Uplift fast rejection to new solver

13 files changed, 419 insertions, 402 deletions

diff --git a/compiler/rustc_hir_typeck/src/method/suggest.rs b/compiler/rustc_hir_typeck/src/method/suggest.rs
index a385bc70e35..57f0da195c7 100644
--- a/compiler/rustc_hir_typeck/src/method/suggest.rs
+++ b/compiler/rustc_hir_typeck/src/method/suggest.rs
@@ -2129,7 +2129,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let target_ty = self
                 .autoderef(sugg_span, rcvr_ty)
                 .find(|(rcvr_ty, _)| {
-                    DeepRejectCtxt { treat_obligation_params: TreatParams::AsCandidateKey }
+                    DeepRejectCtxt::new(self.tcx, TreatParams::ForLookup)
                         .types_may_unify(*rcvr_ty, impl_ty)
                 })
                 .map_or(impl_ty, |(ty, _)| ty)
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index ed1ec55bc8e..7fd2f4c9105 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -373,17 +373,6 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
             .map(|assoc_item| assoc_item.def_id)
     }
 
-    fn args_may_unify_deep(
-        self,
-        obligation_args: ty::GenericArgsRef<'tcx>,
-        impl_args: ty::GenericArgsRef<'tcx>,
-    ) -> bool {
-        ty::fast_reject::DeepRejectCtxt {
-            treat_obligation_params: ty::fast_reject::TreatParams::ForLookup,
-        }
-        .args_may_unify(obligation_args, impl_args)
-    }
-
     // This implementation is a bit different from `TyCtxt::for_each_relevant_impl`,
     // since we want to skip over blanket impls for non-rigid aliases, and also we
     // only want to consider types that *actually* unify with float/int vars.
diff --git a/compiler/rustc_middle/src/ty/fast_reject.rs b/compiler/rustc_middle/src/ty/fast_reject.rs
index 923667e609b..0413cfa5a63 100644
--- a/compiler/rustc_middle/src/ty/fast_reject.rs
+++ b/compiler/rustc_middle/src/ty/fast_reject.rs
@@ -1,369 +1,9 @@
-use crate::mir::Mutability;
-use crate::ty::GenericArgKind;
-use crate::ty::{self, GenericArgsRef, Ty, TyCtxt, TypeVisitableExt};
 use rustc_hir::def_id::DefId;
-use rustc_macros::{HashStable, TyDecodable, TyEncodable};
-use std::fmt::Debug;
-use std::hash::Hash;
-use std::iter;
 
-/// See `simplify_type`.
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, TyEncodable, TyDecodable, HashStable)]
-pub enum SimplifiedType {
-    Bool,
-    Char,
-    Int(ty::IntTy),
-    Uint(ty::UintTy),
-    Float(ty::FloatTy),
-    Adt(DefId),
-    Foreign(DefId),
-    Str,
-    Array,
-    Slice,
-    Ref(Mutability),
-    Ptr(Mutability),
-    Never,
-    Tuple(usize),
-    /// A trait object, all of whose components are markers
-    /// (e.g., `dyn Send + Sync`).
-    MarkerTraitObject,
-    Trait(DefId),
-    Closure(DefId),
-    Coroutine(DefId),
-    CoroutineWitness(DefId),
-    Function(usize),
-    Placeholder,
-    Error,
-}
+use super::TyCtxt;
 
-/// Generic parameters are pretty much just bound variables, e.g.
-/// the type of `fn foo<'a, T>(x: &'a T) -> u32 { ... }` can be thought of as
-/// `for<'a, T> fn(&'a T) -> u32`.
-///
-/// Typecheck of `foo` has to succeed for all possible generic arguments, so
-/// during typeck, we have to treat its generic parameters as if they
-/// were placeholders.
-///
-/// But when calling `foo` we only have to provide a specific generic argument.
-/// In that case the generic parameters are instantiated with inference variables.
-/// As we use `simplify_type` before that instantiation happens, we just treat
-/// generic parameters as if they were inference variables in that case.
-#[derive(PartialEq, Eq, Debug, Clone, Copy)]
-pub enum TreatParams {
-    /// Treat parameters as infer vars. This is the correct mode for caching
-    /// an impl's type for lookup.
-    AsCandidateKey,
-    /// Treat parameters as placeholders in the given environment. This is the
-    /// correct mode for *lookup*, as during candidate selection.
-    ///
-    /// This also treats projections with inference variables as infer vars
-    /// since they could be further normalized.
-    ForLookup,
-}
+pub use rustc_type_ir::fast_reject::*;
 
-/// Tries to simplify a type by only returning the outermost injective¹ layer, if one exists.
-///
-/// **This function should only be used if you need to store or retrieve the type from some
-/// hashmap. If you want to quickly decide whether two types may unify, use the [DeepRejectCtxt]
-/// instead.**
-///
-/// The idea is to get something simple that we can use to quickly decide if two types could unify,
-/// for example during method lookup. If this function returns `Some(x)` it can only unify with
-/// types for which this method returns either `Some(x)` as well or `None`.
-///
-/// A special case here are parameters and projections, which are only injective
-/// if they are treated as placeholders.
-///
-/// For example when storing impls based on their simplified self type, we treat
-/// generic parameters as if they were inference variables. We must not simplify them here,
-/// as they can unify with any other type.
-///
-/// With projections we have to be even more careful, as treating them as placeholders
-/// is only correct if they are fully normalized.
-///
-/// ¹ meaning that if the outermost layers are different, then the whole types are also different.
-pub fn simplify_type<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    ty: Ty<'tcx>,
-    treat_params: TreatParams,
-) -> Option<SimplifiedType> {
-    match *ty.kind() {
-        ty::Bool => Some(SimplifiedType::Bool),
-        ty::Char => Some(SimplifiedType::Char),
-        ty::Int(int_type) => Some(SimplifiedType::Int(int_type)),
-        ty::Uint(uint_type) => Some(SimplifiedType::Uint(uint_type)),
-        ty::Float(float_type) => Some(SimplifiedType::Float(float_type)),
-        ty::Adt(def, _) => Some(SimplifiedType::Adt(def.did())),
-        ty::Str => Some(SimplifiedType::Str),
-        ty::Array(..) => Some(SimplifiedType::Array),
-        ty::Slice(..) => Some(SimplifiedType::Slice),
-        ty::Pat(ty, ..) => simplify_type(tcx, ty, treat_params),
-        ty::RawPtr(_, mutbl) => Some(SimplifiedType::Ptr(mutbl)),
-        ty::Dynamic(trait_info, ..) => match trait_info.principal_def_id() {
-            Some(principal_def_id) if !tcx.trait_is_auto(principal_def_id) => {
-                Some(SimplifiedType::Trait(principal_def_id))
-            }
-            _ => Some(SimplifiedType::MarkerTraitObject),
-        },
-        ty::Ref(_, _, mutbl) => Some(SimplifiedType::Ref(mutbl)),
-        ty::FnDef(def_id, _) | ty::Closure(def_id, _) | ty::CoroutineClosure(def_id, _) => {
-            Some(SimplifiedType::Closure(def_id))
-        }
-        ty::Coroutine(def_id, _) => Some(SimplifiedType::Coroutine(def_id)),
-        ty::CoroutineWitness(def_id, _) => Some(SimplifiedType::CoroutineWitness(def_id)),
-        ty::Never => Some(SimplifiedType::Never),
-        ty::Tuple(tys) => Some(SimplifiedType::Tuple(tys.len())),
-        ty::FnPtr(f) => Some(SimplifiedType::Function(f.skip_binder().inputs().len())),
-        ty::Placeholder(..) => Some(SimplifiedType::Placeholder),
-        ty::Param(_) => match treat_params {
-            TreatParams::ForLookup => Some(SimplifiedType::Placeholder),
-            TreatParams::AsCandidateKey => None,
-        },
-        ty::Alias(..) => match treat_params {
-            // When treating `ty::Param` as a placeholder, projections also
-            // don't unify with anything else as long as they are fully normalized.
-            // FIXME(-Znext-solver): Can remove this `if` and always simplify to `Placeholder`
-            // when the new solver is enabled by default.
-            TreatParams::ForLookup if !ty.has_non_region_infer() => {
-                Some(SimplifiedType::Placeholder)
-            }
-            TreatParams::ForLookup | TreatParams::AsCandidateKey => None,
-        },
-        ty::Foreign(def_id) => Some(SimplifiedType::Foreign(def_id)),
-        ty::Error(_) => Some(SimplifiedType::Error),
-        ty::Bound(..) | ty::Infer(_) => None,
-    }
-}
+pub type DeepRejectCtxt<'tcx> = rustc_type_ir::fast_reject::DeepRejectCtxt<TyCtxt<'tcx>>;
 
-impl SimplifiedType {
-    pub fn def(self) -> Option<DefId> {
-        match self {
-            SimplifiedType::Adt(d)
-            | SimplifiedType::Foreign(d)
-            | SimplifiedType::Trait(d)
-            | SimplifiedType::Closure(d)
-            | SimplifiedType::Coroutine(d)
-            | SimplifiedType::CoroutineWitness(d) => Some(d),
-            _ => None,
-        }
-    }
-}
-
-/// Given generic arguments from an obligation and an impl,
-/// could these two be unified after replacing parameters in the
-/// the impl with inference variables.
-///
-/// For obligations, parameters won't be replaced by inference
-/// variables and only unify with themselves. We treat them
-/// the same way we treat placeholders.
-///
-/// We also use this function during coherence. For coherence the
-/// impls only have to overlap for some value, so we treat parameters
-/// on both sides like inference variables. This behavior is toggled
-/// using the `treat_obligation_params` field.
-#[derive(Debug, Clone, Copy)]
-pub struct DeepRejectCtxt {
-    pub treat_obligation_params: TreatParams,
-}
-
-impl DeepRejectCtxt {
-    pub fn args_may_unify<'tcx>(
-        self,
-        obligation_args: GenericArgsRef<'tcx>,
-        impl_args: GenericArgsRef<'tcx>,
-    ) -> bool {
-        iter::zip(obligation_args, impl_args).all(|(obl, imp)| {
-            match (obl.unpack(), imp.unpack()) {
-                // We don't fast reject based on regions.
-                (GenericArgKind::Lifetime(_), GenericArgKind::Lifetime(_)) => true,
-                (GenericArgKind::Type(obl), GenericArgKind::Type(imp)) => {
-                    self.types_may_unify(obl, imp)
-                }
-                (GenericArgKind::Const(obl), GenericArgKind::Const(imp)) => {
-                    self.consts_may_unify(obl, imp)
-                }
-                _ => bug!("kind mismatch: {obl} {imp}"),
-            }
-        })
-    }
-
-    pub fn types_may_unify<'tcx>(self, obligation_ty: Ty<'tcx>, impl_ty: Ty<'tcx>) -> bool {
-        match impl_ty.kind() {
-            // Start by checking whether the type in the impl may unify with
-            // pretty much everything. Just return `true` in that case.
-            ty::Param(_) | ty::Error(_) | ty::Alias(..) => return true,
-            // These types only unify with inference variables or their own
-            // variant.
-            ty::Bool
-            | ty::Char
-            | ty::Int(_)
-            | ty::Uint(_)
-            | ty::Float(_)
-            | ty::Adt(..)
-            | ty::Str
-            | ty::Array(..)
-            | ty::Slice(..)
-            | ty::RawPtr(..)
-            | ty::Dynamic(..)
-            | ty::Pat(..)
-            | ty::Ref(..)
-            | ty::Never
-            | ty::Tuple(..)
-            | ty::FnPtr(..)
-            | ty::Foreign(..) => debug_assert!(impl_ty.is_known_rigid()),
-            ty::FnDef(..)
-            | ty::Closure(..)
-            | ty::CoroutineClosure(..)
-            | ty::Coroutine(..)
-            | ty::CoroutineWitness(..)
-            | ty::Placeholder(..)
-            | ty::Bound(..)
-            | ty::Infer(_) => bug!("unexpected impl_ty: {impl_ty}"),
-        }
-
-        let k = impl_ty.kind();
-        match *obligation_ty.kind() {
-            // Purely rigid types, use structural equivalence.
-            ty::Bool
-            | ty::Char
-            | ty::Int(_)
-            | ty::Uint(_)
-            | ty::Float(_)
-            | ty::Str
-            | ty::Never
-            | ty::Foreign(_) => obligation_ty == impl_ty,
-            ty::Ref(_, obl_ty, obl_mutbl) => match k {
-                &ty::Ref(_, impl_ty, impl_mutbl) => {
-                    obl_mutbl == impl_mutbl && self.types_may_unify(obl_ty, impl_ty)
-                }
-                _ => false,
-            },
-            ty::Adt(obl_def, obl_args) => match k {
-                &ty::Adt(impl_def, impl_args) => {
-                    obl_def == impl_def && self.args_may_unify(obl_args, impl_args)
-                }
-                _ => false,
-            },
-            ty::Pat(obl_ty, _) => {
-                // FIXME(pattern_types): take pattern into account
-                matches!(k, &ty::Pat(impl_ty, _) if self.types_may_unify(obl_ty, impl_ty))
-            }
-            ty::Slice(obl_ty) => {
-                matches!(k, &ty::Slice(impl_ty) if self.types_may_unify(obl_ty, impl_ty))
-            }
-            ty::Array(obl_ty, obl_len) => match k {
-                &ty::Array(impl_ty, impl_len) => {
-                    self.types_may_unify(obl_ty, impl_ty)
-                        && self.consts_may_unify(obl_len, impl_len)
-                }
-                _ => false,
-            },
-            ty::Tuple(obl) => match k {
-                &ty::Tuple(imp) => {
-                    obl.len() == imp.len()
-                        && iter::zip(obl, imp).all(|(obl, imp)| self.types_may_unify(obl, imp))
-                }
-                _ => false,
-            },
-            ty::RawPtr(obl_ty, obl_mutbl) => match *k {
-                ty::RawPtr(imp_ty, imp_mutbl) => {
-                    obl_mutbl == imp_mutbl && self.types_may_unify(obl_ty, imp_ty)
-                }
-                _ => false,
-            },
-            ty::Dynamic(obl_preds, ..) => {
-                // Ideally we would walk the existential predicates here or at least
-                // compare their length. But considering that the relevant `Relate` impl
-                // actually sorts and deduplicates these, that doesn't work.
-                matches!(k, ty::Dynamic(impl_preds, ..) if
-                    obl_preds.principal_def_id() == impl_preds.principal_def_id()
-                )
-            }
-            ty::FnPtr(obl_sig) => match k {
-                ty::FnPtr(impl_sig) => {
-                    let ty::FnSig { inputs_and_output, c_variadic, safety, abi } =
-                        obl_sig.skip_binder();
-                    let impl_sig = impl_sig.skip_binder();
-
-                    abi == impl_sig.abi
-                        && c_variadic == impl_sig.c_variadic
-                        && safety == impl_sig.safety
-                        && inputs_and_output.len() == impl_sig.inputs_and_output.len()
-                        && iter::zip(inputs_and_output, impl_sig.inputs_and_output)
-                            .all(|(obl, imp)| self.types_may_unify(obl, imp))
-                }
-                _ => false,
-            },
-
-            // Impls cannot contain these types as these cannot be named directly.
-            ty::FnDef(..) | ty::Closure(..) | ty::CoroutineClosure(..) | ty::Coroutine(..) => false,
-
-            // Placeholder types don't unify with anything on their own
-            ty::Placeholder(..) | ty::Bound(..) => false,
-
-            // Depending on the value of `treat_obligation_params`, we either
-            // treat generic parameters like placeholders or like inference variables.
-            ty::Param(_) => match self.treat_obligation_params {
-                TreatParams::ForLookup => false,
-                TreatParams::AsCandidateKey => true,
-            },
-
-            ty::Infer(ty::IntVar(_)) => impl_ty.is_integral(),
-
-            ty::Infer(ty::FloatVar(_)) => impl_ty.is_floating_point(),
-
-            ty::Infer(_) => true,
-
-            // As we're walking the whole type, it may encounter projections
-            // inside of binders and what not, so we're just going to assume that
-            // projections can unify with other stuff.
-            //
-            // Looking forward to lazy normalization this is the safer strategy anyways.
-            ty::Alias(..) => true,
-
-            ty::Error(_) => true,
-
-            ty::CoroutineWitness(..) => {
-                bug!("unexpected obligation type: {:?}", obligation_ty)
-            }
-        }
-    }
-
-    pub fn consts_may_unify(self, obligation_ct: ty::Const<'_>, impl_ct: ty::Const<'_>) -> bool {
-        let impl_val = match impl_ct.kind() {
-            ty::ConstKind::Expr(_)
-            | ty::ConstKind::Param(_)
-            | ty::ConstKind::Unevaluated(_)
-            | ty::ConstKind::Error(_) => {
-                return true;
-            }
-            ty::ConstKind::Value(_, impl_val) => impl_val,
-            ty::ConstKind::Infer(_) | ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
-                bug!("unexpected impl arg: {:?}", impl_ct)
-            }
-        };
-
-        match obligation_ct.kind() {
-            ty::ConstKind::Param(_) => match self.treat_obligation_params {
-                TreatParams::ForLookup => false,
-                TreatParams::AsCandidateKey => true,
-            },
-
-            // Placeholder consts don't unify with anything on their own
-            ty::ConstKind::Placeholder(_) => false,
-
-            // As we don't necessarily eagerly evaluate constants,
-            // they might unify with any value.
-            ty::ConstKind::Expr(_) | ty::ConstKind::Unevaluated(_) | ty::ConstKind::Error(_) => {
-                true
-            }
-            ty::ConstKind::Value(_, obl_val) => obl_val == impl_val,
-
-            ty::ConstKind::Infer(_) => true,
-
-            ty::ConstKind::Bound(..) => {
-                bug!("unexpected obl const: {:?}", obligation_ct)
-            }
-        }
-    }
-}
+pub type SimplifiedType = rustc_type_ir::fast_reject::SimplifiedType<DefId>;
diff --git a/compiler/rustc_middle/src/ty/impls_ty.rs b/compiler/rustc_middle/src/ty/impls_ty.rs
index efcf428c213..9be7370a1c2 100644
--- a/compiler/rustc_middle/src/ty/impls_ty.rs
+++ b/compiler/rustc_middle/src/ty/impls_ty.rs
@@ -4,7 +4,6 @@
 use crate::middle::region;
 use crate::mir;
 use crate::ty;
-use crate::ty::fast_reject::SimplifiedType;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::HashingControls;
@@ -57,18 +56,6 @@ where
     }
 }
 
-impl<'a> ToStableHashKey<StableHashingContext<'a>> for SimplifiedType {
-    type KeyType = Fingerprint;
-
-    #[inline]
-    fn to_stable_hash_key(&self, hcx: &StableHashingContext<'a>) -> Fingerprint {
-        let mut hasher = StableHasher::new();
-        let mut hcx: StableHashingContext<'a> = hcx.clone();
-        self.hash_stable(&mut hcx, &mut hasher);
-        hasher.finish()
-    }
-}
-
 impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ty::GenericArg<'tcx> {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
         self.unpack().hash_stable(hcx, hasher);
diff --git a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
index 4e8cb4384f4..69219c06e57 100644
--- a/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs
@@ -3,6 +3,7 @@ mod inherent;
 mod opaque_types;
 mod weak_types;
 
+use rustc_type_ir::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
 use rustc_type_ir::Upcast as _;
@@ -144,7 +145,7 @@ where
 
         let goal_trait_ref = goal.predicate.alias.trait_ref(cx);
         let impl_trait_ref = cx.impl_trait_ref(impl_def_id);
-        if !ecx.cx().args_may_unify_deep(
+        if !DeepRejectCtxt::new(ecx.cx(), TreatParams::ForLookup).args_may_unify(
             goal.predicate.alias.trait_ref(cx).args,
             impl_trait_ref.skip_binder().args,
         ) {
diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
index 2bc9d35c2b0..33050e5c019 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -2,6 +2,7 @@
 
 use rustc_ast_ir::Movability;
 use rustc_type_ir::data_structures::IndexSet;
+use rustc_type_ir::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
 use rustc_type_ir::visit::TypeVisitableExt as _;
@@ -46,7 +47,8 @@ where
         let cx = ecx.cx();
 
         let impl_trait_ref = cx.impl_trait_ref(impl_def_id);
-        if !cx.args_may_unify_deep(goal.predicate.trait_ref.args, impl_trait_ref.skip_binder().args)
+        if !DeepRejectCtxt::new(ecx.cx(), TreatParams::ForLookup)
+            .args_may_unify(goal.predicate.trait_ref.args, impl_trait_ref.skip_binder().args)
         {
             return Err(NoSolution);
         }
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index a4177d8a93f..9f0d84e7d45 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -121,7 +121,7 @@ pub fn overlapping_impls(
     // 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.
-    let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsCandidateKey };
+    let drcx = DeepRejectCtxt::new(tcx, TreatParams::AsCandidateKey);
     let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
     let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
     let may_overlap = match (impl1_ref, impl2_ref) {
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 e36a9ca8bd1..4c3d833b0f9 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -571,7 +571,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
-        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
+        let drcx = DeepRejectCtxt::new(self.tcx(), TreatParams::ForLookup);
         let obligation_args = obligation.predicate.skip_binder().trait_ref.args;
         self.tcx().for_each_relevant_impl(
             obligation.predicate.def_id(),
diff --git a/compiler/rustc_type_ir/src/fast_reject.rs b/compiler/rustc_type_ir/src/fast_reject.rs
new file mode 100644
index 00000000000..0810fa5c558
--- /dev/null
+++ b/compiler/rustc_type_ir/src/fast_reject.rs
@@ -0,0 +1,397 @@
+use std::fmt::Debug;
+use std::hash::Hash;
+use std::iter;
+use std::marker::PhantomData;
+
+use rustc_ast_ir::Mutability;
+#[cfg(feature = "nightly")]
+use rustc_data_structures::fingerprint::Fingerprint;
+#[cfg(feature = "nightly")]
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher, ToStableHashKey};
+#[cfg(feature = "nightly")]
+use rustc_macros::{HashStable_NoContext, TyDecodable, TyEncodable};
+
+use crate::inherent::*;
+use crate::visit::TypeVisitableExt as _;
+use crate::{self as ty, Interner};
+
+/// See `simplify_type`.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[cfg_attr(feature = "nightly", derive(TyEncodable, TyDecodable, HashStable_NoContext))]
+pub enum SimplifiedType<DefId> {
+    Bool,
+    Char,
+    Int(ty::IntTy),
+    Uint(ty::UintTy),
+    Float(ty::FloatTy),
+    Adt(DefId),
+    Foreign(DefId),
+    Str,
+    Array,
+    Slice,
+    Ref(Mutability),
+    Ptr(Mutability),
+    Never,
+    Tuple(usize),
+    /// A trait object, all of whose components are markers
+    /// (e.g., `dyn Send + Sync`).
+    MarkerTraitObject,
+    Trait(DefId),
+    Closure(DefId),
+    Coroutine(DefId),
+    CoroutineWitness(DefId),
+    Function(usize),
+    Placeholder,
+    Error,
+}
+
+#[cfg(feature = "nightly")]
+impl<HCX: Clone, DefId: HashStable<HCX>> ToStableHashKey<HCX> for SimplifiedType<DefId> {
+    type KeyType = Fingerprint;
+
+    #[inline]
+    fn to_stable_hash_key(&self, hcx: &HCX) -> Fingerprint {
+        let mut hasher = StableHasher::new();
+        let mut hcx: HCX = hcx.clone();
+        self.hash_stable(&mut hcx, &mut hasher);
+        hasher.finish()
+    }
+}
+
+/// Generic parameters are pretty much just bound variables, e.g.
+/// the type of `fn foo<'a, T>(x: &'a T) -> u32 { ... }` can be thought of as
+/// `for<'a, T> fn(&'a T) -> u32`.
+///
+/// Typecheck of `foo` has to succeed for all possible generic arguments, so
+/// during typeck, we have to treat its generic parameters as if they
+/// were placeholders.
+///
+/// But when calling `foo` we only have to provide a specific generic argument.
+/// In that case the generic parameters are instantiated with inference variables.
+/// As we use `simplify_type` before that instantiation happens, we just treat
+/// generic parameters as if they were inference variables in that case.
+#[derive(PartialEq, Eq, Debug, Clone, Copy)]
+pub enum TreatParams {
+    /// Treat parameters as infer vars. This is the correct mode for caching
+    /// an impl's type for lookup.
+    AsCandidateKey,
+    /// Treat parameters as placeholders in the given environment. This is the
+    /// correct mode for *lookup*, as during candidate selection.
+    ///
+    /// This also treats projections with inference variables as infer vars
+    /// since they could be further normalized.
+    ForLookup,
+}
+
+/// Tries to simplify a type by only returning the outermost injective¹ layer, if one exists.
+///
+/// **This function should only be used if you need to store or retrieve the type from some
+/// hashmap. If you want to quickly decide whether two types may unify, use the [DeepRejectCtxt]
+/// instead.**
+///
+/// The idea is to get something simple that we can use to quickly decide if two types could unify,
+/// for example during method lookup. If this function returns `Some(x)` it can only unify with
+/// types for which this method returns either `Some(x)` as well or `None`.
+///
+/// A special case here are parameters and projections, which are only injective
+/// if they are treated as placeholders.
+///
+/// For example when storing impls based on their simplified self type, we treat
+/// generic parameters as if they were inference variables. We must not simplify them here,
+/// as they can unify with any other type.
+///
+/// With projections we have to be even more careful, as treating them as placeholders
+/// is only correct if they are fully normalized.
+///
+/// ¹ meaning that if the outermost layers are different, then the whole types are also different.
+pub fn simplify_type<I: Interner>(
+    tcx: I,
+    ty: I::Ty,
+    treat_params: TreatParams,
+) -> Option<SimplifiedType<I::DefId>> {
+    match ty.kind() {
+        ty::Bool => Some(SimplifiedType::Bool),
+        ty::Char => Some(SimplifiedType::Char),
+        ty::Int(int_type) => Some(SimplifiedType::Int(int_type)),
+        ty::Uint(uint_type) => Some(SimplifiedType::Uint(uint_type)),
+        ty::Float(float_type) => Some(SimplifiedType::Float(float_type)),
+        ty::Adt(def, _) => Some(SimplifiedType::Adt(def.def_id())),
+        ty::Str => Some(SimplifiedType::Str),
+        ty::Array(..) => Some(SimplifiedType::Array),
+        ty::Slice(..) => Some(SimplifiedType::Slice),
+        ty::Pat(ty, ..) => simplify_type(tcx, ty, treat_params),
+        ty::RawPtr(_, mutbl) => Some(SimplifiedType::Ptr(mutbl)),
+        ty::Dynamic(trait_info, ..) => match trait_info.principal_def_id() {
+            Some(principal_def_id) if !tcx.trait_is_auto(principal_def_id) => {
+                Some(SimplifiedType::Trait(principal_def_id))
+            }
+            _ => Some(SimplifiedType::MarkerTraitObject),
+        },
+        ty::Ref(_, _, mutbl) => Some(SimplifiedType::Ref(mutbl)),
+        ty::FnDef(def_id, _) | ty::Closure(def_id, _) | ty::CoroutineClosure(def_id, _) => {
+            Some(SimplifiedType::Closure(def_id))
+        }
+        ty::Coroutine(def_id, _) => Some(SimplifiedType::Coroutine(def_id)),
+        ty::CoroutineWitness(def_id, _) => Some(SimplifiedType::CoroutineWitness(def_id)),
+        ty::Never => Some(SimplifiedType::Never),
+        ty::Tuple(tys) => Some(SimplifiedType::Tuple(tys.len())),
+        ty::FnPtr(f) => Some(SimplifiedType::Function(f.skip_binder().inputs().len())),
+        ty::Placeholder(..) => Some(SimplifiedType::Placeholder),
+        ty::Param(_) => match treat_params {
+            TreatParams::ForLookup => Some(SimplifiedType::Placeholder),
+            TreatParams::AsCandidateKey => None,
+        },
+        ty::Alias(..) => match treat_params {
+            // When treating `ty::Param` as a placeholder, projections also
+            // don't unify with anything else as long as they are fully normalized.
+            // FIXME(-Znext-solver): Can remove this `if` and always simplify to `Placeholder`
+            // when the new solver is enabled by default.
+            TreatParams::ForLookup if !ty.has_non_region_infer() => {
+                Some(SimplifiedType::Placeholder)
+            }
+            TreatParams::ForLookup | TreatParams::AsCandidateKey => None,
+        },
+        ty::Foreign(def_id) => Some(SimplifiedType::Foreign(def_id)),
+        ty::Error(_) => Some(SimplifiedType::Error),
+        ty::Bound(..) | ty::Infer(_) => None,
+    }
+}
+
+impl<DefId> SimplifiedType<DefId> {
+    pub fn def(self) -> Option<DefId> {
+        match self {
+            SimplifiedType::Adt(d)
+            | SimplifiedType::Foreign(d)
+            | SimplifiedType::Trait(d)
+            | SimplifiedType::Closure(d)
+            | SimplifiedType::Coroutine(d)
+            | SimplifiedType::CoroutineWitness(d) => Some(d),
+            _ => None,
+        }
+    }
+}
+
+/// Given generic arguments from an obligation and an impl,
+/// could these two be unified after replacing parameters in the
+/// the impl with inference variables.
+///
+/// For obligations, parameters won't be replaced by inference
+/// variables and only unify with themselves. We treat them
+/// the same way we treat placeholders.
+///
+/// We also use this function during coherence. For coherence the
+/// impls only have to overlap for some value, so we treat parameters
+/// on both sides like inference variables. This behavior is toggled
+/// using the `treat_obligation_params` field.
+#[derive(Debug, Clone, Copy)]
+pub struct DeepRejectCtxt<I: Interner> {
+    treat_obligation_params: TreatParams,
+    _interner: PhantomData<I>,
+}
+
+impl<I: Interner> DeepRejectCtxt<I> {
+    pub fn new(_interner: I, treat_obligation_params: TreatParams) -> Self {
+        DeepRejectCtxt { treat_obligation_params, _interner: PhantomData }
+    }
+
+    pub fn args_may_unify(
+        self,
+        obligation_args: I::GenericArgs,
+        impl_args: I::GenericArgs,
+    ) -> bool {
+        iter::zip(obligation_args.iter(), impl_args.iter()).all(|(obl, imp)| {
+            match (obl.kind(), imp.kind()) {
+                // We don't fast reject based on regions.
+                (ty::GenericArgKind::Lifetime(_), ty::GenericArgKind::Lifetime(_)) => true,
+                (ty::GenericArgKind::Type(obl), ty::GenericArgKind::Type(imp)) => {
+                    self.types_may_unify(obl, imp)
+                }
+                (ty::GenericArgKind::Const(obl), ty::GenericArgKind::Const(imp)) => {
+                    self.consts_may_unify(obl, imp)
+                }
+                _ => panic!("kind mismatch: {obl:?} {imp:?}"),
+            }
+        })
+    }
+
+    pub fn types_may_unify(self, obligation_ty: I::Ty, impl_ty: I::Ty) -> bool {
+        match impl_ty.kind() {
+            // Start by checking whether the type in the impl may unify with
+            // pretty much everything. Just return `true` in that case.
+            ty::Param(_) | ty::Error(_) | ty::Alias(..) => return true,
+            // These types only unify with inference variables or their own
+            // variant.
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Adt(..)
+            | ty::Str
+            | ty::Array(..)
+            | ty::Slice(..)
+            | ty::RawPtr(..)
+            | ty::Dynamic(..)
+            | ty::Pat(..)
+            | ty::Ref(..)
+            | ty::Never
+            | ty::Tuple(..)
+            | ty::FnPtr(..)
+            | ty::Foreign(..) => debug_assert!(impl_ty.is_known_rigid()),
+            ty::FnDef(..)
+            | ty::Closure(..)
+            | ty::CoroutineClosure(..)
+            | ty::Coroutine(..)
+            | ty::CoroutineWitness(..)
+            | ty::Placeholder(..)
+            | ty::Bound(..)
+            | ty::Infer(_) => panic!("unexpected impl_ty: {impl_ty:?}"),
+        }
+
+        let k = impl_ty.kind();
+        match obligation_ty.kind() {
+            // Purely rigid types, use structural equivalence.
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Str
+            | ty::Never
+            | ty::Foreign(_) => obligation_ty == impl_ty,
+            ty::Ref(_, obl_ty, obl_mutbl) => match k {
+                ty::Ref(_, impl_ty, impl_mutbl) => {
+                    obl_mutbl == impl_mutbl && self.types_may_unify(obl_ty, impl_ty)
+                }
+                _ => false,
+            },
+            ty::Adt(obl_def, obl_args) => match k {
+                ty::Adt(impl_def, impl_args) => {
+                    obl_def == impl_def && self.args_may_unify(obl_args, impl_args)
+                }
+                _ => false,
+            },
+            ty::Pat(obl_ty, _) => {
+                // FIXME(pattern_types): take pattern into account
+                matches!(k, ty::Pat(impl_ty, _) if self.types_may_unify(obl_ty, impl_ty))
+            }
+            ty::Slice(obl_ty) => {
+                matches!(k, ty::Slice(impl_ty) if self.types_may_unify(obl_ty, impl_ty))
+            }
+            ty::Array(obl_ty, obl_len) => match k {
+                ty::Array(impl_ty, impl_len) => {
+                    self.types_may_unify(obl_ty, impl_ty)
+                        && self.consts_may_unify(obl_len, impl_len)
+                }
+                _ => false,
+            },
+            ty::Tuple(obl) => match k {
+                ty::Tuple(imp) => {
+                    obl.len() == imp.len()
+                        && iter::zip(obl.iter(), imp.iter())
+                            .all(|(obl, imp)| self.types_may_unify(obl, imp))
+                }
+                _ => false,
+            },
+            ty::RawPtr(obl_ty, obl_mutbl) => match k {
+                ty::RawPtr(imp_ty, imp_mutbl) => {
+                    obl_mutbl == imp_mutbl && self.types_may_unify(obl_ty, imp_ty)
+                }
+                _ => false,
+            },
+            ty::Dynamic(obl_preds, ..) => {
+                // Ideally we would walk the existential predicates here or at least
+                // compare their length. But considering that the relevant `Relate` impl
+                // actually sorts and deduplicates these, that doesn't work.
+                matches!(k, ty::Dynamic(impl_preds, ..) if
+                    obl_preds.principal_def_id() == impl_preds.principal_def_id()
+                )
+            }
+            ty::FnPtr(obl_sig) => match k {
+                ty::FnPtr(impl_sig) => {
+                    let ty::FnSig { inputs_and_output, c_variadic, safety, abi } =
+                        obl_sig.skip_binder();
+                    let impl_sig = impl_sig.skip_binder();
+
+                    abi == impl_sig.abi
+                        && c_variadic == impl_sig.c_variadic
+                        && safety == impl_sig.safety
+                        && inputs_and_output.len() == impl_sig.inputs_and_output.len()
+                        && iter::zip(inputs_and_output.iter(), impl_sig.inputs_and_output.iter())
+                            .all(|(obl, imp)| self.types_may_unify(obl, imp))
+                }
+                _ => false,
+            },
+
+            // Impls cannot contain these types as these cannot be named directly.
+            ty::FnDef(..) | ty::Closure(..) | ty::CoroutineClosure(..) | ty::Coroutine(..) => false,
+
+            // Placeholder types don't unify with anything on their own
+            ty::Placeholder(..) | ty::Bound(..) => false,
+
+            // Depending on the value of `treat_obligation_params`, we either
+            // treat generic parameters like placeholders or like inference variables.
+            ty::Param(_) => match self.treat_obligation_params {
+                TreatParams::ForLookup => false,
+                TreatParams::AsCandidateKey => true,
+            },
+
+            ty::Infer(ty::IntVar(_)) => impl_ty.is_integral(),
+
+            ty::Infer(ty::FloatVar(_)) => impl_ty.is_floating_point(),
+
+            ty::Infer(_) => true,
+
+            // As we're walking the whole type, it may encounter projections
+            // inside of binders and what not, so we're just going to assume that
+            // projections can unify with other stuff.
+            //
+            // Looking forward to lazy normalization this is the safer strategy anyways.
+            ty::Alias(..) => true,
+
+            ty::Error(_) => true,
+
+            ty::CoroutineWitness(..) => {
+                panic!("unexpected obligation type: {:?}", obligation_ty)
+            }
+        }
+    }
+
+    pub fn consts_may_unify(self, obligation_ct: I::Const, impl_ct: I::Const) -> bool {
+        let impl_val = match impl_ct.kind() {
+            ty::ConstKind::Expr(_)
+            | ty::ConstKind::Param(_)
+            | ty::ConstKind::Unevaluated(_)
+            | ty::ConstKind::Error(_) => {
+                return true;
+            }
+            ty::ConstKind::Value(_, impl_val) => impl_val,
+            ty::ConstKind::Infer(_) | ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => {
+                panic!("unexpected impl arg: {:?}", impl_ct)
+            }
+        };
+
+        match obligation_ct.kind() {
+            ty::ConstKind::Param(_) => match self.treat_obligation_params {
+                TreatParams::ForLookup => false,
+                TreatParams::AsCandidateKey => true,
+            },
+
+            // Placeholder consts don't unify with anything on their own
+            ty::ConstKind::Placeholder(_) => false,
+
+            // As we don't necessarily eagerly evaluate constants,
+            // they might unify with any value.
+            ty::ConstKind::Expr(_) | ty::ConstKind::Unevaluated(_) | ty::ConstKind::Error(_) => {
+                true
+            }
+            ty::ConstKind::Value(_, obl_val) => obl_val == impl_val,
+
+            ty::ConstKind::Infer(_) => true,
+
+            ty::ConstKind::Bound(..) => {
+                panic!("unexpected obl const: {:?}", obligation_ct)
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_type_ir/src/inherent.rs b/compiler/rustc_type_ir/src/inherent.rs
index a4e1a97d505..ffe16964ae5 100644
--- a/compiler/rustc_type_ir/src/inherent.rs
+++ b/compiler/rustc_type_ir/src/inherent.rs
@@ -120,6 +120,14 @@ pub trait Ty<I: Interner<Ty = Self>>:
         matches!(self.kind(), ty::Infer(ty::TyVar(_)))
     }
 
+    fn is_floating_point(self) -> bool {
+        matches!(self.kind(), ty::Float(_) | ty::Infer(ty::FloatVar(_)))
+    }
+
+    fn is_integral(self) -> bool {
+        matches!(self.kind(), ty::Infer(ty::IntVar(_)) | ty::Int(_) | ty::Uint(_))
+    }
+
     fn is_fn_ptr(self) -> bool {
         matches!(self.kind(), ty::FnPtr(_))
     }
diff --git a/compiler/rustc_type_ir/src/interner.rs b/compiler/rustc_type_ir/src/interner.rs
index 6665158c7cd..eaa3ab7ce43 100644
--- a/compiler/rustc_type_ir/src/interner.rs
+++ b/compiler/rustc_type_ir/src/interner.rs
@@ -222,13 +222,6 @@ pub trait Interner:
 
     fn associated_type_def_ids(self, def_id: Self::DefId) -> impl IntoIterator<Item = Self::DefId>;
 
-    // FIXME: move `fast_reject` into `rustc_type_ir`.
-    fn args_may_unify_deep(
-        self,
-        obligation_args: Self::GenericArgs,
-        impl_args: Self::GenericArgs,
-    ) -> bool;
-
     fn for_each_relevant_impl(
         self,
         trait_def_id: Self::DefId,
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index 9b8ca5efdda..960ebf78cfe 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -21,6 +21,7 @@ pub mod visit;
 pub mod codec;
 pub mod data_structures;
 pub mod error;
+pub mod fast_reject;
 pub mod fold;
 pub mod inherent;
 pub mod ir_print;
diff --git a/src/librustdoc/html/render/write_shared.rs b/src/librustdoc/html/render/write_shared.rs
index c806bf1cc66..8fd56eae37f 100644
--- a/src/librustdoc/html/render/write_shared.rs
+++ b/src/librustdoc/html/render/write_shared.rs
@@ -507,8 +507,7 @@ else if (window.initSearch) window.initSearch(searchIndex);
                 // Be aware of `tests/rustdoc/type-alias/deeply-nested-112515.rs` which might regress.
                 let Some(impl_did) = impl_item_id.as_def_id() else { continue };
                 let for_ty = self.cx.tcx().type_of(impl_did).skip_binder();
-                let reject_cx =
-                    DeepRejectCtxt { treat_obligation_params: TreatParams::AsCandidateKey };
+                let reject_cx = DeepRejectCtxt::new(self.cx.tcx(), TreatParams::AsCandidateKey);
                 if !reject_cx.types_may_unify(aliased_ty, for_ty) {
                     continue;
                 }