Auto merge of #2738 - RalfJung:rustup, r=RalfJung

Rustup

21 files changed, 1509 insertions, 159 deletions

diff --git a/compiler/rustc_trait_selection/src/lib.rs b/compiler/rustc_trait_selection/src/lib.rs
index 975ff31a607..a30d1df4ede 100644
--- a/compiler/rustc_trait_selection/src/lib.rs
+++ b/compiler/rustc_trait_selection/src/lib.rs
@@ -19,6 +19,7 @@
 #![feature(let_chains)]
 #![feature(if_let_guard)]
 #![feature(never_type)]
+#![feature(result_option_inspect)]
 #![feature(type_alias_impl_trait)]
 #![recursion_limit = "512"] // For rustdoc
 
@@ -37,4 +38,5 @@ extern crate smallvec;
 pub mod autoderef;
 pub mod errors;
 pub mod infer;
+pub mod solve;
 pub mod traits;
diff --git a/compiler/rustc_trait_selection/src/solve/assembly.rs b/compiler/rustc_trait_selection/src/solve/assembly.rs
new file mode 100644
index 00000000000..e9ddad11ff2
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/assembly.rs
@@ -0,0 +1,150 @@
+//! Code shared by trait and projection goals for candidate assembly.
+
+use super::infcx_ext::InferCtxtExt;
+use super::{
+    fixme_instantiate_canonical_query_response, CanonicalGoal, CanonicalResponse, Certainty,
+    EvalCtxt, Goal,
+};
+use rustc_hir::def_id::DefId;
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::infer::{
+    canonical::{CanonicalVarValues, OriginalQueryValues},
+    InferCtxt,
+};
+use rustc_infer::traits::query::NoSolution;
+use rustc_middle::ty::TypeFoldable;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::DUMMY_SP;
+use std::fmt::Debug;
+
+/// A candidate is a possible way to prove a goal.
+///
+/// It consists of both the `source`, which describes how that goal would be proven,
+/// and the `result` when using the given `source`.
+///
+/// For the list of possible candidates, please look at the documentation of
+/// [super::trait_goals::CandidateSource] and [super::project_goals::CandidateSource].
+#[derive(Debug, Clone)]
+pub(super) struct Candidate<'tcx, G: GoalKind<'tcx>> {
+    pub(super) source: G::CandidateSource,
+    pub(super) result: CanonicalResponse<'tcx>,
+}
+
+pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy {
+    type CandidateSource: Debug + Copy;
+
+    fn self_ty(self) -> Ty<'tcx>;
+
+    fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self;
+
+    fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId;
+
+    fn consider_impl_candidate(
+        acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
+        goal: Goal<'tcx, Self>,
+        impl_def_id: DefId,
+    );
+}
+
+/// An abstraction which correctly deals with the canonical results for candidates.
+///
+/// It also deduplicates the behavior between trait and projection predicates.
+pub(super) struct AssemblyCtxt<'a, 'tcx, G: GoalKind<'tcx>> {
+    pub(super) cx: &'a mut EvalCtxt<'tcx>,
+    pub(super) infcx: &'a InferCtxt<'tcx>,
+    var_values: CanonicalVarValues<'tcx>,
+    candidates: Vec<Candidate<'tcx, G>>,
+}
+
+impl<'a, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'tcx, G> {
+    pub(super) fn assemble_and_evaluate_candidates(
+        cx: &'a mut EvalCtxt<'tcx>,
+        goal: CanonicalGoal<'tcx, G>,
+    ) -> Vec<Candidate<'tcx, G>> {
+        let (ref infcx, goal, var_values) =
+            cx.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
+        let mut acx = AssemblyCtxt { cx, infcx, var_values, candidates: Vec::new() };
+
+        acx.assemble_candidates_after_normalizing_self_ty(goal);
+
+        acx.assemble_impl_candidates(goal);
+
+        acx.candidates
+    }
+
+    pub(super) fn try_insert_candidate(
+        &mut self,
+        source: G::CandidateSource,
+        certainty: Certainty,
+    ) {
+        match self.infcx.make_canonical_response(self.var_values.clone(), certainty) {
+            Ok(result) => self.candidates.push(Candidate { source, result }),
+            Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
+        }
+    }
+
+    /// If the self type of a goal is a projection, computing the relevant candidates is difficult.
+    ///
+    /// To deal with this, we first try to normalize the self type and add the candidates for the normalized
+    /// self type to the list of candidates in case that succeeds. Note that we can't just eagerly return in
+    /// this case as projections as self types add `
+    fn assemble_candidates_after_normalizing_self_ty(&mut self, goal: Goal<'tcx, G>) {
+        let tcx = self.cx.tcx;
+        // FIXME: We also have to normalize opaque types, not sure where to best fit that in.
+        let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.self_ty().kind() else {
+            return
+        };
+        self.infcx.probe(|_| {
+            let normalized_ty = self.infcx.next_ty_infer();
+            let normalizes_to_goal = goal.with(
+                tcx,
+                ty::Binder::dummy(ty::ProjectionPredicate {
+                    projection_ty,
+                    term: normalized_ty.into(),
+                }),
+            );
+            let normalization_certainty =
+                match self.cx.evaluate_goal(&self.infcx, normalizes_to_goal) {
+                    Ok((_, certainty)) => certainty,
+                    Err(NoSolution) => return,
+                };
+
+            // NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
+            // This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
+            let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
+            let mut orig_values = OriginalQueryValues::default();
+            let goal = self.infcx.canonicalize_query(goal, &mut orig_values);
+            let normalized_candidates =
+                AssemblyCtxt::assemble_and_evaluate_candidates(self.cx, goal);
+
+            // Map each candidate from being canonical wrt the current inference context to being
+            // canonical wrt the caller.
+            for Candidate { source, result } in normalized_candidates {
+                self.infcx.probe(|_| {
+                    let candidate_certainty = fixme_instantiate_canonical_query_response(
+                        &self.infcx,
+                        &orig_values,
+                        result,
+                    );
+
+                    // FIXME: This is a bit scary if the `normalizes_to_goal` overflows.
+                    //
+                    // If we have an ambiguous candidate it hides that normalization
+                    // caused an overflow which may cause issues.
+                    self.try_insert_candidate(
+                        source,
+                        normalization_certainty.unify_and(candidate_certainty),
+                    )
+                })
+            }
+        })
+    }
+
+    fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, G>) {
+        self.cx.tcx.for_each_relevant_impl(
+            goal.predicate.trait_def_id(self.cx.tcx),
+            goal.predicate.self_ty(),
+            |impl_def_id| G::consider_impl_candidate(self, goal, impl_def_id),
+        );
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/solve/cache.rs b/compiler/rustc_trait_selection/src/solve/cache.rs
new file mode 100644
index 00000000000..993b7989066
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/cache.rs
@@ -0,0 +1,257 @@
+//! This module both handles the global cache which stores "finished" goals,
+//! and the provisional cache which contains partially computed goals.
+//!
+//! The provisional cache is necessary when dealing with coinductive cycles.
+//!
+//! For more information about the provisional cache and coinduction in general,
+//! check out the relevant section of the rustc-dev-guide.
+//!
+//! FIXME(@lcnr): Write that section, feel free to ping me if you need help here
+//! before then or if I still haven't done that before January 2023.
+use super::overflow::OverflowData;
+use super::CanonicalGoal;
+use super::{EvalCtxt, QueryResult};
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_middle::ty::TyCtxt;
+use std::{cmp::Ordering, collections::hash_map::Entry};
+
+#[derive(Debug, Clone)]
+struct ProvisionalEntry<'tcx> {
+    // In case we have a coinductive cycle, this is the
+    // the currently least restrictive result of this goal.
+    response: QueryResult<'tcx>,
+    // The lowest element on the stack on which this result
+    // relies on. Starts out as just being the depth at which
+    // we've proven this obligation, but gets lowered to the
+    // depth of another goal if we rely on it in a cycle.
+    depth: usize,
+}
+
+struct StackElem<'tcx> {
+    goal: CanonicalGoal<'tcx>,
+    has_been_used: bool,
+}
+
+/// The cache used for goals which are currently in progress or which depend
+/// on in progress results.
+///
+/// Once we're done with a goal we can store it in the global trait solver
+/// cache of the `TyCtxt`. For goals which we're currently proving, or which
+/// have only been proven via a coinductive cycle using a goal still on our stack
+/// we have to use this separate data structure.
+///
+/// The current data structure is not perfect, so there may still be room for
+/// improvement here. We have the following requirements:
+///
+/// ## Is there is a provisional entry for the given goal:
+///
+/// ```ignore (for syntax highlighting)
+/// self.entries.get(goal)
+/// ```
+///
+/// ## Get all goals on the stack involved in a cycle:
+///
+/// ```ignore (for syntax highlighting)
+/// let entry = self.entries.get(goal).unwrap();
+/// let involved_goals = self.stack.iter().skip(entry.depth);
+/// ```
+///
+/// ## Capping the depth of all entries
+///
+/// Needed whenever we encounter a cycle. The current implementation always
+/// iterates over all entries instead of only the ones with a larger depth.
+/// Changing this may result in notable performance improvements.
+///
+/// ```ignore (for syntax highlighting)
+/// let cycle_depth = self.entries.get(goal).unwrap().depth;
+/// for e in &mut self.entries {
+///     e.depth = e.depth.min(cycle_depth);
+/// }
+/// ```
+///
+/// ## Checking whether we have to rerun the current goal
+///
+/// A goal has to be rerun if its provisional result was used in a cycle
+/// and that result is different from its final result. We update
+/// [StackElem::has_been_used] for the deepest stack element involved in a cycle.
+///
+/// ## Moving all finished goals into the global cache
+///
+/// If `stack_elem.has_been_used` is true, iterate over all entries, moving the ones
+/// with equal depth. If not, simply move this single entry.
+pub(super) struct ProvisionalCache<'tcx> {
+    stack: Vec<StackElem<'tcx>>,
+    entries: FxHashMap<CanonicalGoal<'tcx>, ProvisionalEntry<'tcx>>,
+}
+
+impl<'tcx> ProvisionalCache<'tcx> {
+    pub(super) fn empty() -> ProvisionalCache<'tcx> {
+        ProvisionalCache { stack: Vec::new(), entries: Default::default() }
+    }
+
+    pub(super) fn current_depth(&self) -> usize {
+        self.stack.len()
+    }
+}
+
+impl<'tcx> EvalCtxt<'tcx> {
+    /// Tries putting the new goal on the stack, returning an error if it is already cached.
+    ///
+    /// This correctly updates the provisional cache if there is a cycle.
+    pub(super) fn try_push_stack(
+        &mut self,
+        goal: CanonicalGoal<'tcx>,
+    ) -> Result<(), QueryResult<'tcx>> {
+        // FIXME: start by checking the global cache
+
+        // Look at the provisional cache to check for cycles.
+        let cache = &mut self.provisional_cache;
+        match cache.entries.entry(goal) {
+            // No entry, simply push this goal on the stack after dealing with overflow.
+            Entry::Vacant(v) => {
+                if self.overflow_data.has_overflow(cache.stack.len()) {
+                    return Err(self.deal_with_overflow());
+                }
+
+                v.insert(ProvisionalEntry {
+                    response: fixme_response_yes_no_constraints(),
+                    depth: cache.stack.len(),
+                });
+                cache.stack.push(StackElem { goal, has_been_used: false });
+                Ok(())
+            }
+            // We have a nested goal which relies on a goal `root` deeper in the stack.
+            //
+            // We first store that we may have to rerun `evaluate_goal` for `root` in case the
+            // provisional response is not equal to the final response. We also update the depth
+            // of all goals which recursively depend on our current goal to depend on `root`
+            // instead.
+            //
+            // Finally we can return either the provisional response for that goal if we have a
+            // coinductive cycle or an ambiguous result if the cycle is inductive.
+            Entry::Occupied(entry) => {
+                // FIXME: `ProvisionalEntry` should be `Copy`.
+                let entry = entry.get().clone();
+                cache.stack[entry.depth].has_been_used = true;
+                for provisional_entry in cache.entries.values_mut() {
+                    provisional_entry.depth = provisional_entry.depth.min(entry.depth);
+                }
+
+                // NOTE: The goals on the stack aren't the only goals involved in this cycle.
+                // We can also depend on goals which aren't part of the stack but coinductively
+                // depend on the stack themselves. We already checked whether all the goals
+                // between these goals and their root on the stack. This means that as long as
+                // each goal in a cycle is checked for coinductivity by itself simply checking
+                // the stack is enough.
+                if cache.stack[entry.depth..]
+                    .iter()
+                    .all(|g| g.goal.value.predicate.is_coinductive(self.tcx))
+                {
+                    Err(entry.response)
+                } else {
+                    Err(fixme_response_maybe_no_constraints())
+                }
+            }
+        }
+    }
+
+    /// We cannot simply store the result of [EvalCtxt::compute_goal] as we have to deal with
+    /// coinductive cycles.
+    ///
+    /// When we encounter a coinductive cycle, we have to prove the final result of that cycle
+    /// while we are still computing that result. Because of this we continously recompute the
+    /// cycle until the result of the previous iteration is equal to the final result, at which
+    /// point we are done.
+    ///
+    /// This function returns `true` if we were able to finalize the goal and `false` if it has
+    /// updated the provisional cache and we have to recompute the current goal.
+    ///
+    /// FIXME: Refer to the rustc-dev-guide entry once it exists.
+    pub(super) fn try_finalize_goal(
+        &mut self,
+        actual_goal: CanonicalGoal<'tcx>,
+        response: QueryResult<'tcx>,
+    ) -> bool {
+        let cache = &mut self.provisional_cache;
+        let StackElem { goal, has_been_used } = cache.stack.pop().unwrap();
+        assert_eq!(goal, actual_goal);
+
+        let provisional_entry = cache.entries.get_mut(&goal).unwrap();
+        // Check whether the current stack entry is the root of a cycle.
+        //
+        // If so, we either move all participants of that cycle to the global cache
+        // or, in case the provisional response used in the cycle is not equal to the
+        // final response, have to recompute the goal after updating the provisional
+        // response to the final response of this iteration.
+        if has_been_used {
+            if provisional_entry.response == response {
+                // We simply drop all entries according to an immutable condition, so
+                // query instability is not a concern here.
+                #[allow(rustc::potential_query_instability)]
+                cache.entries.retain(|goal, entry| match entry.depth.cmp(&cache.stack.len()) {
+                    Ordering::Less => true,
+                    Ordering::Equal => {
+                        Self::try_move_finished_goal_to_global_cache(
+                            self.tcx,
+                            &mut self.overflow_data,
+                            &cache.stack,
+                            // FIXME: these should be `Copy` :(
+                            goal.clone(),
+                            entry.response.clone(),
+                        );
+                        false
+                    }
+                    Ordering::Greater => bug!("entry with greater depth than the current leaf"),
+                });
+
+                true
+            } else {
+                provisional_entry.response = response;
+                cache.stack.push(StackElem { goal, has_been_used: false });
+                false
+            }
+        } else {
+            Self::try_move_finished_goal_to_global_cache(
+                self.tcx,
+                &mut self.overflow_data,
+                &cache.stack,
+                goal,
+                response,
+            );
+            cache.entries.remove(&goal);
+            true
+        }
+    }
+
+    fn try_move_finished_goal_to_global_cache(
+        tcx: TyCtxt<'tcx>,
+        overflow_data: &mut OverflowData,
+        stack: &[StackElem<'tcx>],
+        goal: CanonicalGoal<'tcx>,
+        response: QueryResult<'tcx>,
+    ) {
+        // We move goals to the global cache if we either did not hit an overflow or if it's
+        // the root goal as that will now always hit the same overflow limit.
+        //
+        // NOTE: We cannot move any non-root goals to the global cache even if their final result
+        // isn't impacted by the overflow as that goal still has unstable query dependencies
+        // because it didn't go its full depth.
+        //
+        // FIXME(@lcnr): We could still cache subtrees which are not impacted by overflow though.
+        // Tracking that info correctly isn't trivial, so I haven't implemented it for now.
+        let should_cache_globally = !overflow_data.did_overflow() || stack.is_empty();
+        if should_cache_globally {
+            // FIXME: move the provisional entry to the global cache.
+            let _ = (tcx, goal, response);
+        }
+    }
+}
+
+fn fixme_response_yes_no_constraints<'tcx>() -> QueryResult<'tcx> {
+    unimplemented!()
+}
+
+fn fixme_response_maybe_no_constraints<'tcx>() -> QueryResult<'tcx> {
+    unimplemented!()
+}
diff --git a/compiler/rustc_trait_selection/src/solve/fulfill.rs b/compiler/rustc_trait_selection/src/solve/fulfill.rs
new file mode 100644
index 00000000000..80115d78d88
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/fulfill.rs
@@ -0,0 +1,92 @@
+use std::mem;
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_infer::{
+    infer::InferCtxt,
+    traits::{query::NoSolution, FulfillmentError, PredicateObligation, TraitEngine},
+};
+use rustc_middle::ty;
+
+use super::{Certainty, EvalCtxt};
+
+/// A trait engine using the new trait solver.
+///
+/// This is mostly identical to how `evaluate_all` works inside of the
+/// solver, except that the requirements are slightly different.
+///
+/// Unlike `evaluate_all` it is possible to add new obligations later on
+/// and we also have to track diagnostics information by using `Obligation`
+/// instead of `Goal`.
+///
+/// It is also likely that we want to use slightly different datastructures
+/// here as this will have to deal with far more root goals than `evaluate_all`.
+pub struct FulfillmentCtxt<'tcx> {
+    obligations: Vec<PredicateObligation<'tcx>>,
+}
+
+impl<'tcx> FulfillmentCtxt<'tcx> {
+    pub fn new() -> FulfillmentCtxt<'tcx> {
+        FulfillmentCtxt { obligations: Vec::new() }
+    }
+}
+
+impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
+    fn register_predicate_obligation(
+        &mut self,
+        _infcx: &InferCtxt<'tcx>,
+        obligation: PredicateObligation<'tcx>,
+    ) {
+        self.obligations.push(obligation);
+    }
+
+    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
+        let errors = self.select_where_possible(infcx);
+        if !errors.is_empty() {
+            return errors;
+        }
+
+        if self.obligations.is_empty() {
+            Vec::new()
+        } else {
+            unimplemented!("ambiguous obligations")
+        }
+    }
+
+    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
+        let errors = Vec::new();
+        for i in 0.. {
+            if !infcx.tcx.recursion_limit().value_within_limit(i) {
+                unimplemented!("overflow")
+            }
+
+            let mut has_changed = false;
+            for o in mem::take(&mut self.obligations) {
+                let mut cx = EvalCtxt::new(infcx.tcx);
+                let (changed, certainty) = match cx.evaluate_goal(infcx, o.clone().into()) {
+                    Ok(result) => result,
+                    Err(NoSolution) => unimplemented!("error"),
+                };
+
+                has_changed |= changed;
+                match certainty {
+                    Certainty::Yes => {}
+                    Certainty::Maybe(_) => self.obligations.push(o),
+                }
+            }
+
+            if !has_changed {
+                break;
+            }
+        }
+
+        errors
+    }
+
+    fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
+        self.obligations.clone()
+    }
+
+    fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
+        unimplemented!("Should be moved out of `TraitEngine`")
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/solve/infcx_ext.rs b/compiler/rustc_trait_selection/src/solve/infcx_ext.rs
new file mode 100644
index 00000000000..436f4eea662
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/infcx_ext.rs
@@ -0,0 +1,55 @@
+use rustc_infer::infer::canonical::CanonicalVarValues;
+use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use rustc_infer::infer::InferCtxt;
+use rustc_infer::traits::query::NoSolution;
+use rustc_middle::ty::Ty;
+use rustc_span::DUMMY_SP;
+
+use crate::solve::ExternalConstraints;
+
+use super::{Certainty, QueryResult, Response};
+
+/// Methods used inside of the canonical queries of the solver.
+pub(super) trait InferCtxtExt<'tcx> {
+    fn next_ty_infer(&self) -> Ty<'tcx>;
+
+    fn make_canonical_response(
+        &self,
+        var_values: CanonicalVarValues<'tcx>,
+        certainty: Certainty,
+    ) -> QueryResult<'tcx>;
+}
+
+impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
+    fn next_ty_infer(&self) -> Ty<'tcx> {
+        self.next_ty_var(TypeVariableOrigin {
+            kind: TypeVariableOriginKind::MiscVariable,
+            span: DUMMY_SP,
+        })
+    }
+
+    fn make_canonical_response(
+        &self,
+        var_values: CanonicalVarValues<'tcx>,
+        certainty: Certainty,
+    ) -> QueryResult<'tcx> {
+        let external_constraints = take_external_constraints(self)?;
+
+        Ok(self.canonicalize_response(Response { var_values, external_constraints, certainty }))
+    }
+}
+
+#[instrument(level = "debug", skip(infcx), ret)]
+fn take_external_constraints<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+) -> Result<ExternalConstraints<'tcx>, NoSolution> {
+    let region_obligations = infcx.take_registered_region_obligations();
+    let opaque_types = infcx.take_opaque_types_for_query_response();
+    Ok(ExternalConstraints {
+        // FIXME: Now that's definitely wrong :)
+        //
+        // Should also do the leak check here I think
+        regions: drop(region_obligations),
+        opaque_types,
+    })
+}
diff --git a/compiler/rustc_trait_selection/src/solve/mod.rs b/compiler/rustc_trait_selection/src/solve/mod.rs
new file mode 100644
index 00000000000..7f5e3208f4e
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/mod.rs
@@ -0,0 +1,309 @@
+//! The new trait solver, currently still WIP.
+//!
+//! As a user of the trait system, you can use `TyCtxt::evaluate_goal` to
+//! interact with this solver.
+//!
+//! For a high-level overview of how this solver works, check out the relevant
+//! section of the rustc-dev-guide.
+//!
+//! FIXME(@lcnr): Write that section. If you read this before then ask me
+//! about it on zulip.
+
+// FIXME: Instead of using `infcx.canonicalize_query` we have to add a new routine which
+// preserves universes and creates a unique var (in the highest universe) for each
+// appearance of a region.
+
+// FIXME: `CanonicalVarValues` should be interned and `Copy`.
+
+// FIXME: uses of `infcx.at` need to enable deferred projection equality once that's implemented.
+
+use std::mem;
+
+use rustc_infer::infer::canonical::OriginalQueryValues;
+use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
+use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::Obligation;
+use rustc_middle::infer::canonical::{Canonical, CanonicalVarValues};
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{RegionOutlivesPredicate, ToPredicate, TypeOutlivesPredicate};
+use rustc_span::DUMMY_SP;
+
+use self::infcx_ext::InferCtxtExt;
+
+mod assembly;
+mod cache;
+mod fulfill;
+mod infcx_ext;
+mod overflow;
+mod project_goals;
+mod trait_goals;
+
+pub use fulfill::FulfillmentCtxt;
+
+/// A goal is a statement, i.e. `predicate`, we want to prove
+/// given some assumptions, i.e. `param_env`.
+///
+/// Most of the time the `param_env` contains the `where`-bounds of the function
+/// we're currently typechecking while the `predicate` is some trait bound.
+#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
+pub struct Goal<'tcx, P> {
+    param_env: ty::ParamEnv<'tcx>,
+    predicate: P,
+}
+
+impl<'tcx, P> Goal<'tcx, P> {
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        predicate: impl ToPredicate<'tcx, P>,
+    ) -> Goal<'tcx, P> {
+        Goal { param_env, predicate: predicate.to_predicate(tcx) }
+    }
+
+    /// Updates the goal to one with a different `predicate` but the same `param_env`.
+    fn with<Q>(self, tcx: TyCtxt<'tcx>, predicate: impl ToPredicate<'tcx, Q>) -> Goal<'tcx, Q> {
+        Goal { param_env: self.param_env, predicate: predicate.to_predicate(tcx) }
+    }
+}
+
+impl<'tcx, P> From<Obligation<'tcx, P>> for Goal<'tcx, P> {
+    fn from(obligation: Obligation<'tcx, P>) -> Goal<'tcx, P> {
+        Goal { param_env: obligation.param_env, predicate: obligation.predicate }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable)]
+pub struct Response<'tcx> {
+    pub var_values: CanonicalVarValues<'tcx>,
+    /// Additional constraints returned by this query.
+    pub external_constraints: ExternalConstraints<'tcx>,
+    pub certainty: Certainty,
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
+pub enum Certainty {
+    Yes,
+    Maybe(MaybeCause),
+}
+
+impl Certainty {
+    /// When proving multiple goals using **AND**, e.g. nested obligations for an impl,
+    /// use this function to unify the certainty of these goals
+    pub fn unify_and(self, other: Certainty) -> Certainty {
+        match (self, other) {
+            (Certainty::Yes, Certainty::Yes) => Certainty::Yes,
+            (Certainty::Yes, Certainty::Maybe(_)) => other,
+            (Certainty::Maybe(_), Certainty::Yes) => self,
+            (Certainty::Maybe(MaybeCause::Overflow), Certainty::Maybe(MaybeCause::Overflow)) => {
+                Certainty::Maybe(MaybeCause::Overflow)
+            }
+            // If at least one of the goals is ambiguous, hide the overflow as the ambiguous goal
+            // may still result in failure.
+            (Certainty::Maybe(MaybeCause::Ambiguity), Certainty::Maybe(_))
+            | (Certainty::Maybe(_), Certainty::Maybe(MaybeCause::Ambiguity)) => {
+                Certainty::Maybe(MaybeCause::Ambiguity)
+            }
+        }
+    }
+}
+
+/// Why we failed to evaluate a goal.
+#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
+pub enum MaybeCause {
+    /// We failed due to ambiguity. This ambiguity can either
+    /// be a true ambiguity, i.e. there are multiple different answers,
+    /// or we hit a case where we just don't bother, e.g. `?x: Trait` goals.
+    Ambiguity,
+    /// We gave up due to an overflow, most often by hitting the recursion limit.
+    Overflow,
+}
+
+/// Additional constraints returned on success.
+#[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable)]
+pub struct ExternalConstraints<'tcx> {
+    // FIXME: implement this.
+    regions: (),
+    opaque_types: Vec<(Ty<'tcx>, Ty<'tcx>)>,
+}
+
+type CanonicalGoal<'tcx, T = ty::Predicate<'tcx>> = Canonical<'tcx, Goal<'tcx, T>>;
+type CanonicalResponse<'tcx> = Canonical<'tcx, Response<'tcx>>;
+/// The result of evaluating a canonical query.
+///
+/// FIXME: We use a different type than the existing canonical queries. This is because
+/// we need to add a `Certainty` for `overflow` and may want to restructure this code without
+/// having to worry about changes to currently used code. Once we've made progress on this
+/// solver, merge the two responses again.
+pub type QueryResult<'tcx> = Result<CanonicalResponse<'tcx>, NoSolution>;
+
+pub trait TyCtxtExt<'tcx> {
+    fn evaluate_goal(self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx>;
+}
+
+impl<'tcx> TyCtxtExt<'tcx> for TyCtxt<'tcx> {
+    fn evaluate_goal(self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx> {
+        let mut cx = EvalCtxt::new(self);
+        cx.evaluate_canonical_goal(goal)
+    }
+}
+
+struct EvalCtxt<'tcx> {
+    tcx: TyCtxt<'tcx>,
+
+    provisional_cache: cache::ProvisionalCache<'tcx>,
+    overflow_data: overflow::OverflowData,
+}
+
+impl<'tcx> EvalCtxt<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>) -> EvalCtxt<'tcx> {
+        EvalCtxt {
+            tcx,
+            provisional_cache: cache::ProvisionalCache::empty(),
+            overflow_data: overflow::OverflowData::new(tcx),
+        }
+    }
+
+    /// Recursively evaluates `goal`, returning whether any inference vars have
+    /// been constrained and the certainty of the result.
+    fn evaluate_goal(
+        &mut self,
+        infcx: &InferCtxt<'tcx>,
+        goal: Goal<'tcx, ty::Predicate<'tcx>>,
+    ) -> Result<(bool, Certainty), NoSolution> {
+        let mut orig_values = OriginalQueryValues::default();
+        let canonical_goal = infcx.canonicalize_query(goal, &mut orig_values);
+        let canonical_response = self.evaluate_canonical_goal(canonical_goal)?;
+        Ok((
+            true, // FIXME: check whether `var_values` are an identity substitution.
+            fixme_instantiate_canonical_query_response(infcx, &orig_values, canonical_response),
+        ))
+    }
+
+    fn evaluate_canonical_goal(&mut self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx> {
+        match self.try_push_stack(goal) {
+            Ok(()) => {}
+            // Our goal is already on the stack, eager return.
+            Err(response) => return response,
+        }
+
+        // We may have to repeatedly recompute the goal in case of coinductive cycles,
+        // check out the `cache` module for more information.
+        //
+        // FIXME: Similar to `evaluate_all`, this has to check for overflow.
+        loop {
+            let result = self.compute_goal(goal);
+
+            // FIXME: `Response` should be `Copy`
+            if self.try_finalize_goal(goal, result.clone()) {
+                return result;
+            }
+        }
+    }
+
+    fn compute_goal(&mut self, canonical_goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx> {
+        // WARNING: We're looking at a canonical value without instantiating it here.
+        //
+        // We have to be incredibly careful to not change the order of bound variables or
+        // remove any. As we go from `Goal<'tcx, Predicate>` to `Goal` with the variants
+        // of `PredicateKind` this is the case and it is and faster than instantiating and
+        // recanonicalizing.
+        let Goal { param_env, predicate } = canonical_goal.value;
+        if let Some(kind) = predicate.kind().no_bound_vars() {
+            match kind {
+                ty::PredicateKind::Clause(ty::Clause::Trait(predicate)) => self.compute_trait_goal(
+                    canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
+                ),
+                ty::PredicateKind::Clause(ty::Clause::Projection(predicate)) => self
+                    .compute_projection_goal(
+                        canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
+                    ),
+                ty::PredicateKind::Clause(ty::Clause::TypeOutlives(predicate)) => self
+                    .compute_type_outlives_goal(
+                        canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
+                    ),
+                ty::PredicateKind::Clause(ty::Clause::RegionOutlives(predicate)) => self
+                    .compute_region_outlives_goal(
+                        canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
+                    ),
+                // FIXME: implement these predicates :)
+                ty::PredicateKind::WellFormed(_)
+                | ty::PredicateKind::ObjectSafe(_)
+                | ty::PredicateKind::ClosureKind(_, _, _)
+                | ty::PredicateKind::Subtype(_)
+                | ty::PredicateKind::Coerce(_)
+                | ty::PredicateKind::ConstEvaluatable(_)
+                | ty::PredicateKind::ConstEquate(_, _)
+                | ty::PredicateKind::TypeWellFormedFromEnv(_)
+                | ty::PredicateKind::Ambiguous => unimplemented!(),
+            }
+        } else {
+            let (infcx, goal, var_values) =
+                self.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
+            let kind = infcx.replace_bound_vars_with_placeholders(goal.predicate.kind());
+            let goal = goal.with(self.tcx, ty::Binder::dummy(kind));
+            let (_, certainty) = self.evaluate_goal(&infcx, goal)?;
+            infcx.make_canonical_response(var_values, certainty)
+        }
+    }
+
+    fn compute_type_outlives_goal(
+        &mut self,
+        _goal: CanonicalGoal<'tcx, TypeOutlivesPredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        todo!()
+    }
+
+    fn compute_region_outlives_goal(
+        &mut self,
+        _goal: CanonicalGoal<'tcx, RegionOutlivesPredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        todo!()
+    }
+}
+
+impl<'tcx> EvalCtxt<'tcx> {
+    fn evaluate_all(
+        &mut self,
+        infcx: &InferCtxt<'tcx>,
+        mut goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
+    ) -> Result<Certainty, NoSolution> {
+        let mut new_goals = Vec::new();
+        self.repeat_while_none(|this| {
+            let mut has_changed = Err(Certainty::Yes);
+            for goal in goals.drain(..) {
+                let (changed, certainty) = match this.evaluate_goal(infcx, goal) {
+                    Ok(result) => result,
+                    Err(NoSolution) => return Some(Err(NoSolution)),
+                };
+
+                if changed {
+                    has_changed = Ok(());
+                }
+
+                match certainty {
+                    Certainty::Yes => {}
+                    Certainty::Maybe(_) => {
+                        new_goals.push(goal);
+                        has_changed = has_changed.map_err(|c| c.unify_and(certainty));
+                    }
+                }
+            }
+
+            match has_changed {
+                Ok(()) => {
+                    mem::swap(&mut new_goals, &mut goals);
+                    None
+                }
+                Err(certainty) => Some(Ok(certainty)),
+            }
+        })
+    }
+}
+
+fn fixme_instantiate_canonical_query_response<'tcx>(
+    _: &InferCtxt<'tcx>,
+    _: &OriginalQueryValues<'tcx>,
+    _: CanonicalResponse<'tcx>,
+) -> Certainty {
+    unimplemented!()
+}
diff --git a/compiler/rustc_trait_selection/src/solve/overflow.rs b/compiler/rustc_trait_selection/src/solve/overflow.rs
new file mode 100644
index 00000000000..8d73a83aec9
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/overflow.rs
@@ -0,0 +1,80 @@
+use rustc_infer::traits::query::NoSolution;
+use rustc_middle::ty::TyCtxt;
+use rustc_session::Limit;
+
+use super::{Certainty, EvalCtxt, MaybeCause, QueryResult};
+
+/// When detecting a solver overflow, we return ambiguity. Overflow can be
+/// *hidden* by either a fatal error in an **AND** or a trivial success in an **OR**.
+///
+/// This is in issue in case of exponential blowup, e.g. if each goal on the stack
+/// has multiple nested (overflowing) candidates. To deal with this, we reduce the limit
+/// used by the solver when hitting the default limit for the first time.
+///
+/// FIXME: Get tests where always using the `default_limit` results in a hang and refer
+/// to them here. We can also improve the overflow strategy if necessary.
+pub(super) struct OverflowData {
+    default_limit: Limit,
+    current_limit: Limit,
+    /// When proving an **AND** we have to repeatedly iterate over the yet unproven goals.
+    ///
+    /// Because of this each iteration also increases the depth in addition to the stack
+    /// depth.
+    additional_depth: usize,
+}
+
+impl OverflowData {
+    pub(super) fn new(tcx: TyCtxt<'_>) -> OverflowData {
+        let default_limit = tcx.recursion_limit();
+        OverflowData { default_limit, current_limit: default_limit, additional_depth: 0 }
+    }
+
+    #[inline]
+    pub(super) fn did_overflow(&self) -> bool {
+        self.default_limit.0 != self.current_limit.0
+    }
+
+    #[inline]
+    pub(super) fn has_overflow(&self, depth: usize) -> bool {
+        self.current_limit.value_within_limit(depth + self.additional_depth)
+    }
+
+    /// Updating the current limit when hitting overflow.
+    fn deal_with_overflow(&mut self) {
+        // When first hitting overflow we reduce the overflow limit
+        // for all future goals to prevent hangs if there's an exponental
+        // blowup.
+        self.current_limit.0 = self.default_limit.0 / 8;
+    }
+}
+
+impl<'tcx> EvalCtxt<'tcx> {
+    pub(super) fn deal_with_overflow(&mut self) -> QueryResult<'tcx> {
+        self.overflow_data.deal_with_overflow();
+        fixme_response_overflow_no_constraints()
+    }
+
+    /// A `while`-loop which tracks overflow.
+    pub(super) fn repeat_while_none(
+        &mut self,
+        mut loop_body: impl FnMut(&mut Self) -> Option<Result<Certainty, NoSolution>>,
+    ) -> Result<Certainty, NoSolution> {
+        let start_depth = self.overflow_data.additional_depth;
+        let depth = self.provisional_cache.current_depth();
+        while !self.overflow_data.has_overflow(depth) {
+            if let Some(result) = loop_body(self) {
+                self.overflow_data.additional_depth = start_depth;
+                return result;
+            }
+
+            self.overflow_data.additional_depth += 1;
+        }
+        self.overflow_data.additional_depth = start_depth;
+        self.overflow_data.deal_with_overflow();
+        Ok(Certainty::Maybe(MaybeCause::Overflow))
+    }
+}
+
+fn fixme_response_overflow_no_constraints<'tcx>() -> QueryResult<'tcx> {
+    unimplemented!()
+}
diff --git a/compiler/rustc_trait_selection/src/solve/project_goals.rs b/compiler/rustc_trait_selection/src/solve/project_goals.rs
new file mode 100644
index 00000000000..b50f42c4d94
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/project_goals.rs
@@ -0,0 +1,244 @@
+use crate::traits::{specialization_graph, translate_substs};
+
+use super::assembly::{self, AssemblyCtxt};
+use super::{CanonicalGoal, EvalCtxt, Goal, QueryResult};
+use rustc_errors::ErrorGuaranteed;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::DefId;
+use rustc_infer::infer::{InferCtxt, InferOk};
+use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::specialization_graph::LeafDef;
+use rustc_infer::traits::{ObligationCause, Reveal};
+use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
+use rustc_middle::ty::ProjectionPredicate;
+use rustc_middle::ty::TypeVisitable;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::DUMMY_SP;
+use std::iter;
+
+#[allow(dead_code)] // FIXME: implement and use all variants.
+#[derive(Debug, Clone, Copy)]
+pub(super) enum CandidateSource {
+    Impl(DefId),
+    ParamEnv(usize),
+    Builtin,
+}
+
+type Candidate<'tcx> = assembly::Candidate<'tcx, ProjectionPredicate<'tcx>>;
+
+impl<'tcx> EvalCtxt<'tcx> {
+    pub(super) fn compute_projection_goal(
+        &mut self,
+        goal: CanonicalGoal<'tcx, ProjectionPredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        let candidates = AssemblyCtxt::assemble_and_evaluate_candidates(self, goal);
+        self.merge_project_candidates(candidates)
+    }
+
+    fn merge_project_candidates(
+        &mut self,
+        mut candidates: Vec<Candidate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        match candidates.len() {
+            0 => return Err(NoSolution),
+            1 => return Ok(candidates.pop().unwrap().result),
+            _ => {}
+        }
+
+        if candidates.len() > 1 {
+            let mut i = 0;
+            'outer: while i < candidates.len() {
+                for j in (0..candidates.len()).filter(|&j| i != j) {
+                    if self.project_candidate_should_be_dropped_in_favor_of(
+                        &candidates[i],
+                        &candidates[j],
+                    ) {
+                        debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
+                        candidates.swap_remove(i);
+                        continue 'outer;
+                    }
+                }
+
+                debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
+                // If there are *STILL* multiple candidates, give up
+                // and report ambiguity.
+                i += 1;
+                if i > 1 {
+                    debug!("multiple matches, ambig");
+                    // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
+                    unimplemented!();
+                }
+            }
+        }
+
+        Ok(candidates.pop().unwrap().result)
+    }
+
+    fn project_candidate_should_be_dropped_in_favor_of(
+        &self,
+        candidate: &Candidate<'tcx>,
+        other: &Candidate<'tcx>,
+    ) -> bool {
+        // FIXME: implement this
+        match (candidate.source, other.source) {
+            (CandidateSource::Impl(_), _)
+            | (CandidateSource::ParamEnv(_), _)
+            | (CandidateSource::Builtin, _) => unimplemented!(),
+        }
+    }
+}
+
+impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
+    type CandidateSource = CandidateSource;
+
+    fn self_ty(self) -> Ty<'tcx> {
+        self.self_ty()
+    }
+
+    fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
+        self.with_self_ty(tcx, self_ty)
+    }
+
+    fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
+        self.trait_def_id(tcx)
+    }
+
+    fn consider_impl_candidate(
+        acx: &mut AssemblyCtxt<'_, 'tcx, ProjectionPredicate<'tcx>>,
+        goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
+        impl_def_id: DefId,
+    ) {
+        let tcx = acx.cx.tcx;
+        let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
+        let impl_trait_ref = tcx.bound_impl_trait_ref(impl_def_id).unwrap();
+        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
+        if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
+            .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
+        {
+            return;
+        }
+
+        acx.infcx.probe(|_| {
+            let impl_substs = acx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
+            let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
+
+            let Ok(InferOk { obligations, .. }) = acx
+                .infcx
+                .at(&ObligationCause::dummy(), goal.param_env)
+                .define_opaque_types(false)
+                .eq(goal_trait_ref, impl_trait_ref)
+                .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
+            else {
+                return
+            };
+
+            let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
+            let Ok(trait_ref_certainty) = acx.cx.evaluate_all(acx.infcx, nested_goals) else { return };
+
+            let Some(assoc_def) = fetch_eligible_assoc_item_def(
+                acx.infcx,
+                goal.param_env,
+                goal_trait_ref,
+                goal.predicate.def_id(),
+                impl_def_id
+            ) else {
+                return
+            };
+
+            if !assoc_def.item.defaultness(tcx).has_value() {
+                tcx.sess.delay_span_bug(
+                    tcx.def_span(assoc_def.item.def_id),
+                    "missing value for assoc item in impl",
+                );
+            }
+
+            // Getting the right substitutions here is complex, e.g. given:
+            // - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
+            // - the applicable impl `impl<T> Trait<i32> for Vec<T>`
+            // - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
+            //
+            // We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
+            // to `[u32, u64]`.
+            //
+            // And then map these substs to the substs of the defining impl of `Assoc`, going
+            // from `[u32, u64]` to `[u32, i32, u64]`.
+            let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
+                tcx,
+                goal_trait_ref.def_id,
+                impl_trait_ref.substs,
+            );
+            let substs = translate_substs(
+                acx.infcx,
+                goal.param_env,
+                impl_def_id,
+                impl_substs_with_gat,
+                assoc_def.defining_node,
+            );
+
+            // Finally we construct the actual value of the associated type.
+            let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
+            let ty = tcx.bound_type_of(assoc_def.item.def_id);
+            let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
+                let identity_substs = ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
+                let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
+                let kind =
+                    ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
+                ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
+            } else {
+                ty.map_bound(|ty| ty.into())
+            };
+
+            let Ok(InferOk { obligations, .. }) = acx
+                .infcx
+                .at(&ObligationCause::dummy(), goal.param_env)
+                .define_opaque_types(false)
+                .eq(goal.predicate.term,  term.subst(tcx, substs))
+                .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
+            else {
+                return
+            };
+
+            let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
+            let Ok(rhs_certainty) = acx.cx.evaluate_all(acx.infcx, nested_goals) else { return };
+
+            let certainty = trait_ref_certainty.unify_and(rhs_certainty);
+            acx.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
+        })
+    }
+}
+
+/// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
+///
+/// FIXME: We should merge these 3 implementations as it's likely that they otherwise
+/// diverge.
+#[instrument(level = "debug", skip(infcx, param_env), ret)]
+fn fetch_eligible_assoc_item_def<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    goal_trait_ref: ty::TraitRef<'tcx>,
+    trait_assoc_def_id: DefId,
+    impl_def_id: DefId,
+) -> Option<LeafDef> {
+    let node_item = specialization_graph::assoc_def(infcx.tcx, impl_def_id, trait_assoc_def_id)
+        .map_err(|ErrorGuaranteed { .. }| ())
+        .ok()?;
+
+    let eligible = if node_item.is_final() {
+        // Non-specializable items are always projectable.
+        true
+    } else {
+        // Only reveal a specializable default if we're past type-checking
+        // and the obligation is monomorphic, otherwise passes such as
+        // transmute checking and polymorphic MIR optimizations could
+        // get a result which isn't correct for all monomorphizations.
+        if param_env.reveal() == Reveal::All {
+            let poly_trait_ref = infcx.resolve_vars_if_possible(goal_trait_ref);
+            !poly_trait_ref.still_further_specializable()
+        } else {
+            debug!(?node_item.item.def_id, "not eligible due to default");
+            false
+        }
+    };
+
+    if eligible { Some(node_item) } else { None }
+}
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
new file mode 100644
index 00000000000..10b45a77dab
--- /dev/null
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -0,0 +1,180 @@
+//! Dealing with trait goals, i.e. `T: Trait<'a, U>`.
+
+use std::iter;
+
+use super::assembly::{self, AssemblyCtxt};
+use super::{CanonicalGoal, EvalCtxt, Goal, QueryResult};
+use rustc_hir::def_id::DefId;
+use rustc_infer::infer::InferOk;
+use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::ObligationCause;
+use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
+use rustc_middle::ty::TraitPredicate;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::DUMMY_SP;
+
+#[allow(dead_code)] // FIXME: implement and use all variants.
+#[derive(Debug, Clone, Copy)]
+pub(super) enum CandidateSource {
+    /// Some user-defined impl with the given `DefId`.
+    Impl(DefId),
+    /// The n-th caller bound in the `param_env` of our goal.
+    ///
+    /// This is pretty much always a bound from the `where`-clauses of the
+    /// currently checked item.
+    ParamEnv(usize),
+    /// A bound on the `self_ty` in case it is a projection or an opaque type.
+    ///
+    /// # Examples
+    ///
+    /// ```ignore (for syntax highlighting)
+    /// trait Trait {
+    ///     type Assoc: OtherTrait;
+    /// }
+    /// ```
+    ///
+    /// We know that `<Whatever as Trait>::Assoc: OtherTrait` holds by looking at
+    /// the bounds on `Trait::Assoc`.
+    AliasBound(usize),
+    /// A builtin implementation for some specific traits, used in cases
+    /// where we cannot rely an ordinary library implementations.
+    ///
+    /// The most notable examples are `Sized`, `Copy` and `Clone`. This is also
+    /// used for the `DiscriminantKind` and `Pointee` trait, both of which have
+    /// an associated type.
+    Builtin,
+    /// An automatic impl for an auto trait, e.g. `Send`. These impls recursively look
+    /// at the constituent types of the `self_ty` to check whether the auto trait
+    /// is implemented for those.
+    AutoImpl,
+}
+
+type Candidate<'tcx> = assembly::Candidate<'tcx, TraitPredicate<'tcx>>;
+
+impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
+    type CandidateSource = CandidateSource;
+
+    fn self_ty(self) -> Ty<'tcx> {
+        self.self_ty()
+    }
+
+    fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
+        self.with_self_ty(tcx, self_ty)
+    }
+
+    fn trait_def_id(self, _: TyCtxt<'tcx>) -> DefId {
+        self.def_id()
+    }
+
+    fn consider_impl_candidate(
+        acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
+        goal: Goal<'tcx, TraitPredicate<'tcx>>,
+        impl_def_id: DefId,
+    ) {
+        let impl_trait_ref = acx.cx.tcx.bound_impl_trait_ref(impl_def_id).unwrap();
+        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
+        if iter::zip(goal.predicate.trait_ref.substs, impl_trait_ref.skip_binder().substs)
+            .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
+        {
+            return;
+        }
+
+        acx.infcx.probe(|_| {
+            let impl_substs = acx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
+            let impl_trait_ref = impl_trait_ref.subst(acx.cx.tcx, impl_substs);
+
+            let Ok(InferOk { obligations, .. }) = acx
+                .infcx
+                .at(&ObligationCause::dummy(), goal.param_env)
+                .define_opaque_types(false)
+                .eq(goal.predicate.trait_ref, impl_trait_ref)
+                .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
+            else {
+                return
+            };
+
+            let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
+
+            let Ok(certainty) = acx.cx.evaluate_all(acx.infcx, nested_goals) else { return };
+            acx.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
+        })
+    }
+}
+
+impl<'tcx> EvalCtxt<'tcx> {
+    pub(super) fn compute_trait_goal(
+        &mut self,
+        goal: CanonicalGoal<'tcx, TraitPredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        let candidates = AssemblyCtxt::assemble_and_evaluate_candidates(self, goal);
+        self.merge_trait_candidates_discard_reservation_impls(candidates)
+    }
+
+    #[instrument(level = "debug", skip(self), ret)]
+    pub(super) fn merge_trait_candidates_discard_reservation_impls(
+        &mut self,
+        mut candidates: Vec<Candidate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        match candidates.len() {
+            0 => return Err(NoSolution),
+            1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
+            _ => {}
+        }
+
+        if candidates.len() > 1 {
+            let mut i = 0;
+            'outer: while i < candidates.len() {
+                for j in (0..candidates.len()).filter(|&j| i != j) {
+                    if self.trait_candidate_should_be_dropped_in_favor_of(
+                        &candidates[i],
+                        &candidates[j],
+                    ) {
+                        debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
+                        candidates.swap_remove(i);
+                        continue 'outer;
+                    }
+                }
+
+                debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
+                // If there are *STILL* multiple candidates, give up
+                // and report ambiguity.
+                i += 1;
+                if i > 1 {
+                    debug!("multiple matches, ambig");
+                    // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
+                    unimplemented!();
+                }
+            }
+        }
+
+        Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
+    }
+
+    fn trait_candidate_should_be_dropped_in_favor_of(
+        &self,
+        candidate: &Candidate<'tcx>,
+        other: &Candidate<'tcx>,
+    ) -> bool {
+        // FIXME: implement this
+        match (candidate.source, other.source) {
+            (CandidateSource::Impl(_), _)
+            | (CandidateSource::ParamEnv(_), _)
+            | (CandidateSource::AliasBound(_), _)
+            | (CandidateSource::Builtin, _)
+            | (CandidateSource::AutoImpl, _) => unimplemented!(),
+        }
+    }
+
+    fn discard_reservation_impl(&self, candidate: Candidate<'tcx>) -> Candidate<'tcx> {
+        if let CandidateSource::Impl(def_id) = candidate.source {
+            if let ty::ImplPolarity::Reservation = self.tcx.impl_polarity(def_id) {
+                debug!("Selected reservation impl");
+                // FIXME: reduce candidate to ambiguous
+                // FIXME: replace `var_values` with identity, yeet external constraints.
+                unimplemented!()
+            }
+        }
+
+        candidate
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index aef2f8ff991..948632ccc6c 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -159,13 +159,12 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             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)
+            .evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds)
             .unwrap_or_else(|| {
                 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
             });
@@ -247,7 +246,6 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         user_env: ty::ParamEnv<'tcx>,
         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
-        only_projections: bool,
     ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
         let tcx = infcx.tcx;
 
@@ -322,7 +320,6 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                         fresh_preds,
                         &mut predicates,
                         &mut select,
-                        only_projections,
                     ) {
                         return None;
                     }
@@ -600,7 +597,6 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
         predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
         selcx: &mut SelectionContext<'_, 'tcx>,
-        only_projections: bool,
     ) -> bool {
         let dummy_cause = ObligationCause::dummy();
 
@@ -744,7 +740,6 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                                     fresh_preds,
                                     predicates,
                                     selcx,
-                                    only_projections,
                                 ) {
                                     return false;
                                 }
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 7c569621cfe..26757965c95 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -66,13 +66,13 @@ pub fn add_placeholder_note(err: &mut Diagnostic) {
 /// with a suitably-freshened `ImplHeader` with those types
 /// substituted. Otherwise, returns `None`.
 #[instrument(skip(tcx, skip_leak_check), level = "debug")]
-pub fn overlapping_impls<'tcx>(
-    tcx: TyCtxt<'tcx>,
+pub fn overlapping_impls(
+    tcx: TyCtxt<'_>,
     impl1_def_id: DefId,
     impl2_def_id: DefId,
     skip_leak_check: SkipLeakCheck,
     overlap_mode: OverlapMode,
-) -> Option<OverlapResult<'tcx>> {
+) -> 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.
@@ -283,7 +283,7 @@ fn implicit_negative<'cx, 'tcx>(
 
 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
 /// where-clauses) If so, return true, they are disjoint and false otherwise.
-fn negative_impl<'tcx>(tcx: TyCtxt<'tcx>, impl1_def_id: DefId, impl2_def_id: DefId) -> bool {
+fn negative_impl(tcx: TyCtxt<'_>, impl1_def_id: DefId, impl2_def_id: DefId) -> bool {
     debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
 
     // Create an infcx, taking the predicates of impl1 as assumptions:
diff --git a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
index 7c9fde27420..f8efe9bfa9f 100644
--- a/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
+++ b/compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
@@ -138,10 +138,10 @@ pub fn is_const_evaluatable<'tcx>(
                 } 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?"),
-                    );
+                    let guar = infcx
+                        .tcx
+                        .sess
+                        .delay_span_bug(span, "Missing value for constant, but no error reported?");
                     NotConstEvaluatable::Error(guar)
                 };
 
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
index cb373d65772..27c207528c7 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
@@ -14,21 +14,27 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
     fn tag(&self) -> &'static str {
         "CollectAllMismatches"
     }
+
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.infcx.tcx
     }
+
     fn intercrate(&self) -> bool {
         false
     }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
+
     fn a_is_expected(&self) -> bool {
         true
-    } // irrelevant
+    }
+
     fn mark_ambiguous(&mut self) {
         bug!()
     }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _: ty::Variance,
@@ -38,6 +44,7 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
     ) -> RelateResult<'tcx, T> {
         self.relate(a, b)
     }
+
     fn regions(
         &mut self,
         a: ty::Region<'tcx>,
@@ -45,15 +52,20 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
         Ok(a)
     }
+
     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        if a == b || matches!(a.kind(), ty::Infer(_)) || matches!(b.kind(), ty::Infer(_)) {
-            return Ok(a);
-        }
-        relate::super_relate_tys(self, a, b).or_else(|e| {
-            self.errors.push(e);
-            Ok(a)
+        self.infcx.probe(|_| {
+            if a.is_ty_infer() || b.is_ty_infer() {
+                Ok(a)
+            } else {
+                self.infcx.super_combine_tys(self, a, b).or_else(|e| {
+                    self.errors.push(e);
+                    Ok(a)
+                })
+            }
         })
     }
+
     fn consts(
         &mut self,
         a: ty::Const<'tcx>,
@@ -64,6 +76,7 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
         }
         relate::super_relate_consts(self, a, b) // could do something similar here for constants!
     }
+
     fn binders<T: Relate<'tcx>>(
         &mut self,
         a: ty::Binder<'tcx, T>,
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 2dd2c568bab..8f317beaa77 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -226,7 +226,7 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
             let arg_length = arguments.len();
             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
             match (arg_length, arguments.get(0)) {
-                (1, Some(&ArgKind::Tuple(_, ref fields))) => {
+                (1, Some(ArgKind::Tuple(_, fields))) => {
                     format!("a single {}-tuple as argument", fields.len())
                 }
                 _ => format!(
@@ -1574,7 +1574,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     &error.obligation.cause,
                     expected_found.expected,
                     expected_found.found,
-                    err.clone(),
+                    *err,
                 )
                 .emit();
             }
@@ -1583,7 +1583,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     &error.obligation.cause,
                     expected_found.expected,
                     expected_found.found,
-                    err.clone(),
+                    *err,
                 );
                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
                 if let ObligationCauseCode::BindingObligation(..)
@@ -1735,8 +1735,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 values.map(|(_, is_normalized_ty_expected, normalized_ty, expected_ty)| {
                     infer::ValuePairs::Terms(ExpectedFound::new(
                         is_normalized_ty_expected,
-                        normalized_ty.into(),
-                        expected_ty.into(),
+                        normalized_ty,
+                        expected_ty,
                     ))
                 }),
                 err,
@@ -2332,9 +2332,9 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 // 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!(">")));
+                                suggestions.push((generic_arg.span_ext.shrink_to_hi(), ">".to_string()));
                             } else {
-                                suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
+                                suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), ">".to_string()));
                             }
                             err.multipart_suggestion(
                                 message,
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 d47a5ea3e37..9656bfbf4ec 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -335,7 +335,7 @@ pub trait TypeErrCtxtExt<'tcx> {
         err: &mut Diagnostic,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     );
-    fn function_argument_obligation(
+    fn note_function_argument_obligation(
         &self,
         arg_hir_id: HirId,
         err: &mut Diagnostic,
@@ -1789,7 +1789,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         self.note_conflicting_closure_bounds(cause, &mut err);
 
         if let Some(found_node) = found_node {
-            hint_missing_borrow(span, found_span, found, expected, found_node, &mut err);
+            hint_missing_borrow(span, found, expected, found_node, &mut err);
         }
 
         err
@@ -2344,28 +2344,33 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
             GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
-                // `Some(ref_ty)` if `target_ty` is `&T` and `T` fails to impl `Sync`
-                let refers_to_non_sync = match target_ty.kind() {
-                    ty::Ref(_, ref_ty, _) => match self.evaluate_obligation(&obligation) {
-                        Ok(eval) if !eval.may_apply() => Some(ref_ty),
+                // `Some((ref_ty, is_mut))` if `target_ty` is `&T` or `&mut T` and fails to impl `Send`
+                let non_send = match target_ty.kind() {
+                    ty::Ref(_, ref_ty, mutability) => match self.evaluate_obligation(&obligation) {
+                        Ok(eval) if !eval.may_apply() => Some((ref_ty, mutability.is_mut())),
                         _ => None,
                     },
                     _ => None,
                 };
 
-                let (span_label, span_note) = match refers_to_non_sync {
-                    // if `target_ty` is `&T` and `T` fails to impl `Sync`,
-                    // include suggestions to make `T: Sync` so that `&T: Send`
-                    Some(ref_ty) => (
-                        format!(
-                            "has type `{}` which {}, because `{}` is not `Sync`",
-                            target_ty, trait_explanation, ref_ty
-                        ),
-                        format!(
-                            "captured value {} because `&` references cannot be sent unless their referent is `Sync`",
-                            trait_explanation
-                        ),
-                    ),
+                let (span_label, span_note) = match non_send {
+                    // if `target_ty` is `&T` or `&mut T` and fails to impl `Send`,
+                    // include suggestions to make `T: Sync` so that `&T: Send`,
+                    // or to make `T: Send` so that `&mut T: Send`
+                    Some((ref_ty, is_mut)) => {
+                        let ref_ty_trait = if is_mut { "Send" } else { "Sync" };
+                        let ref_kind = if is_mut { "&mut" } else { "&" };
+                        (
+                            format!(
+                                "has type `{}` which {}, because `{}` is not `{}`",
+                                target_ty, trait_explanation, ref_ty, ref_ty_trait
+                            ),
+                            format!(
+                                "captured value {} because `{}` references cannot be sent unless their referent is `{}`",
+                                trait_explanation, ref_kind, ref_ty_trait
+                            ),
+                        )
+                    }
                     None => (
                         format!("has type `{}` which {}", target_ty, trait_explanation),
                         format!("captured value {}", trait_explanation),
@@ -2740,7 +2745,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             }
                             ty::Closure(def_id, _) => err.span_note(
                                 self.tcx.def_span(def_id),
-                                &format!("required because it's used within this closure"),
+                                "required because it's used within this closure",
                             ),
                             _ => err.note(&msg),
                         };
@@ -2904,7 +2909,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 ref parent_code,
                 ..
             } => {
-                self.function_argument_obligation(
+                self.note_function_argument_obligation(
                     arg_hir_id,
                     err,
                     parent_code,
@@ -3136,23 +3141,20 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             );
         }
     }
-    fn function_argument_obligation(
+    fn note_function_argument_obligation(
         &self,
         arg_hir_id: HirId,
         err: &mut Diagnostic,
         parent_code: &ObligationCauseCode<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
-        predicate: ty::Predicate<'tcx>,
+        failed_pred: ty::Predicate<'tcx>,
         call_hir_id: HirId,
     ) {
         let tcx = self.tcx;
         let hir = tcx.hir();
-        if let Some(Node::Expr(expr)) = hir.find(arg_hir_id) {
-            let parent_id = hir.get_parent_item(arg_hir_id);
-            let typeck_results: &TypeckResults<'tcx> = match &self.typeck_results {
-                Some(t) if t.hir_owner == parent_id => t,
-                _ => self.tcx.typeck(parent_id.def_id),
-            };
+        if let Some(Node::Expr(expr)) = hir.find(arg_hir_id)
+            && let Some(typeck_results) = &self.typeck_results
+        {
             if let hir::Expr { kind: hir::ExprKind::Block(..), .. } = expr {
                 let expr = expr.peel_blocks();
                 let ty = typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error());
@@ -3177,37 +3179,29 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             let mut type_diffs = vec![];
 
             if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = parent_code.deref()
-                && let predicates = self.tcx.predicates_of(def_id).instantiate_identity(self.tcx)
-                && let Some(pred) = predicates.predicates.get(*idx)
+                && let Some(node_substs) = typeck_results.node_substs_opt(call_hir_id)
+                && let where_clauses = self.tcx.predicates_of(def_id).instantiate(self.tcx, node_substs)
+                && let Some(where_pred) = where_clauses.predicates.get(*idx)
             {
-                if let Ok(trait_pred) = pred.kind().try_map_bound(|pred| match pred {
-                    ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) => Ok(trait_pred),
-                    _ => Err(()),
-                })
-                    && let Ok(trait_predicate) = predicate.kind().try_map_bound(|pred| match pred {
-                        ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) => Ok(trait_pred),
-                        _ => Err(()),
-                    })
+                if let Some(where_pred) = where_pred.to_opt_poly_trait_pred()
+                    && let Some(failed_pred) = failed_pred.to_opt_poly_trait_pred()
                 {
                     let mut c = CollectAllMismatches {
                         infcx: self.infcx,
                         param_env,
                         errors: vec![],
                     };
-                    if let Ok(_) = c.relate(trait_pred, trait_predicate) {
+                    if let Ok(_) = c.relate(where_pred, failed_pred) {
                         type_diffs = c.errors;
                     }
-                } else if let ty::PredicateKind::Clause(
-                    ty::Clause::Projection(proj)
-                ) = pred.kind().skip_binder()
-                    && let ty::PredicateKind::Clause(
-                        ty::Clause::Projection(projection)
-                    ) = predicate.kind().skip_binder()
+                } else if let Some(where_pred) = where_pred.to_opt_poly_projection_pred()
+                    && let Some(failed_pred) = failed_pred.to_opt_poly_projection_pred()
+                    && let Some(found) = failed_pred.skip_binder().term.ty()
                 {
                     type_diffs = vec![
                         Sorts(ty::error::ExpectedFound {
-                            expected: self.tcx.mk_ty(ty::Alias(ty::Projection, proj.projection_ty)),
-                            found: projection.term.ty().unwrap(),
+                            expected: self.tcx.mk_ty(ty::Alias(ty::Projection, where_pred.skip_binder().projection_ty)),
+                            found,
                         }),
                     ];
                 }
@@ -3222,9 +3216,9 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 // If the expression we're calling on is a binding, we want to point at the
                 // `let` when talking about the type. Otherwise we'll point at every part
                 // of the method chain with the type.
-                self.point_at_chain(binding_expr, typeck_results, type_diffs, param_env, err);
+                self.point_at_chain(binding_expr, &typeck_results, type_diffs, param_env, err);
             } else {
-                self.point_at_chain(expr, typeck_results, type_diffs, param_env, err);
+                self.point_at_chain(expr, &typeck_results, type_diffs, param_env, err);
             }
         }
         let call_node = hir.find(call_hir_id);
@@ -3386,7 +3380,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
             err.span_note(
                 multi_span,
-                format!("the method call chain might not have had the expected associated types"),
+                "the method call chain might not have had the expected associated types",
             );
         }
     }
@@ -3455,7 +3449,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 /// Add a hint to add a missing borrow or remove an unnecessary one.
 fn hint_missing_borrow<'tcx>(
     span: Span,
-    found_span: Span,
     found: Ty<'tcx>,
     expected: Ty<'tcx>,
     found_node: Node<'_>,
@@ -3474,13 +3467,12 @@ fn hint_missing_borrow<'tcx>(
         }
     };
 
-    let fn_decl = found_node
-        .fn_decl()
-        .unwrap_or_else(|| span_bug!(found_span, "found node must be a function"));
+    // This could be a variant constructor, for example.
+    let Some(fn_decl) = found_node.fn_decl() else { return; };
 
     let arg_spans = fn_decl.inputs.iter().map(|ty| ty.span);
 
-    fn get_deref_type_and_refs<'tcx>(mut ty: Ty<'tcx>) -> (Ty<'tcx>, usize) {
+    fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
         let mut refs = 0;
 
         while let ty::Ref(_, new_ty, _) = ty.kind() {
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 2566d793d78..c30531fa906 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -484,10 +484,7 @@ fn subst_and_check_impossible_predicates<'tcx>(
 ///
 /// 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 {
+fn is_impossible_method(tcx: TyCtxt<'_>, (impl_def_id, trait_item_def_id): (DefId, DefId)) -> bool {
     struct ReferencesOnlyParentGenerics<'tcx> {
         tcx: TyCtxt<'tcx>,
         generics: &'tcx ty::Generics,
diff --git a/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
index e1092a788e3..f2c5f730b31 100644
--- a/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
+++ b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
@@ -34,7 +34,7 @@ impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
     /// argument types are well-formed. This may imply certain relationships
     /// between generic parameters. For example:
     /// ```
-    /// fn foo<'a,T>(x: &'a T) {}
+    /// fn foo<T>(x: &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`.
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 84d7244c1db..5276da2e49c 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -25,7 +25,6 @@ use rustc_data_structures::sso::SsoHashSet;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def::DefKind;
-use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::at::At;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
@@ -1553,7 +1552,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                 // NOTE: This should be kept in sync with the similar code in
                 // `rustc_ty_utils::instance::resolve_associated_item()`.
                 let node_item =
-                    assoc_def(selcx, impl_data.impl_def_id, obligation.predicate.def_id)
+                    specialization_graph::assoc_def(selcx.tcx(), impl_data.impl_def_id, obligation.predicate.def_id)
                         .map_err(|ErrorGuaranteed { .. }| ())?;
 
                 if node_item.is_final() {
@@ -2113,7 +2112,7 @@ fn confirm_impl_candidate<'cx, 'tcx>(
     let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
 
     let param_env = obligation.param_env;
-    let Ok(assoc_ty) = assoc_def(selcx, impl_def_id, assoc_item_id) else {
+    let Ok(assoc_ty) = specialization_graph::assoc_def(tcx, impl_def_id, assoc_item_id) else {
         return Progress { term: tcx.ty_error().into(), obligations: nested };
     };
 
@@ -2210,7 +2209,7 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
     let mut obligations = data.nested;
 
     let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.def_id);
-    let Ok(leaf_def) = assoc_def(selcx, data.impl_def_id, trait_fn_def_id) else {
+    let Ok(leaf_def) = specialization_graph::assoc_def(tcx, 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() {
@@ -2347,58 +2346,6 @@ fn assoc_ty_own_obligations<'cx, 'tcx>(
     }
 }
 
-/// Locate the definition of an associated type in the specialization hierarchy,
-/// starting from the given impl.
-///
-/// Based on the "projection mode", this lookup may in fact only examine the
-/// topmost impl. See the comments for `Reveal` for more details.
-fn assoc_def(
-    selcx: &SelectionContext<'_, '_>,
-    impl_def_id: DefId,
-    assoc_def_id: DefId,
-) -> Result<specialization_graph::LeafDef, ErrorGuaranteed> {
-    let tcx = selcx.tcx();
-    let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
-    let trait_def = tcx.trait_def(trait_def_id);
-
-    // This function may be called while we are still building the
-    // specialization graph that is queried below (via TraitDef::ancestors()),
-    // so, in order to avoid unnecessary infinite recursion, we manually look
-    // for the associated item at the given impl.
-    // If there is no such item in that impl, this function will fail with a
-    // cycle error if the specialization graph is currently being built.
-    if let Some(&impl_item_id) = tcx.impl_item_implementor_ids(impl_def_id).get(&assoc_def_id) {
-        let item = tcx.associated_item(impl_item_id);
-        let impl_node = specialization_graph::Node::Impl(impl_def_id);
-        return Ok(specialization_graph::LeafDef {
-            item: *item,
-            defining_node: impl_node,
-            finalizing_node: if item.defaultness(tcx).is_default() {
-                None
-            } else {
-                Some(impl_node)
-            },
-        });
-    }
-
-    let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
-    if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_def_id) {
-        Ok(assoc_item)
-    } else {
-        // This is saying that neither the trait nor
-        // the impl contain a definition for this
-        // associated type.  Normally this situation
-        // could only arise through a compiler bug --
-        // if the user wrote a bad item name, it
-        // should have failed in astconv.
-        bug!(
-            "No associated type `{}` for {}",
-            tcx.item_name(assoc_def_id),
-            tcx.def_path_str(impl_def_id)
-        )
-    }
-}
-
 pub(crate) trait ProjectionCacheKeyExt<'cx, 'tcx>: Sized {
     fn from_poly_projection_predicate(
         selcx: &mut SelectionContext<'cx, 'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 792933096b1..81e8f9e914c 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1171,19 +1171,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     where
         I: Iterator<Item = ty::Predicate<'tcx>>,
     {
-        cycle.all(|predicate| self.coinductive_predicate(predicate))
-    }
-
-    fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
-        let result = match predicate.kind().skip_binder() {
-            ty::PredicateKind::Clause(ty::Clause::Trait(ref data)) => {
-                self.tcx().trait_is_coinductive(data.def_id())
-            }
-            ty::PredicateKind::WellFormed(_) => true,
-            _ => false,
-        };
-        debug!(?predicate, ?result, "coinductive_predicate");
-        result
+        cycle.all(|predicate| predicate.is_coinductive(self.tcx()))
     }
 
     /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
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 4546c953393..02b06677740 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
@@ -1,6 +1,7 @@
 use super::OverlapError;
 
 use crate::traits;
+use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::fast_reject::{self, SimplifiedType, TreatParams};
 use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
@@ -379,3 +380,51 @@ impl<'tcx> GraphExt<'tcx> for Graph {
         self.children.entry(parent).or_default().insert_blindly(tcx, child);
     }
 }
+
+/// Locate the definition of an associated type in the specialization hierarchy,
+/// starting from the given impl.
+pub(crate) fn assoc_def(
+    tcx: TyCtxt<'_>,
+    impl_def_id: DefId,
+    assoc_def_id: DefId,
+) -> Result<LeafDef, ErrorGuaranteed> {
+    let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
+    let trait_def = tcx.trait_def(trait_def_id);
+
+    // This function may be called while we are still building the
+    // specialization graph that is queried below (via TraitDef::ancestors()),
+    // so, in order to avoid unnecessary infinite recursion, we manually look
+    // for the associated item at the given impl.
+    // If there is no such item in that impl, this function will fail with a
+    // cycle error if the specialization graph is currently being built.
+    if let Some(&impl_item_id) = tcx.impl_item_implementor_ids(impl_def_id).get(&assoc_def_id) {
+        let &item = tcx.associated_item(impl_item_id);
+        let impl_node = Node::Impl(impl_def_id);
+        return Ok(LeafDef {
+            item,
+            defining_node: impl_node,
+            finalizing_node: if item.defaultness(tcx).is_default() {
+                None
+            } else {
+                Some(impl_node)
+            },
+        });
+    }
+
+    let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
+    if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_def_id) {
+        Ok(assoc_item)
+    } else {
+        // This is saying that neither the trait nor
+        // the impl contain a definition for this
+        // associated type.  Normally this situation
+        // could only arise through a compiler bug --
+        // if the user wrote a bad item name, it
+        // should have failed in astconv.
+        bug!(
+            "No associated type `{}` for {}",
+            tcx.item_name(assoc_def_id),
+            tcx.def_path_str(impl_def_id)
+        )
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/traits/vtable.rs b/compiler/rustc_trait_selection/src/traits/vtable.rs
index 41ce6cdf789..5ec9c2a24cd 100644
--- a/compiler/rustc_trait_selection/src/traits/vtable.rs
+++ b/compiler/rustc_trait_selection/src/traits/vtable.rs
@@ -191,7 +191,7 @@ fn dump_vtable_entries<'tcx>(
     });
 }
 
-fn own_existential_vtable_entries<'tcx>(tcx: TyCtxt<'tcx>, trait_def_id: DefId) -> &'tcx [DefId] {
+fn own_existential_vtable_entries(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &[DefId] {
     let trait_methods = tcx
         .associated_items(trait_def_id)
         .in_definition_order()