary_span,
-                values,
+                values.map(|(_, is_normalized_ty_expected, normalized_ty, term)| {
+                    infer::ValuePairs::Terms(ExpectedFound::new(
+                        is_normalized_ty_expected,
+                        normalized_ty,
+                        term,
+                    ))
+                }),
                 err,
                 true,
                 false,
@@ -1606,6 +1693,33 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         });
     }
 
+    fn maybe_detailed_projection_msg(
+        &self,
+        pred: ty::ProjectionPredicate<'tcx>,
+        normalized_ty: ty::Term<'tcx>,
+        expected_ty: ty::Term<'tcx>,
+    ) -> Option<String> {
+        let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
+        let self_ty = pred.projection_ty.self_ty();
+
+        if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
+            Some(format!(
+                "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
+                fn_kind = self_ty.prefix_string(self.tcx)
+            ))
+        } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
+            Some(format!(
+                "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
+            ))
+        } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
+            Some(format!(
+                "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
+            ))
+        } else {
+            None
+        }
+    }
+
     fn fuzzy_match_tys(
         &self,
         mut a: Ty<'tcx>,
@@ -1731,13 +1845,21 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 return false;
             }
             if candidates.len() == 1 {
+                let ty_desc = match candidates[0].self_ty().kind() {
+                    ty::FnPtr(_) => Some("fn pointer"),
+                    _ => None,
+                };
+                let the_desc = match ty_desc {
+                    Some(desc) => format!(" implemented for {} `", desc),
+                    None => " implemented for `".to_string(),
+                };
                 err.highlighted_help(vec![
                     (
                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
                         Style::NoStyle,
                     ),
                     ("is".to_string(), Style::Highlight),
-                    (" implemented for `".to_string(), Style::NoStyle),
+                    (the_desc, Style::NoStyle),
                     (candidates[0].self_ty().to_string(), Style::Highlight),
                     ("`".to_string(), Style::NoStyle),
                 ]);
@@ -1804,7 +1926,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                         // types, too.
                         self.tcx
                             .visibility(def.did())
-                            .is_accessible_from(body_id.owner.to_def_id(), self.tcx)
+                            .is_accessible_from(body_id.owner.def_id, self.tcx)
                     } else {
                         true
                     }
@@ -1814,16 +1936,11 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         }
 
         let normalize = |candidate| {
-            self.tcx.infer_ctxt().enter(|ref infcx| {
-                let normalized = infcx
-                    .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
-                    .normalize(candidate)
-                    .ok();
-                match normalized {
-                    Some(normalized) => normalized.value,
-                    None => candidate,
-                }
-            })
+            let infcx = self.tcx.infer_ctxt().build();
+            infcx
+                .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
+                .normalize(candidate)
+                .map_or(candidate, |normalized| normalized.value)
         };
 
         // Sort impl candidates so that ordering is consistent for UI tests.
@@ -1940,7 +2057,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         let predicate = self.resolve_vars_if_possible(obligation.predicate);
         let span = obligation.cause.span;
 
-        debug!(?predicate, obligation.cause.code = tracing::field::debug(&obligation.cause.code()));
+        debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
 
         // Ambiguity errors are often caused as fallout from earlier errors.
         // We ignore them if this `infcx` is tainted in some cases below.
@@ -1997,7 +2114,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 // Pick the first substitution that still contains inference variables as the one
                 // we're going to emit an error for. If there are none (see above), fall back to
                 // a more general error.
-                let subst = data.trait_ref.substs.iter().find(|s| s.has_infer_types_or_consts());
+                let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
 
                 let mut err = if let Some(subst) = subst {
                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
@@ -2033,13 +2150,11 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                     }
                 }
 
-                if let ObligationCauseCode::ItemObligation(def_id) = *obligation.cause.code() {
+                if let ObligationCauseCode::ItemObligation(def_id) | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code() {
                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
-                } else if let (
-                    Ok(ref snippet),
-                    &ObligationCauseCode::BindingObligation(def_id, _),
-                ) =
-                    (self.tcx.sess.source_map().span_to_snippet(span), obligation.cause.code())
+                } else if let Ok(snippet) = &self.tcx.sess.source_map().span_to_snippet(span)
+                    && let ObligationCauseCode::BindingObligation(def_id, _) | ObligationCauseCode::ExprBindingObligation(def_id, ..)
+                        = *obligation.cause.code()
                 {
                     let generics = self.tcx.generics_of(def_id);
                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
@@ -2119,12 +2234,12 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                         && let [
                             ..,
                             trait_path_segment @ hir::PathSegment {
-                                res: Some(rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id)),
+                                res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
                                 ..
                             },
                             hir::PathSegment {
                                 ident: assoc_item_name,
-                                res: Some(rustc_hir::def::Res::Def(_, item_id)),
+                                res: rustc_hir::def::Res::Def(_, item_id),
                                 ..
                             }
                         ] = path.segments
@@ -2169,13 +2284,22 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                                     trait_impls.non_blanket_impls().len()
                                 )
                             };
-
+                            let mut suggestions = vec![(
+                                trait_path_segment.ident.span.shrink_to_lo(),
+                                format!("<{} as ", self.tcx.type_of(impl_def_id))
+                            )];
+                            if let Some(generic_arg) = trait_path_segment.args {
+                                let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
+                                // get rid of :: between Trait and <type>
+                                // must be '::' between them, otherwise the parser won't accept the code
+                                suggestions.push((between_span, "".to_string(),));
+                                suggestions.push((generic_arg.span_ext.shrink_to_hi(), format!(">")));
+                            } else {
+                                suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
+                            }
                             err.multipart_suggestion(
                                 message,
-                                vec![
-                                    (trait_path_segment.ident.span.shrink_to_lo(), format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())),
-                                    (trait_path_segment.ident.span.shrink_to_hi(), format!(">"))
-                                ],
+                                suggestions,
                                 Applicability::MaybeIncorrect
                             );
                         }
@@ -2220,7 +2344,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                     .substs
                     .iter()
                     .chain(Some(data.term.into_arg()))
-                    .find(|g| g.has_infer_types_or_consts());
+                    .find(|g| g.has_non_region_infer());
                 if let Some(subst) = subst {
                     let mut err = self.emit_inference_failure_err(
                         body_id,
@@ -2249,7 +2373,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 if predicate.references_error() || self.is_tainted_by_errors() {
                     return;
                 }
-                let subst = data.substs.iter().find(|g| g.has_infer_types_or_consts());
+                let subst = data.walk().find(|g| g.is_non_region_infer());
                 if let Some(subst) = subst {
                     let err = self.emit_inference_failure_err(
                         body_id,
@@ -2389,10 +2513,10 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
     fn predicate_can_apply(
         &self,
         param_env: ty::ParamEnv<'tcx>,
-        pred: ty::PolyTraitRef<'tcx>,
+        pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
         struct ParamToVarFolder<'a, 'tcx> {
-            infcx: &'a InferCtxt<'a, 'tcx>,
+            infcx: &'a InferCtxt<'tcx>,
             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
         }
 
@@ -2433,7 +2557,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
             let obligation = Obligation::new(
                 ObligationCause::dummy(),
                 param_env,
-                cleaned_pred.without_const().to_predicate(selcx.tcx()),
+                cleaned_pred.to_predicate(selcx.tcx()),
             );
 
             self.predicate_may_hold(&obligation)
@@ -2462,15 +2586,10 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         err: &mut Diagnostic,
         obligation: &PredicateObligation<'tcx>,
     ) {
-        let (
-            ty::PredicateKind::Trait(pred),
-            &ObligationCauseCode::BindingObligation(item_def_id, span),
-        ) = (
-            obligation.predicate.kind().skip_binder(),
-            obligation.cause.code().peel_derives(),
-        )  else {
-            return;
-        };
+        let ty::PredicateKind::Trait(pred) = obligation.predicate.kind().skip_binder() else { return; };
+        let (ObligationCauseCode::BindingObligation(item_def_id, span)
+        | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
+            = *obligation.cause.code().peel_derives() else { return; };
         debug!(?pred, ?item_def_id, ?span);
 
         let (Some(node), true) = (
@@ -2483,12 +2602,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
     }
 
     #[instrument(level = "debug", skip_all)]
-    fn maybe_suggest_unsized_generics<'hir>(
-        &self,
-        err: &mut Diagnostic,
-        span: Span,
-        node: Node<'hir>,
-    ) {
+    fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
         let Some(generics) = node.generics() else {
             return;
         };
@@ -2539,11 +2653,11 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         );
     }
 
-    fn maybe_indirection_for_unsized<'hir>(
+    fn maybe_indirection_for_unsized(
         &self,
         err: &mut Diagnostic,
-        item: &'hir Item<'hir>,
-        param: &'hir GenericParam<'hir>,
+        item: &Item<'tcx>,
+        param: &GenericParam<'tcx>,
     ) -> bool {
         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
@@ -2643,82 +2757,6 @@ impl<'v> Visitor<'v> for FindTypeParam {
     }
 }
 
-pub fn recursive_type_with_infinite_size_error<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    type_def_id: DefId,
-    spans: Vec<(Span, Option<hir::HirId>)>,
-) {
-    assert!(type_def_id.is_local());
-    let span = tcx.def_span(type_def_id);
-    let path = tcx.def_path_str(type_def_id);
-    let mut err =
-        struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
-    err.span_label(span, "recursive type has infinite size");
-    for &(span, _) in &spans {
-        err.span_label(span, "recursive without indirection");
-    }
-    let msg = format!(
-        "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
-        path,
-    );
-    if spans.len() <= 4 {
-        // FIXME(compiler-errors): This suggestion might be erroneous if Box is shadowed
-        err.multipart_suggestion(
-            &msg,
-            spans
-                .into_iter()
-                .flat_map(|(span, field_id)| {
-                    if let Some(generic_span) = get_option_generic_from_field_id(tcx, field_id) {
-                        // If we match an `Option` and can grab the span of the Option's generic, then
-                        // suggest boxing the generic arg for a non-null niche optimization.
-                        vec![
-                            (generic_span.shrink_to_lo(), "Box<".to_string()),
-                            (generic_span.shrink_to_hi(), ">".to_string()),
-                        ]
-                    } else {
-                        vec![
-                            (span.shrink_to_lo(), "Box<".to_string()),
-                            (span.shrink_to_hi(), ">".to_string()),
-                        ]
-                    }
-                })
-                .collect(),
-            Applicability::HasPlaceholders,
-        );
-    } else {
-        err.help(&msg);
-    }
-    err.emit();
-}
-
-/// Extract the span for the generic type `T` of `Option<T>` in a field definition
-fn get_option_generic_from_field_id(tcx: TyCtxt<'_>, field_id: Option<hir::HirId>) -> Option<Span> {
-    let node = tcx.hir().find(field_id?);
-
-    // Expect a field from our field_id
-    let Some(hir::Node::Field(field_def)) = node
-        else { bug!("Expected HirId corresponding to FieldDef, found: {:?}", node) };
-
-    // Match a type that is a simple QPath with no Self
-    let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = &field_def.ty.kind
-        else { return None };
-
-    // Check if the path we're checking resolves to Option
-    let hir::def::Res::Def(_, did) = path.res
-        else { return None };
-
-    // Bail if this path doesn't describe `::core::option::Option`
-    if !tcx.is_diagnostic_item(sym::Option, did) {
-        return None;
-    }
-
-    // Match a single generic arg in the 0th path segment
-    let generic_arg = path.segments.last()?.args?.args.get(0)?;
-
-    // Take the span out of the type, if it's a type
-    if let hir::GenericArg::Type(generic_ty) = generic_arg { Some(generic_ty.span) } else { None }
-}
-
 /// Summarizes information
 #[derive(Clone)]
 pub enum ArgKind {
@@ -2763,3 +2801,8 @@ impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
         }
     }
 }
+
+pub enum DefIdOrName {
+    DefId(DefId),
+    Name(&'static str),
+}
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
index e6907637c..5eef54c63 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
@@ -1,17 +1,17 @@
 use super::{
     ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote, PredicateObligation,
 };
-use crate::infer::InferCtxt;
+use crate::infer::error_reporting::TypeErrCtxt;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
-use rustc_middle::ty::subst::{Subst, SubstsRef};
+use rustc_middle::ty::SubstsRef;
 use rustc_middle::ty::{self, GenericParamDefKind};
 use rustc_span::symbol::sym;
 use std::iter;
 
 use super::InferCtxtPrivExt;
 
-pub trait InferCtxtExt<'tcx> {
+pub trait TypeErrCtxtExt<'tcx> {
     /*private*/
     fn impl_similar_to(
         &self,
@@ -29,7 +29,7 @@ pub trait InferCtxtExt<'tcx> {
     ) -> OnUnimplementedNote;
 }
 
-impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
+impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     fn impl_similar_to(
         &self,
         trait_ref: ty::PolyTraitRef<'tcx>,
@@ -143,7 +143,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         }
 
         if let ObligationCauseCode::ItemObligation(item)
-        | ObligationCauseCode::BindingObligation(item, _) = *obligation.cause.code()
+        | ObligationCauseCode::BindingObligation(item, _)
+        | ObligationCauseCode::ExprItemObligation(item, ..)
+        | ObligationCauseCode::ExprBindingObligation(item, ..) = *obligation.cause.code()
         {
             // FIXME: maybe also have some way of handling methods
             // from other traits? That would require name resolution,
@@ -162,6 +164,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
         }
 
+        if let ObligationCauseCode::MainFunctionType = obligation.cause.code() {
+            flags.push((sym::cause, Some("MainFunctionType".to_string())));
+        }
+
         // Add all types without trimmed paths.
         ty::print::with_no_trimmed_paths!({
             let generics = self.tcx.generics_of(def_id);
@@ -254,7 +260,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     }
                 }
             }
-            if let ty::Dynamic(traits, _) = self_ty.kind() {
+            if let ty::Dynamic(traits, _, _) = self_ty.kind() {
                 for t in traits.iter() {
                     if let ty::ExistentialPredicate::Trait(trait_ref) = t.skip_binder() {
                         flags.push((sym::_Self, Some(self.tcx.def_path_str(trait_ref.def_id))))
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 219413121..8c41d9d24 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -1,5 +1,5 @@
 use super::{
-    EvaluationResult, Obligation, ObligationCause, ObligationCauseCode, PredicateObligation,
+    DefIdOrName, Obligation, ObligationCause, ObligationCauseCode, PredicateObligation,
     SelectionContext,
 };
 
@@ -7,6 +7,7 @@ use crate::autoderef::Autoderef;
 use crate::infer::InferCtxt;
 use crate::traits::normalize_to;
 
+use hir::def::CtorOf;
 use hir::HirId;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -20,18 +21,17 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
-use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::infer::error_reporting::TypeErrCtxt;
+use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_middle::hir::map;
 use rustc_middle::ty::{
     self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
     GeneratorDiagnosticData, GeneratorInteriorTypeCause, Infer, InferTy, IsSuggestable,
-    ProjectionPredicate, ToPredicate, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
-    TypeVisitable,
+    ToPredicate, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitable,
 };
 use rustc_middle::ty::{TypeAndMut, TypeckResults};
-use rustc_session::Limit;
-use rustc_span::def_id::LOCAL_CRATE;
-use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{BytePos, DesugaringKind, ExpnKind, Span, DUMMY_SP};
 use rustc_target::spec::abi;
 use std::fmt;
@@ -63,7 +63,7 @@ impl<'tcx, 'a> GeneratorData<'tcx, 'a> {
     // meet an obligation
     fn try_get_upvar_span<F>(
         &self,
-        infer_context: &InferCtxt<'a, 'tcx>,
+        infer_context: &InferCtxt<'tcx>,
         generator_did: DefId,
         ty_matches: F,
     ) -> Option<GeneratorInteriorOrUpvar>
@@ -169,12 +169,12 @@ impl<'tcx, 'a> GeneratorData<'tcx, 'a> {
 }
 
 // This trait is public to expose the diagnostics methods to clippy.
-pub trait InferCtxtExt<'tcx> {
+pub trait TypeErrCtxtExt<'tcx> {
     fn suggest_restricting_param_bound(
         &self,
         err: &mut Diagnostic,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
-        proj_pred: Option<ty::PolyProjectionPredicate<'tcx>>,
+        associated_item: Option<(&'static str, Ty<'tcx>)>,
         body_id: hir::HirId,
     );
 
@@ -256,8 +256,15 @@ pub trait InferCtxtExt<'tcx> {
         found_span: Option<Span>,
         found: ty::PolyTraitRef<'tcx>,
         expected: ty::PolyTraitRef<'tcx>,
+        cause: &ObligationCauseCode<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 
+    fn note_conflicting_closure_bounds(
+        &self,
+        cause: &ObligationCauseCode<'tcx>,
+        err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
+    );
+
     fn suggest_fully_qualified_path(
         &self,
         err: &mut Diagnostic,
@@ -297,8 +304,6 @@ pub trait InferCtxtExt<'tcx> {
     ) where
         T: fmt::Display;
 
-    fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
-
     /// Suggest to await before try: future? => future.await?
     fn suggest_await_before_try(
         &self,
@@ -462,12 +467,12 @@ fn suggest_restriction<'tcx>(
     }
 }
 
-impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
+impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     fn suggest_restricting_param_bound(
         &self,
         mut err: &mut Diagnostic,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
-        proj_pred: Option<ty::PolyProjectionPredicate<'tcx>>,
+        associated_ty: Option<(&'static str, Ty<'tcx>)>,
         body_id: hir::HirId,
     ) {
         let trait_pred = self.resolve_numeric_literals_with_default(trait_pred);
@@ -604,21 +609,18 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         trait_pred.print_modifiers_and_trait_path().to_string()
                     );
 
-                    if let Some(proj_pred) = proj_pred {
-                        let ProjectionPredicate { projection_ty, term } = proj_pred.skip_binder();
-                        let item = self.tcx.associated_item(projection_ty.item_def_id);
-
+                    if let Some((name, term)) = associated_ty {
                         // FIXME: this case overlaps with code in TyCtxt::note_and_explain_type_err.
                         // That should be extracted into a helper function.
                         if constraint.ends_with('>') {
                             constraint = format!(
-                                "{}, {}={}>",
+                                "{}, {} = {}>",
                                 &constraint[..constraint.len() - 1],
-                                item.name,
+                                name,
                                 term
                             );
                         } else {
-                            constraint.push_str(&format!("<{}={}>", item.name, term));
+                            constraint.push_str(&format!("<{} = {}>", name, term));
                         }
                     }
 
@@ -648,7 +650,13 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     ..
                 }) if !param_ty => {
                     // Missing generic type parameter bound.
-                    if suggest_arbitrary_trait_bound(self.tcx, generics, &mut err, trait_pred) {
+                    if suggest_arbitrary_trait_bound(
+                        self.tcx,
+                        generics,
+                        &mut err,
+                        trait_pred,
+                        associated_ty,
+                    ) {
                         return;
                     }
                 }
@@ -657,7 +665,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 _ => {}
             }
 
-            hir_id = self.tcx.hir().local_def_id_to_hir_id(self.tcx.hir().get_parent_item(hir_id));
+            hir_id = self.tcx.hir().get_parent_item(hir_id).into();
         }
     }
 
@@ -671,11 +679,15 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
         // It only make sense when suggesting dereferences for arguments
-        let ObligationCauseCode::FunctionArgumentObligation { .. } = obligation.cause.code() else {
-            return false;
-        };
-        let param_env = obligation.param_env;
-        let body_id = obligation.cause.body_id;
+        let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, .. } = obligation.cause.code()
+            else { return false; };
+        let Some(typeck_results) = &self.typeck_results
+            else { return false; };
+        let hir::Node::Expr(expr) = self.tcx.hir().get(*arg_hir_id)
+            else { return false; };
+        let Some(arg_ty) = typeck_results.expr_ty_adjusted_opt(expr)
+            else { return false; };
+
         let span = obligation.cause.span;
         let mut real_trait_pred = trait_pred;
         let mut code = obligation.cause.code();
@@ -685,11 +697,25 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 real_trait_pred = parent_trait_pred;
             }
 
-            // Skipping binder here, remapping below
-            let real_ty = real_trait_pred.self_ty().skip_binder();
+            let real_ty = real_trait_pred.self_ty();
+            // We `erase_late_bound_regions` here because `make_subregion` does not handle
+            // `ReLateBound`, and we don't particularly care about the regions.
+            if self
+                .can_eq(obligation.param_env, self.tcx.erase_late_bound_regions(real_ty), arg_ty)
+                .is_err()
+            {
+                continue;
+            }
 
-            if let ty::Ref(region, base_ty, mutbl) = *real_ty.kind() {
-                let mut autoderef = Autoderef::new(self, param_env, body_id, span, base_ty, span);
+            if let ty::Ref(region, base_ty, mutbl) = *real_ty.skip_binder().kind() {
+                let mut autoderef = Autoderef::new(
+                    self,
+                    obligation.param_env,
+                    obligation.cause.body_id,
+                    span,
+                    base_ty,
+                    span,
+                );
                 if let Some(steps) = autoderef.find_map(|(ty, steps)| {
                     // Re-add the `&`
                     let ty = self.tcx.mk_ref(region, TypeAndMut { ty, mutbl });
@@ -697,24 +723,29 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     // Remapping bound vars here
                     let real_trait_pred_and_ty =
                         real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, ty));
-                    let obligation = self
-                        .mk_trait_obligation_with_new_self_ty(param_env, real_trait_pred_and_ty);
+                    let obligation = self.mk_trait_obligation_with_new_self_ty(
+                        obligation.param_env,
+                        real_trait_pred_and_ty,
+                    );
                     Some(steps).filter(|_| self.predicate_may_hold(&obligation))
                 }) {
                     if steps > 0 {
-                        if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(span) {
-                            // Don't care about `&mut` because `DerefMut` is used less
-                            // often and user will not expect autoderef happens.
-                            if src.starts_with('&') && !src.starts_with("&mut ") {
-                                let derefs = "*".repeat(steps);
-                                err.span_suggestion(
-                                    span,
-                                    "consider dereferencing here",
-                                    format!("&{}{}", derefs, &src[1..]),
-                                    Applicability::MachineApplicable,
-                                );
-                                return true;
-                            }
+                        // Don't care about `&mut` because `DerefMut` is used less
+                        // often and user will not expect autoderef happens.
+                        if let Some(hir::Node::Expr(hir::Expr {
+                            kind:
+                                hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, expr),
+                            ..
+                        })) = self.tcx.hir().find(*arg_hir_id)
+                        {
+                            let derefs = "*".repeat(steps);
+                            err.span_suggestion_verbose(
+                                expr.span.shrink_to_lo(),
+                                "consider dereferencing here",
+                                derefs,
+                                Applicability::MachineApplicable,
+                            );
+                            return true;
                         }
                     }
                 } else if real_trait_pred != trait_pred {
@@ -724,7 +755,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     let real_trait_pred_and_base_ty =
                         real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, base_ty));
                     let obligation = self.mk_trait_obligation_with_new_self_ty(
-                        param_env,
+                        obligation.param_env,
                         real_trait_pred_and_base_ty,
                     );
                     if self.predicate_may_hold(&obligation) {
@@ -750,7 +781,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             // Get the local name of this closure. This can be inaccurate because
             // of the possibility of reassignment, but this should be good enough.
             match &kind {
-                hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, None) => {
+                hir::PatKind::Binding(hir::BindingAnnotation::NONE, _, ident, None) => {
                     Some(ident.name)
                 }
                 _ => {
@@ -783,75 +814,138 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         err: &mut Diagnostic,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
-        // Skipping binder here, remapping below
-        let self_ty = trait_pred.self_ty().skip_binder();
-
-        let (def_id, output_ty, callable) = match *self_ty.kind() {
-            ty::Closure(def_id, substs) => (def_id, substs.as_closure().sig().output(), "closure"),
-            ty::FnDef(def_id, _) => (def_id, self_ty.fn_sig(self.tcx).output(), "function"),
-            _ => return false,
-        };
-        let msg = format!("use parentheses to call the {}", callable);
+        if let ty::PredicateKind::Trait(trait_pred) = obligation.predicate.kind().skip_binder()
+            && Some(trait_pred.def_id()) == self.tcx.lang_items().sized_trait()
+        {
+            // Don't suggest calling to turn an unsized type into a sized type
+            return false;
+        }
 
-        // "We should really create a single list of bound vars from the combined vars
-        // from the predicate and function, but instead we just liberate the function bound vars"
-        let output_ty = self.tcx.liberate_late_bound_regions(def_id, output_ty);
+        // This is duplicated from `extract_callable_info` in typeck, which
+        // relies on autoderef, so we can't use it here.
+        let found = trait_pred.self_ty().skip_binder().peel_refs();
+        let Some((def_id_or_name, output, inputs)) = (match *found.kind()
+        {
+            ty::FnPtr(fn_sig) => {
+                Some((DefIdOrName::Name("function pointer"), fn_sig.output(), fn_sig.inputs()))
+            }
+            ty::FnDef(def_id, _) => {
+                let fn_sig = found.fn_sig(self.tcx);
+                Some((DefIdOrName::DefId(def_id), fn_sig.output(), fn_sig.inputs()))
+            }
+            ty::Closure(def_id, substs) => {
+                let fn_sig = substs.as_closure().sig();
+                Some((
+                    DefIdOrName::DefId(def_id),
+                    fn_sig.output(),
+                    fn_sig.inputs().map_bound(|inputs| &inputs[1..]),
+                ))
+            }
+            ty::Opaque(def_id, substs) => {
+                self.tcx.bound_item_bounds(def_id).subst(self.tcx, substs).iter().find_map(|pred| {
+                    if let ty::PredicateKind::Projection(proj) = pred.kind().skip_binder()
+                    && Some(proj.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output()
+                    // args tuple will always be substs[1]
+                    && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
+                    {
+                        Some((
+                            DefIdOrName::DefId(def_id),
+                            pred.kind().rebind(proj.term.ty().unwrap()),
+                            pred.kind().rebind(args.as_slice()),
+                        ))
+                    } else {
+                        None
+                    }
+                })
+            }
+            ty::Dynamic(data, _, ty::Dyn) => {
+                data.iter().find_map(|pred| {
+                    if let ty::ExistentialPredicate::Projection(proj) = pred.skip_binder()
+                    && Some(proj.item_def_id) == self.tcx.lang_items().fn_once_output()
+                    // for existential projection, substs are shifted over by 1
+                    && let ty::Tuple(args) = proj.substs.type_at(0).kind()
+                    {
+                        Some((
+                            DefIdOrName::Name("trait object"),
+                            pred.rebind(proj.term.ty().unwrap()),
+                            pred.rebind(args.as_slice()),
+                        ))
+                    } else {
+                        None
+                    }
+                })
+            }
+            ty::Param(_) => {
+                obligation.param_env.caller_bounds().iter().find_map(|pred| {
+                    if let ty::PredicateKind::Projection(proj) = pred.kind().skip_binder()
+                    && Some(proj.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output()
+                    && proj.projection_ty.self_ty() == found
+                    // args tuple will always be substs[1]
+                    && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
+                    {
+                        Some((
+                            DefIdOrName::Name("type parameter"),
+                            pred.kind().rebind(proj.term.ty().unwrap()),
+                            pred.kind().rebind(args.as_slice()),
+                        ))
+                    } else {
+                        None
+                    }
+                })
+            }
+            _ => None,
+        }) else { return false; };
+        let output = self.replace_bound_vars_with_fresh_vars(
+            obligation.cause.span,
+            LateBoundRegionConversionTime::FnCall,
+            output,
+        );
+        let inputs = inputs.skip_binder().iter().map(|ty| {
+            self.replace_bound_vars_with_fresh_vars(
+                obligation.cause.span,
+                LateBoundRegionConversionTime::FnCall,
+                inputs.rebind(*ty),
+            )
+        });
 
         // Remapping bound vars here
-        let trait_pred_and_self = trait_pred.map_bound(|trait_pred| (trait_pred, output_ty));
+        let trait_pred_and_self = trait_pred.map_bound(|trait_pred| (trait_pred, output));
 
         let new_obligation =
             self.mk_trait_obligation_with_new_self_ty(obligation.param_env, trait_pred_and_self);
-
-        match self.evaluate_obligation(&new_obligation) {
-            Ok(
-                EvaluationResult::EvaluatedToOk
-                | EvaluationResult::EvaluatedToOkModuloRegions
-                | EvaluationResult::EvaluatedToOkModuloOpaqueTypes
-                | EvaluationResult::EvaluatedToAmbig,
-            ) => {}
-            _ => return false,
+        if !self.predicate_must_hold_modulo_regions(&new_obligation) {
+            return false;
         }
-        let hir = self.tcx.hir();
+
         // Get the name of the callable and the arguments to be used in the suggestion.
-        let (snippet, sugg) = match hir.get_if_local(def_id) {
-            Some(hir::Node::Expr(hir::Expr {
-                kind: hir::ExprKind::Closure(hir::Closure { fn_decl, fn_decl_span, .. }),
-                ..
-            })) => {
-                err.span_label(*fn_decl_span, "consider calling this closure");
-                let Some(name) = self.get_closure_name(def_id, err, &msg) else {
-                    return false;
-                };
-                let args = fn_decl.inputs.iter().map(|_| "_").collect::<Vec<_>>().join(", ");
-                let sugg = format!("({})", args);
-                (format!("{}{}", name, sugg), sugg)
-            }
-            Some(hir::Node::Item(hir::Item {
-                ident,
-                kind: hir::ItemKind::Fn(.., body_id),
-                ..
-            })) => {
-                err.span_label(ident.span, "consider calling this function");
-                let body = hir.body(*body_id);
-                let args = body
-                    .params
-                    .iter()
-                    .map(|arg| match &arg.pat.kind {
-                        hir::PatKind::Binding(_, _, ident, None)
-                        // FIXME: provide a better suggestion when encountering `SelfLower`, it
-                        // should suggest a method call.
-                        if ident.name != kw::SelfLower => ident.to_string(),
-                        _ => "_".to_string(),
-                    })
-                    .collect::<Vec<_>>()
-                    .join(", ");
-                let sugg = format!("({})", args);
-                (format!("{}{}", ident, sugg), sugg)
-            }
-            _ => return false,
+        let hir = self.tcx.hir();
+
+        let msg = match def_id_or_name {
+            DefIdOrName::DefId(def_id) => match self.tcx.def_kind(def_id) {
+                DefKind::Ctor(CtorOf::Struct, _) => {
+                    "use parentheses to construct this tuple struct".to_string()
+                }
+                DefKind::Ctor(CtorOf::Variant, _) => {
+                    "use parentheses to construct this tuple variant".to_string()
+                }
+                kind => format!("use parentheses to call this {}", kind.descr(def_id)),
+            },
+            DefIdOrName::Name(name) => format!("use parentheses to call this {name}"),
         };
+
+        let args = inputs
+            .map(|ty| {
+                if ty.is_suggestable(self.tcx, false) {
+                    format!("/* {ty} */")
+                } else {
+                    "/* value */".to_string()
+                }
+            })
+            .collect::<Vec<_>>()
+            .join(", ");
+
         if matches!(obligation.cause.code(), ObligationCauseCode::FunctionArgumentObligation { .. })
+            && obligation.cause.span.can_be_used_for_suggestions()
         {
             // When the obligation error has been ensured to have been caused by
             // an argument, the `obligation.cause.span` points at the expression
@@ -860,11 +954,36 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             err.span_suggestion_verbose(
                 obligation.cause.span.shrink_to_hi(),
                 &msg,
-                sugg,
+                format!("({args})"),
                 Applicability::HasPlaceholders,
             );
-        } else {
-            err.help(&format!("{}: `{}`", msg, snippet));
+        } else if let DefIdOrName::DefId(def_id) = def_id_or_name {
+            let name = match hir.get_if_local(def_id) {
+                Some(hir::Node::Expr(hir::Expr {
+                    kind: hir::ExprKind::Closure(hir::Closure { fn_decl_span, .. }),
+                    ..
+                })) => {
+                    err.span_label(*fn_decl_span, "consider calling this closure");
+                    let Some(name) = self.get_closure_name(def_id, err, &msg) else {
+                        return false;
+                    };
+                    name.to_string()
+                }
+                Some(hir::Node::Item(hir::Item { ident, kind: hir::ItemKind::Fn(..), .. })) => {
+                    err.span_label(ident.span, "consider calling this function");
+                    ident.to_string()
+                }
+                Some(hir::Node::Ctor(..)) => {
+                    let name = self.tcx.def_path_str(def_id);
+                    err.span_label(
+                        self.tcx.def_span(def_id),
+                        format!("consider calling the constructor for `{}`", name),
+                    );
+                    name
+                }
+                _ => return false,
+            };
+            err.help(&format!("{msg}: `{name}({args})`"));
         }
         true
     }
@@ -882,6 +1001,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             obligation.cause.code()
         {
             &parent_code
+        } else if let ObligationCauseCode::ItemObligation(_)
+        | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
+        {
+            obligation.cause.code()
         } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
             span.ctxt().outer_expn_data().kind
         {
@@ -906,102 +1029,121 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let param_env = obligation.param_env;
 
         // Try to apply the original trait binding obligation by borrowing.
-        let mut try_borrowing =
-            |old_pred: ty::PolyTraitPredicate<'tcx>, blacklist: &[DefId]| -> bool {
-                if blacklist.contains(&old_pred.def_id()) {
-                    return false;
-                }
-                // We map bounds to `&T` and `&mut T`
-                let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
-                    (
-                        trait_pred,
-                        self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
-                    )
-                });
-                let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
+        let mut try_borrowing = |old_pred: ty::PolyTraitPredicate<'tcx>,
+                                 blacklist: &[DefId]|
+         -> bool {
+            if blacklist.contains(&old_pred.def_id()) {
+                return false;
+            }
+            // We map bounds to `&T` and `&mut T`
+            let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
+                (
+                    trait_pred,
+                    self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
+                )
+            });
+            let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
+                (
+                    trait_pred,
+                    self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
+                )
+            });
+
+            let mk_result = |trait_pred_and_new_ty| {
+                let obligation =
+                    self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
+                self.predicate_must_hold_modulo_regions(&obligation)
+            };
+            let imm_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_imm_ref);
+            let mut_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_mut_ref);
+
+            let (ref_inner_ty_satisfies_pred, ref_inner_ty_mut) =
+                if let ObligationCauseCode::ItemObligation(_) | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
+                    && let ty::Ref(_, ty, mutability) = old_pred.self_ty().skip_binder().kind()
+                {
                     (
-                        trait_pred,
-                        self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
+                        mk_result(old_pred.map_bound(|trait_pred| (trait_pred, *ty))),
+                        matches!(mutability, hir::Mutability::Mut),
                     )
-                });
-
-                let mk_result = |trait_pred_and_new_ty| {
-                    let obligation =
-                        self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
-                    self.predicate_must_hold_modulo_regions(&obligation)
+                } else {
+                    (false, false)
                 };
-                let imm_result = mk_result(trait_pred_and_imm_ref);
-                let mut_result = mk_result(trait_pred_and_mut_ref);
-
-                if imm_result || mut_result {
-                    if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
-                        // We have a very specific type of error, where just borrowing this argument
-                        // might solve the problem. In cases like this, the important part is the
-                        // original type obligation, not the last one that failed, which is arbitrary.
-                        // Because of this, we modify the error to refer to the original obligation and
-                        // return early in the caller.
-
-                        let msg = format!("the trait bound `{}` is not satisfied", old_pred);
-                        if has_custom_message {
-                            err.note(&msg);
-                        } else {
-                            err.message =
-                                vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
-                        }
-                        if snippet.starts_with('&') {
-                            // This is already a literal borrow and the obligation is failing
-                            // somewhere else in the obligation chain. Do not suggest non-sense.
-                            return false;
-                        }
-                        err.span_label(
-                            span,
+
+            if imm_ref_self_ty_satisfies_pred
+                || mut_ref_self_ty_satisfies_pred
+                || ref_inner_ty_satisfies_pred
+            {
+                if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                    // We don't want a borrowing suggestion on the fields in structs,
+                    // ```
+                    // struct Foo {
+                    //  the_foos: Vec<Foo>
+                    // }
+                    // ```
+                    if !matches!(
+                        span.ctxt().outer_expn_data().kind,
+                        ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
+                    ) {
+                        return false;
+                    }
+                    if snippet.starts_with('&') {
+                        // This is already a literal borrow and the obligation is failing
+                        // somewhere else in the obligation chain. Do not suggest non-sense.
+                        return false;
+                    }
+                    // We have a very specific type of error, where just borrowing this argument
+                    // might solve the problem. In cases like this, the important part is the
+                    // original type obligation, not the last one that failed, which is arbitrary.
+                    // Because of this, we modify the error to refer to the original obligation and
+                    // return early in the caller.
+
+                    let msg = format!("the trait bound `{}` is not satisfied", old_pred);
+                    if has_custom_message {
+                        err.note(&msg);
+                    } else {
+                        err.message =
+                            vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
+                    }
+                    err.span_label(
+                        span,
+                        format!(
+                            "the trait `{}` is not implemented for `{}`",
+                            old_pred.print_modifiers_and_trait_path(),
+                            old_pred.self_ty().skip_binder(),
+                        ),
+                    );
+
+                    if imm_ref_self_ty_satisfies_pred && mut_ref_self_ty_satisfies_pred {
+                        err.span_suggestions(
+                            span.shrink_to_lo(),
+                            "consider borrowing here",
+                            ["&".to_string(), "&mut ".to_string()].into_iter(),
+                            Applicability::MaybeIncorrect,
+                        );
+                    } else {
+                        let is_mut = mut_ref_self_ty_satisfies_pred || ref_inner_ty_mut;
+                        err.span_suggestion_verbose(
+                            span.shrink_to_lo(),
                             &format!(
-                                "expected an implementor of trait `{}`",
-                                old_pred.print_modifiers_and_trait_path(),
+                                "consider{} borrowing here",
+                                if is_mut { " mutably" } else { "" }
                             ),
+                            format!("&{}", if is_mut { "mut " } else { "" }),
+                            Applicability::MaybeIncorrect,
                         );
-
-                        // This if is to prevent a special edge-case
-                        if matches!(
-                            span.ctxt().outer_expn_data().kind,
-                            ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
-                        ) {
-                            // We don't want a borrowing suggestion on the fields in structs,
-                            // ```
-                            // struct Foo {
-                            //  the_foos: Vec<Foo>
-                            // }
-                            // ```
-
-                            if imm_result && mut_result {
-                                err.span_suggestions(
-                                    span.shrink_to_lo(),
-                                    "consider borrowing here",
-                                    ["&".to_string(), "&mut ".to_string()].into_iter(),
-                                    Applicability::MaybeIncorrect,
-                                );
-                            } else {
-                                err.span_suggestion_verbose(
-                                    span.shrink_to_lo(),
-                                    &format!(
-                                        "consider{} borrowing here",
-                                        if mut_result { " mutably" } else { "" }
-                                    ),
-                                    format!("&{}", if mut_result { "mut " } else { "" }),
-                                    Applicability::MaybeIncorrect,
-                                );
-                            }
-                        }
-                        return true;
                     }
+                    return true;
                 }
-                return false;
-            };
+            }
+            return false;
+        };
 
         if let ObligationCauseCode::ImplDerivedObligation(cause) = &*code {
             try_borrowing(cause.derived.parent_trait_pred, &[])
         } else if let ObligationCauseCode::BindingObligation(_, _)
-        | ObligationCauseCode::ItemObligation(_) = code
+        | ObligationCauseCode::ItemObligation(_)
+        | ObligationCauseCode::ExprItemObligation(..)
+        | ObligationCauseCode::ExprBindingObligation(..) = code
         {
             try_borrowing(poly_trait_pred, &never_suggest_borrow)
         } else {
@@ -1017,7 +1159,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         self_ty: Ty<'tcx>,
         object_ty: Ty<'tcx>,
     ) {
-        let ty::Dynamic(predicates, _) = object_ty.kind() else { return; };
+        let ty::Dynamic(predicates, _, ty::Dyn) = object_ty.kind() else { return; };
         let self_ref_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, self_ty);
 
         for predicate in predicates.iter() {
@@ -1110,8 +1252,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     // and if not maybe suggest doing something else? If we kept the expression around we
                     // could also check if it is an fn call (very likely) and suggest changing *that*, if
                     // it is from the local crate.
-                    err.span_suggestion_verbose(
-                        expr.span.shrink_to_hi().with_hi(span.hi()),
+                    err.span_suggestion(
+                        span,
                         "remove the `.await`",
                         "",
                         Applicability::MachineApplicable,
@@ -1126,8 +1268,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                                 &format!("this call returns `{}`", pred.self_ty()),
                             );
                         }
-                        if let Some(typeck_results) =
-                            self.in_progress_typeck_results.map(|t| t.borrow())
+                        if let Some(typeck_results) = &self.typeck_results
                             && let ty = typeck_results.expr_ty_adjusted(base)
                             && let ty::FnDef(def_id, _substs) = ty.kind()
                             && let Some(hir::Node::Item(hir::Item { ident, span, vis_span, .. })) =
@@ -1181,7 +1322,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 return;
             }
             let trait_pred = self.resolve_vars_if_possible(trait_pred);
-            if trait_pred.has_infer_types_or_consts() {
+            if trait_pred.has_non_region_infer() {
                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
                 // unresolved bindings.
                 return;
@@ -1250,8 +1391,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             && let Some(stmt) = blk.stmts.last()
             && let hir::StmtKind::Semi(expr) = stmt.kind
             // Only suggest this if the expression behind the semicolon implements the predicate
-            && let Some(typeck_results) = self.in_progress_typeck_results
-            && let Some(ty) = typeck_results.borrow().expr_ty_opt(expr)
+            && let Some(typeck_results) = &self.typeck_results
+            && let Some(ty) = typeck_results.expr_ty_opt(expr)
             && self.predicate_may_hold(&self.mk_trait_obligation_with_new_self_ty(
                 obligation.param_env, trait_pred.map_bound(|trait_pred| (trait_pred, ty))
             ))
@@ -1315,7 +1456,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let trait_pred = self.resolve_vars_if_possible(trait_pred);
         let ty = trait_pred.skip_binder().self_ty();
         let is_object_safe = match ty.kind() {
-            ty::Dynamic(predicates, _) => {
+            ty::Dynamic(predicates, _, ty::Dyn) => {
                 // If the `dyn Trait` is not object safe, do not suggest `Box<dyn Trait>`.
                 predicates
                     .principal_def_id()
@@ -1340,7 +1481,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let mut visitor = ReturnsVisitor::default();
         visitor.visit_body(&body);
 
-        let typeck_results = self.in_progress_typeck_results.map(|t| t.borrow()).unwrap();
+        let typeck_results = self.typeck_results.as_ref().unwrap();
         let Some(liberated_sig) = typeck_results.liberated_fn_sigs().get(fn_hir_id).copied() else { return false; };
 
         let ret_types = visitor
@@ -1375,7 +1516,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let mut spans_and_needs_box = vec![];
 
         match liberated_sig.output().kind() {
-            ty::Dynamic(predicates, _) => {
+            ty::Dynamic(predicates, _, ty::Dyn) => {
                 let cause = ObligationCause::misc(ret_ty.span, fn_hir_id);
                 let param_env = ty::ParamEnv::empty();
 
@@ -1523,7 +1664,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             // Point at all the `return`s in the function as they have failed trait bounds.
             let mut visitor = ReturnsVisitor::default();
             visitor.visit_body(&body);
-            let typeck_results = self.in_progress_typeck_results.map(|t| t.borrow()).unwrap();
+            let typeck_results = self.typeck_results.as_ref().unwrap();
             for expr in &visitor.returns {
                 if let Some(returned_ty) = typeck_results.node_type_opt(expr.hir_id) {
                     let ty = self.resolve_vars_if_possible(returned_ty);
@@ -1539,34 +1680,41 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         found_span: Option<Span>,
         found: ty::PolyTraitRef<'tcx>,
         expected: ty::PolyTraitRef<'tcx>,
+        cause: &ObligationCauseCode<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         pub(crate) fn build_fn_sig_ty<'tcx>(
-            tcx: TyCtxt<'tcx>,
+            infcx: &InferCtxt<'tcx>,
             trait_ref: ty::PolyTraitRef<'tcx>,
         ) -> Ty<'tcx> {
             let inputs = trait_ref.skip_binder().substs.type_at(1);
             let sig = match inputs.kind() {
                 ty::Tuple(inputs)
-                    if tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some() =>
+                    if infcx.tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some() =>
                 {
-                    tcx.mk_fn_sig(
+                    infcx.tcx.mk_fn_sig(
                         inputs.iter(),
-                        tcx.mk_ty_infer(ty::TyVar(ty::TyVid::from_u32(0))),
+                        infcx.next_ty_var(TypeVariableOrigin {
+                            span: DUMMY_SP,
+                            kind: TypeVariableOriginKind::MiscVariable,
+                        }),
                         false,
                         hir::Unsafety::Normal,
                         abi::Abi::Rust,
                     )
                 }
-                _ => tcx.mk_fn_sig(
+                _ => infcx.tcx.mk_fn_sig(
                     std::iter::once(inputs),
-                    tcx.mk_ty_infer(ty::TyVar(ty::TyVid::from_u32(0))),
+                    infcx.next_ty_var(TypeVariableOrigin {
+                        span: DUMMY_SP,
+                        kind: TypeVariableOriginKind::MiscVariable,
+                    }),
                     false,
                     hir::Unsafety::Normal,
                     abi::Abi::Rust,
                 ),
             };
 
-            tcx.mk_fn_ptr(trait_ref.rebind(sig))
+            infcx.tcx.mk_fn_ptr(trait_ref.rebind(sig))
         }
 
         let argument_kind = match expected.skip_binder().self_ty().kind() {
@@ -1586,18 +1734,76 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let found_span = found_span.unwrap_or(span);
         err.span_label(found_span, "found signature defined here");
 
-        let expected = build_fn_sig_ty(self.tcx, expected);
-        let found = build_fn_sig_ty(self.tcx, found);
+        let expected = build_fn_sig_ty(self, expected);
+        let found = build_fn_sig_ty(self, found);
 
-        let (expected_str, found_str) =
-            self.tcx.infer_ctxt().enter(|infcx| infcx.cmp(expected, found));
+        let (expected_str, found_str) = self.cmp(expected, found);
 
         let signature_kind = format!("{argument_kind} signature");
         err.note_expected_found(&signature_kind, expected_str, &signature_kind, found_str);
 
+        self.note_conflicting_closure_bounds(cause, &mut err);
+
         err
     }
 
+    // Add a note if there are two `Fn`-family bounds that have conflicting argument
+    // requirements, which will always cause a closure to have a type error.
+    fn note_conflicting_closure_bounds(
+        &self,
+        cause: &ObligationCauseCode<'tcx>,
+        err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
+    ) {
+        // First, look for an `ExprBindingObligation`, which means we can get
+        // the unsubstituted predicate list of the called function. And check
+        // that the predicate that we failed to satisfy is a `Fn`-like trait.
+        if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = cause
+            && let predicates = self.tcx.predicates_of(def_id).instantiate_identity(self.tcx)
+            && let Some(pred) = predicates.predicates.get(*idx)
+            && let ty::PredicateKind::Trait(trait_pred) = pred.kind().skip_binder()
+            && ty::ClosureKind::from_def_id(self.tcx, trait_pred.def_id()).is_some()
+        {
+            let expected_self =
+                self.tcx.anonymize_late_bound_regions(pred.kind().rebind(trait_pred.self_ty()));
+            let expected_substs = self
+                .tcx
+                .anonymize_late_bound_regions(pred.kind().rebind(trait_pred.trait_ref.substs));
+
+            // Find another predicate whose self-type is equal to the expected self type,
+            // but whose substs don't match.
+            let other_pred = std::iter::zip(&predicates.predicates, &predicates.spans)
+                .enumerate()
+                .find(|(other_idx, (pred, _))| match pred.kind().skip_binder() {
+                    ty::PredicateKind::Trait(trait_pred)
+                        if ty::ClosureKind::from_def_id(self.tcx, trait_pred.def_id())
+                            .is_some()
+                            && other_idx != idx
+                            // Make sure that the self type matches
+                            // (i.e. constraining this closure)
+                            && expected_self
+                                == self.tcx.anonymize_late_bound_regions(
+                                    pred.kind().rebind(trait_pred.self_ty()),
+                                )
+                            // But the substs don't match (i.e. incompatible args)
+                            && expected_substs
+                                != self.tcx.anonymize_late_bound_regions(
+                                    pred.kind().rebind(trait_pred.trait_ref.substs),
+                                ) =>
+                    {
+                        true
+                    }
+                    _ => false,
+                });
+            // If we found one, then it's very likely the cause of the error.
+            if let Some((_, (_, other_pred_span))) = other_pred {
+                err.span_note(
+                    *other_pred_span,
+                    "closure inferred to have a different signature due to this bound",
+                );
+            }
+        }
+    }
+
     fn suggest_fully_qualified_path(
         &self,
         err: &mut Diagnostic,
@@ -1786,12 +1992,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         let span = self.tcx.def_span(generator_did);
 
-        let in_progress_typeck_results = self.in_progress_typeck_results.map(|t| t.borrow());
         let generator_did_root = self.tcx.typeck_root_def_id(generator_did);
         debug!(
             ?generator_did,
             ?generator_did_root,
-            in_progress_typeck_results.hir_owner = ?in_progress_typeck_results.as_ref().map(|t| t.hir_owner),
+            typeck_results.hir_owner = ?self.typeck_results.as_ref().map(|t| t.hir_owner),
             ?span,
         );
 
@@ -1846,7 +2051,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         // type-checking; otherwise, get them by performing a query.  This is needed to avoid
         // cycles. If we can't use resolved types because the generator comes from another crate,
         // we still provide a targeted error but without all the relevant spans.
-        let generator_data: Option<GeneratorData<'tcx, '_>> = match &in_progress_typeck_results {
+        let generator_data: Option<GeneratorData<'tcx, '_>> = match &self.typeck_results {
             Some(t) if t.hir_owner.to_def_id() == generator_did_root => {
                 Some(GeneratorData::Local(&t))
             }
@@ -2201,7 +2406,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             | ObligationCauseCode::QuestionMark
             | ObligationCauseCode::CheckAssociatedTypeBounds { .. }
             | ObligationCauseCode::LetElse
-            | ObligationCauseCode::BinOp { .. } => {}
+            | ObligationCauseCode::BinOp { .. }
+            | ObligationCauseCode::AscribeUserTypeProvePredicate(..) => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");
             }
@@ -2223,11 +2429,13 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     region, object_ty,
                 ));
             }
-            ObligationCauseCode::ItemObligation(_item_def_id) => {
+            ObligationCauseCode::ItemObligation(_)
+            | ObligationCauseCode::ExprItemObligation(..) => {
                 // We hold the `DefId` of the item introducing the obligation, but displaying it
                 // doesn't add user usable information. It always point at an associated item.
             }
-            ObligationCauseCode::BindingObligation(item_def_id, span) => {
+            ObligationCauseCode::BindingObligation(item_def_id, span)
+            | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..) => {
                 let item_name = tcx.def_path_str(item_def_id);
                 let mut multispan = MultiSpan::from(span);
                 if let Some(ident) = tcx.opt_item_ident(item_def_id) {
@@ -2537,9 +2745,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 parent_trait_pred.remap_constness_diag(param_env);
                 let parent_def_id = parent_trait_pred.def_id();
                 let msg = format!(
-                    "required because of the requirements on the impl of `{}` for `{}`",
-                    parent_trait_pred.print_modifiers_and_trait_path(),
-                    parent_trait_pred.skip_binder().self_ty()
+                    "required for `{}` to implement `{}`",
+                    parent_trait_pred.skip_binder().self_ty(),
+                    parent_trait_pred.print_modifiers_and_trait_path()
                 );
                 let mut is_auto_trait = false;
                 match self.tcx.hir().get_if_local(data.impl_def_id) {
@@ -2608,9 +2816,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         pluralize!(count)
                     ));
                     err.note(&format!(
-                        "required because of the requirements on the impl of `{}` for `{}`",
-                        parent_trait_pred.print_modifiers_and_trait_path(),
-                        parent_trait_pred.skip_binder().self_ty()
+                        "required for `{}` to implement `{}`",
+                        parent_trait_pred.skip_binder().self_ty(),
+                        parent_trait_pred.print_modifiers_and_trait_path()
                     ));
                 }
                 // #74711: avoid a stack overflow
@@ -2649,19 +2857,18 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 if let Some(Node::Expr(expr @ hir::Expr { kind: hir::ExprKind::Block(..), .. })) =
                     hir.find(arg_hir_id)
                 {
-                    let in_progress_typeck_results =
-                        self.in_progress_typeck_results.map(|t| t.borrow());
                     let parent_id = hir.get_parent_item(arg_hir_id);
-                    let typeck_results: &TypeckResults<'tcx> = match &in_progress_typeck_results {
+                    let typeck_results: &TypeckResults<'tcx> = match &self.typeck_results {
                         Some(t) if t.hir_owner == parent_id => t,
-                        _ => self.tcx.typeck(parent_id),
+                        _ => self.tcx.typeck(parent_id.def_id),
                     };
-                    let ty = typeck_results.expr_ty_adjusted(expr);
-                    let span = expr.peel_blocks().span;
+                    let expr = expr.peel_blocks();
+                    let ty = typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error());
+                    let span = expr.span;
                     if Some(span) != err.span.primary_span() {
                         err.span_label(
                             span,
-                            &if ty.references_error() {
+                            if ty.references_error() {
                                 String::new()
                             } else {
                                 format!("this tail expression is of type `{:?}`", ty)
@@ -2738,19 +2945,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         }
     }
 
-    fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
-        let suggested_limit = match self.tcx.recursion_limit() {
-            Limit(0) => Limit(2),
-            limit => limit * 2,
-        };
-        err.help(&format!(
-            "consider increasing the recursion limit by adding a \
-             `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
-            suggested_limit,
-            self.tcx.crate_name(LOCAL_CRATE),
-        ));
-    }
-
     #[instrument(
         level = "debug", skip(self, err), fields(trait_pred.self_ty = ?trait_pred.self_ty())
     )]
@@ -2832,19 +3026,15 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             ObligationCauseCode::BinOp { rhs_span: Some(span), is_lit, .. } if *is_lit => span,
             _ => return,
         };
-        match (
-            trait_ref.skip_binder().self_ty().kind(),
-            trait_ref.skip_binder().substs.type_at(1).kind(),
-        ) {
-            (ty::Float(_), ty::Infer(InferTy::IntVar(_))) => {
-                err.span_suggestion_verbose(
-                    rhs_span.shrink_to_hi(),
-                    "consider using a floating-point literal by writing it with `.0`",
-                    ".0",
-                    Applicability::MaybeIncorrect,
-                );
-            }
-            _ => {}
+        if let ty::Float(_) = trait_ref.skip_binder().self_ty().kind()
+            && let ty::Infer(InferTy::IntVar(_)) = trait_ref.skip_binder().substs.type_at(1).kind()
+        {
+            err.span_suggestion_verbose(
+                rhs_span.shrink_to_hi(),
+                "consider using a floating-point literal by writing it with `.0`",
+                ".0",
+                Applicability::MaybeIncorrect,
+            );
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 556ef466c..a417e1440 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -25,10 +25,9 @@ use super::Unimplemented;
 use super::{FulfillmentError, FulfillmentErrorCode};
 use super::{ObligationCause, PredicateObligation};
 
-use crate::traits::error_reporting::InferCtxtExt as _;
 use crate::traits::project::PolyProjectionObligation;
 use crate::traits::project::ProjectionCacheKeyExt as _;
-use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
 
 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
     /// Note that we include both the `ParamEnv` and the `Predicate`,
@@ -103,7 +102,7 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
     }
 
     /// Attempts to select obligations using `selcx`.
-    fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select(&mut self, selcx: SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
         let _enter = span.enter();
 
@@ -135,10 +134,10 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
     /// `SomeTrait` or a where-clause that lets us unify `$0` with
     /// something concrete. If this fails, we'll unify `$0` with
     /// `projection_ty` again.
-    #[tracing::instrument(level = "debug", skip(self, infcx, param_env, cause))]
+    #[instrument(level = "debug", skip(self, infcx, param_env, cause))]
     fn normalize_projection_type(
         &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
+        infcx: &InferCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         projection_ty: ty::ProjectionTy<'tcx>,
         cause: ObligationCause<'tcx>,
@@ -166,7 +165,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 
     fn register_predicate_obligation(
         &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
+        infcx: &InferCtxt<'tcx>,
         obligation: PredicateObligation<'tcx>,
     ) {
         // this helps to reduce duplicate errors, as well as making
@@ -183,7 +182,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
     }
 
-    fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
+    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
         {
             let errors = self.select_where_possible(infcx);
             if !errors.is_empty() {
@@ -194,12 +193,9 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
     }
 
-    fn select_where_possible(
-        &mut self,
-        infcx: &InferCtxt<'_, 'tcx>,
-    ) -> Vec<FulfillmentError<'tcx>> {
-        let mut selcx = SelectionContext::new(infcx);
-        self.select(&mut selcx)
+    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
+        let selcx = SelectionContext::new(infcx);
+        self.select(selcx)
     }
 
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
@@ -211,8 +207,8 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
     }
 }
 
-struct FulfillProcessor<'a, 'b, 'tcx> {
-    selcx: &'a mut SelectionContext<'b, 'tcx>,
+struct FulfillProcessor<'a, 'tcx> {
+    selcx: SelectionContext<'a, 'tcx>,
 }
 
 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
@@ -221,9 +217,10 @@ fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligatio
         .collect()
 }
 
-impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
+impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
     type Obligation = PendingPredicateObligation<'tcx>;
     type Error = FulfillmentErrorCode<'tcx>;
+    type OUT = Outcome<Self::Obligation, Self::Error>;
 
     /// Identifies whether a predicate obligation needs processing.
     ///
@@ -279,7 +276,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 
         debug!(?obligation, "pre-resolve");
 
-        if obligation.predicate.has_infer_types_or_consts() {
+        if obligation.predicate.has_non_region_infer() {
             obligation.predicate =
                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
         }
@@ -291,7 +288,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
         if obligation.predicate.has_projections() {
             let mut obligations = Vec::new();
             let predicate = crate::traits::project::try_normalize_with_depth_to(
-                self.selcx,
+                &mut self.selcx,
                 obligation.param_env,
                 obligation.cause.clone(),
                 obligation.recursion_depth + 1,
@@ -358,7 +355,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                 }
 
                 ty::PredicateKind::RegionOutlives(data) => {
-                    if infcx.considering_regions || data.has_placeholders() {
+                    if infcx.considering_regions {
                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
                     }
 
@@ -427,16 +424,14 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                         obligation.param_env,
                         Binder::dummy(subtype),
                     ) {
-                        None => {
+                        Err((a, b)) => {
                             // None means that both are unresolved.
-                            pending_obligation.stalled_on = vec![
-                                TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
-                                TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
-                            ];
+                            pending_obligation.stalled_on =
+                                vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
                             ProcessResult::Unchanged
                         }
-                        Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
-                        Some(Err(err)) => {
+                        Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
+                        Ok(Err(err)) => {
                             let expected_found =
                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
@@ -453,16 +448,14 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                         obligation.param_env,
                         Binder::dummy(coerce),
                     ) {
-                        None => {
+                        Err((a, b)) => {
                             // None means that both are unresolved.
-                            pending_obligation.stalled_on = vec![
-                                TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
-                                TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
-                            ];
+                            pending_obligation.stalled_on =
+                                vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
                             ProcessResult::Unchanged
                         }
-                        Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
-                        Some(Err(err)) => {
+                        Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
+                        Ok(Err(err)) => {
                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
                                 expected_found,
@@ -483,9 +476,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                         Err(NotConstEvaluatable::MentionsInfer) => {
                             pending_obligation.stalled_on.clear();
                             pending_obligation.stalled_on.extend(
-                                uv.substs
-                                    .iter()
-                                    .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
+                                uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
                             );
                             ProcessResult::Unchanged
                         }
@@ -499,23 +490,20 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                 }
 
                 ty::PredicateKind::ConstEquate(c1, c2) => {
+                    assert!(
+                        self.selcx.tcx().features().generic_const_exprs,
+                        "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
+                    );
                     debug!(?c1, ?c2, "equating consts");
-                    let tcx = self.selcx.tcx();
-                    if tcx.features().generic_const_exprs {
-                        // FIXME: we probably should only try to unify abstract constants
-                        // if the constants depend on generic parameters.
-                        //
-                        // Let's just see where this breaks :shrug:
-                        if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
-                            (c1.kind(), c2.kind())
-                        {
-                            if infcx.try_unify_abstract_consts(
-                                a.shrink(),
-                                b.shrink(),
-                                obligation.param_env,
-                            ) {
-                                return ProcessResult::Changed(vec![]);
-                            }
+                    // FIXME: we probably should only try to unify abstract constants
+                    // if the constants depend on generic parameters.
+                    //
+                    // Let's just see where this breaks :shrug:
+                    if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
+                        (c1.kind(), c2.kind())
+                    {
+                        if infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
+                            return ProcessResult::Changed(vec![]);
                         }
                     }
 
@@ -577,7 +565,7 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
                             )
                         }
                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
-                            if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
+                            if c1.has_non_region_infer() || c2.has_non_region_infer() {
                                 ProcessResult::Unchanged
                             } else {
                                 // Two different constants using generic parameters ~> error.
@@ -597,23 +585,26 @@ impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
         }
     }
 
+    #[inline(never)]
     fn process_backedge<'c, I>(
         &mut self,
         cycle: I,
         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
-    ) where
+    ) -> Result<(), FulfillmentErrorCode<'tcx>>
+    where
         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
     {
         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
             debug!("process_child_obligations: coinductive match");
+            Ok(())
         } else {
             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
-            self.selcx.infcx().report_overflow_error_cycle(&cycle);
+            Err(FulfillmentErrorCode::CodeCycle(cycle))
         }
     }
 }
 
-impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
+impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
     fn process_trait_obligation(
         &mut self,
@@ -648,7 +639,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                 // information about the types in the trait.
                 stalled_on.clear();
                 stalled_on.extend(substs_infer_vars(
-                    self.selcx,
+                    &self.selcx,
                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
                 ));
 
@@ -700,12 +691,12 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
             }
         }
 
-        match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
+        match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
             ProjectAndUnifyResult::FailedNormalization => {
                 stalled_on.clear();
                 stalled_on.extend(substs_infer_vars(
-                    self.selcx,
+                    &self.selcx,
                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
                 ));
                 ProcessResult::Unchanged
@@ -723,7 +714,7 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 
 /// Returns the set of inference variables contained in `substs`.
 fn substs_infer_vars<'a, 'tcx>(
-    selcx: &mut SelectionContext<'a, 'tcx>,
+    selcx: &SelectionContext<'a, 'tcx>,
     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
     selcx
@@ -731,11 +722,11 @@ fn substs_infer_vars<'a, 'tcx>(
         .resolve_vars_if_possible(substs)
         .skip_binder() // ok because this check doesn't care about regions
         .iter()
-        .filter(|arg| arg.has_infer_types_or_consts())
+        .filter(|arg| arg.has_non_region_infer())
         .flat_map(|arg| {
             let mut walker = arg.walk();
             while let Some(c) = walker.next() {
-                if !c.has_infer_types_or_consts() {
+                if !c.has_non_region_infer() {
                     walker.visited.remove(&c);
                     walker.skip_current_subtree();
                 }
diff --git a/compiler/rustc_trait_selection/src/traits/misc.rs b/compiler/rustc_trait_selection/src/traits/misc.rs
index dd2769c71..be603c609 100644
--- a/compiler/rustc_trait_selection/src/traits/misc.rs
+++ b/compiler/rustc_trait_selection/src/traits/misc.rs
@@ -7,7 +7,7 @@ use rustc_hir as hir;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
 
-use crate::traits::error_reporting::InferCtxtExt;
+use crate::traits::error_reporting::TypeErrCtxtExt;
 
 #[derive(Clone)]
 pub enum CopyImplementationError<'tcx> {
@@ -23,66 +23,64 @@ pub fn can_type_implement_copy<'tcx>(
     parent_cause: ObligationCause<'tcx>,
 ) -> Result<(), CopyImplementationError<'tcx>> {
     // FIXME: (@jroesch) float this code up
-    tcx.infer_ctxt().enter(|infcx| {
-        let (adt, substs) = match self_type.kind() {
-            // These types used to have a builtin impl.
-            // Now libcore provides that impl.
-            ty::Uint(_)
-            | ty::Int(_)
-            | ty::Bool
-            | ty::Float(_)
-            | ty::Char
-            | ty::RawPtr(..)
-            | ty::Never
-            | ty::Ref(_, _, hir::Mutability::Not)
-            | ty::Array(..) => return Ok(()),
+    let infcx = tcx.infer_ctxt().build();
+    let (adt, substs) = match self_type.kind() {
+        // These types used to have a builtin impl.
+        // Now libcore provides that impl.
+        ty::Uint(_)
+        | ty::Int(_)
+        | ty::Bool
+        | ty::Float(_)
+        | ty::Char
+        | ty::RawPtr(..)
+        | ty::Never
+        | ty::Ref(_, _, hir::Mutability::Not)
+        | ty::Array(..) => return Ok(()),
 
-            ty::Adt(adt, substs) => (adt, substs),
+        ty::Adt(adt, substs) => (adt, substs),
 
-            _ => return Err(CopyImplementationError::NotAnAdt),
-        };
+        _ => return Err(CopyImplementationError::NotAnAdt),
+    };
 
-        let mut infringing = Vec::new();
-        for variant in adt.variants() {
-            for field in &variant.fields {
-                let ty = field.ty(tcx, substs);
-                if ty.references_error() {
-                    continue;
-                }
-                let span = tcx.def_span(field.did);
-                // FIXME(compiler-errors): This gives us better spans for bad
-                // projection types like in issue-50480.
-                // If the ADT has substs, point to the cause we are given.
-                // If it does not, then this field probably doesn't normalize
-                // to begin with, and point to the bad field's span instead.
-                let cause = if field
-                    .ty(tcx, traits::InternalSubsts::identity_for_item(tcx, adt.did()))
-                    .has_param_types_or_consts()
-                {
-                    parent_cause.clone()
-                } else {
-                    ObligationCause::dummy_with_span(span)
-                };
-                let ctx = traits::FulfillmentContext::new();
-                match traits::fully_normalize(&infcx, ctx, cause, param_env, ty) {
-                    Ok(ty) => {
-                        if !infcx.type_is_copy_modulo_regions(param_env, ty, span) {
-                            infringing.push((field, ty));
-                        }
-                    }
-                    Err(errors) => {
-                        infcx.report_fulfillment_errors(&errors, None, false);
-                    }
-                };
+    let mut infringing = Vec::new();
+    for variant in adt.variants() {
+        for field in &variant.fields {
+            let ty = field.ty(tcx, substs);
+            if ty.references_error() {
+                continue;
             }
+            let span = tcx.def_span(field.did);
+            // FIXME(compiler-errors): This gives us better spans for bad
+            // projection types like in issue-50480.
+            // If the ADT has substs, point to the cause we are given.
+            // If it does not, then this field probably doesn't normalize
+            // to begin with, and point to the bad field's span instead.
+            let cause = if field
+                .ty(tcx, traits::InternalSubsts::identity_for_item(tcx, adt.did()))
+                .has_non_region_param()
+            {
+                parent_cause.clone()
+            } else {
+                ObligationCause::dummy_with_span(span)
+            };
+            match traits::fully_normalize(&infcx, cause, param_env, ty) {
+                Ok(ty) => {
+                    if !infcx.type_is_copy_modulo_regions(param_env, ty, span) {
+                        infringing.push((field, ty));
+                    }
+                }
+                Err(errors) => {
+                    infcx.err_ctxt().report_fulfillment_errors(&errors, None, false);
+                }
+            };
         }
-        if !infringing.is_empty() {
-            return Err(CopyImplementationError::InfrigingFields(infringing));
-        }
-        if adt.has_dtor(tcx) {
-            return Err(CopyImplementationError::HasDestructor);
-        }
+    }
+    if !infringing.is_empty() {
+        return Err(CopyImplementationError::InfrigingFields(infringing));
+    }
+    if adt.has_dtor(tcx) {
+        return Err(CopyImplementationError::HasDestructor);
+    }
 
-        Ok(())
-    })
+    Ok(())
 }
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 9c6bb0731..0bf54c096 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -13,6 +13,7 @@ mod fulfill;
 pub mod misc;
 mod object_safety;
 mod on_unimplemented;
+pub mod outlives_bounds;
 mod project;
 pub mod query;
 pub(crate) mod relationships;
@@ -22,18 +23,22 @@ mod structural_match;
 mod util;
 pub mod wf;
 
+use crate::errors::DumpVTableEntries;
 use crate::infer::outlives::env::OutlivesEnvironment;
 use crate::infer::{InferCtxt, TyCtxtInferExt};
-use crate::traits::error_reporting::InferCtxtExt as _;
+use crate::traits::error_reporting::TypeErrCtxtExt as _;
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
+use rustc_infer::traits::TraitEngineExt as _;
 use rustc_middle::ty::fold::TypeFoldable;
-use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
 use rustc_middle::ty::visit::TypeVisitable;
-use rustc_middle::ty::{self, GenericParamDefKind, ToPredicate, Ty, TyCtxt, VtblEntry};
+use rustc_middle::ty::{
+    self, DefIdTree, GenericParamDefKind, ToPredicate, Ty, TyCtxt, TypeSuperVisitable, VtblEntry,
+};
+use rustc_middle::ty::{InternalSubsts, SubstsRef};
 use rustc_span::{sym, Span};
 use smallvec::SmallVec;
 
@@ -113,11 +118,21 @@ pub enum TraitQueryMode {
 
 /// Creates predicate obligations from the generic bounds.
 pub fn predicates_for_generics<'tcx>(
-    cause: ObligationCause<'tcx>,
+    cause: impl Fn(usize, Span) -> ObligationCause<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     generic_bounds: ty::InstantiatedPredicates<'tcx>,
 ) -> impl Iterator<Item = PredicateObligation<'tcx>> {
-    util::predicates_for_generics(cause, 0, param_env, generic_bounds)
+    let generic_bounds = generic_bounds;
+    debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);
+
+    std::iter::zip(generic_bounds.predicates, generic_bounds.spans).enumerate().map(
+        move |(idx, (predicate, span))| Obligation {
+            cause: cause(idx, span),
+            recursion_depth: 0,
+            param_env,
+            predicate,
+        },
+    )
 }
 
 /// Determines whether the type `ty` is known to meet `bound` and
@@ -125,8 +140,8 @@ pub fn predicates_for_generics<'tcx>(
 /// `bound` or is not known to meet bound (note that this is
 /// conservative towards *no impl*, which is the opposite of the
 /// `evaluate` methods).
-pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
-    infcx: &InferCtxt<'a, 'tcx>,
+pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
+    infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     ty: Ty<'tcx>,
     def_id: DefId,
@@ -155,28 +170,26 @@ pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
         result
     );
 
-    if result && ty.has_infer_types_or_consts() {
+    if result && ty.has_non_region_infer() {
         // Because of inference "guessing", selection can sometimes claim
         // to succeed while the success requires a guess. To ensure
         // this function's result remains infallible, we must confirm
         // that guess. While imperfect, I believe this is sound.
 
-        // The handling of regions in this area of the code is terrible,
-        // see issue #29149. We should be able to improve on this with
-        // NLL.
-        let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
-
         // We can use a dummy node-id here because we won't pay any mind
         // to region obligations that arise (there shouldn't really be any
         // anyhow).
         let cause = ObligationCause::misc(span, hir::CRATE_HIR_ID);
 
-        fulfill_cx.register_bound(infcx, param_env, ty, def_id, cause);
+        // The handling of regions in this area of the code is terrible,
+        // see issue #29149. We should be able to improve on this with
+        // NLL.
+        let errors = fully_solve_bound(infcx, cause, param_env, ty, def_id);
 
         // Note: we only assume something is `Copy` if we can
         // *definitively* show that it implements `Copy`. Otherwise,
         // assume it is move; linear is always ok.
-        match fulfill_cx.select_all_or_error(infcx).as_slice() {
+        match &errors[..] {
             [] => {
                 debug!(
                     "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} success",
@@ -221,56 +234,51 @@ fn do_normalize_predicates<'tcx>(
     // by wfcheck anyway, so I'm not sure we have to check
     // them here too, and we will remove this function when
     // we move over to lazy normalization *anyway*.
-    tcx.infer_ctxt().ignoring_regions().enter(|infcx| {
-        let fulfill_cx = FulfillmentContext::new();
-        let predicates =
-            match fully_normalize(&infcx, fulfill_cx, cause, elaborated_env, predicates) {
-                Ok(predicates) => predicates,
-                Err(errors) => {
-                    let reported = infcx.report_fulfillment_errors(&errors, None, false);
-                    return Err(reported);
-                }
-            };
+    let infcx = tcx.infer_ctxt().ignoring_regions().build();
+    let predicates = match fully_normalize(&infcx, cause, elaborated_env, predicates) {
+        Ok(predicates) => predicates,
+        Err(errors) => {
+            let reported = infcx.err_ctxt().report_fulfillment_errors(&errors, None, false);
+            return Err(reported);
+        }
+    };
+
+    debug!("do_normalize_predictes: normalized predicates = {:?}", predicates);
 
-        debug!("do_normalize_predictes: normalized predicates = {:?}", predicates);
+    // We can use the `elaborated_env` here; the region code only
+    // cares about declarations like `'a: 'b`.
+    let outlives_env = OutlivesEnvironment::new(elaborated_env);
 
-        // We can use the `elaborated_env` here; the region code only
-        // cares about declarations like `'a: 'b`.
-        let outlives_env = OutlivesEnvironment::new(elaborated_env);
+    // FIXME: It's very weird that we ignore region obligations but apparently
+    // still need to use `resolve_regions` as we need the resolved regions in
+    // the normalized predicates.
+    let errors = infcx.resolve_regions(&outlives_env);
+    if !errors.is_empty() {
+        tcx.sess.delay_span_bug(
+            span,
+            format!("failed region resolution while normalizing {elaborated_env:?}: {errors:?}"),
+        );
+    }
 
-        // FIXME: It's very weird that we ignore region obligations but apparently
-        // still need to use `resolve_regions` as we need the resolved regions in
-        // the normalized predicates.
-        let errors = infcx.resolve_regions(&outlives_env);
-        if !errors.is_empty() {
-            tcx.sess.delay_span_bug(
+    match infcx.fully_resolve(predicates) {
+        Ok(predicates) => Ok(predicates),
+        Err(fixup_err) => {
+            // If we encounter a fixup error, it means that some type
+            // variable wound up unconstrained. I actually don't know
+            // if this can happen, and I certainly don't expect it to
+            // happen often, but if it did happen it probably
+            // represents a legitimate failure due to some kind of
+            // unconstrained variable.
+            //
+            // @lcnr: Let's still ICE here for now. I want a test case
+            // for that.
+            span_bug!(
                 span,
-                format!(
-                    "failed region resolution while normalizing {elaborated_env:?}: {errors:?}"
-                ),
+                "inference variables in normalized parameter environment: {}",
+                fixup_err
             );
         }
-
-        match infcx.fully_resolve(predicates) {
-            Ok(predicates) => Ok(predicates),
-            Err(fixup_err) => {
-                // If we encounter a fixup error, it means that some type
-                // variable wound up unconstrained. I actually don't know
-                // if this can happen, and I certainly don't expect it to
-                // happen often, but if it did happen it probably
-                // represents a legitimate failure due to some kind of
-                // unconstrained variable.
-                //
-                // @lcnr: Let's still ICE here for now. I want a test case
-                // for that.
-                span_bug!(
-                    span,
-                    "inference variables in normalized parameter environment: {}",
-                    fixup_err
-                );
-            }
-        }
-    })
+    }
 }
 
 // FIXME: this is gonna need to be removed ...
@@ -381,9 +389,9 @@ pub fn normalize_param_env_or_error<'tcx>(
     )
 }
 
-pub fn fully_normalize<'a, 'tcx, T>(
-    infcx: &InferCtxt<'a, 'tcx>,
-    mut fulfill_cx: FulfillmentContext<'tcx>,
+/// Normalize a type and process all resulting obligations, returning any errors
+pub fn fully_normalize<'tcx, T>(
+    infcx: &InferCtxt<'tcx>,
     cause: ObligationCause<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     value: T,
@@ -399,8 +407,10 @@ where
         "fully_normalize: normalized_value={:?} obligations={:?}",
         normalized_value, obligations
     );
+
+    let mut fulfill_cx = FulfillmentContext::new();
     for obligation in obligations {
-        fulfill_cx.register_predicate_obligation(selcx.infcx(), obligation);
+        fulfill_cx.register_predicate_obligation(infcx, obligation);
     }
 
     debug!("fully_normalize: select_all_or_error start");
@@ -414,6 +424,43 @@ where
     Ok(resolved_value)
 }
 
+/// Process an obligation (and any nested obligations that come from it) to
+/// completion, returning any errors
+pub fn fully_solve_obligation<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    obligation: PredicateObligation<'tcx>,
+) -> Vec<FulfillmentError<'tcx>> {
+    let mut engine = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
+    engine.register_predicate_obligation(infcx, obligation);
+    engine.select_all_or_error(infcx)
+}
+
+/// Process a set of obligations (and any nested obligations that come from them)
+/// to completion
+pub fn fully_solve_obligations<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    obligations: impl IntoIterator<Item = PredicateObligation<'tcx>>,
+) -> Vec<FulfillmentError<'tcx>> {
+    let mut engine = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
+    engine.register_predicate_obligations(infcx, obligations);
+    engine.select_all_or_error(infcx)
+}
+
+/// Process a bound (and any nested obligations that come from it) to completion.
+/// This is a convenience function for traits that have no generic arguments, such
+/// as auto traits, and builtin traits like Copy or Sized.
+pub fn fully_solve_bound<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    cause: ObligationCause<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    ty: Ty<'tcx>,
+    bound: DefId,
+) -> Vec<FulfillmentError<'tcx>> {
+    let mut engine = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
+    engine.register_bound(infcx, param_env, ty, bound, cause);
+    engine.select_all_or_error(infcx)
+}
+
 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
 /// returns true, then either normalize encountered an error or one of the predicates did not
 /// hold. Used when creating vtables to check for unsatisfiable methods.
@@ -423,31 +470,20 @@ pub fn impossible_predicates<'tcx>(
 ) -> bool {
     debug!("impossible_predicates(predicates={:?})", predicates);
 
-    let result = tcx.infer_ctxt().enter(|infcx| {
-        // HACK: Set tainted by errors to gracefully exit in case of overflow.
-        infcx.set_tainted_by_errors();
-
-        let param_env = ty::ParamEnv::reveal_all();
-        let mut selcx = SelectionContext::new(&infcx);
-        let mut fulfill_cx = FulfillmentContext::new();
-        let cause = ObligationCause::dummy();
-        let Normalized { value: predicates, obligations } =
-            normalize(&mut selcx, param_env, cause.clone(), predicates);
-        for obligation in obligations {
-            fulfill_cx.register_predicate_obligation(&infcx, obligation);
-        }
-        for predicate in predicates {
-            let obligation = Obligation::new(cause.clone(), param_env, predicate);
-            fulfill_cx.register_predicate_obligation(&infcx, obligation);
-        }
-
-        let errors = fulfill_cx.select_all_or_error(&infcx);
+    let infcx = tcx.infer_ctxt().build();
+    let param_env = ty::ParamEnv::reveal_all();
+    let ocx = ObligationCtxt::new(&infcx);
+    let predicates = ocx.normalize(ObligationCause::dummy(), param_env, predicates);
+    for predicate in predicates {
+        let obligation = Obligation::new(ObligationCause::dummy(), param_env, predicate);
+        ocx.register_obligation(obligation);
+    }
+    let errors = ocx.select_all_or_error();
 
-        // Clean up after ourselves
-        let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
+    // Clean up after ourselves
+    let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
 
-        !errors.is_empty()
-    });
+    let result = !errors.is_empty();
     debug!("impossible_predicates = {:?}", result);
     result
 }
@@ -474,6 +510,82 @@ fn subst_and_check_impossible_predicates<'tcx>(
     result
 }
 
+/// Checks whether a trait's method is impossible to call on a given impl.
+///
+/// This only considers predicates that reference the impl's generics, and not
+/// those that reference the method's generics.
+fn is_impossible_method<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    (impl_def_id, trait_item_def_id): (DefId, DefId),
+) -> bool {
+    struct ReferencesOnlyParentGenerics<'tcx> {
+        tcx: TyCtxt<'tcx>,
+        generics: &'tcx ty::Generics,
+        trait_item_def_id: DefId,
+    }
+    impl<'tcx> ty::TypeVisitor<'tcx> for ReferencesOnlyParentGenerics<'tcx> {
+        type BreakTy = ();
+        fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+            // If this is a parameter from the trait item's own generics, then bail
+            if let ty::Param(param) = t.kind()
+                && let param_def_id = self.generics.type_param(param, self.tcx).def_id
+                && self.tcx.parent(param_def_id) == self.trait_item_def_id
+            {
+                return ControlFlow::BREAK;
+            }
+            t.super_visit_with(self)
+        }
+        fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::ReEarlyBound(param) = r.kind()
+                && let param_def_id = self.generics.region_param(&param, self.tcx).def_id
+                && self.tcx.parent(param_def_id) == self.trait_item_def_id
+            {
+                return ControlFlow::BREAK;
+            }
+            r.super_visit_with(self)
+        }
+        fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+            if let ty::ConstKind::Param(param) = ct.kind()
+                && let param_def_id = self.generics.const_param(&param, self.tcx).def_id
+                && self.tcx.parent(param_def_id) == self.trait_item_def_id
+            {
+                return ControlFlow::BREAK;
+            }
+            ct.super_visit_with(self)
+        }
+    }
+
+    let generics = tcx.generics_of(trait_item_def_id);
+    let predicates = tcx.predicates_of(trait_item_def_id);
+    let impl_trait_ref =
+        tcx.impl_trait_ref(impl_def_id).expect("expected impl to correspond to trait");
+    let param_env = tcx.param_env(impl_def_id);
+
+    let mut visitor = ReferencesOnlyParentGenerics { tcx, generics, trait_item_def_id };
+    let predicates_for_trait = predicates.predicates.iter().filter_map(|(pred, span)| {
+        if pred.visit_with(&mut visitor).is_continue() {
+            Some(Obligation::new(
+                ObligationCause::dummy_with_span(*span),
+                param_env,
+                ty::EarlyBinder(*pred).subst(tcx, impl_trait_ref.substs),
+            ))
+        } else {
+            None
+        }
+    });
+
+    let infcx = tcx.infer_ctxt().ignoring_regions().build();
+    for obligation in predicates_for_trait {
+        // Ignore overflow error, to be conservative.
+        if let Ok(result) = infcx.evaluate_obligation(&obligation)
+            && !result.may_apply()
+        {
+            return true;
+        }
+    }
+    false
+}
+
 #[derive(Clone, Debug)]
 enum VtblSegment<'tcx> {
     MetadataDSA,
@@ -645,16 +757,16 @@ fn dump_vtable_entries<'tcx>(
     trait_ref: ty::PolyTraitRef<'tcx>,
     entries: &[VtblEntry<'tcx>],
 ) {
-    let msg = format!("vtable entries for `{}`: {:#?}", trait_ref, entries);
-    tcx.sess.struct_span_err(sp, &msg).emit();
+    tcx.sess.emit_err(DumpVTableEntries {
+        span: sp,
+        trait_ref,
+        entries: format!("{:#?}", entries),
+    });
 }
 
-fn own_existential_vtable_entries<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    trait_ref: ty::PolyExistentialTraitRef<'tcx>,
-) -> &'tcx [DefId] {
+fn own_existential_vtable_entries<'tcx>(tcx: TyCtxt<'tcx>, trait_def_id: DefId) -> &'tcx [DefId] {
     let trait_methods = tcx
-        .associated_items(trait_ref.def_id())
+        .associated_items(trait_def_id)
         .in_definition_order()
         .filter(|item| item.kind == ty::AssocKind::Fn);
     // Now list each method's DefId (for within its trait).
@@ -663,7 +775,7 @@ fn own_existential_vtable_entries<'tcx>(
         let def_id = trait_method.def_id;
 
         // Some methods cannot be called on an object; skip those.
-        if !is_vtable_safe_method(tcx, trait_ref.def_id(), &trait_method) {
+        if !is_vtable_safe_method(tcx, trait_def_id, &trait_method) {
             debug!("own_existential_vtable_entry: not vtable safe");
             return None;
         }
@@ -695,7 +807,7 @@ fn vtable_entries<'tcx>(
 
                 // Lookup the shape of vtable for the trait.
                 let own_existential_entries =
-                    tcx.own_existential_vtable_entries(existential_trait_ref);
+                    tcx.own_existential_vtable_entries(existential_trait_ref.def_id());
 
                 let own_entries = own_existential_entries.iter().copied().map(|def_id| {
                     debug!("vtable_entries: trait_method={:?}", def_id);
@@ -831,10 +943,9 @@ pub fn vtable_trait_upcasting_coercion_new_vptr_slot<'tcx>(
         }),
     );
 
-    let implsrc = tcx.infer_ctxt().enter(|infcx| {
-        let mut selcx = SelectionContext::new(&infcx);
-        selcx.select(&obligation).unwrap()
-    });
+    let infcx = tcx.infer_ctxt().build();
+    let mut selcx = SelectionContext::new(&infcx);
+    let implsrc = selcx.select(&obligation).unwrap();
 
     let Some(ImplSource::TraitUpcasting(implsrc_traitcasting)) = implsrc else {
         bug!();
@@ -849,11 +960,12 @@ pub fn provide(providers: &mut ty::query::Providers) {
     *providers = ty::query::Providers {
         specialization_graph_of: specialize::specialization_graph_provider,
         specializes: specialize::specializes,
-        codegen_fulfill_obligation: codegen::codegen_fulfill_obligation,
+        codegen_select_candidate: codegen::codegen_select_candidate,
         own_existential_vtable_entries,
         vtable_entries,
         vtable_trait_upcasting_coercion_new_vptr_slot,
         subst_and_check_impossible_predicates,
+        is_impossible_method,
         try_unify_abstract_consts: |tcx, param_env_and| {
             let (param_env, (a, b)) = param_env_and.into_parts();
             const_evaluatable::try_unify_abstract_consts(tcx, (a, b), param_env)
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 612f51309..0bb25a74d 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -8,19 +8,20 @@
 //!   - not reference the erased type `Self` except for in this receiver;
 //!   - not have generic type parameters.
 
-use super::elaborate_predicates;
+use super::{elaborate_predicates, elaborate_trait_ref};
 
 use crate::infer::TyCtxtInferExt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{self, Obligation, ObligationCause};
-use rustc_errors::{FatalError, MultiSpan};
+use hir::def::DefKind;
+use rustc_errors::{DelayDm, FatalError, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst};
-use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
 use rustc_middle::ty::{
     self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
 };
+use rustc_middle::ty::{GenericArg, InternalSubsts};
 use rustc_middle::ty::{Predicate, ToPredicate};
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;
@@ -163,37 +164,42 @@ fn lint_object_unsafe_trait(
 ) {
     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
     // It's also hard to get a use site span, so we use the method definition span.
-    tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
-        let mut err = lint.build(&format!(
-            "the trait `{}` cannot be made into an object",
-            tcx.def_path_str(trait_def_id)
-        ));
-        let node = tcx.hir().get_if_local(trait_def_id);
-        let mut spans = MultiSpan::from_span(span);
-        if let Some(hir::Node::Item(item)) = node {
-            spans.push_span_label(item.ident.span, "this trait cannot be made into an object...");
-            spans.push_span_label(span, format!("...because {}", violation.error_msg()));
-        } else {
-            spans.push_span_label(
-                span,
-                format!(
-                    "the trait cannot be made into an object because {}",
-                    violation.error_msg()
-                ),
+    tcx.struct_span_lint_hir(
+        WHERE_CLAUSES_OBJECT_SAFETY,
+        hir::CRATE_HIR_ID,
+        span,
+        DelayDm(|| format!("the trait `{}` cannot be made into an object", tcx.def_path_str(trait_def_id))),
+        |err| {
+            let node = tcx.hir().get_if_local(trait_def_id);
+            let mut spans = MultiSpan::from_span(span);
+            if let Some(hir::Node::Item(item)) = node {
+                spans.push_span_label(
+                    item.ident.span,
+                    "this trait cannot be made into an object...",
+                );
+                spans.push_span_label(span, format!("...because {}", violation.error_msg()));
+            } else {
+                spans.push_span_label(
+                    span,
+                    format!(
+                        "the trait cannot be made into an object because {}",
+                        violation.error_msg()
+                    ),
+                );
+            };
+            err.span_note(
+                spans,
+                "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
+                call to be resolvable dynamically; for more information visit \
+                <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
             );
-        };
-        err.span_note(
-            spans,
-            "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
-             call to be resolvable dynamically; for more information visit \
-             <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
-        );
-        if node.is_some() {
-            // Only provide the help if its a local trait, otherwise it's not
-            violation.solution(&mut err);
-        }
-        err.emit();
-    });
+            if node.is_some() {
+                // Only provide the help if its a local trait, otherwise it's not
+                violation.solution(err);
+            }
+            err
+        },
+    );
 }
 
 fn sized_trait_bound_spans<'tcx>(
@@ -431,6 +437,9 @@ fn virtual_call_violation_for_method<'tcx>(
     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
         return Some(MethodViolationCode::ReferencesSelfOutput);
     }
+    if contains_illegal_impl_trait_in_trait(tcx, sig.output()) {
+        return Some(MethodViolationCode::ReferencesImplTraitInTrait);
+    }
 
     // We can't monomorphize things like `fn foo<A>(...)`.
     let own_counts = tcx.generics_of(method.def_id).own_counts();
@@ -438,19 +447,6 @@ fn virtual_call_violation_for_method<'tcx>(
         return Some(MethodViolationCode::Generic);
     }
 
-    if tcx
-        .predicates_of(method.def_id)
-        .predicates
-        .iter()
-        // A trait object can't claim to live more than the concrete type,
-        // so outlives predicates will always hold.
-        .cloned()
-        .filter(|(p, _)| p.to_opt_type_outlives().is_none())
-        .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
-    {
-        return Some(MethodViolationCode::WhereClauseReferencesSelf);
-    }
-
     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 
     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
@@ -529,6 +525,21 @@ fn virtual_call_violation_for_method<'tcx>(
         }
     }
 
+    // NOTE: This check happens last, because it results in a lint, and not a
+    // hard error.
+    if tcx
+        .predicates_of(method.def_id)
+        .predicates
+        .iter()
+        // A trait object can't claim to live more than the concrete type,
+        // so outlives predicates will always hold.
+        .cloned()
+        .filter(|(p, _)| p.to_opt_type_outlives().is_none())
+        .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
+    {
+        return Some(MethodViolationCode::WhereClauseReferencesSelf);
+    }
+
     None
 }
 
@@ -556,51 +567,44 @@ fn receiver_for_self_ty<'tcx>(
 /// Creates the object type for the current trait. For example,
 /// if the current trait is `Deref`, then this will be
 /// `dyn Deref<Target = Self::Target> + 'static`.
+#[instrument(level = "trace", skip(tcx), ret)]
 fn object_ty_for_trait<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_def_id: DefId,
     lifetime: ty::Region<'tcx>,
 ) -> Ty<'tcx> {
-    debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
-
     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
+    debug!(?trait_ref);
 
     let trait_predicate = trait_ref.map_bound(|trait_ref| {
         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
     });
-
-    let mut associated_types = traits::supertraits(tcx, trait_ref)
-        .flat_map(|super_trait_ref| {
-            tcx.associated_items(super_trait_ref.def_id())
-                .in_definition_order()
-                .map(move |item| (super_trait_ref, item))
-        })
-        .filter(|(_, item)| item.kind == ty::AssocKind::Type)
-        .collect::<Vec<_>>();
-
-    // existential predicates need to be in a specific order
-    associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
-
-    let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
-        // We *can* get bound lifetimes here in cases like
-        // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
-        super_trait_ref.map_bound(|super_trait_ref| {
-            ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
-                term: tcx.mk_projection(item.def_id, super_trait_ref.substs).into(),
-                item_def_id: item.def_id,
-                substs: super_trait_ref.substs,
-            })
+    debug!(?trait_predicate);
+
+    let mut elaborated_predicates: Vec<_> = elaborate_trait_ref(tcx, trait_ref)
+        .filter_map(|obligation| {
+            debug!(?obligation);
+            let pred = obligation.predicate.to_opt_poly_projection_pred()?;
+            Some(pred.map_bound(|p| {
+                ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
+                    item_def_id: p.projection_ty.item_def_id,
+                    substs: p.projection_ty.substs,
+                    term: p.term,
+                })
+            }))
         })
-    });
+        .collect();
+    // NOTE: Since #37965, the existential predicates list has depended on the
+    // list of predicates to be sorted. This is mostly to enforce that the primary
+    // predicate comes first.
+    elaborated_predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
+    elaborated_predicates.dedup();
 
     let existential_predicates = tcx
-        .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
+        .mk_poly_existential_predicates(iter::once(trait_predicate).chain(elaborated_predicates));
+    debug!(?existential_predicates);
 
-    let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
-
-    debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
-
-    object_ty
+    tcx.mk_dynamic(existential_predicates, lifetime, ty::Dyn)
 }
 
 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
@@ -725,10 +729,9 @@ fn receiver_is_dispatchable<'tcx>(
         Obligation::new(ObligationCause::dummy(), param_env, predicate)
     };
 
-    tcx.infer_ctxt().enter(|ref infcx| {
-        // the receiver is dispatchable iff the obligation holds
-        infcx.predicate_must_hold_modulo_regions(&obligation)
-    })
+    let infcx = tcx.infer_ctxt().build();
+    // the receiver is dispatchable iff the obligation holds
+    infcx.predicate_must_hold_modulo_regions(&obligation)
 }
 
 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
@@ -793,6 +796,12 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
                         ControlFlow::CONTINUE
                     }
                 }
+                ty::Projection(ref data)
+                    if self.tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder =>
+                {
+                    // We'll deny these later in their own pass
+                    ControlFlow::CONTINUE
+                }
                 ty::Projection(ref data) => {
                     // This is a projected type `<Foo as SomeTrait>::X`.
 
@@ -828,21 +837,14 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
             }
         }
 
-        fn visit_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
+        fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
             // Constants can only influence object safety if they reference `Self`.
             // This is only possible for unevaluated constants, so we walk these here.
             //
-            // If `AbstractConst::new` returned an error we already failed compilation
+            // If `AbstractConst::from_const` returned an error we already failed compilation
             // so we don't have to emit an additional error here.
-            //
-            // We currently recurse into abstract consts here but do not recurse in
-            // `is_const_evaluatable`. This means that the object safety check is more
-            // liberal than the const eval check.
-            //
-            // This shouldn't really matter though as we can't really use any
-            // constants which are not considered const evaluatable.
             use rustc_middle::ty::abstract_const::Node;
-            if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
+            if let Ok(Some(ct)) = AbstractConst::from_const(self.tcx, ct) {
                 walk_abstract_const(self.tcx, ct, |node| match node.root(self.tcx) {
                     Node::Leaf(leaf) => self.visit_const(leaf),
                     Node::Cast(_, _, ty) => self.visit_ty(ty),
@@ -851,7 +853,7 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
                     }
                 })
             } else {
-                ControlFlow::CONTINUE
+                ct.super_visit_with(self)
             }
         }
     }
@@ -861,6 +863,22 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
         .is_break()
 }
 
+pub fn contains_illegal_impl_trait_in_trait<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: ty::Binder<'tcx, Ty<'tcx>>,
+) -> bool {
+    // FIXME(RPITIT): Perhaps we should use a visitor here?
+    ty.skip_binder().walk().any(|arg| {
+        if let ty::GenericArgKind::Type(ty) = arg.unpack()
+            && let ty::Projection(proj) = ty.kind()
+        {
+            tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
+        } else {
+            false
+        }
+    })
+}
+
 pub fn provide(providers: &mut ty::query::Providers) {
     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 }
diff --git a/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs
index 9227bbf01..4a4f34b76 100644
--- a/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs
+++ b/compiler/rustc_trait_selection/src/traits/on_unimplemented.rs
@@ -8,6 +8,10 @@ use rustc_parse_format::{ParseMode, Parser, Piece, Position};
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
 
+use crate::errors::{
+    EmptyOnClauseInOnUnimplemented, InvalidOnClauseInOnUnimplemented, NoValueInOnUnimplemented,
+};
+
 #[derive(Clone, Debug)]
 pub struct OnUnimplementedFormatString(Symbol);
 
@@ -18,7 +22,7 @@ pub struct OnUnimplementedDirective {
     pub message: Option<OnUnimplementedFormatString>,
     pub label: Option<OnUnimplementedFormatString>,
     pub note: Option<OnUnimplementedFormatString>,
-    pub enclosing_scope: Option<OnUnimplementedFormatString>,
+    pub parent_label: Option<OnUnimplementedFormatString>,
     pub append_const_msg: Option<Option<Symbol>>,
 }
 
@@ -27,7 +31,7 @@ pub struct OnUnimplementedNote {
     pub message: Option<String>,
     pub label: Option<String>,
     pub note: Option<String>,
-    pub enclosing_scope: Option<String>,
+    pub parent_label: Option<String>,
     /// Append a message for `~const Trait` errors. `None` means not requested and
     /// should fallback to a generic message, `Some(None)` suggests using the default
     /// appended message, `Some(Some(s))` suggests use the `s` message instead of the
@@ -35,21 +39,6 @@ pub struct OnUnimplementedNote {
     pub append_const_msg: Option<Option<Symbol>>,
 }
 
-fn parse_error(
-    tcx: TyCtxt<'_>,
-    span: Span,
-    message: &str,
-    label: &str,
-    note: Option<&str>,
-) -> ErrorGuaranteed {
-    let mut diag = struct_span_err!(tcx.sess, span, E0232, "{}", message);
-    diag.span_label(span, label);
-    if let Some(note) = note {
-        diag.note(note);
-    }
-    diag.emit()
-}
-
 impl<'tcx> OnUnimplementedDirective {
     fn parse(
         tcx: TyCtxt<'tcx>,
@@ -70,25 +59,9 @@ impl<'tcx> OnUnimplementedDirective {
         } else {
             let cond = item_iter
                 .next()
-                .ok_or_else(|| {
-                    parse_error(
-                        tcx,
-                        span,
-                        "empty `on`-clause in `#[rustc_on_unimplemented]`",
-                        "empty on-clause here",
-                        None,
-                    )
-                })?
+                .ok_or_else(|| tcx.sess.emit_err(EmptyOnClauseInOnUnimplemented { span }))?
                 .meta_item()
-                .ok_or_else(|| {
-                    parse_error(
-                        tcx,
-                        span,
-                        "invalid `on`-clause in `#[rustc_on_unimplemented]`",
-                        "invalid on-clause here",
-                        None,
-                    )
-                })?;
+                .ok_or_else(|| tcx.sess.emit_err(InvalidOnClauseInOnUnimplemented { span }))?;
             attr::eval_condition(cond, &tcx.sess.parse_sess, Some(tcx.features()), &mut |cfg| {
                 if let Some(value) = cfg.value && let Err(guar) = parse_value(value) {
                     errored = Some(guar);
@@ -101,7 +74,7 @@ impl<'tcx> OnUnimplementedDirective {
         let mut message = None;
         let mut label = None;
         let mut note = None;
-        let mut enclosing_scope = None;
+        let mut parent_label = None;
         let mut subcommands = vec![];
         let mut append_const_msg = None;
 
@@ -121,9 +94,9 @@ impl<'tcx> OnUnimplementedDirective {
                     note = parse_value(note_)?;
                     continue;
                 }
-            } else if item.has_name(sym::enclosing_scope) && enclosing_scope.is_none() {
-                if let Some(enclosing_scope_) = item.value_str() {
-                    enclosing_scope = parse_value(enclosing_scope_)?;
+            } else if item.has_name(sym::parent_label) && parent_label.is_none() {
+                if let Some(parent_label_) = item.value_str() {
+                    parent_label = parse_value(parent_label_)?;
                     continue;
                 }
             } else if item.has_name(sym::on)
@@ -150,13 +123,7 @@ impl<'tcx> OnUnimplementedDirective {
             }
 
             // nothing found
-            parse_error(
-                tcx,
-                item.span(),
-                "this attribute must have a valid value",
-                "expected value here",
-                Some(r#"eg `#[rustc_on_unimplemented(message="foo")]`"#),
-            );
+            tcx.sess.emit_err(NoValueInOnUnimplemented { span: item.span() });
         }
 
         if let Some(reported) = errored {
@@ -168,7 +135,7 @@ impl<'tcx> OnUnimplementedDirective {
                 message,
                 label,
                 note,
-                enclosing_scope,
+                parent_label,
                 append_const_msg,
             })
         }
@@ -193,7 +160,7 @@ impl<'tcx> OnUnimplementedDirective {
                     attr.span,
                 )?),
                 note: None,
-                enclosing_scope: None,
+                parent_label: None,
                 append_const_msg: None,
             }))
         } else {
@@ -214,7 +181,7 @@ impl<'tcx> OnUnimplementedDirective {
         let mut message = None;
         let mut label = None;
         let mut note = None;
-        let mut enclosing_scope = None;
+        let mut parent_label = None;
         let mut append_const_msg = None;
         info!("evaluate({:?}, trait_ref={:?}, options={:?})", self, trait_ref, options);
 
@@ -250,8 +217,8 @@ impl<'tcx> OnUnimplementedDirective {
                 note = Some(note_.clone());
             }
 
-            if let Some(ref enclosing_scope_) = command.enclosing_scope {
-                enclosing_scope = Some(enclosing_scope_.clone());
+            if let Some(ref parent_label_) = command.parent_label {
+                parent_label = Some(parent_label_.clone());
             }
 
             append_const_msg = command.append_const_msg;
@@ -261,7 +228,7 @@ impl<'tcx> OnUnimplementedDirective {
             label: label.map(|l| l.format(tcx, trait_ref, &options_map)),
             message: message.map(|m| m.format(tcx, trait_ref, &options_map)),
             note: note.map(|n| n.format(tcx, trait_ref, &options_map)),
-            enclosing_scope: enclosing_scope.map(|e_s| e_s.format(tcx, trait_ref, &options_map)),
+            parent_label: parent_label.map(|e_s| e_s.format(tcx, trait_ref, &options_map)),
             append_const_msg,
         }
     }
diff --git a/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
new file mode 100644
index 000000000..108dae092
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
@@ -0,0 +1,115 @@
+use crate::infer::InferCtxt;
+use crate::traits::query::type_op::{self, TypeOp, TypeOpOutput};
+use crate::traits::query::NoSolution;
+use crate::traits::{ObligationCause, TraitEngine, TraitEngineExt};
+use rustc_data_structures::fx::FxHashSet;
+use rustc_hir as hir;
+use rustc_hir::HirId;
+use rustc_middle::ty::{self, ParamEnv, Ty};
+
+pub use rustc_middle::traits::query::OutlivesBound;
+
+type Bounds<'a, 'tcx: 'a> = impl Iterator<Item = OutlivesBound<'tcx>> + 'a;
+pub trait InferCtxtExt<'a, 'tcx> {
+    fn implied_outlives_bounds(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        body_id: hir::HirId,
+        ty: Ty<'tcx>,
+    ) -> Vec<OutlivesBound<'tcx>>;
+
+    fn implied_bounds_tys(
+        &'a self,
+        param_env: ty::ParamEnv<'tcx>,
+        body_id: hir::HirId,
+        tys: FxHashSet<Ty<'tcx>>,
+    ) -> Bounds<'a, 'tcx>;
+}
+
+impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
+    /// Implied bounds are region relationships that we deduce
+    /// automatically. The idea is that (e.g.) a caller must check that a
+    /// function's argument types are well-formed immediately before
+    /// calling that fn, and hence the *callee* can assume that its
+    /// argument types are well-formed. This may imply certain relationships
+    /// between generic parameters. For example:
+    /// ```
+    /// fn foo<'a,T>(x: &'a T) {}
+    /// ```
+    /// can only be called with a `'a` and `T` such that `&'a T` is WF.
+    /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
+    ///
+    /// # Parameters
+    ///
+    /// - `param_env`, the where-clauses in scope
+    /// - `body_id`, the body-id to use when normalizing assoc types.
+    ///   Note that this may cause outlives obligations to be injected
+    ///   into the inference context with this body-id.
+    /// - `ty`, the type that we are supposed to assume is WF.
+    #[instrument(level = "debug", skip(self, param_env, body_id), ret)]
+    fn implied_outlives_bounds(
+        &self,
+        param_env: ty::ParamEnv<'tcx>,
+        body_id: hir::HirId,
+        ty: Ty<'tcx>,
+    ) -> Vec<OutlivesBound<'tcx>> {
+        let span = self.tcx.hir().span(body_id);
+        let result = param_env
+            .and(type_op::implied_outlives_bounds::ImpliedOutlivesBounds { ty })
+            .fully_perform(self);
+        let result = match result {
+            Ok(r) => r,
+            Err(NoSolution) => {
+                self.tcx.sess.delay_span_bug(
+                    span,
+                    "implied_outlives_bounds failed to solve all obligations",
+                );
+                return vec![];
+            }
+        };
+
+        let TypeOpOutput { output, constraints, .. } = result;
+
+        if let Some(constraints) = constraints {
+            debug!(?constraints);
+            // Instantiation may have produced new inference variables and constraints on those
+            // variables. Process these constraints.
+            let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(self.tcx);
+            let cause = ObligationCause::misc(span, body_id);
+            for &constraint in &constraints.outlives {
+                let obligation = self.query_outlives_constraint_to_obligation(
+                    constraint,
+                    cause.clone(),
+                    param_env,
+                );
+                fulfill_cx.register_predicate_obligation(self, obligation);
+            }
+            if !constraints.member_constraints.is_empty() {
+                span_bug!(span, "{:#?}", constraints.member_constraints);
+            }
+            let errors = fulfill_cx.select_all_or_error(self);
+            if !errors.is_empty() {
+                self.tcx.sess.delay_span_bug(
+                    span,
+                    "implied_outlives_bounds failed to solve obligations from instantiation",
+                );
+            }
+        };
+
+        output
+    }
+
+    fn implied_bounds_tys(
+        &'a self,
+        param_env: ParamEnv<'tcx>,
+        body_id: HirId,
+        tys: FxHashSet<Ty<'tcx>>,
+    ) -> Bounds<'a, 'tcx> {
+        tys.into_iter()
+            .map(move |ty| {
+                let ty = self.resolve_vars_if_possible(ty);
+                self.implied_outlives_bounds(param_env, body_id, ty)
+            })
+            .flatten()
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index c4e80e1ba..e4284b9d3 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -18,7 +18,7 @@ use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
 
 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
-use crate::traits::error_reporting::InferCtxtExt as _;
+use crate::traits::error_reporting::TypeErrCtxtExt as _;
 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use crate::traits::select::ProjectionMatchesProjection;
 use rustc_data_structures::sso::SsoHashSet;
@@ -30,8 +30,8 @@ use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
-use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::visit::{MaxUniverse, TypeVisitable};
+use rustc_middle::ty::DefIdTree;
 use rustc_middle::ty::{self, Term, ToPredicate, Ty, TyCtxt};
 use rustc_span::symbol::sym;
 
@@ -62,7 +62,8 @@ enum ProjectionCandidate<'tcx> {
     /// From a where-clause in the env or object type
     ParamEnv(ty::PolyProjectionPredicate<'tcx>),
 
-    /// From the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
+    /// From the definition of `Trait` when you have something like
+    /// `<<A as Trait>::B as Trait2>::C`.
     TraitDef(ty::PolyProjectionPredicate<'tcx>),
 
     /// Bounds specified on an object type
@@ -70,6 +71,16 @@ enum ProjectionCandidate<'tcx> {
 
     /// From an "impl" (or a "pseudo-impl" returned by select)
     Select(Selection<'tcx>),
+
+    ImplTraitInTrait(ImplTraitInTraitCandidate<'tcx>),
+}
+
+#[derive(PartialEq, Eq, Debug)]
+enum ImplTraitInTraitCandidate<'tcx> {
+    // The `impl Trait` from a trait function's default body
+    Trait,
+    // A concrete type provided from a trait's `impl Trait` from an impl
+    Impl(ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>),
 }
 
 enum ProjectionCandidateSet<'tcx> {
@@ -231,7 +242,7 @@ pub(super) fn poly_project_and_unify_type<'cx, 'tcx>(
 /// If successful, this may result in additional obligations.
 ///
 /// See [poly_project_and_unify_type] for an explanation of the return value.
-#[tracing::instrument(level = "debug", skip(selcx))]
+#[instrument(level = "debug", skip(selcx))]
 fn project_and_unify_type<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionObligation<'tcx>,
@@ -253,7 +264,7 @@ fn project_and_unify_type<'cx, 'tcx>(
     };
     debug!(?normalized, ?obligations, "project_and_unify_type result");
     let actual = obligation.predicate.term;
-    // For an example where this is neccessary see src/test/ui/impl-trait/nested-return-type2.rs
+    // For an example where this is necessary see src/test/ui/impl-trait/nested-return-type2.rs
     // This allows users to omit re-mentioning all bounds on an associated type and just use an
     // `impl Trait` for the assoc type to add more bounds.
     let InferOk { value: actual, obligations: new } =
@@ -511,7 +522,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                                 self.param_env,
                                 ty,
                             );
-                            self.selcx.infcx().report_overflow_error(&obligation, true);
+                            self.selcx.infcx().err_ctxt().report_overflow_error(&obligation, true);
                         }
 
                         let substs = substs.fold_with(self);
@@ -552,7 +563,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                     )
                     .ok()
                     .flatten()
-                    .unwrap_or_else(|| ty::Term::Ty(ty.super_fold_with(self)))
+                    .unwrap_or_else(|| ty.super_fold_with(self).into())
                 };
                 debug!(
                     ?self.depth,
@@ -620,19 +631,33 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     fn fold_const(&mut self, constant: ty::Const<'tcx>) -> ty::Const<'tcx> {
-        if self.selcx.tcx().lazy_normalization() || !self.eager_inference_replacement {
+        let tcx = self.selcx.tcx();
+        if tcx.lazy_normalization() {
             constant
         } else {
             let constant = constant.super_fold_with(self);
-            debug!(?constant);
-            debug!("self.param_env: {:?}", self.param_env);
-            constant.eval(self.selcx.tcx(), self.param_env)
+            debug!(?constant, ?self.param_env);
+            with_replaced_escaping_bound_vars(
+                self.selcx.infcx(),
+                &mut self.universes,
+                constant,
+                |constant| constant.eval(tcx, self.param_env),
+            )
+        }
+    }
+
+    #[inline]
+    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        if p.allow_normalization() && needs_normalization(&p, self.param_env.reveal()) {
+            p.super_fold_with(self)
+        } else {
+            p
         }
     }
 }
 
 pub struct BoundVarReplacer<'me, 'tcx> {
-    infcx: &'me InferCtxt<'me, 'tcx>,
+    infcx: &'me InferCtxt<'tcx>,
     // These three maps track the bound variable that were replaced by placeholders. It might be
     // nice to remove these since we already have the `kind` in the placeholder; we really just need
     // the `var` (but we *could* bring that into scope if we were to track them as we pass them).
@@ -647,11 +672,46 @@ pub struct BoundVarReplacer<'me, 'tcx> {
     universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
 }
 
+/// Executes `f` on `value` after replacing all escaping bound variables with placeholders
+/// and then replaces these placeholders with the original bound variables in the result.
+///
+/// In most places, bound variables should be replaced right when entering a binder, making
+/// this function unnecessary. However, normalization currently does not do that, so we have
+/// to do this lazily.
+///
+/// You should not add any additional uses of this function, at least not without first
+/// discussing it with t-types.
+///
+/// FIXME(@lcnr): We may even consider experimenting with eagerly replacing bound vars during
+/// normalization as well, at which point this function will be unnecessary and can be removed.
+pub fn with_replaced_escaping_bound_vars<'a, 'tcx, T: TypeFoldable<'tcx>, R: TypeFoldable<'tcx>>(
+    infcx: &'a InferCtxt<'tcx>,
+    universe_indices: &'a mut Vec<Option<ty::UniverseIndex>>,
+    value: T,
+    f: impl FnOnce(T) -> R,
+) -> R {
+    if value.has_escaping_bound_vars() {
+        let (value, mapped_regions, mapped_types, mapped_consts) =
+            BoundVarReplacer::replace_bound_vars(infcx, universe_indices, value);
+        let result = f(value);
+        PlaceholderReplacer::replace_placeholders(
+            infcx,
+            mapped_regions,
+            mapped_types,
+            mapped_consts,
+            universe_indices,
+            result,
+        )
+    } else {
+        f(value)
+    }
+}
+
 impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
     /// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
     /// use a binding level above `universe_indices.len()`, we fail.
     pub fn replace_bound_vars<T: TypeFoldable<'tcx>>(
-        infcx: &'me InferCtxt<'me, 'tcx>,
+        infcx: &'me InferCtxt<'tcx>,
         universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
         value: T,
     ) -> (
@@ -771,7 +831,7 @@ impl<'tcx> TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
 
 // The inverse of `BoundVarReplacer`: replaces placeholders with the bound vars from which they came.
 pub struct PlaceholderReplacer<'me, 'tcx> {
-    infcx: &'me InferCtxt<'me, 'tcx>,
+    infcx: &'me InferCtxt<'tcx>,
     mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
     mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
     mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
@@ -781,7 +841,7 @@ pub struct PlaceholderReplacer<'me, 'tcx> {
 
 impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
     pub fn replace_placeholders<T: TypeFoldable<'tcx>>(
-        infcx: &'me InferCtxt<'me, 'tcx>,
+        infcx: &'me InferCtxt<'tcx>,
         mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
         mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
         mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
@@ -1182,7 +1242,7 @@ impl<'tcx> Progress<'tcx> {
 ///
 /// IMPORTANT:
 /// - `obligation` must be fully normalized
-#[tracing::instrument(level = "info", skip(selcx))]
+#[instrument(level = "info", skip(selcx))]
 fn project<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
@@ -1201,6 +1261,8 @@ fn project<'cx, 'tcx>(
 
     let mut candidates = ProjectionCandidateSet::None;
 
+    assemble_candidate_for_impl_trait_in_trait(selcx, obligation, &mut candidates);
+
     // Make sure that the following procedures are kept in order. ParamEnv
     // needs to be first because it has highest priority, and Select checks
     // the return value of push_candidate which assumes it's ran at last.
@@ -1239,6 +1301,63 @@ fn project<'cx, 'tcx>(
     }
 }
 
+/// If the predicate's item is an `ImplTraitPlaceholder`, we do a select on the
+/// corresponding trait ref. If this yields an `impl`, then we're able to project
+/// to a concrete type, since we have an `impl`'s method  to provide the RPITIT.
+fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
+    selcx: &mut SelectionContext<'cx, 'tcx>,
+    obligation: &ProjectionTyObligation<'tcx>,
+    candidate_set: &mut ProjectionCandidateSet<'tcx>,
+) {
+    let tcx = selcx.tcx();
+    if tcx.def_kind(obligation.predicate.item_def_id) == DefKind::ImplTraitPlaceholder {
+        let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.item_def_id);
+        // If we are trying to project an RPITIT with trait's default `Self` parameter,
+        // then we must be within a default trait body.
+        if obligation.predicate.self_ty()
+            == ty::InternalSubsts::identity_for_item(tcx, obligation.predicate.item_def_id)
+                .type_at(0)
+            && tcx.associated_item(trait_fn_def_id).defaultness(tcx).has_value()
+        {
+            candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(
+                ImplTraitInTraitCandidate::Trait,
+            ));
+            return;
+        }
+
+        let trait_def_id = tcx.parent(trait_fn_def_id);
+        let trait_substs =
+            obligation.predicate.substs.truncate_to(tcx, tcx.generics_of(trait_def_id));
+        // FIXME(named-returns): Binders
+        let trait_predicate =
+            ty::Binder::dummy(ty::TraitRef { def_id: trait_def_id, substs: trait_substs })
+                .to_poly_trait_predicate();
+
+        let _ =
+            selcx.infcx().commit_if_ok(|_| match selcx.select(&obligation.with(trait_predicate)) {
+                Ok(Some(super::ImplSource::UserDefined(data))) => {
+                    candidate_set.push_candidate(ProjectionCandidate::ImplTraitInTrait(
+                        ImplTraitInTraitCandidate::Impl(data),
+                    ));
+                    Ok(())
+                }
+                Ok(None) => {
+                    candidate_set.mark_ambiguous();
+                    return Err(());
+                }
+                Ok(Some(_)) => {
+                    // Don't know enough about the impl to provide a useful signature
+                    return Err(());
+                }
+                Err(e) => {
+                    debug!(error = ?e, "selection error");
+                    candidate_set.mark_error(e);
+                    return Err(());
+                }
+            });
+    }
+}
+
 /// The first thing we have to do is scan through the parameter
 /// environment to see whether there are any projection predicates
 /// there that can answer this question.
@@ -1257,7 +1376,7 @@ fn assemble_candidates_from_param_env<'cx, 'tcx>(
     );
 }
 
-/// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
+/// In the case of a nested projection like `<<A as Foo>::FooT as Bar>::BarT`, we may find
 /// that the definition of `Foo` has some clues:
 ///
 /// ```ignore (illustrative)
@@ -1344,7 +1463,7 @@ fn assemble_candidates_from_object_ty<'cx, 'tcx>(
     );
 }
 
-#[tracing::instrument(
+#[instrument(
     level = "debug",
     skip(selcx, candidate_set, ctor, env_predicates, potentially_unnormalized_candidates)
 )]
@@ -1378,7 +1497,7 @@ fn assemble_candidates_from_predicates<'cx, 'tcx>(
                     candidate_set.push_candidate(ctor(data));
 
                     if potentially_unnormalized_candidates
-                        && !obligation.predicate.has_infer_types_or_consts()
+                        && !obligation.predicate.has_non_region_infer()
                     {
                         // HACK: Pick the first trait def candidate for a fully
                         // inferred predicate. This is to allow duplicates that
@@ -1395,12 +1514,17 @@ fn assemble_candidates_from_predicates<'cx, 'tcx>(
     }
 }
 
-#[tracing::instrument(level = "debug", skip(selcx, obligation, candidate_set))]
+#[instrument(level = "debug", skip(selcx, obligation, candidate_set))]
 fn assemble_candidates_from_impls<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
     candidate_set: &mut ProjectionCandidateSet<'tcx>,
 ) {
+    // Can't assemble candidate from impl for RPITIT
+    if selcx.tcx().def_kind(obligation.predicate.item_def_id) == DefKind::ImplTraitPlaceholder {
+        return;
+    }
+
     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
     // start out by selecting the predicate `T as TraitRef<...>`:
     let poly_trait_ref = ty::Binder::dummy(obligation.predicate.trait_ref(selcx.tcx()));
@@ -1676,6 +1800,18 @@ fn confirm_candidate<'cx, 'tcx>(
         ProjectionCandidate::Select(impl_source) => {
             confirm_select_candidate(selcx, obligation, impl_source)
         }
+        ProjectionCandidate::ImplTraitInTrait(ImplTraitInTraitCandidate::Impl(data)) => {
+            confirm_impl_trait_in_trait_candidate(selcx, obligation, data)
+        }
+        // If we're projecting an RPITIT for a default trait body, that's just
+        // the same def-id, but as an opaque type (with regular RPIT semantics).
+        ProjectionCandidate::ImplTraitInTrait(ImplTraitInTraitCandidate::Trait) => Progress {
+            term: selcx
+                .tcx()
+                .mk_opaque(obligation.predicate.item_def_id, obligation.predicate.substs)
+                .into(),
+            obligations: vec![],
+        },
     };
 
     // When checking for cycle during evaluation, we compare predicates with
@@ -2021,15 +2157,15 @@ fn confirm_impl_candidate<'cx, 'tcx>(
         let identity_substs =
             crate::traits::InternalSubsts::identity_for_item(tcx, assoc_ty.item.def_id);
         let did = ty::WithOptConstParam::unknown(assoc_ty.item.def_id);
-        let kind = ty::ConstKind::Unevaluated(ty::Unevaluated::new(did, identity_substs));
+        let kind = ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
         ty.map_bound(|ty| tcx.mk_const(ty::ConstS { ty, kind }).into())
     } else {
         ty.map_bound(|ty| ty.into())
     };
-    if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
+    if !check_substs_compatible(tcx, &assoc_ty.item, substs) {
         let err = tcx.ty_error_with_message(
             obligation.cause.span,
-            "impl item and trait item have different parameter counts",
+            "impl item and trait item have different parameters",
         );
         Progress { term: err.into(), obligations: nested }
     } else {
@@ -2038,10 +2174,125 @@ fn confirm_impl_candidate<'cx, 'tcx>(
     }
 }
 
+// Verify that the trait item and its implementation have compatible substs lists
+fn check_substs_compatible<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    assoc_ty: &ty::AssocItem,
+    substs: ty::SubstsRef<'tcx>,
+) -> bool {
+    fn check_substs_compatible_inner<'tcx>(
+        tcx: TyCtxt<'tcx>,
+        generics: &'tcx ty::Generics,
+        args: &'tcx [ty::GenericArg<'tcx>],
+    ) -> bool {
+        if generics.count() != args.len() {
+            return false;
+        }
+
+        let (parent_args, own_args) = args.split_at(generics.parent_count);
+
+        if let Some(parent) = generics.parent
+            && let parent_generics = tcx.generics_of(parent)
+            && !check_substs_compatible_inner(tcx, parent_generics, parent_args) {
+            return false;
+        }
+
+        for (param, arg) in std::iter::zip(&generics.params, own_args) {
+            match (&param.kind, arg.unpack()) {
+                (ty::GenericParamDefKind::Type { .. }, ty::GenericArgKind::Type(_))
+                | (ty::GenericParamDefKind::Lifetime, ty::GenericArgKind::Lifetime(_))
+                | (ty::GenericParamDefKind::Const { .. }, ty::GenericArgKind::Const(_)) => {}
+                _ => return false,
+            }
+        }
+
+        true
+    }
+
+    check_substs_compatible_inner(tcx, tcx.generics_of(assoc_ty.def_id), substs.as_slice())
+}
+
+fn confirm_impl_trait_in_trait_candidate<'tcx>(
+    selcx: &mut SelectionContext<'_, 'tcx>,
+    obligation: &ProjectionTyObligation<'tcx>,
+    data: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
+) -> Progress<'tcx> {
+    let tcx = selcx.tcx();
+    let mut obligations = data.nested;
+
+    let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.item_def_id);
+    let Ok(leaf_def) = assoc_def(selcx, data.impl_def_id, trait_fn_def_id) else {
+        return Progress { term: tcx.ty_error().into(), obligations };
+    };
+    if !leaf_def.item.defaultness(tcx).has_value() {
+        return Progress { term: tcx.ty_error().into(), obligations };
+    }
+
+    // Use the default `impl Trait` for the trait, e.g., for a default trait body
+    if leaf_def.item.container == ty::AssocItemContainer::TraitContainer {
+        return Progress {
+            term: tcx
+                .mk_opaque(obligation.predicate.item_def_id, obligation.predicate.substs)
+                .into(),
+            obligations,
+        };
+    }
+
+    let impl_fn_def_id = leaf_def.item.def_id;
+    // Rebase from {trait}::{fn}::{opaque} to {impl}::{fn}::{opaque},
+    // since `data.substs` are the impl substs.
+    let impl_fn_substs =
+        obligation.predicate.substs.rebase_onto(tcx, tcx.parent(trait_fn_def_id), data.substs);
+
+    let cause = ObligationCause::new(
+        obligation.cause.span,
+        obligation.cause.body_id,
+        super::ItemObligation(impl_fn_def_id),
+    );
+    let predicates = normalize_with_depth_to(
+        selcx,
+        obligation.param_env,
+        cause.clone(),
+        obligation.recursion_depth + 1,
+        tcx.predicates_of(impl_fn_def_id).instantiate(tcx, impl_fn_substs),
+        &mut obligations,
+    );
+    obligations.extend(std::iter::zip(predicates.predicates, predicates.spans).map(
+        |(pred, span)| {
+            Obligation::with_depth(
+                ObligationCause::new(
+                    obligation.cause.span,
+                    obligation.cause.body_id,
+                    if span.is_dummy() {
+                        super::ItemObligation(impl_fn_def_id)
+                    } else {
+                        super::BindingObligation(impl_fn_def_id, span)
+                    },
+                ),
+                obligation.recursion_depth + 1,
+                obligation.param_env,
+                pred,
+            )
+        },
+    ));
+
+    let ty = super::normalize_to(
+        selcx,
+        obligation.param_env,
+        cause.clone(),
+        tcx.bound_trait_impl_trait_tys(impl_fn_def_id)
+            .map_bound(|tys| {
+                tys.map_or_else(|_| tcx.ty_error(), |tys| tys[&obligation.predicate.item_def_id])
+            })
+            .subst(tcx, impl_fn_substs),
+        &mut obligations,
+    );
+
+    Progress { term: ty.into(), obligations }
+}
+
 // Get obligations corresponding to the predicates from the where-clause of the
 // associated type itself.
-// Note: `feature(generic_associated_types)` is required to write such
-// predicates, even for non-generic associated types.
 fn assoc_ty_own_obligations<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs b/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
index 32669e23d..c84f128dd 100644
--- a/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
@@ -31,7 +31,7 @@ pub trait InferCtxtExt<'tcx> {
     ) -> EvaluationResult;
 }
 
-impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
+impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
     /// Evaluates whether the predicate can be satisfied (by any means)
     /// in the given `ParamEnv`.
     fn predicate_may_hold(&self, obligation: &PredicateObligation<'tcx>) -> bool {
diff --git a/compiler/rustc_trait_selection/src/traits/query/normalize.rs b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
index 449d7a7b4..58e4597b7 100644
--- a/compiler/rustc_trait_selection/src/traits/query/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
@@ -5,17 +5,16 @@
 use crate::infer::at::At;
 use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::{InferCtxt, InferOk};
-use crate::traits::error_reporting::InferCtxtExt;
-use crate::traits::project::needs_normalization;
+use crate::traits::error_reporting::TypeErrCtxtExt;
+use crate::traits::project::{needs_normalization, BoundVarReplacer, PlaceholderReplacer};
 use crate::traits::{Obligation, ObligationCause, PredicateObligation, Reveal};
 use rustc_data_structures::sso::SsoHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::traits::Normalized;
-use rustc_middle::mir;
 use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable};
-use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitable};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
+use rustc_span::DUMMY_SP;
 
 use std::ops::ControlFlow;
 
@@ -48,10 +47,11 @@ impl<'cx, 'tcx> AtExt<'tcx> for At<'cx, 'tcx> {
         T: TypeFoldable<'tcx>,
     {
         debug!(
-            "normalize::<{}>(value={:?}, param_env={:?})",
+            "normalize::<{}>(value={:?}, param_env={:?}, cause={:?})",
             std::any::type_name::<T>(),
             value,
             self.param_env,
+            self.cause,
         );
         if !needs_normalization(&value, self.param_env.reveal()) {
             return Ok(Normalized { value, obligations: vec![] });
@@ -154,7 +154,7 @@ impl<'tcx> TypeVisitor<'tcx> for MaxEscapingBoundVarVisitor {
 }
 
 struct QueryNormalizer<'cx, 'tcx> {
-    infcx: &'cx InferCtxt<'cx, 'tcx>,
+    infcx: &'cx InferCtxt<'tcx>,
     cause: &'cx ObligationCause<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     obligations: Vec<PredicateObligation<'tcx>>,
@@ -213,7 +213,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                                 self.param_env,
                                 ty,
                             );
-                            self.infcx.report_overflow_error(&obligation, true);
+                            self.infcx.err_ctxt().report_overflow_error(&obligation, true);
                         }
 
                         let generic_ty = self.tcx().bound_type_of(def_id);
@@ -254,7 +254,15 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 let result = tcx.normalize_projection_ty(c_data)?;
                 // We don't expect ambiguity.
                 if result.is_ambiguous() {
-                    bug!("unexpected ambiguity: {:?} {:?}", c_data, result);
+                    // Rustdoc normalizes possibly not well-formed types, so only
+                    // treat this as a bug if we're not in rustdoc.
+                    if !tcx.sess.opts.actually_rustdoc {
+                        tcx.sess.delay_span_bug(
+                            DUMMY_SP,
+                            format!("unexpected ambiguity: {:?} {:?}", c_data, result),
+                        );
+                    }
+                    return Err(NoSolution);
                 }
                 let InferOk { value: result, obligations } =
                     self.infcx.instantiate_query_response_and_region_obligations(
@@ -266,7 +274,15 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 debug!("QueryNormalizer: result = {:#?}", result);
                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
                 self.obligations.extend(obligations);
-                Ok(result.normalized_ty)
+
+                let res = result.normalized_ty;
+                // `tcx.normalize_projection_ty` may normalize to a type that still has
+                // unevaluated consts, so keep normalizing here if that's the case.
+                if res != ty && res.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
+                    Ok(res.try_super_fold_with(self)?)
+                } else {
+                    Ok(res)
+                }
             }
 
             ty::Projection(data) => {
@@ -275,11 +291,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 let tcx = self.infcx.tcx;
                 let infcx = self.infcx;
                 let (data, mapped_regions, mapped_types, mapped_consts) =
-                    crate::traits::project::BoundVarReplacer::replace_bound_vars(
-                        infcx,
-                        &mut self.universes,
-                        data,
-                    );
+                    BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
                 let data = data.try_fold_with(self)?;
 
                 let mut orig_values = OriginalQueryValues::default();
@@ -293,7 +305,15 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 let result = tcx.normalize_projection_ty(c_data)?;
                 // We don't expect ambiguity.
                 if result.is_ambiguous() {
-                    bug!("unexpected ambiguity: {:?} {:?}", c_data, result);
+                    // Rustdoc normalizes possibly not well-formed types, so only
+                    // treat this as a bug if we're not in rustdoc.
+                    if !tcx.sess.opts.actually_rustdoc {
+                        tcx.sess.delay_span_bug(
+                            DUMMY_SP,
+                            format!("unexpected ambiguity: {:?} {:?}", c_data, result),
+                        );
+                    }
+                    return Err(NoSolution);
                 }
                 let InferOk { value: result, obligations } =
                     self.infcx.instantiate_query_response_and_region_obligations(
@@ -305,18 +325,26 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 debug!("QueryNormalizer: result = {:#?}", result);
                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
                 self.obligations.extend(obligations);
-                Ok(crate::traits::project::PlaceholderReplacer::replace_placeholders(
+                let res = PlaceholderReplacer::replace_placeholders(
                     infcx,
                     mapped_regions,
                     mapped_types,
                     mapped_consts,
                     &self.universes,
                     result.normalized_ty,
-                ))
+                );
+                // `tcx.normalize_projection_ty` may normalize to a type that still has
+                // unevaluated consts, so keep normalizing here if that's the case.
+                if res != ty && res.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
+                    Ok(res.try_super_fold_with(self)?)
+                } else {
+                    Ok(res)
+                }
             }
 
             _ => ty.try_super_fold_with(self),
         })()?;
+
         self.cache.insert(ty, res);
         Ok(res)
     }
@@ -326,29 +354,24 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
         constant: ty::Const<'tcx>,
     ) -> Result<ty::Const<'tcx>, Self::Error> {
         let constant = constant.try_super_fold_with(self)?;
-        Ok(constant.eval(self.infcx.tcx, self.param_env))
+        debug!(?constant, ?self.param_env);
+        Ok(crate::traits::project::with_replaced_escaping_bound_vars(
+            self.infcx,
+            &mut self.universes,
+            constant,
+            |constant| constant.eval(self.infcx.tcx, self.param_env),
+        ))
     }
 
-    fn try_fold_mir_const(
+    #[inline]
+    fn try_fold_predicate(
         &mut self,
-        constant: mir::ConstantKind<'tcx>,
-    ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
-        Ok(match constant {
-            mir::ConstantKind::Ty(c) => {
-                let const_folded = c.try_super_fold_with(self)?;
-                match const_folded.kind() {
-                    ty::ConstKind::Value(valtree) => {
-                        let tcx = self.infcx.tcx;
-                        let ty = const_folded.ty();
-                        let const_val = tcx.valtree_to_const_val((ty, valtree));
-                        debug!(?ty, ?valtree, ?const_val);
-
-                        mir::ConstantKind::Val(const_val, ty)
-                    }
-                    _ => mir::ConstantKind::Ty(const_folded),
-                }
-            }
-            mir::ConstantKind::Val(_, _) => constant.try_super_fold_with(self)?,
-        })
+        p: ty::Predicate<'tcx>,
+    ) -> Result<ty::Predicate<'tcx>, Self::Error> {
+        if p.allow_normalization() && needs_normalization(&p, self.param_env.reveal()) {
+            p.try_super_fold_with(self)
+        } else {
+            Ok(p)
+        }
     }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
index c99564936..6bf3ed0d0 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
@@ -1,11 +1,9 @@
 use crate::infer::canonical::query_response;
 use crate::infer::{InferCtxt, InferOk};
-use crate::traits::engine::TraitEngineExt as _;
+use crate::traits;
 use crate::traits::query::type_op::TypeOpOutput;
 use crate::traits::query::Fallible;
-use crate::traits::TraitEngine;
 use rustc_infer::infer::region_constraints::RegionConstraintData;
-use rustc_infer::traits::TraitEngineExt as _;
 use rustc_span::source_map::DUMMY_SP;
 
 use std::fmt;
@@ -18,16 +16,16 @@ pub struct CustomTypeOp<F, G> {
 impl<F, G> CustomTypeOp<F, G> {
     pub fn new<'tcx, R>(closure: F, description: G) -> Self
     where
-        F: FnOnce(&InferCtxt<'_, 'tcx>) -> Fallible<InferOk<'tcx, R>>,
+        F: FnOnce(&InferCtxt<'tcx>) -> Fallible<InferOk<'tcx, R>>,
         G: Fn() -> String,
     {
         CustomTypeOp { closure, description }
     }
 }
 
-impl<'tcx, F, R, G> super::TypeOp<'tcx> for CustomTypeOp<F, G>
+impl<'tcx, F, R: fmt::Debug, G> super::TypeOp<'tcx> for CustomTypeOp<F, G>
 where
-    F: for<'a, 'cx> FnOnce(&'a InferCtxt<'cx, 'tcx>) -> Fallible<InferOk<'tcx, R>>,
+    F: for<'a, 'cx> FnOnce(&'a InferCtxt<'tcx>) -> Fallible<InferOk<'tcx, R>>,
     G: Fn() -> String,
 {
     type Output = R;
@@ -38,7 +36,7 @@ where
     /// Processes the operation and all resulting obligations,
     /// returning the final result along with any region constraints
     /// (they will be given over to the NLL region solver).
-    fn fully_perform(self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
+    fn fully_perform(self, infcx: &InferCtxt<'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
         if cfg!(debug_assertions) {
             info!("fully_perform({:?})", self);
         }
@@ -59,11 +57,9 @@ where
 /// Executes `op` and then scrapes out all the "old style" region
 /// constraints that result, creating query-region-constraints.
 pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
-    infcx: &InferCtxt<'_, 'tcx>,
+    infcx: &InferCtxt<'tcx>,
     op: impl FnOnce() -> Fallible<InferOk<'tcx, R>>,
 ) -> Fallible<(TypeOpOutput<'tcx, Op>, RegionConstraintData<'tcx>)> {
-    let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
-
     // During NLL, we expect that nobody will register region
     // obligations **except** as part of a custom type op (and, at the
     // end of each custom type op, we scrape out the region
@@ -77,8 +73,7 @@ pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     );
 
     let InferOk { value, obligations } = infcx.commit_if_ok(|_| op())?;
-    fulfill_cx.register_predicate_obligations(infcx, obligations);
-    let errors = fulfill_cx.select_all_or_error(infcx);
+    let errors = traits::fully_solve_obligations(infcx, obligations);
     if !errors.is_empty() {
         infcx.tcx.sess.diagnostic().delay_span_bug(
             DUMMY_SP,
@@ -94,8 +89,8 @@ pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
         infcx.tcx,
         region_obligations
             .iter()
-            .map(|r_o| (r_o.sup_type, r_o.sub_region))
-            .map(|(ty, r)| (infcx.resolve_vars_if_possible(ty), r)),
+            .map(|r_o| (r_o.sup_type, r_o.sub_region, r_o.origin.to_constraint_category()))
+            .map(|(ty, r, cc)| (infcx.resolve_vars_if_possible(ty), r, cc)),
         &region_constraint_data,
     );
 
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs
index 578e1d00c..29ae8ae6b 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs
@@ -26,13 +26,13 @@ pub use rustc_middle::traits::query::type_op::*;
 /// extract out the resulting region constraints (or an error if it
 /// cannot be completed).
 pub trait TypeOp<'tcx>: Sized + fmt::Debug {
-    type Output;
+    type Output: fmt::Debug;
     type ErrorInfo;
 
     /// Processes the operation and all resulting obligations,
     /// returning the final result along with any region constraints
     /// (they will be given over to the NLL region solver).
-    fn fully_perform(self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>>;
+    fn fully_perform(self, infcx: &InferCtxt<'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>>;
 }
 
 /// The output from performing a type op
@@ -78,7 +78,7 @@ pub trait QueryTypeOp<'tcx>: fmt::Debug + Copy + TypeFoldable<'tcx> + 'tcx {
 
     fn fully_perform_into(
         query_key: ParamEnvAnd<'tcx, Self>,
-        infcx: &InferCtxt<'_, 'tcx>,
+        infcx: &InferCtxt<'tcx>,
         output_query_region_constraints: &mut QueryRegionConstraints<'tcx>,
     ) -> Fallible<(
         Self::QueryResponse,
@@ -120,7 +120,7 @@ where
     type Output = Q::QueryResponse;
     type ErrorInfo = Canonical<'tcx, ParamEnvAnd<'tcx, Q>>;
 
-    fn fully_perform(self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
+    fn fully_perform(self, infcx: &InferCtxt<'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
         let mut region_constraints = QueryRegionConstraints::default();
         let (output, error_info, mut obligations, _) =
             Q::fully_perform_into(self, infcx, &mut region_constraints)?;
diff --git a/compiler/rustc_trait_selection/src/traits/relationships.rs b/compiler/rustc_trait_selection/src/traits/relationships.rs
index 8148e2b78..8cf500a46 100644
--- a/compiler/rustc_trait_selection/src/traits/relationships.rs
+++ b/compiler/rustc_trait_selection/src/traits/relationships.rs
@@ -6,7 +6,7 @@ use rustc_middle::ty::{self, ToPredicate};
 
 pub(crate) fn update<'tcx, T>(
     engine: &mut T,
-    infcx: &InferCtxt<'_, 'tcx>,
+    infcx: &InferCtxt<'tcx>,
     obligation: &PredicateObligation<'tcx>,
 ) where
     T: TraitEngine<'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index a60ce0f34..4c5bc3339 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -6,9 +6,10 @@
 //!
 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
 use hir::LangItem;
+use rustc_errors::DelayDm;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
-use rustc_infer::traits::TraitEngine;
+use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
 use rustc_lint_defs::builtin::DEREF_INTO_DYN_SUPERTRAIT;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -28,7 +29,7 @@ use super::SelectionCandidate::{self, *};
 use super::{EvaluatedCandidate, SelectionCandidateSet, SelectionContext, TraitObligationStack};
 
 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
-    #[instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self), ret)]
     pub(super) fn candidate_from_obligation<'o>(
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
@@ -48,7 +49,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if let Some(c) =
             self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
         {
-            debug!(candidate = ?c, "CACHE HIT");
+            debug!("CACHE HIT");
             return c;
         }
 
@@ -61,7 +62,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let (candidate, dep_node) =
             self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
 
-        debug!(?candidate, "CACHE MISS");
+        debug!("CACHE MISS");
         self.insert_candidate_cache(
             stack.obligation.param_env,
             cache_fresh_trait_pred,
@@ -75,7 +76,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
-        if let Some(conflict) = self.is_knowable(stack) {
+        if let Err(conflict) = self.is_knowable(stack) {
             debug!("coherence stage: not knowable");
             if self.intercrate_ambiguity_causes.is_some() {
                 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
@@ -173,7 +174,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
 
-        let needs_infer = stack.obligation.predicate.has_infer_types_or_consts();
+        let needs_infer = stack.obligation.predicate.has_non_region_infer();
 
         // If there are STILL multiple candidates, we can further
         // reduce the list by dropping duplicates -- including
@@ -309,6 +310,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // User-defined transmutability impls are permitted.
                 self.assemble_candidates_from_impls(obligation, &mut candidates);
                 self.assemble_candidates_for_transmutability(obligation, &mut candidates);
+            } else if lang_items.tuple_trait() == Some(def_id) {
+                self.assemble_candidate_for_tuple(obligation, &mut candidates);
             } else {
                 if lang_items.clone_trait() == Some(def_id) {
                     // Same builtin conditions as `Copy`, i.e., every type which has builtin support
@@ -337,7 +340,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(candidates)
     }
 
-    #[tracing::instrument(level = "debug", skip(self, candidates))]
+    #[instrument(level = "debug", skip(self, candidates))]
     fn assemble_candidates_from_projected_tys(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -360,14 +363,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .infcx
             .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));
 
-        candidates.vec.extend(result.into_iter().map(ProjectionCandidate));
+        candidates
+            .vec
+            .extend(result.into_iter().map(|(idx, constness)| ProjectionCandidate(idx, constness)));
     }
 
     /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
     /// supplied to find out whether it is listed among them.
     ///
     /// Never affects the inference environment.
-    #[tracing::instrument(level = "debug", skip(self, stack, candidates))]
+    #[instrument(level = "debug", skip(self, stack, candidates))]
     fn assemble_candidates_from_caller_bounds<'o>(
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
@@ -620,7 +625,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     }
                 }
 
-                _ => candidates.vec.push(AutoImplCandidate(def_id)),
+                _ => candidates.vec.push(AutoImplCandidate),
             }
         }
     }
@@ -706,8 +711,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn need_migrate_deref_output_trait_object(
         &mut self,
         ty: Ty<'tcx>,
-        cause: &traits::ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
+        cause: &ObligationCause<'tcx>,
     ) -> Option<(Ty<'tcx>, DefId)> {
         let tcx = self.tcx();
         if tcx.features().trait_upcasting {
@@ -729,24 +734,27 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return None;
         }
 
-        let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot();
-        let normalized_ty = fulfillcx.normalize_projection_type(
-            &self.infcx,
+        let ty = traits::normalize_projection_type(
+            self,
             param_env,
             ty::ProjectionTy {
                 item_def_id: tcx.lang_items().deref_target()?,
                 substs: trait_ref.substs,
             },
             cause.clone(),
-        );
-
-        let ty::Dynamic(data, ..) = normalized_ty.kind() else {
-            return None;
-        };
-
-        let def_id = data.principal_def_id()?;
-
-        return Some((normalized_ty, def_id));
+            0,
+            // We're *intentionally* throwing these away,
+            // since we don't actually use them.
+            &mut vec![],
+        )
+        .ty()
+        .unwrap();
+
+        if let ty::Dynamic(data, ..) = ty.kind() {
+            Some((ty, data.principal_def_id()?))
+        } else {
+            None
+        }
     }
 
     /// Searches for unsizing that might apply to `obligation`.
@@ -809,8 +817,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         if let Some((deref_output_ty, deref_output_trait_did)) = self
                             .need_migrate_deref_output_trait_object(
                                 source,
-                                &obligation.cause,
                                 obligation.param_env,
+                                &obligation.cause,
                             )
                         {
                             if deref_output_trait_did == target_trait_did {
@@ -818,13 +826,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                     DEREF_INTO_DYN_SUPERTRAIT,
                                     obligation.cause.body_id,
                                     obligation.cause.span,
-                                    |lint| {
-                                        lint.build(&format!(
-                                            "`{}` implements `Deref` with supertrait `{}` as output",
-                                            source,
-                                            deref_output_ty
-                                        )).emit();
-                                    },
+                                    DelayDm(|| format!(
+                                        "`{}` implements `Deref` with supertrait `{}` as output",
+                                        source, deref_output_ty
+                                    )),
+                                    |lint| lint,
                                 );
                                 return;
                             }
@@ -877,17 +883,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
     }
 
-    #[tracing::instrument(level = "debug", skip(self, obligation, candidates))]
+    #[instrument(level = "debug", skip(self, obligation, candidates))]
     fn assemble_candidates_for_transmutability(
         &mut self,
         obligation: &TraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        if obligation.has_param_types_or_consts() {
+        if obligation.has_non_region_param() {
             return;
         }
 
-        if obligation.has_infer_types_or_consts() {
+        if obligation.has_non_region_infer() {
             candidates.ambiguous = true;
             return;
         }
@@ -895,7 +901,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidates.vec.push(TransmutabilityCandidate);
     }
 
-    #[tracing::instrument(level = "debug", skip(self, obligation, candidates))]
+    #[instrument(level = "debug", skip(self, obligation, candidates))]
     fn assemble_candidates_for_trait_alias(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -908,13 +914,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let def_id = obligation.predicate.def_id();
 
         if self.tcx().is_trait_alias(def_id) {
-            candidates.vec.push(TraitAliasCandidate(def_id));
+            candidates.vec.push(TraitAliasCandidate);
         }
     }
 
     /// Assembles the trait which are built-in to the language itself:
     /// `Copy`, `Clone` and `Sized`.
-    #[tracing::instrument(level = "debug", skip(self, candidates))]
+    #[instrument(level = "debug", skip(self, candidates))]
     fn assemble_builtin_bound_candidates(
         &mut self,
         conditions: BuiltinImplConditions<'tcx>,
@@ -1006,4 +1012,46 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         }
     }
+
+    fn assemble_candidate_for_tuple(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) {
+        let self_ty = self.infcx().shallow_resolve(obligation.self_ty().skip_binder());
+        match self_ty.kind() {
+            ty::Tuple(_) => {
+                candidates.vec.push(BuiltinCandidate { has_nested: false });
+            }
+            ty::Infer(ty::TyVar(_)) => {
+                candidates.ambiguous = true;
+            }
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Adt(_, _)
+            | ty::Foreign(_)
+            | ty::Str
+            | ty::Array(_, _)
+            | ty::Slice(_)
+            | ty::RawPtr(_)
+            | ty::Ref(_, _, _)
+            | ty::FnDef(_, _)
+            | ty::FnPtr(_)
+            | ty::Dynamic(_, _, _)
+            | ty::Closure(_, _)
+            | ty::Generator(_, _, _)
+            | ty::GeneratorWitness(_)
+            | ty::Never
+            | ty::Projection(_)
+            | ty::Opaque(_, _)
+            | ty::Param(_)
+            | ty::Bound(_, _)
+            | ty::Error(_)
+            | ty::Infer(_)
+            | ty::Placeholder(_) => {}
+        }
+    }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 2a1099fc8..ed22058c6 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -11,9 +11,10 @@ use rustc_hir::lang_items::LangItem;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_infer::infer::InferOk;
 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
-use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst, SubstsRef};
-use rustc_middle::ty::{self, GenericParamDefKind, Ty, TyCtxt};
-use rustc_middle::ty::{ToPolyTraitRef, ToPredicate};
+use rustc_middle::ty::{
+    self, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
+    ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
+};
 use rustc_span::def_id::DefId;
 
 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
@@ -63,15 +64,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
             }
 
-            AutoImplCandidate(trait_def_id) => {
-                let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
+            AutoImplCandidate => {
+                let data = self.confirm_auto_impl_candidate(obligation);
                 ImplSource::AutoImpl(data)
             }
 
-            ProjectionCandidate(idx) => {
+            ProjectionCandidate(idx, constness) => {
                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
-                // FIXME(jschievink): constness
-                ImplSource::Param(obligations, ty::BoundConstness::NotConst)
+                ImplSource::Param(obligations, constness)
             }
 
             ObjectCandidate(idx) => {
@@ -100,8 +100,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
             PointeeCandidate => ImplSource::Pointee(ImplSourcePointeeData),
 
-            TraitAliasCandidate(alias_def_id) => {
-                let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
+            TraitAliasCandidate => {
+                let data = self.confirm_trait_alias_candidate(obligation);
                 ImplSource::TraitAlias(data)
             }
 
@@ -279,29 +279,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let predicate = obligation.predicate;
 
         let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
-        let bool_at = |i| {
-            predicate
-                .skip_binder()
-                .trait_ref
-                .substs
-                .const_at(i)
-                .try_eval_bool(self.tcx(), obligation.param_env)
-                .unwrap_or(true)
-        };
+        let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
 
         let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
-            src: p.trait_ref.substs.type_at(1),
             dst: p.trait_ref.substs.type_at(0),
+            src: p.trait_ref.substs.type_at(1),
         });
 
         let scope = type_at(2).skip_binder();
 
-        let assume = rustc_transmute::Assume {
-            alignment: bool_at(3),
-            lifetimes: bool_at(4),
-            validity: bool_at(5),
-            visibility: bool_at(6),
-        };
+        let Some(assume) =
+            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
+                return Err(Unimplemented);
+            };
 
         let cause = obligation.cause.clone();
 
@@ -325,13 +315,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn confirm_auto_impl_candidate(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-        trait_def_id: DefId,
     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
-        debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
+        debug!(?obligation, "confirm_auto_impl_candidate");
 
         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
         let types = self.constituent_types_for_ty(self_ty);
-        self.vtable_auto_impl(obligation, trait_def_id, types)
+        self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
     }
 
     /// See `confirm_auto_impl_candidate`.
@@ -629,17 +618,47 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         )
         .map_bound(|(trait_ref, _)| trait_ref);
 
-        let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+        let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+
+        // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
+        let cause = obligation.derived_cause(BuiltinDerivedObligation);
+        // The binder on the Fn obligation is "less" important than the one on
+        // the signature, as evidenced by how we treat it during projection.
+        // The safe thing to do here is to liberate it, though, which should
+        // have no worse effect than skipping the binder here.
+        let liberated_fn_ty =
+            self.infcx.replace_bound_vars_with_placeholders(obligation.predicate.rebind(self_ty));
+        let output_ty = self
+            .infcx
+            .replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
+        let output_ty = normalize_with_depth_to(
+            self,
+            obligation.param_env,
+            cause.clone(),
+            obligation.recursion_depth,
+            output_ty,
+            &mut nested,
+        );
+        let tr = ty::Binder::dummy(ty::TraitRef::new(
+            self.tcx().require_lang_item(LangItem::Sized, None),
+            self.tcx().mk_substs_trait(output_ty, &[]),
+        ));
+        nested.push(Obligation::new(
+            cause,
+            obligation.param_env,
+            tr.to_poly_trait_predicate().to_predicate(self.tcx()),
+        ));
+
         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
     }
 
     fn confirm_trait_alias_candidate(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-        alias_def_id: DefId,
     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
-        debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
+        debug!(?obligation, "confirm_trait_alias_candidate");
 
+        let alias_def_id = obligation.predicate.def_id();
         let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
         let trait_ref = predicate.trait_ref;
         let trait_def_id = trait_ref.def_id;
@@ -794,7 +813,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let upcast_trait_ref;
         match (source.kind(), target.kind()) {
             // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
-            (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
+            (&ty::Dynamic(ref data_a, r_a, repr_a), &ty::Dynamic(ref data_b, r_b, repr_b))
+                if repr_a == repr_b =>
+            {
                 // See `assemble_candidates_for_unsizing` for more info.
                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
                 let principal_a = data_a.principal().unwrap();
@@ -820,7 +841,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             .map(ty::Binder::dummy),
                     );
                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
-                let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
+                let source_trait = tcx.mk_dynamic(existential_predicates, r_b, repr_b);
 
                 // Require that the traits involved in this upcast are **equal**;
                 // only the **lifetime bound** is changed.
@@ -898,7 +919,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let mut nested = vec![];
         match (source.kind(), target.kind()) {
             // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
-            (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
+            (&ty::Dynamic(ref data_a, r_a, ty::Dyn), &ty::Dynamic(ref data_b, r_b, ty::Dyn)) => {
                 // See `assemble_candidates_for_unsizing` for more info.
                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
                 let iter = data_a
@@ -917,7 +938,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             .map(ty::Binder::dummy),
                     );
                 let existential_predicates = tcx.mk_poly_existential_predicates(iter);
-                let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
+                let source_trait = tcx.mk_dynamic(existential_predicates, r_b, ty::Dyn);
 
                 // Require that the traits involved in this upcast are **equal**;
                 // only the **lifetime bound** is changed.
@@ -944,7 +965,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             // `T` -> `Trait`
-            (_, &ty::Dynamic(ref data, r)) => {
+            (_, &ty::Dynamic(ref data, r, ty::Dyn)) => {
                 let mut object_dids = data.auto_traits().chain(data.principal_def_id());
                 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
                     return Err(TraitNotObjectSafe(did));
@@ -1047,9 +1068,25 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     return Err(Unimplemented);
                 }
 
-                // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.
-                let source_tail = tail_field_ty.subst(tcx, substs_a);
-                let target_tail = tail_field_ty.subst(tcx, substs_b);
+                // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
+                // normalizing in the process, since `type_of` returns something directly from
+                // astconv (which means it's un-normalized).
+                let source_tail = normalize_with_depth_to(
+                    self,
+                    obligation.param_env,
+                    obligation.cause.clone(),
+                    obligation.recursion_depth + 1,
+                    tail_field_ty.subst(tcx, substs_a),
+                    &mut nested,
+                );
+                let target_tail = normalize_with_depth_to(
+                    self,
+                    obligation.param_env,
+                    obligation.cause.clone(),
+                    obligation.recursion_depth + 1,
+                    tail_field_ty.subst(tcx, substs_b),
+                    &mut nested,
+                );
 
                 // Check that the source struct with the target's
                 // unsizing parameters is equal to the target.
@@ -1188,6 +1225,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | ty::Never
                 | ty::Foreign(_) => {}
 
+                // `ManuallyDrop` is trivially drop
+                ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
+
                 // These types are built-in, so we can fast-track by registering
                 // nested predicates for their constituent type(s)
                 ty::Array(ty, _) | ty::Slice(ty) => {
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index c01ac1979..9ebff4892 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -20,7 +20,7 @@ use super::{
 };
 
 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
-use crate::traits::error_reporting::InferCtxtExt;
+use crate::traits::error_reporting::TypeErrCtxtExt;
 use crate::traits::project::ProjectAndUnifyResult;
 use crate::traits::project::ProjectionCacheKeyExt;
 use crate::traits::ProjectionCacheKey;
@@ -35,9 +35,8 @@ use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::fold::BottomUpFolder;
-use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::relate::TypeRelation;
-use rustc_middle::ty::subst::{Subst, SubstsRef};
+use rustc_middle::ty::SubstsRef;
 use rustc_middle::ty::{self, EarlyBinder, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitable};
 use rustc_span::symbol::sym;
@@ -94,7 +93,7 @@ impl IntercrateAmbiguityCause {
 }
 
 pub struct SelectionContext<'cx, 'tcx> {
-    infcx: &'cx InferCtxt<'cx, 'tcx>,
+    infcx: &'cx InferCtxt<'tcx>,
 
     /// Freshener used specifically for entries on the obligation
     /// stack. This ensures that all entries on the stack at one time
@@ -215,7 +214,7 @@ enum BuiltinImplConditions<'tcx> {
 }
 
 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
-    pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
+    pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
         SelectionContext {
             infcx,
             freshener: infcx.freshener_keep_static(),
@@ -225,28 +224,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: true,
-            intercrate_ambiguity_causes: None,
-            query_mode: TraitQueryMode::Standard,
-        }
+    pub fn intercrate(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
+        SelectionContext { intercrate: true, ..SelectionContext::new(infcx) }
     }
 
     pub fn with_query_mode(
-        infcx: &'cx InferCtxt<'cx, 'tcx>,
+        infcx: &'cx InferCtxt<'tcx>,
         query_mode: TraitQueryMode,
     ) -> SelectionContext<'cx, 'tcx> {
         debug!(?query_mode, "with_query_mode");
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: false,
-            intercrate_ambiguity_causes: None,
-            query_mode,
-        }
+        SelectionContext { query_mode, ..SelectionContext::new(infcx) }
     }
 
     /// Enables tracking of intercrate ambiguity causes. See
@@ -266,7 +253,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.intercrate_ambiguity_causes.take().unwrap_or_default()
     }
 
-    pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {
+    pub fn infcx(&self) -> &'cx InferCtxt<'tcx> {
         self.infcx
     }
 
@@ -295,7 +282,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
     /// type environment by performing unification.
-    #[instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self), ret)]
     pub fn select(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -325,10 +312,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Err(SelectionError::Overflow(OverflowError::Canonical))
             }
             Err(e) => Err(e),
-            Ok(candidate) => {
-                debug!(?candidate, "confirmed");
-                Ok(Some(candidate))
-            }
+            Ok(candidate) => Ok(Some(candidate)),
         }
     }
 
@@ -435,6 +419,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         level = "debug",
         skip(self, previous_stack),
         fields(previous_stack = ?previous_stack.head())
+        ret,
     )]
     fn evaluate_predicate_recursively<'o>(
         &mut self,
@@ -450,7 +435,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             None => self.check_recursion_limit(&obligation, &obligation)?,
         }
 
-        let result = ensure_sufficient_stack(|| {
+        ensure_sufficient_stack(|| {
             let bound_predicate = obligation.predicate.kind();
             match bound_predicate.skip_binder() {
                 ty::PredicateKind::Trait(t) => {
@@ -464,15 +449,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let p = bound_predicate.rebind(p);
                     // Does this code ever run?
                     match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
-                        Some(Ok(InferOk { mut obligations, .. })) => {
+                        Ok(Ok(InferOk { mut obligations, .. })) => {
                             self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
                             self.evaluate_predicates_recursively(
                                 previous_stack,
                                 obligations.into_iter(),
                             )
                         }
-                        Some(Err(_)) => Ok(EvaluatedToErr),
-                        None => Ok(EvaluatedToAmbig),
+                        Ok(Err(_)) => Ok(EvaluatedToErr),
+                        Err(..) => Ok(EvaluatedToAmbig),
                     }
                 }
 
@@ -480,15 +465,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let p = bound_predicate.rebind(p);
                     // Does this code ever run?
                     match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
-                        Some(Ok(InferOk { mut obligations, .. })) => {
+                        Ok(Ok(InferOk { mut obligations, .. })) => {
                             self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
                             self.evaluate_predicates_recursively(
                                 previous_stack,
                                 obligations.into_iter(),
                             )
                         }
-                        Some(Err(_)) => Ok(EvaluatedToErr),
-                        None => Ok(EvaluatedToAmbig),
+                        Ok(Err(_)) => Ok(EvaluatedToErr),
+                        Err(..) => Ok(EvaluatedToAmbig),
                     }
                 }
 
@@ -691,23 +676,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
 
                 ty::PredicateKind::ConstEquate(c1, c2) => {
+                    assert!(
+                        self.tcx().features().generic_const_exprs,
+                        "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
+                    );
                     debug!(?c1, ?c2, "evaluate_predicate_recursively: equating consts");
 
-                    if self.tcx().features().generic_const_exprs {
-                        // FIXME: we probably should only try to unify abstract constants
-                        // if the constants depend on generic parameters.
-                        //
-                        // Let's just see where this breaks :shrug:
-                        if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
-                            (c1.kind(), c2.kind())
-                        {
-                            if self.infcx.try_unify_abstract_consts(
-                                a.shrink(),
-                                b.shrink(),
-                                obligation.param_env,
-                            ) {
-                                return Ok(EvaluatedToOk);
-                            }
+                    // FIXME: we probably should only try to unify abstract constants
+                    // if the constants depend on generic parameters.
+                    //
+                    // Let's just see where this breaks :shrug:
+                    if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
+                        (c1.kind(), c2.kind())
+                    {
+                        if self.infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
+                            return Ok(EvaluatedToOk);
                         }
                     }
 
@@ -747,7 +730,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             )
                         }
                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
-                            if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
+                            if c1.has_non_region_infer() || c2.has_non_region_infer() {
                                 Ok(EvaluatedToAmbig)
                             } else {
                                 // Two different constants using generic parameters ~> error.
@@ -760,14 +743,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     bug!("TypeWellFormedFromEnv is only used for chalk")
                 }
             }
-        });
-
-        debug!("finished: {:?} from {:?}", result, obligation);
-
-        result
+        })
     }
 
-    #[instrument(skip(self, previous_stack), level = "debug")]
+    #[instrument(skip(self, previous_stack), level = "debug", ret)]
     fn evaluate_trait_predicate_recursively<'o>(
         &mut self,
         previous_stack: TraitObligationStackList<'o, 'tcx>,
@@ -798,12 +777,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // If a trait predicate is in the (local or global) evaluation cache,
         // then we know it holds without cycles.
         if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
-            debug!(?result, "CACHE HIT");
+            debug!("CACHE HIT");
             return Ok(result);
         }
 
         if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
-            debug!(?result, "PROVISIONAL CACHE HIT");
+            debug!("PROVISIONAL CACHE HIT");
             stack.update_reached_depth(result.reached_depth);
             return Ok(result.result);
         }
@@ -826,11 +805,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let reached_depth = stack.reached_depth.get();
         if reached_depth >= stack.depth {
-            debug!(?result, "CACHE MISS");
+            debug!("CACHE MISS");
             self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
             stack.cache().on_completion(stack.dfn);
         } else {
-            debug!(?result, "PROVISIONAL");
+            debug!("PROVISIONAL");
             debug!(
                 "caching provisionally because {:?} \
                  is a cycle participant (at depth {}, reached depth {})",
@@ -849,7 +828,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// must be met of course). One obvious case this comes up is
     /// marker traits like `Send`. Think of a linked list:
     ///
-    ///    struct List<T> { data: T, next: Option<Box<List<T>>> }
+    ///     struct List<T> { data: T, next: Option<Box<List<T>>> }
     ///
     /// `Box<List<T>>` will be `Send` if `T` is `Send` and
     /// `Option<Box<List<T>>>` is `Send`, and in turn
@@ -936,38 +915,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let unbound_input_types =
             stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
 
-        if stack.obligation.polarity() != ty::ImplPolarity::Negative {
-            // This check was an imperfect workaround for a bug in the old
-            // intercrate mode; it should be removed when that goes away.
-            if unbound_input_types && self.intercrate {
-                debug!("evaluate_stack --> unbound argument, intercrate -->  ambiguous",);
-                // Heuristics: show the diagnostics when there are no candidates in crate.
-                if self.intercrate_ambiguity_causes.is_some() {
-                    debug!("evaluate_stack: intercrate_ambiguity_causes is some");
-                    if let Ok(candidate_set) = self.assemble_candidates(stack) {
-                        if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
-                            let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
-                            let self_ty = trait_ref.self_ty();
-                            let cause = with_no_trimmed_paths!({
-                                IntercrateAmbiguityCause::DownstreamCrate {
-                                    trait_desc: trait_ref.print_only_trait_path().to_string(),
-                                    self_desc: if self_ty.has_concrete_skeleton() {
-                                        Some(self_ty.to_string())
-                                    } else {
-                                        None
-                                    },
-                                }
-                            });
-
-                            debug!(?cause, "evaluate_stack: pushing cause");
-                            self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
-                        }
-                    }
-                }
-                return Ok(EvaluatedToAmbig);
-            }
-        }
-
         if unbound_input_types
             && stack.iter().skip(1).any(|prev| {
                 stack.obligation.param_env == prev.obligation.param_env
@@ -1023,7 +970,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     #[instrument(
         level = "debug",
         skip(self, stack),
-        fields(depth = stack.obligation.recursion_depth)
+        fields(depth = stack.obligation.recursion_depth),
+        ret
     )]
     fn evaluate_candidate<'o>(
         &mut self,
@@ -1056,7 +1004,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             result = result.max(EvaluatedToOkModuloRegions);
         }
 
-        debug!(?result);
         Ok(result)
     }
 
@@ -1150,7 +1097,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             ErrorGuaranteed::unchecked_claim_error_was_emitted(),
                         ));
                     }
-                    self.infcx.report_overflow_error(error_obligation, true);
+                    self.infcx.err_ctxt().report_overflow_error(error_obligation, true);
                 }
                 TraitQueryMode::Canonical => {
                     return Err(OverflowError::Canonical);
@@ -1202,8 +1149,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     ImplCandidate(def_id) if tcx.constness(def_id) == hir::Constness::Const => {}
                     // const param
                     ParamCandidate(trait_pred) if trait_pred.is_const_if_const() => {}
+                    // const projection
+                    ProjectionCandidate(_, ty::BoundConstness::ConstIfConst) => {}
                     // auto trait impl
-                    AutoImplCandidate(..) => {}
+                    AutoImplCandidate => {}
                     // generator, this will raise error in other places
                     // or ignore error with const_async_blocks feature
                     GeneratorCandidate => {}
@@ -1265,11 +1214,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(Some(candidate))
     }
 
-    fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
+    fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
         debug!("is_knowable(intercrate={:?})", self.intercrate);
 
         if !self.intercrate || stack.obligation.polarity() == ty::ImplPolarity::Negative {
-            return None;
+            return Ok(());
         }
 
         let obligation = &stack.obligation;
@@ -1405,11 +1354,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// a projection, look at the bounds of `T::Bar`, see if we can find a
     /// `Baz` bound. We return indexes into the list returned by
     /// `tcx.item_bounds` for any applicable bounds.
-    #[instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self), ret)]
     fn match_projection_obligation_against_definition_bounds(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-    ) -> smallvec::SmallVec<[usize; 2]> {
+    ) -> smallvec::SmallVec<[(usize, ty::BoundConstness); 2]> {
         let poly_trait_predicate = self.infcx().resolve_vars_if_possible(obligation.predicate);
         let placeholder_trait_predicate =
             self.infcx().replace_bound_vars_with_placeholders(poly_trait_predicate);
@@ -1435,7 +1384,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // unnecessary ambiguity.
         let mut distinct_normalized_bounds = FxHashSet::default();
 
-        let matching_bounds = bounds
+        bounds
             .iter()
             .enumerate()
             .filter_map(|(idx, bound)| {
@@ -1457,15 +1406,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             _ => false,
                         }
                     }) {
-                        return Some(idx);
+                        return Some((idx, pred.constness));
                     }
                 }
                 None
             })
-            .collect();
-
-        debug!(?matching_bounds);
-        matching_bounds
+            .collect()
     }
 
     /// Equates the trait in `obligation` with trait bound. If the two traits
@@ -1576,7 +1522,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             if !generics.params.is_empty()
                 && obligation.predicate.substs[generics.parent_count..]
                     .iter()
-                    .any(|&p| p.has_infer_types_or_consts() && self.infcx.shallow_resolve(p) != p)
+                    .any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
             {
                 ProjectionMatchesProjection::Ambiguous
             } else {
@@ -1618,12 +1564,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
-        // `DiscriminantKindCandidate`, and `ConstDestructCandidate` to anything else.
+        // `DiscriminantKindCandidate`, `ConstDestructCandidate`
+        // to anything else.
         //
         // This is a fix for #53123 and prevents winnowing from accidentally extending the
         // lifetime of a variable.
         match (&other.candidate, &victim.candidate) {
-            (_, AutoImplCandidate(..)) | (AutoImplCandidate(..), _) => {
+            (_, AutoImplCandidate) | (AutoImplCandidate, _) => {
                 bug!(
                     "default implementations shouldn't be recorded \
                     when there are other valid candidates"
@@ -1691,11 +1638,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
-                | TraitAliasCandidate(..)
+                | TraitAliasCandidate
                 | ObjectCandidate(_)
-                | ProjectionCandidate(_),
+                | ProjectionCandidate(..),
             ) => !is_global(cand),
-            (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
+            (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
                 is_global(cand)
@@ -1709,7 +1656,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
                 ParamCandidate(ref cand),
             ) => {
                 // Prefer these to a global where-clause bound
@@ -1717,20 +1664,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 is_global(cand) && other.evaluation.must_apply_modulo_regions()
             }
 
-            (ProjectionCandidate(i), ProjectionCandidate(j))
+            (ProjectionCandidate(i, _), ProjectionCandidate(j, _))
             | (ObjectCandidate(i), ObjectCandidate(j)) => {
                 // Arbitrarily pick the lower numbered candidate for backwards
                 // compatibility reasons. Don't let this affect inference.
                 i < j && !needs_infer
             }
-            (ObjectCandidate(_), ProjectionCandidate(_))
-            | (ProjectionCandidate(_), ObjectCandidate(_)) => {
+            (ObjectCandidate(_), ProjectionCandidate(..))
+            | (ProjectionCandidate(..), ObjectCandidate(_)) => {
                 bug!("Have both object and projection candidate")
             }
 
             // Arbitrarily give projection and object candidates priority.
             (
-                ObjectCandidate(_) | ProjectionCandidate(_),
+                ObjectCandidate(_) | ProjectionCandidate(..),
                 ImplCandidate(..)
                 | ClosureCandidate
                 | GeneratorCandidate
@@ -1739,7 +1686,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
             ) => true,
 
             (
@@ -1751,18 +1698,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
-                | TraitAliasCandidate(..),
-                ObjectCandidate(_) | ProjectionCandidate(_),
+                | TraitAliasCandidate,
+                ObjectCandidate(_) | ProjectionCandidate(..),
             ) => false,
 
             (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
                 // See if we can toss out `victim` based on specialization.
-                // This requires us to know *for sure* that the `other` impl applies
-                // i.e., `EvaluatedToOk`.
+                // While this requires us to know *for sure* that the `other` impl applies
+                // we still use modulo regions here.
                 //
-                // FIXME(@lcnr): Using `modulo_regions` here seems kind of scary
-                // to me but is required for `std` to compile, so I didn't change it
-                // for now.
+                // This is fine as specialization currently assumes that specializing
+                // impls have to be always applicable, meaning that the only allowed
+                // region constraints may be constraints also present on the default impl.
                 let tcx = self.tcx();
                 if other.evaluation.must_apply_modulo_regions() {
                     if tcx.specializes((other_def, victim_def)) {
@@ -1832,7 +1779,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
                 ImplCandidate(_)
                 | ClosureCandidate
                 | GeneratorCandidate
@@ -1841,7 +1788,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
             ) => false,
         }
     }
@@ -1871,6 +1818,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             | ty::Array(..)
             | ty::Closure(..)
             | ty::Never
+            | ty::Dynamic(_, _, ty::DynStar)
             | ty::Error(_) => {
                 // safe for everything
                 Where(ty::Binder::dummy(Vec::new()))
@@ -1937,8 +1885,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ty::Dynamic(..)
             | ty::Str
             | ty::Slice(..)
-            | ty::Generator(..)
-            | ty::GeneratorWitness(..)
+            | ty::Generator(_, _, hir::Movability::Static)
             | ty::Foreign(..)
             | ty::Ref(_, _, hir::Mutability::Mut) => None,
 
@@ -1947,6 +1894,43 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Where(obligation.predicate.rebind(tys.iter().collect()))
             }
 
+            ty::Generator(_, substs, hir::Movability::Movable) => {
+                if self.tcx().features().generator_clone {
+                    let resolved_upvars =
+                        self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
+                    let resolved_witness =
+                        self.infcx.shallow_resolve(substs.as_generator().witness());
+                    if resolved_upvars.is_ty_var() || resolved_witness.is_ty_var() {
+                        // Not yet resolved.
+                        Ambiguous
+                    } else {
+                        let all = substs
+                            .as_generator()
+                            .upvar_tys()
+                            .chain(iter::once(substs.as_generator().witness()))
+                            .collect::<Vec<_>>();
+                        Where(obligation.predicate.rebind(all))
+                    }
+                } else {
+                    None
+                }
+            }
+
+            ty::GeneratorWitness(binder) => {
+                let witness_tys = binder.skip_binder();
+                for witness_ty in witness_tys.iter() {
+                    let resolved = self.infcx.shallow_resolve(witness_ty);
+                    if resolved.is_ty_var() {
+                        return Ambiguous;
+                    }
+                }
+                // (*) binder moved here
+                let all_vars = self.tcx().mk_bound_variable_kinds(
+                    obligation.predicate.bound_vars().iter().chain(binder.bound_vars().iter()),
+                );
+                Where(ty::Binder::bind_with_vars(witness_tys.to_vec(), all_vars))
+            }
+
             ty::Closure(_, substs) => {
                 // (*) binder moved here
                 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
@@ -2153,7 +2137,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    #[tracing::instrument(level = "debug", skip(self))]
+    #[instrument(level = "debug", skip(self), ret)]
     fn match_impl(
         &mut self,
         impl_def_id: DefId,
@@ -2194,17 +2178,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .at(&cause, obligation.param_env)
             .define_opaque_types(false)
             .eq(placeholder_obligation_trait_ref, impl_trait_ref)
-            .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
+            .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{e}`"))?;
         nested_obligations.extend(obligations);
 
         if !self.intercrate
             && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
         {
-            debug!("match_impl: reservation impls only apply in intercrate mode");
+            debug!("reservation impls only apply in intercrate mode");
             return Err(());
         }
 
-        debug!(?impl_substs, ?nested_obligations, "match_impl: success");
         Ok(Normalized { value: impl_substs, obligations: nested_obligations })
     }
 
@@ -2335,7 +2318,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// impl or trait. The obligations are substituted and fully
     /// normalized. This is used when confirming an impl or default
     /// impl.
-    #[tracing::instrument(level = "debug", skip(self, cause, param_env))]
+    #[instrument(level = "debug", skip(self, cause, param_env))]
     fn impl_or_trait_obligations(
         &mut self,
         cause: &ObligationCause<'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index 6223c5ea3..c89165858 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -12,22 +12,21 @@
 pub mod specialization_graph;
 use specialization_graph::GraphExt;
 
+use crate::errors::NegativePositiveConflict;
 use crate::infer::{InferCtxt, InferOk, TyCtxtInferExt};
 use crate::traits::select::IntercrateAmbiguityCause;
-use crate::traits::{
-    self, coherence, FutureCompatOverlapErrorKind, ObligationCause, TraitEngine, TraitEngineExt,
-};
+use crate::traits::{self, coherence, FutureCompatOverlapErrorKind, ObligationCause};
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
-use rustc_errors::{struct_span_err, EmissionGuarantee, LintDiagnosticBuilder};
+use rustc_errors::{struct_span_err, DiagnosticBuilder, EmissionGuarantee};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
 use rustc_middle::ty::{self, ImplSubject, TyCtxt};
+use rustc_middle::ty::{InternalSubsts, SubstsRef};
 use rustc_session::lint::builtin::COHERENCE_LEAK_CHECK;
 use rustc_session::lint::builtin::ORDER_DEPENDENT_TRAIT_OBJECTS;
 use rustc_span::{Span, DUMMY_SP};
 
+use super::util;
 use super::SelectionContext;
-use super::{util, FulfillmentContext};
 
 /// Information pertinent to an overlapping impl error.
 #[derive(Debug)]
@@ -74,8 +73,8 @@ pub struct OverlapError {
 /// through associated type projection. We deal with such cases by using
 /// *fulfillment* to relate the two impls, requiring that all projections are
 /// resolved.
-pub fn translate_substs<'a, 'tcx>(
-    infcx: &InferCtxt<'a, 'tcx>,
+pub fn translate_substs<'tcx>(
+    infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     source_impl: DefId,
     source_substs: SubstsRef<'tcx>,
@@ -150,14 +149,9 @@ pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId,
     let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id).unwrap();
 
     // Create an infcx, taking the predicates of impl1 as assumptions:
-    tcx.infer_ctxt().enter(|infcx| {
-        let impl1_trait_ref = match traits::fully_normalize(
-            &infcx,
-            FulfillmentContext::new(),
-            ObligationCause::dummy(),
-            penv,
-            impl1_trait_ref,
-        ) {
+    let infcx = tcx.infer_ctxt().build();
+    let impl1_trait_ref =
+        match traits::fully_normalize(&infcx, ObligationCause::dummy(), penv, impl1_trait_ref) {
             Ok(impl1_trait_ref) => impl1_trait_ref,
             Err(_errors) => {
                 tcx.sess.delay_span_bug(
@@ -168,9 +162,8 @@ pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId,
             }
         };
 
-        // Attempt to prove that impl2 applies, given all of the above.
-        fulfill_implication(&infcx, penv, impl1_trait_ref, impl2_def_id).is_ok()
-    })
+    // Attempt to prove that impl2 applies, given all of the above.
+    fulfill_implication(&infcx, penv, impl1_trait_ref, impl2_def_id).is_ok()
 }
 
 /// Attempt to fulfill all obligations of `target_impl` after unification with
@@ -178,8 +171,8 @@ pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId,
 /// generics of `target_impl`, including both those needed to unify with
 /// `source_trait_ref` and those whose identity is determined via a where
 /// clause in the impl.
-fn fulfill_implication<'a, 'tcx>(
-    infcx: &InferCtxt<'a, 'tcx>,
+fn fulfill_implication<'tcx>(
+    infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     source_trait_ref: ty::TraitRef<'tcx>,
     target_impl: DefId,
@@ -211,11 +204,8 @@ fn fulfill_implication<'a, 'tcx>(
     // (which are packed up in penv)
 
     infcx.save_and_restore_in_snapshot_flag(|infcx| {
-        let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
-        for oblig in obligations.chain(more_obligations) {
-            fulfill_cx.register_predicate_obligation(&infcx, oblig);
-        }
-        match fulfill_cx.select_all_or_error(infcx).as_slice() {
+        let errors = traits::fully_solve_obligations(&infcx, obligations.chain(more_obligations));
+        match &errors[..] {
             [] => {
                 debug!(
                     "fulfill_implication: an impl for {:?} specializes {:?}",
@@ -333,35 +323,13 @@ fn report_negative_positive_conflict(
     positive_impl_def_id: DefId,
     sg: &mut specialization_graph::Graph,
 ) {
-    let impl_span = tcx.def_span(local_impl_def_id);
-
-    let mut err = struct_span_err!(
-        tcx.sess,
-        impl_span,
-        E0751,
-        "found both positive and negative implementation of trait `{}`{}:",
-        overlap.trait_desc,
-        overlap.self_desc.clone().map_or_else(String::new, |ty| format!(" for type `{}`", ty))
-    );
-
-    match tcx.span_of_impl(negative_impl_def_id) {
-        Ok(span) => {
-            err.span_label(span, "negative implementation here");
-        }
-        Err(cname) => {
-            err.note(&format!("negative implementation in crate `{}`", cname));
-        }
-    }
-
-    match tcx.span_of_impl(positive_impl_def_id) {
-        Ok(span) => {
-            err.span_label(span, "positive implementation here");
-        }
-        Err(cname) => {
-            err.note(&format!("positive implementation in crate `{}`", cname));
-        }
-    }
-
+    let mut err = tcx.sess.create_err(NegativePositiveConflict {
+        impl_span: tcx.def_span(local_impl_def_id),
+        trait_desc: &overlap.trait_desc,
+        self_desc: &overlap.self_desc,
+        negative_impl_span: tcx.span_of_impl(negative_impl_def_id),
+        positive_impl_span: tcx.span_of_impl(positive_impl_def_id),
+    });
     sg.has_errored = Some(err.emit());
 }
 
@@ -377,26 +345,12 @@ fn report_conflicting_impls(
     // Work to be done after we've built the DiagnosticBuilder. We have to define it
     // now because the struct_lint methods don't return back the DiagnosticBuilder
     // that's passed in.
-    fn decorate<G: EmissionGuarantee>(
+    fn decorate<'a, 'b, G: EmissionGuarantee>(
         tcx: TyCtxt<'_>,
         overlap: OverlapError,
-        used_to_be_allowed: Option<FutureCompatOverlapErrorKind>,
         impl_span: Span,
-        err: LintDiagnosticBuilder<'_, G>,
-    ) -> G {
-        let msg = format!(
-            "conflicting implementations of trait `{}`{}{}",
-            overlap.trait_desc,
-            overlap
-                .self_desc
-                .clone()
-                .map_or_else(String::new, |ty| { format!(" for type `{}`", ty) }),
-            match used_to_be_allowed {
-                Some(FutureCompatOverlapErrorKind::Issue33140) => ": (E0119)",
-                _ => "",
-            }
-        );
-        let mut err = err.build(&msg);
+        err: &'b mut DiagnosticBuilder<'a, G>,
+    ) -> &'b mut DiagnosticBuilder<'a, G> {
         match tcx.span_of_impl(overlap.with_impl) {
             Ok(span) => {
                 err.span_label(span, "first implementation here");
@@ -411,7 +365,9 @@ fn report_conflicting_impls(
             }
             Err(cname) => {
                 let msg = match to_pretty_impl_header(tcx, overlap.with_impl) {
-                    Some(s) => format!("conflicting implementation in crate `{}`:\n- {}", cname, s),
+                    Some(s) => {
+                        format!("conflicting implementation in crate `{}`:\n- {}", cname, s)
+                    }
                     None => format!("conflicting implementation in crate `{}`", cname),
                 };
                 err.note(&msg);
@@ -419,28 +375,33 @@ fn report_conflicting_impls(
         }
 
         for cause in &overlap.intercrate_ambiguity_causes {
-            cause.add_intercrate_ambiguity_hint(&mut err);
+            cause.add_intercrate_ambiguity_hint(err);
         }
 
         if overlap.involves_placeholder {
-            coherence::add_placeholder_note(&mut err);
+            coherence::add_placeholder_note(err);
         }
-        err.emit()
+        err
     }
 
+    let msg = format!(
+        "conflicting implementations of trait `{}`{}{}",
+        overlap.trait_desc,
+        overlap.self_desc.as_deref().map_or_else(String::new, |ty| format!(" for type `{ty}`")),
+        match used_to_be_allowed {
+            Some(FutureCompatOverlapErrorKind::Issue33140) => ": (E0119)",
+            _ => "",
+        }
+    );
+
     match used_to_be_allowed {
         None => {
             let reported = if overlap.with_impl.is_local()
                 || tcx.orphan_check_impl(impl_def_id).is_ok()
             {
-                let err = struct_span_err!(tcx.sess, impl_span, E0119, "");
-                Some(decorate(
-                    tcx,
-                    overlap,
-                    used_to_be_allowed,
-                    impl_span,
-                    LintDiagnosticBuilder::new(err),
-                ))
+                let mut err = struct_span_err!(tcx.sess, impl_span, E0119, "{msg}",);
+                decorate(tcx, overlap, impl_span, &mut err);
+                Some(err.emit())
             } else {
                 Some(tcx.sess.delay_span_bug(impl_span, "impl should have failed the orphan check"))
             };
@@ -455,9 +416,8 @@ fn report_conflicting_impls(
                 lint,
                 tcx.hir().local_def_id_to_hir_id(impl_def_id),
                 impl_span,
-                |ldb| {
-                    decorate(tcx, overlap, used_to_be_allowed, impl_span, ldb);
-                },
+                msg,
+                |err| decorate(tcx, overlap, impl_span, err),
             );
         }
     };
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs b/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
index fcb73b43f..63f89a33e 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
@@ -137,9 +137,8 @@ impl ChildrenExt<'_> for Children {
                     impl_def_id,
                     traits::SkipLeakCheck::default(),
                     overlap_mode,
-                    |_| true,
-                    || false,
-                );
+                )
+                .is_some();
 
                 let error = create_overlap_error(overlap);
 
@@ -162,34 +161,29 @@ impl ChildrenExt<'_> for Children {
                 impl_def_id,
                 traits::SkipLeakCheck::Yes,
                 overlap_mode,
-                |overlap| {
-                    if let Some(overlap_kind) =
-                        tcx.impls_are_allowed_to_overlap(impl_def_id, possible_sibling)
-                    {
-                        match overlap_kind {
-                            ty::ImplOverlapKind::Permitted { marker: _ } => {}
-                            ty::ImplOverlapKind::Issue33140 => {
-                                *last_lint_mut = Some(FutureCompatOverlapError {
-                                    error: create_overlap_error(overlap),
-                                    kind: FutureCompatOverlapErrorKind::Issue33140,
-                                });
-                            }
+            )
+            .map_or(Ok((false, false)), |overlap| {
+                if let Some(overlap_kind) =
+                    tcx.impls_are_allowed_to_overlap(impl_def_id, possible_sibling)
+                {
+                    match overlap_kind {
+                        ty::ImplOverlapKind::Permitted { marker: _ } => {}
+                        ty::ImplOverlapKind::Issue33140 => {
+                            *last_lint_mut = Some(FutureCompatOverlapError {
+                                error: create_overlap_error(overlap),
+                                kind: FutureCompatOverlapErrorKind::Issue33140,
+                            });
                         }
-
-                        return Ok((false, false));
                     }
 
-                    let le = tcx.specializes((impl_def_id, possible_sibling));
-                    let ge = tcx.specializes((possible_sibling, impl_def_id));
+                    return Ok((false, false));
+                }
 
-                    if le == ge {
-                        report_overlap_error(overlap, last_lint_mut)
-                    } else {
-                        Ok((le, ge))
-                    }
-                },
-                || Ok((false, false)),
-            )?;
+                let le = tcx.specializes((impl_def_id, possible_sibling));
+                let ge = tcx.specializes((possible_sibling, impl_def_id));
+
+                if le == ge { report_overlap_error(overlap, last_lint_mut) } else { Ok((le, ge)) }
+            })?;
 
             if le && !ge {
                 debug!(
diff --git a/compiler/rustc_trait_selection/src/traits/structural_match.rs b/compiler/rustc_trait_selection/src/traits/structural_match.rs
index 5829a0f92..932dbbb81 100644
--- a/compiler/rustc_trait_selection/src/traits/structural_match.rs
+++ b/compiler/rustc_trait_selection/src/traits/structural_match.rs
@@ -68,7 +68,7 @@ pub fn search_for_adt_const_param_violation<'tcx>(
 /// Note that this does *not* recursively check if the substructure of `adt_ty`
 /// implements the traits.
 fn type_marked_structural<'tcx>(
-    infcx: &InferCtxt<'_, 'tcx>,
+    infcx: &InferCtxt<'tcx>,
     adt_ty: Ty<'tcx>,
     cause: ObligationCause<'tcx>,
 ) -> bool {
@@ -265,9 +265,8 @@ impl<'tcx> TypeVisitor<'tcx> for Search<'tcx> {
 
 pub fn provide(providers: &mut Providers) {
     providers.has_structural_eq_impls = |tcx, ty| {
-        tcx.infer_ctxt().enter(|infcx| {
-            let cause = ObligationCause::dummy();
-            type_marked_structural(&infcx, ty, cause)
-        })
+        let infcx = tcx.infer_ctxt().build();
+        let cause = ObligationCause::dummy();
+        type_marked_structural(&infcx, ty, cause)
     };
 }
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index d25006016..ed47d2f83 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -5,14 +5,12 @@ use smallvec::SmallVec;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def_id::DefId;
-use rustc_middle::ty::subst::{GenericArg, Subst, SubstsRef};
 use rustc_middle::ty::{self, ImplSubject, ToPredicate, Ty, TyCtxt, TypeVisitable};
+use rustc_middle::ty::{GenericArg, SubstsRef};
 
 use super::{Normalized, Obligation, ObligationCause, PredicateObligation, SelectionContext};
 pub use rustc_infer::traits::{self, util::*};
 
-use std::iter;
-
 ///////////////////////////////////////////////////////////////////////////
 // `TraitAliasExpander` iterator
 ///////////////////////////////////////////////////////////////////////////
@@ -210,7 +208,7 @@ pub fn impl_subject_and_oblig<'a, 'tcx>(
     let Normalized { value: predicates, obligations: normalization_obligations2 } =
         super::normalize(selcx, param_env, ObligationCause::dummy(), predicates);
     let impl_obligations =
-        predicates_for_generics(ObligationCause::dummy(), 0, param_env, predicates);
+        super::predicates_for_generics(|_, _| ObligationCause::dummy(), param_env, predicates);
 
     let impl_obligations = impl_obligations
         .chain(normalization_obligations1.into_iter())
@@ -219,27 +217,6 @@ pub fn impl_subject_and_oblig<'a, 'tcx>(
     (subject, impl_obligations)
 }
 
-pub fn predicates_for_generics<'tcx>(
-    cause: ObligationCause<'tcx>,
-    recursion_depth: usize,
-    param_env: ty::ParamEnv<'tcx>,
-    generic_bounds: ty::InstantiatedPredicates<'tcx>,
-) -> impl Iterator<Item = PredicateObligation<'tcx>> {
-    debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);
-
-    iter::zip(generic_bounds.predicates, generic_bounds.spans).map(move |(predicate, span)| {
-        let cause = match *cause.code() {
-            traits::ItemObligation(def_id) if !span.is_dummy() => traits::ObligationCause::new(
-                cause.span,
-                cause.body_id,
-                traits::BindingObligation(def_id, span),
-            ),
-            _ => cause.clone(),
-        };
-        Obligation { cause, recursion_depth, param_env, predicate }
-    })
-}
-
 pub fn predicate_for_trait_ref<'tcx>(
     tcx: TyCtxt<'tcx>,
     cause: ObligationCause<'tcx>,
@@ -291,10 +268,7 @@ pub fn count_own_vtable_entries<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::PolyTraitRef<'tcx>,
 ) -> usize {
-    let existential_trait_ref =
-        trait_ref.map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
-    let existential_trait_ref = tcx.erase_regions(existential_trait_ref);
-    tcx.own_existential_vtable_entries(existential_trait_ref).len()
+    tcx.own_existential_vtable_entries(trait_ref.def_id()).len()
 }
 
 /// Given an upcast trait object described by `object`, returns the
@@ -305,15 +279,10 @@ pub fn get_vtable_index_of_object_method<'tcx, N>(
     object: &super::ImplSourceObjectData<'tcx, N>,
     method_def_id: DefId,
 ) -> Option<usize> {
-    let existential_trait_ref = object
-        .upcast_trait_ref
-        .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
-    let existential_trait_ref = tcx.erase_regions(existential_trait_ref);
-
     // Count number of methods preceding the one we are selecting and
     // add them to the total offset.
     if let Some(index) = tcx
-        .own_existential_vtable_entries(existential_trait_ref)
+        .own_existential_vtable_entries(object.upcast_trait_ref.def_id())
         .iter()
         .copied()
         .position(|def_id| def_id == method_def_id)
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 414857f0a..fc0a9f690 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -14,8 +14,8 @@ use std::iter;
 /// inference variable, returns `None`, because we are not able to
 /// make any progress at all. This is to prevent "livelock" where we
 /// say "$0 is WF if $0 is WF".
-pub fn obligations<'a, 'tcx>(
-    infcx: &InferCtxt<'a, 'tcx>,
+pub fn obligations<'tcx>(
+    infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     body_id: hir::HirId,
     recursion_depth: usize,
@@ -31,9 +31,9 @@ pub fn obligations<'a, 'tcx>(
                     if resolved_ty == ty {
                         // No progress, bail out to prevent "livelock".
                         return None;
+                    } else {
+                        resolved_ty
                     }
-
-                    resolved_ty
                 }
                 _ => ty,
             }
@@ -41,16 +41,14 @@ pub fn obligations<'a, 'tcx>(
         }
         GenericArgKind::Const(ct) => {
             match ct.kind() {
-                ty::ConstKind::Infer(infer) => {
-                    let resolved = infcx.shallow_resolve(infer);
-                    if resolved == infer {
+                ty::ConstKind::Infer(_) => {
+                    let resolved = infcx.shallow_resolve(ct);
+                    if resolved == ct {
                         // No progress.
                         return None;
+                    } else {
+                        resolved
                     }
-
-                    infcx
-                        .tcx
-                        .mk_const(ty::ConstS { kind: ty::ConstKind::Infer(resolved), ty: ct.ty() })
                 }
                 _ => ct,
             }
@@ -81,8 +79,8 @@ pub fn obligations<'a, 'tcx>(
 /// well-formed.  For example, if there is a trait `Set` defined like
 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
 /// if `Bar: Eq`.
-pub fn trait_obligations<'a, 'tcx>(
-    infcx: &InferCtxt<'a, 'tcx>,
+pub fn trait_obligations<'tcx>(
+    infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     body_id: hir::HirId,
     trait_pred: &ty::TraitPredicate<'tcx>,
@@ -103,8 +101,9 @@ pub fn trait_obligations<'a, 'tcx>(
     wf.normalize(infcx)
 }
 
-pub fn predicate_obligations<'a, 'tcx>(
-    infcx: &InferCtxt<'a, 'tcx>,
+#[instrument(skip(infcx), ret)]
+pub fn predicate_obligations<'tcx>(
+    infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     body_id: hir::HirId,
     predicate: ty::Predicate<'tcx>,
@@ -131,9 +130,9 @@ pub fn predicate_obligations<'a, 'tcx>(
         }
         ty::PredicateKind::Projection(t) => {
             wf.compute_projection(t.projection_ty);
-            wf.compute(match t.term {
-                ty::Term::Ty(ty) => ty.into(),
-                ty::Term::Const(c) => c.into(),
+            wf.compute(match t.term.unpack() {
+                ty::TermKind::Ty(ty) => ty.into(),
+                ty::TermKind::Const(c) => c.into(),
             })
         }
         ty::PredicateKind::WellFormed(arg) => {
@@ -149,13 +148,8 @@ pub fn predicate_obligations<'a, 'tcx>(
             wf.compute(a.into());
             wf.compute(b.into());
         }
-        ty::PredicateKind::ConstEvaluatable(uv) => {
-            let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
-            wf.out.extend(obligations);
-
-            for arg in uv.substs.iter() {
-                wf.compute(arg);
-            }
+        ty::PredicateKind::ConstEvaluatable(ct) => {
+            wf.compute(ct.into());
         }
         ty::PredicateKind::ConstEquate(c1, c2) => {
             wf.compute(c1.into());
@@ -220,12 +214,14 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
         trait_ref, item, cause, pred
     );
     let (items, impl_def_id) = match item {
-        Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
+        Some(hir::Item { kind: hir::ItemKind::Impl(impl_), owner_id, .. }) => {
+            (impl_.items, *owner_id)
+        }
         _ => return,
     };
     let fix_span =
         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
-            hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
+            hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::Type(ty) => ty.span,
             _ => impl_item_ref.span,
         };
 
@@ -242,7 +238,7 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
                 && let Some(impl_item_span) = items
                     .iter()
-                    .find(|item| item.id.def_id.to_def_id() == impl_item_id)
+                    .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
                     .map(fix_span)
             {
                 cause.span = impl_item_span;
@@ -257,7 +253,7 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
                 && let Some(impl_item_span) = items
                     .iter()
-                    .find(|item| item.id.def_id.to_def_id() == impl_item_id)
+                    .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
                     .map(fix_span)
             {
                 cause.span = impl_item_span;
@@ -276,7 +272,7 @@ impl<'tcx> WfPredicates<'tcx> {
         traits::ObligationCause::new(self.span, self.body_id, code)
     }
 
-    fn normalize(self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
+    fn normalize(self, infcx: &InferCtxt<'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
         let cause = self.cause(traits::WellFormed(None));
         let param_env = self.param_env;
         let mut obligations = Vec::with_capacity(self.out.len());
@@ -393,7 +389,8 @@ impl<'tcx> WfPredicates<'tcx> {
         //     `i32: Clone`
         //     `i32: Copy`
         // ]
-        let obligations = self.nominal_obligations(data.item_def_id, data.substs);
+        // Projection types do not require const predicates.
+        let obligations = self.nominal_obligations_without_const(data.item_def_id, data.substs);
         self.out.extend(obligations);
 
         let tcx = self.tcx();
@@ -436,11 +433,13 @@ impl<'tcx> WfPredicates<'tcx> {
     }
 
     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
+    #[instrument(level = "debug", skip(self))]
     fn compute(&mut self, arg: GenericArg<'tcx>) {
         let mut walker = arg.walk();
         let param_env = self.param_env;
         let depth = self.recursion_depth;
         while let Some(arg) = walker.next() {
+            debug!(?arg, ?self.out);
             let ty = match arg.unpack() {
                 GenericArgKind::Type(ty) => ty,
 
@@ -448,14 +447,14 @@ impl<'tcx> WfPredicates<'tcx> {
                 // obligations are handled by the parent (e.g. `ty::Ref`).
                 GenericArgKind::Lifetime(_) => continue,
 
-                GenericArgKind::Const(constant) => {
-                    match constant.kind() {
+                GenericArgKind::Const(ct) => {
+                    match ct.kind() {
                         ty::ConstKind::Unevaluated(uv) => {
                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
                             self.out.extend(obligations);
 
                             let predicate =
-                                ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
+                                ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(ct))
                                     .to_predicate(self.tcx());
                             let cause = self.cause(traits::WellFormed(None));
                             self.out.push(traits::Obligation::with_depth(
@@ -472,7 +471,7 @@ impl<'tcx> WfPredicates<'tcx> {
                                 cause,
                                 self.recursion_depth,
                                 self.param_env,
-                                ty::Binder::dummy(ty::PredicateKind::WellFormed(constant.into()))
+                                ty::Binder::dummy(ty::PredicateKind::WellFormed(ct.into()))
                                     .to_predicate(self.tcx()),
                             ));
                         }
@@ -490,6 +489,8 @@ impl<'tcx> WfPredicates<'tcx> {
                 }
             };
 
+            debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
+
             match *ty.kind() {
                 ty::Bool
                 | ty::Char
@@ -546,7 +547,7 @@ impl<'tcx> WfPredicates<'tcx> {
                 }
 
                 ty::FnDef(did, substs) => {
-                    let obligations = self.nominal_obligations(did, substs);
+                    let obligations = self.nominal_obligations_without_const(did, substs);
                     self.out.extend(obligations);
                 }
 
@@ -636,7 +637,7 @@ impl<'tcx> WfPredicates<'tcx> {
                     }
                 }
 
-                ty::Dynamic(data, r) => {
+                ty::Dynamic(data, r, _) => {
                     // WfObject
                     //
                     // Here, we defer WF checking due to higher-ranked
@@ -688,6 +689,8 @@ impl<'tcx> WfPredicates<'tcx> {
                     ));
                 }
             }
+
+            debug!(?self.out);
         }
     }
 
@@ -713,7 +716,7 @@ impl<'tcx> WfPredicates<'tcx> {
         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
             .map(|((mut pred, span), origin_def_id)| {
                 let code = if span.is_dummy() {
-                    traits::MiscObligation
+                    traits::ItemObligation(origin_def_id)
                 } else {
                     traits::BindingObligation(origin_def_id, span)
                 };
@@ -843,7 +846,7 @@ pub fn object_region_bounds<'tcx>(
 ///
 /// Requires that trait definitions have been processed so that we can
 /// elaborate predicates and walk supertraits.
-#[instrument(skip(tcx, predicates), level = "debug")]
+#[instrument(skip(tcx, predicates), level = "debug", ret)]
 pub(crate) fn required_region_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
     erased_self_ty: Ty<'tcx>,