]> git.lizzy.rs Git - rust.git/blobdiff - crates/hir_ty/src/lower.rs
Add const generics
[rust.git] / crates / hir_ty / src / lower.rs
index 86678b802f83ae54315069e94f4c7e4b4a43ae48..9e69785ac29c4b4ecae51b574784be3cdc551c2d 100644 (file)
@@ -9,8 +9,11 @@
 use std::{iter, sync::Arc};
 
 use base_db::CrateId;
-use chalk_ir::{cast::Cast, fold::Shift, interner::HasInterner, Mutability, Safety};
+use chalk_ir::{cast::Cast, fold::Shift, Mutability, Safety};
+use hir_def::generics::TypeOrConstParamData;
 use hir_def::intern::Interned;
+use hir_def::path::{ModPath, PathKind};
+use hir_def::type_ref::ConstScalarOrPath;
 use hir_def::{
     adt::StructKind,
     body::{Expander, LowerCtx},
     path::{GenericArg, Path, PathSegment, PathSegments},
     resolver::{HasResolver, Resolver, TypeNs},
     type_ref::{TraitBoundModifier, TraitRef as HirTraitRef, TypeBound, TypeRef},
-    AdtId, AssocContainerId, AssocItemId, ConstId, ConstParamId, EnumId, EnumVariantId, FunctionId,
-    GenericDefId, HasModule, ImplId, LocalFieldId, Lookup, StaticId, StructId, TraitId,
-    TypeAliasId, TypeParamId, UnionId, VariantId,
+    AdtId, AssocItemId, ConstId, EnumId, EnumVariantId, FunctionId, GenericDefId, HasModule,
+    ImplId, ItemContainerId, LocalFieldId, Lookup, StaticId, StructId, TraitId, TypeAliasId,
+    UnionId, VariantId,
 };
+use hir_def::{ConstParamId, TypeOrConstParamId, TypeParamId};
 use hir_expand::{name::Name, ExpandResult};
+use itertools::Either;
 use la_arena::ArenaMap;
 use rustc_hash::FxHashSet;
 use smallvec::SmallVec;
-use stdx::impl_from;
-use syntax::ast;
+use stdx::{impl_from, never};
+use syntax::{ast, SmolStr};
 
+use crate::consteval::{path_to_const, unknown_const_as_generic, unknown_const_usize, usize_const};
+use crate::method_resolution::fallback_bound_vars;
+use crate::utils::Generics;
+use crate::{all_super_traits, make_binders, Const, GenericArgData, ParamKind};
 use crate::{
     consteval,
     db::HirDatabase,
     mapping::ToChalk,
     static_lifetime, to_assoc_type_id, to_chalk_trait_id, to_placeholder_idx,
-    utils::{
-        all_super_trait_refs, associated_type_by_name_including_super_traits, generics, Generics,
-    },
+    utils::{all_super_trait_refs, associated_type_by_name_including_super_traits, generics},
     AliasEq, AliasTy, Binders, BoundVar, CallableSig, DebruijnIndex, DynTy, FnPointer, FnSig,
     FnSubst, ImplTraitId, Interner, PolyFnSig, ProjectionTy, QuantifiedWhereClause,
     QuantifiedWhereClauses, ReturnTypeImplTrait, ReturnTypeImplTraits, Substitution,
@@ -53,7 +60,7 @@ pub struct TyLoweringContext<'a> {
     /// some type params should be represented as placeholders, and others
     /// should be converted to variables. I think in practice, this isn't
     /// possible currently, so this should be fine for now.
-    pub type_param_mode: TypeParamLoweringMode,
+    pub type_param_mode: ParamLoweringMode,
     pub impl_trait_mode: ImplTraitLoweringMode,
     impl_trait_counter: Cell<u16>,
     /// When turning `impl Trait` into opaque types, we have to collect the
@@ -74,7 +81,7 @@ impl<'a> TyLoweringContext<'a> {
     pub fn new(db: &'a dyn HirDatabase, resolver: &'a Resolver) -> Self {
         let impl_trait_counter = Cell::new(0);
         let impl_trait_mode = ImplTraitLoweringMode::Disallowed;
-        let type_param_mode = TypeParamLoweringMode::Placeholder;
+        let type_param_mode = ParamLoweringMode::Placeholder;
         let in_binders = DebruijnIndex::INNERMOST;
         let opaque_type_data = RefCell::new(Vec::new());
         Self {
@@ -95,9 +102,9 @@ pub fn with_debruijn<T>(
         debruijn: DebruijnIndex,
         f: impl FnOnce(&TyLoweringContext) -> T,
     ) -> T {
-        let opaque_ty_data_vec = self.opaque_type_data.replace(Vec::new());
-        let expander = self.expander.replace(None);
-        let unsized_types = self.unsized_types.replace(Default::default());
+        let opaque_ty_data_vec = self.opaque_type_data.take();
+        let expander = self.expander.take();
+        let unsized_types = self.unsized_types.take();
         let new_ctx = Self {
             in_binders: debruijn,
             impl_trait_counter: Cell::new(self.impl_trait_counter.get()),
@@ -126,7 +133,7 @@ pub fn with_impl_trait_mode(self, impl_trait_mode: ImplTraitLoweringMode) -> Sel
         Self { impl_trait_mode, ..self }
     }
 
-    pub fn with_type_param_mode(self, type_param_mode: TypeParamLoweringMode) -> Self {
+    pub fn with_type_param_mode(self, type_param_mode: ParamLoweringMode) -> Self {
         Self { type_param_mode, ..self }
     }
 }
@@ -152,7 +159,7 @@ pub enum ImplTraitLoweringMode {
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
-pub enum TypeParamLoweringMode {
+pub enum ParamLoweringMode {
     Placeholder,
     Variable,
 }
@@ -162,14 +169,23 @@ pub fn lower_ty(&self, type_ref: &TypeRef) -> Ty {
         self.lower_ty_ext(type_ref).0
     }
 
+    fn generics(&self) -> Generics {
+        generics(
+            self.db.upcast(),
+            self.resolver
+                .generic_def()
+                .expect("there should be generics if there's a generic param"),
+        )
+    }
+
     pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
         let mut res = None;
         let ty = match type_ref {
-            TypeRef::Never => TyKind::Never.intern(&Interner),
+            TypeRef::Never => TyKind::Never.intern(Interner),
             TypeRef::Tuple(inner) => {
                 let inner_tys = inner.iter().map(|tr| self.lower_ty(tr));
-                TyKind::Tuple(inner_tys.len(), Substitution::from_iter(&Interner, inner_tys))
-                    .intern(&Interner)
+                TyKind::Tuple(inner_tys.len(), Substitution::from_iter(Interner, inner_tys))
+                    .intern(Interner)
             }
             TypeRef::Path(path) => {
                 let (ty, res_) = self.lower_path(path);
@@ -178,48 +194,54 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
             }
             TypeRef::RawPtr(inner, mutability) => {
                 let inner_ty = self.lower_ty(inner);
-                TyKind::Raw(lower_to_chalk_mutability(*mutability), inner_ty).intern(&Interner)
+                TyKind::Raw(lower_to_chalk_mutability(*mutability), inner_ty).intern(Interner)
             }
             TypeRef::Array(inner, len) => {
                 let inner_ty = self.lower_ty(inner);
+                let const_len = const_or_path_to_chalk(
+                    self.db,
+                    self.resolver,
+                    len,
+                    self.type_param_mode,
+                    || self.generics(),
+                    DebruijnIndex::INNERMOST,
+                );
 
-                let const_len = consteval::usize_const(len.as_usize());
-
-                TyKind::Array(inner_ty, const_len).intern(&Interner)
+                TyKind::Array(inner_ty, const_len).intern(Interner)
             }
             TypeRef::Slice(inner) => {
                 let inner_ty = self.lower_ty(inner);
-                TyKind::Slice(inner_ty).intern(&Interner)
+                TyKind::Slice(inner_ty).intern(Interner)
             }
             TypeRef::Reference(inner, _, mutability) => {
                 let inner_ty = self.lower_ty(inner);
                 let lifetime = static_lifetime();
                 TyKind::Ref(lower_to_chalk_mutability(*mutability), lifetime, inner_ty)
-                    .intern(&Interner)
+                    .intern(Interner)
             }
-            TypeRef::Placeholder => TyKind::Error.intern(&Interner),
+            TypeRef::Placeholder => TyKind::Error.intern(Interner),
             TypeRef::Fn(params, is_varargs) => {
                 let substs = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
-                    Substitution::from_iter(&Interner, params.iter().map(|tr| ctx.lower_ty(tr)))
+                    Substitution::from_iter(Interner, params.iter().map(|(_, tr)| ctx.lower_ty(tr)))
                 });
                 TyKind::Function(FnPointer {
                     num_binders: 0, // FIXME lower `for<'a> fn()` correctly
                     sig: FnSig { abi: (), safety: Safety::Safe, variadic: *is_varargs },
                     substitution: FnSubst(substs),
                 })
-                .intern(&Interner)
+                .intern(Interner)
             }
             TypeRef::DynTrait(bounds) => {
                 let self_ty =
-                    TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(&Interner);
+                    TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(Interner);
                 let bounds = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
                     QuantifiedWhereClauses::from_iter(
-                        &Interner,
+                        Interner,
                         bounds.iter().flat_map(|b| ctx.lower_type_bound(b, self_ty.clone(), false)),
                     )
                 });
-                let bounds = crate::make_only_type_binders(1, bounds);
-                TyKind::Dyn(DynTy { bounds, lifetime: static_lifetime() }).intern(&Interner)
+                let bounds = crate::make_single_type_binders(bounds);
+                TyKind::Dyn(DynTy { bounds, lifetime: static_lifetime() }).intern(Interner)
             }
             TypeRef::ImplTrait(bounds) => {
                 match self.impl_trait_mode {
@@ -236,7 +258,7 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
                         // place even if we encounter more opaque types while
                         // lowering the bounds
                         self.opaque_type_data.borrow_mut().push(ReturnTypeImplTrait {
-                            bounds: crate::make_only_type_binders(1, Vec::new()),
+                            bounds: crate::make_single_type_binders(Vec::new()),
                         });
                         // We don't want to lower the bounds inside the binders
                         // we're currently in, because they don't end up inside
@@ -256,8 +278,8 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
                         let impl_trait_id = ImplTraitId::ReturnTypeImplTrait(func, idx);
                         let opaque_ty_id = self.db.intern_impl_trait_id(impl_trait_id).into();
                         let generics = generics(self.db.upcast(), func.into());
-                        let parameters = generics.bound_vars_subst(self.in_binders);
-                        TyKind::OpaqueType(opaque_ty_id, parameters).intern(&Interner)
+                        let parameters = generics.bound_vars_subst(self.db, self.in_binders);
+                        TyKind::OpaqueType(opaque_ty_id, parameters).intern(Interner)
                     }
                     ImplTraitLoweringMode::Param => {
                         let idx = self.impl_trait_counter.get();
@@ -266,7 +288,7 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
                         if let Some(def) = self.resolver.generic_def() {
                             let generics = generics(self.db.upcast(), def);
                             let param = generics
-                                .iter()
+                                .type_iter()
                                 .filter(|(_, data)| {
                                     data.provenance == TypeParamProvenance::ArgumentImplTrait
                                 })
@@ -274,31 +296,36 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
                                 .map_or(TyKind::Error, |(id, _)| {
                                     TyKind::Placeholder(to_placeholder_idx(self.db, id))
                                 });
-                            param.intern(&Interner)
+                            param.intern(Interner)
                         } else {
-                            TyKind::Error.intern(&Interner)
+                            TyKind::Error.intern(Interner)
                         }
                     }
                     ImplTraitLoweringMode::Variable => {
                         let idx = self.impl_trait_counter.get();
                         // FIXME we're probably doing something wrong here
                         self.impl_trait_counter.set(idx + count_impl_traits(type_ref) as u16);
-                        let (parent_params, self_params, list_params, _impl_trait_params) =
-                            if let Some(def) = self.resolver.generic_def() {
-                                let generics = generics(self.db.upcast(), def);
-                                generics.provenance_split()
-                            } else {
-                                (0, 0, 0, 0)
-                            };
+                        let (
+                            parent_params,
+                            self_params,
+                            list_params,
+                            const_params,
+                            _impl_trait_params,
+                        ) = if let Some(def) = self.resolver.generic_def() {
+                            let generics = generics(self.db.upcast(), def);
+                            generics.provenance_split()
+                        } else {
+                            (0, 0, 0, 0, 0)
+                        };
                         TyKind::BoundVar(BoundVar::new(
                             self.in_binders,
-                            idx as usize + parent_params + self_params + list_params,
+                            idx as usize + parent_params + self_params + list_params + const_params,
                         ))
-                        .intern(&Interner)
+                        .intern(Interner)
                     }
                     ImplTraitLoweringMode::Disallowed => {
                         // FIXME: report error
-                        TyKind::Error.intern(&Interner)
+                        TyKind::Error.intern(Interner)
                     }
                 }
             }
@@ -307,17 +334,12 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
                     let mut expander = self.expander.borrow_mut();
                     if expander.is_some() {
                         (Some(expander), false)
+                    } else if let Some(module_id) = self.resolver.module() {
+                        *expander =
+                            Some(Expander::new(self.db.upcast(), macro_call.file_id, module_id));
+                        (Some(expander), true)
                     } else {
-                        if let Some(module_id) = self.resolver.module() {
-                            *expander = Some(Expander::new(
-                                self.db.upcast(),
-                                macro_call.file_id,
-                                module_id,
-                            ));
-                            (Some(expander), true)
-                        } else {
-                            (None, false)
-                        }
+                        (None, false)
                     }
                 };
                 let ty = if let Some(mut expander) = expander {
@@ -347,9 +369,9 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
                 if recursion_start {
                     *self.expander.borrow_mut() = None;
                 }
-                ty.unwrap_or_else(|| TyKind::Error.intern(&Interner))
+                ty.unwrap_or_else(|| TyKind::Error.intern(Interner))
             }
-            TypeRef::Error => TyKind::Error.intern(&Interner),
+            TypeRef::Error => TyKind::Error.intern(Interner),
         };
         (ty, res)
     }
@@ -357,7 +379,7 @@ pub fn lower_ty_ext(&self, type_ref: &TypeRef) -> (Ty, Option<TypeNs>) {
     /// This is only for `generic_predicates_for_param`, where we can't just
     /// lower the self types of the predicates since that could lead to cycles.
     /// So we just check here if the `type_ref` resolves to a generic param, and which.
-    fn lower_ty_only_param(&self, type_ref: &TypeRef) -> Option<TypeParamId> {
+    fn lower_ty_only_param(&self, type_ref: &TypeRef) -> Option<TypeOrConstParamId> {
         let path = match type_ref {
             TypeRef::Path(path) => path,
             _ => return None,
@@ -373,10 +395,9 @@ fn lower_ty_only_param(&self, type_ref: &TypeRef) -> Option<TypeParamId> {
                 Some((it, None)) => it,
                 _ => return None,
             };
-        if let TypeNs::GenericParam(param_id) = resolution {
-            Some(param_id)
-        } else {
-            None
+        match resolution {
+            TypeNs::GenericParam(param_id) => Some(param_id.into()),
+            _ => None,
         }
     }
 
@@ -396,7 +417,7 @@ pub(crate) fn lower_ty_relative_path(
             }
             _ => {
                 // FIXME report error (ambiguous associated type)
-                (TyKind::Error.intern(&Interner), None)
+                (TyKind::Error.intern(Interner), None)
             }
         }
     }
@@ -410,52 +431,59 @@ pub(crate) fn lower_partly_resolved_path(
     ) -> (Ty, Option<TypeNs>) {
         let ty = match resolution {
             TypeNs::TraitId(trait_) => {
-                let ty = if remaining_segments.len() == 1 {
-                    let trait_ref =
-                        self.lower_trait_ref_from_resolved_path(trait_, resolved_segment, None);
-                    let segment = remaining_segments.first().unwrap();
-                    let found = self
-                        .db
-                        .trait_data(trait_ref.hir_trait_id())
-                        .associated_type_by_name(segment.name);
-                    match found {
-                        Some(associated_ty) => {
-                            // FIXME handle type parameters on the segment
-                            TyKind::Alias(AliasTy::Projection(ProjectionTy {
-                                associated_ty_id: to_assoc_type_id(associated_ty),
-                                substitution: trait_ref.substitution,
-                            }))
-                            .intern(&Interner)
-                        }
-                        None => {
-                            // FIXME: report error (associated type not found)
-                            TyKind::Error.intern(&Interner)
+                let ty = match remaining_segments.len() {
+                    1 => {
+                        let trait_ref =
+                            self.lower_trait_ref_from_resolved_path(trait_, resolved_segment, None);
+                        let segment = remaining_segments.first().unwrap();
+                        let found = self
+                            .db
+                            .trait_data(trait_ref.hir_trait_id())
+                            .associated_type_by_name(segment.name);
+                        match found {
+                            Some(associated_ty) => {
+                                // FIXME handle type parameters on the segment
+                                TyKind::Alias(AliasTy::Projection(ProjectionTy {
+                                    associated_ty_id: to_assoc_type_id(associated_ty),
+                                    substitution: trait_ref.substitution,
+                                }))
+                                .intern(Interner)
+                            }
+                            None => {
+                                // FIXME: report error (associated type not found)
+                                TyKind::Error.intern(Interner)
+                            }
                         }
                     }
-                } else if remaining_segments.len() > 1 {
-                    // FIXME report error (ambiguous associated type)
-                    TyKind::Error.intern(&Interner)
-                } else {
-                    let self_ty = Some(
-                        TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0))
-                            .intern(&Interner),
-                    );
-                    let trait_ref = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
-                        ctx.lower_trait_ref_from_resolved_path(trait_, resolved_segment, self_ty)
-                    });
-                    let dyn_ty = DynTy {
-                        bounds: crate::make_only_type_binders(
-                            1,
-                            QuantifiedWhereClauses::from_iter(
-                                &Interner,
-                                Some(crate::wrap_empty_binders(WhereClause::Implemented(
-                                    trait_ref,
-                                ))),
+                    0 => {
+                        let self_ty = Some(
+                            TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0))
+                                .intern(Interner),
+                        );
+                        let trait_ref = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
+                            ctx.lower_trait_ref_from_resolved_path(
+                                trait_,
+                                resolved_segment,
+                                self_ty,
+                            )
+                        });
+                        let dyn_ty = DynTy {
+                            bounds: crate::make_single_type_binders(
+                                QuantifiedWhereClauses::from_iter(
+                                    Interner,
+                                    Some(crate::wrap_empty_binders(WhereClause::Implemented(
+                                        trait_ref,
+                                    ))),
+                                ),
                             ),
-                        ),
-                        lifetime: static_lifetime(),
-                    };
-                    TyKind::Dyn(dyn_ty).intern(&Interner)
+                            lifetime: static_lifetime(),
+                        };
+                        TyKind::Dyn(dyn_ty).intern(Interner)
+                    }
+                    _ => {
+                        // FIXME report error (ambiguous associated type)
+                        TyKind::Error.intern(Interner)
+                    }
                 };
                 return (ty, None);
             }
@@ -465,31 +493,35 @@ pub(crate) fn lower_partly_resolved_path(
                     self.resolver.generic_def().expect("generics in scope"),
                 );
                 match self.type_param_mode {
-                    TypeParamLoweringMode::Placeholder => {
-                        TyKind::Placeholder(to_placeholder_idx(self.db, param_id))
+                    ParamLoweringMode::Placeholder => {
+                        TyKind::Placeholder(to_placeholder_idx(self.db, param_id.into()))
                     }
-                    TypeParamLoweringMode::Variable => {
-                        let idx = generics.param_idx(param_id).expect("matching generics");
+                    ParamLoweringMode::Variable => {
+                        let idx = generics.param_idx(param_id.into()).expect("matching generics");
                         TyKind::BoundVar(BoundVar::new(self.in_binders, idx))
                     }
                 }
-                .intern(&Interner)
+                .intern(Interner)
             }
             TypeNs::SelfType(impl_id) => {
                 let generics = generics(self.db.upcast(), impl_id.into());
                 let substs = match self.type_param_mode {
-                    TypeParamLoweringMode::Placeholder => generics.type_params_subst(self.db),
-                    TypeParamLoweringMode::Variable => generics.bound_vars_subst(self.in_binders),
+                    ParamLoweringMode::Placeholder => generics.placeholder_subst(self.db),
+                    ParamLoweringMode::Variable => {
+                        generics.bound_vars_subst(self.db, self.in_binders)
+                    }
                 };
-                self.db.impl_self_ty(impl_id).substitute(&Interner, &substs)
+                self.db.impl_self_ty(impl_id).substitute(Interner, &substs)
             }
             TypeNs::AdtSelfType(adt) => {
                 let generics = generics(self.db.upcast(), adt.into());
                 let substs = match self.type_param_mode {
-                    TypeParamLoweringMode::Placeholder => generics.type_params_subst(self.db),
-                    TypeParamLoweringMode::Variable => generics.bound_vars_subst(self.in_binders),
+                    ParamLoweringMode::Placeholder => generics.placeholder_subst(self.db),
+                    ParamLoweringMode::Variable => {
+                        generics.bound_vars_subst(self.db, self.in_binders)
+                    }
                 };
-                self.db.ty(adt.into()).substitute(&Interner, &substs)
+                self.db.ty(adt.into()).substitute(Interner, &substs)
             }
 
             TypeNs::AdtId(it) => self.lower_path_inner(resolved_segment, it.into(), infer_args),
@@ -500,7 +532,7 @@ pub(crate) fn lower_partly_resolved_path(
                 self.lower_path_inner(resolved_segment, it.into(), infer_args)
             }
             // FIXME: report error
-            TypeNs::EnumVariantId(_) => return (TyKind::Error.intern(&Interner), None),
+            TypeNs::EnumVariantId(_) => return (TyKind::Error.intern(Interner), None),
         };
         self.lower_ty_relative_path(ty, Some(resolution), remaining_segments)
     }
@@ -514,7 +546,7 @@ pub(crate) fn lower_path(&self, path: &Path) -> (Ty, Option<TypeNs>) {
         let (resolution, remaining_index) =
             match self.resolver.resolve_path_in_type_ns(self.db.upcast(), path.mod_path()) {
                 Some(it) => it,
-                None => return (TyKind::Error.intern(&Interner), None),
+                None => return (TyKind::Error.intern(Interner), None),
             };
         let (resolved_segment, remaining_segments) = match remaining_index {
             None => (
@@ -527,48 +559,50 @@ pub(crate) fn lower_path(&self, path: &Path) -> (Ty, Option<TypeNs>) {
     }
 
     fn select_associated_type(&self, res: Option<TypeNs>, segment: PathSegment<'_>) -> Ty {
-        if let Some(res) = res {
-            let ty = associated_type_shorthand_candidates(
-                self.db,
-                res,
-                move |name, t, associated_ty| {
-                    if name == segment.name {
-                        let substs = match self.type_param_mode {
-                            TypeParamLoweringMode::Placeholder => {
-                                // if we're lowering to placeholders, we have to put
-                                // them in now
-                                let generics = generics(
-                                    self.db.upcast(),
-                                    self.resolver.generic_def().expect(
-                                        "there should be generics if there's a generic param",
-                                    ),
-                                );
-                                let s = generics.type_params_subst(self.db);
-                                s.apply(t.substitution.clone(), &Interner)
-                            }
-                            TypeParamLoweringMode::Variable => t.substitution.clone(),
-                        };
-                        // We need to shift in the bound vars, since
-                        // associated_type_shorthand_candidates does not do that
-                        let substs = substs.shifted_in_from(&Interner, self.in_binders);
-                        // FIXME handle type parameters on the segment
-                        return Some(
-                            TyKind::Alias(AliasTy::Projection(ProjectionTy {
-                                associated_ty_id: to_assoc_type_id(associated_ty),
-                                substitution: substs,
-                            }))
-                            .intern(&Interner),
-                        );
-                    }
-
+        let (def, res) = match (self.resolver.generic_def(), res) {
+            (Some(def), Some(res)) => (def, res),
+            _ => return TyKind::Error.intern(Interner),
+        };
+        let ty = named_associated_type_shorthand_candidates(
+            self.db,
+            def,
+            res,
+            Some(segment.name.clone()),
+            move |name, t, associated_ty| {
+                if name == segment.name {
+                    let substs = match self.type_param_mode {
+                        ParamLoweringMode::Placeholder => {
+                            // if we're lowering to placeholders, we have to put
+                            // them in now
+                            let generics = generics(
+                                self.db.upcast(),
+                                self.resolver
+                                    .generic_def()
+                                    .expect("there should be generics if there's a generic param"),
+                            );
+                            let s = generics.placeholder_subst(self.db);
+                            s.apply(t.substitution.clone(), Interner)
+                        }
+                        ParamLoweringMode::Variable => t.substitution.clone(),
+                    };
+                    // We need to shift in the bound vars, since
+                    // associated_type_shorthand_candidates does not do that
+                    let substs = substs.shifted_in_from(Interner, self.in_binders);
+                    // FIXME handle type parameters on the segment
+                    Some(
+                        TyKind::Alias(AliasTy::Projection(ProjectionTy {
+                            associated_ty_id: to_assoc_type_id(associated_ty),
+                            substitution: substs,
+                        }))
+                        .intern(Interner),
+                    )
+                } else {
                     None
-                },
-            );
+                }
+            },
+        );
 
-            ty.unwrap_or_else(|| TyKind::Error.intern(&Interner))
-        } else {
-            TyKind::Error.intern(&Interner)
-        }
+        ty.unwrap_or_else(|| TyKind::Error.intern(Interner))
     }
 
     fn lower_path_inner(
@@ -583,7 +617,7 @@ fn lower_path_inner(
             TyDefId::TypeAliasId(it) => Some(it.into()),
         };
         let substs = self.substs_from_path_segment(segment, generic_def, infer_args, None);
-        self.db.ty(typeable).substitute(&Interner, &substs)
+        self.db.ty(typeable).substitute(Interner, &substs)
     }
 
     /// Collect generic arguments from a path into a `Substs`. See also
@@ -611,7 +645,7 @@ pub(super) fn substs_from_path(
                 // `Option::None::<T>` are both allowed (though the former is
                 // preferred). See also `def_ids_for_path_segments` in rustc.
                 let len = path.segments().len();
-                let penultimate = if len >= 2 { path.segments().get(len - 2) } else { None };
+                let penultimate = len.checked_sub(2).and_then(|idx| path.segments().get(idx));
                 let segment = match penultimate {
                     Some(segment) if segment.args_and_bindings.is_some() => segment,
                     _ => last,
@@ -630,46 +664,75 @@ fn substs_from_path_segment(
         explicit_self_ty: Option<Ty>,
     ) -> Substitution {
         let mut substs = Vec::new();
-        let def_generics = def_generic.map(|def| generics(self.db.upcast(), def));
-
-        let (parent_params, self_params, type_params, impl_trait_params) =
-            def_generics.map_or((0, 0, 0, 0), |g| g.provenance_split());
-        let total_len = parent_params + self_params + type_params + impl_trait_params;
-
-        substs.extend(iter::repeat(TyKind::Error.intern(&Interner)).take(parent_params));
+        let def_generics = if let Some(def) = def_generic {
+            generics(self.db.upcast(), def)
+        } else {
+            return Substitution::empty(Interner);
+        };
+        let (parent_params, self_params, type_params, const_params, impl_trait_params) =
+            def_generics.provenance_split();
+        let total_len =
+            parent_params + self_params + type_params + const_params + impl_trait_params;
+
+        let ty_error = GenericArgData::Ty(TyKind::Error.intern(Interner)).intern(Interner);
+        let const_error = GenericArgData::Const(consteval::usize_const(None)).intern(Interner);
+
+        for (_, data) in def_generics.iter().take(parent_params) {
+            match data {
+                TypeOrConstParamData::TypeParamData(_) => substs.push(ty_error.clone()),
+                TypeOrConstParamData::ConstParamData(_) => substs.push(const_error.clone()),
+            }
+        }
 
         let fill_self_params = || {
             substs.extend(
                 explicit_self_ty
                     .into_iter()
-                    .chain(iter::repeat(TyKind::Error.intern(&Interner)))
+                    .map(|x| GenericArgData::Ty(x).intern(Interner))
+                    .chain(iter::repeat(ty_error.clone()))
                     .take(self_params),
             )
         };
-        let mut had_explicit_type_args = false;
+        let mut had_explicit_args = false;
 
         if let Some(generic_args) = &segment.args_and_bindings {
             if !generic_args.has_self_type {
                 fill_self_params();
             }
-            let expected_num =
-                if generic_args.has_self_type { self_params + type_params } else { type_params };
+            let expected_num = if generic_args.has_self_type {
+                self_params + type_params + const_params
+            } else {
+                type_params + const_params
+            };
             let skip = if generic_args.has_self_type && self_params == 0 { 1 } else { 0 };
             // if args are provided, it should be all of them, but we can't rely on that
-            for arg in generic_args
+            for (arg, id) in generic_args
                 .args
                 .iter()
-                .filter(|arg| matches!(arg, GenericArg::Type(_)))
+                .filter(|arg| !matches!(arg, GenericArg::Lifetime(_)))
                 .skip(skip)
                 .take(expected_num)
+                .zip(def_generics.iter_id().skip(skip))
             {
-                match arg {
-                    GenericArg::Type(type_ref) => {
-                        had_explicit_type_args = true;
-                        let ty = self.lower_ty(type_ref);
-                        substs.push(ty);
-                    }
-                    GenericArg::Lifetime(_) => {}
+                if let Some(x) = generic_arg_to_chalk(
+                    self.db,
+                    id,
+                    arg,
+                    &mut (),
+                    |_, type_ref| self.lower_ty(type_ref),
+                    |_, c| {
+                        const_or_path_to_chalk(
+                            self.db,
+                            &self.resolver,
+                            c,
+                            self.type_param_mode,
+                            || self.generics(),
+                            DebruijnIndex::INNERMOST,
+                        )
+                    },
+                ) {
+                    had_explicit_args = true;
+                    substs.push(x);
                 }
             }
         } else {
@@ -679,27 +742,30 @@ fn substs_from_path_segment(
         // handle defaults. In expression or pattern path segments without
         // explicitly specified type arguments, missing type arguments are inferred
         // (i.e. defaults aren't used).
-        if !infer_args || had_explicit_type_args {
+        if !infer_args || had_explicit_args {
             if let Some(def_generic) = def_generic {
                 let defaults = self.db.generic_defaults(def_generic);
                 assert_eq!(total_len, defaults.len());
 
                 for default_ty in defaults.iter().skip(substs.len()) {
                     // each default can depend on the previous parameters
-                    let substs_so_far = Substitution::from_iter(&Interner, substs.clone());
-                    substs.push(default_ty.clone().substitute(&Interner, &substs_so_far));
+                    let substs_so_far = Substitution::from_iter(Interner, substs.clone());
+                    substs.push(default_ty.clone().substitute(Interner, &substs_so_far));
                 }
             }
         }
 
         // add placeholders for args that were not provided
         // FIXME: emit diagnostics in contexts where this is not allowed
-        for _ in substs.len()..total_len {
-            substs.push(TyKind::Error.intern(&Interner));
+        for (_, data) in def_generics.iter().skip(substs.len()) {
+            match data {
+                TypeOrConstParamData::TypeParamData(_) => substs.push(ty_error.clone()),
+                TypeOrConstParamData::ConstParamData(_) => substs.push(const_error.clone()),
+            }
         }
         assert_eq!(substs.len(), total_len);
 
-        Substitution::from_iter(&Interner, substs)
+        Substitution::from_iter(Interner, substs)
     }
 
     fn lower_trait_ref_from_path(
@@ -753,20 +819,22 @@ pub(crate) fn lower_where_predicate(
             | WherePredicate::TypeBound { target, bound } => {
                 let self_ty = match target {
                     WherePredicateTypeTarget::TypeRef(type_ref) => self.lower_ty(type_ref),
-                    WherePredicateTypeTarget::TypeParam(param_id) => {
+                    WherePredicateTypeTarget::TypeOrConstParam(param_id) => {
                         let generic_def = self.resolver.generic_def().expect("generics in scope");
                         let generics = generics(self.db.upcast(), generic_def);
-                        let param_id =
-                            hir_def::TypeParamId { parent: generic_def, local_id: *param_id };
+                        let param_id = hir_def::TypeOrConstParamId {
+                            parent: generic_def,
+                            local_id: *param_id,
+                        };
                         let placeholder = to_placeholder_idx(self.db, param_id);
                         match self.type_param_mode {
-                            TypeParamLoweringMode::Placeholder => TyKind::Placeholder(placeholder),
-                            TypeParamLoweringMode::Variable => {
+                            ParamLoweringMode::Placeholder => TyKind::Placeholder(placeholder),
+                            ParamLoweringMode::Variable => {
                                 let idx = generics.param_idx(param_id).expect("matching generics");
                                 TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, idx))
                             }
                         }
-                        .intern(&Interner)
+                        .intern(Interner)
                     }
                 };
                 self.lower_type_bound(bound, self_ty, ignore_bindings)
@@ -793,7 +861,7 @@ pub(crate) fn lower_type_bound(
                 let sized_trait = self
                     .resolver
                     .krate()
-                    .and_then(|krate| self.db.lang_item(krate, "sized".into()))
+                    .and_then(|krate| self.db.lang_item(krate, SmolStr::new_inline("sized")))
                     .and_then(|lang_item| lang_item.as_trait());
                 // Don't lower associated type bindings as the only possible relaxed trait bound
                 // `?Sized` has no of them.
@@ -837,8 +905,8 @@ fn assoc_type_bindings_from_type_bound(
         };
         last_segment
             .into_iter()
-            .flat_map(|segment| segment.args_and_bindings.into_iter())
-            .flat_map(|args_and_bindings| args_and_bindings.bindings.iter())
+            .filter_map(|segment| segment.args_and_bindings)
+            .flat_map(|args_and_bindings| &args_and_bindings.bindings)
             .flat_map(move |binding| {
                 let found = associated_type_by_name_including_super_traits(
                     self.db,
@@ -846,14 +914,14 @@ fn assoc_type_bindings_from_type_bound(
                     &binding.name,
                 );
                 let (super_trait_ref, associated_ty) = match found {
-                    None => return SmallVec::<[QuantifiedWhereClause; 1]>::new(),
+                    None => return SmallVec::new(),
                     Some(t) => t,
                 };
                 let projection_ty = ProjectionTy {
                     associated_ty_id: to_assoc_type_id(associated_ty),
                     substitution: super_trait_ref.substitution,
                 };
-                let mut preds = SmallVec::with_capacity(
+                let mut preds: SmallVec<[_; 1]> = SmallVec::with_capacity(
                     binding.type_ref.as_ref().map_or(0, |_| 1) + binding.bounds.len(),
                 );
                 if let Some(type_ref) = &binding.type_ref {
@@ -865,7 +933,7 @@ fn assoc_type_bindings_from_type_bound(
                 for bound in &binding.bounds {
                     preds.extend(self.lower_type_bound(
                         bound,
-                        TyKind::Alias(AliasTy::Projection(projection_ty.clone())).intern(&Interner),
+                        TyKind::Alias(AliasTy::Projection(projection_ty.clone())).intern(Interner),
                         false,
                     ));
                 }
@@ -879,8 +947,7 @@ fn lower_impl_trait(
         func: FunctionId,
     ) -> ReturnTypeImplTrait {
         cov_mark::hit!(lower_rpit);
-        let self_ty =
-            TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(&Interner);
+        let self_ty = TyKind::BoundVar(BoundVar::new(DebruijnIndex::INNERMOST, 0)).intern(Interner);
         let predicates = self.with_shifted_in(DebruijnIndex::ONE, |ctx| {
             let mut predicates: Vec<_> = bounds
                 .iter()
@@ -891,12 +958,12 @@ fn lower_impl_trait(
                 let krate = func.lookup(ctx.db.upcast()).module(ctx.db.upcast()).krate();
                 let sized_trait = ctx
                     .db
-                    .lang_item(krate, "sized".into())
+                    .lang_item(krate, SmolStr::new_inline("sized"))
                     .and_then(|lang_item| lang_item.as_trait().map(to_chalk_trait_id));
                 let sized_clause = sized_trait.map(|trait_id| {
                     let clause = WhereClause::Implemented(TraitRef {
                         trait_id,
-                        substitution: Substitution::from1(&Interner, self_ty.clone()),
+                        substitution: Substitution::from1(Interner, self_ty.clone()),
                     });
                     crate::wrap_empty_binders(clause)
                 });
@@ -905,8 +972,7 @@ fn lower_impl_trait(
             }
             predicates
         });
-
-        ReturnTypeImplTrait { bounds: crate::make_only_type_binders(1, predicates) }
+        ReturnTypeImplTrait { bounds: crate::make_single_type_binders(predicates) }
     }
 }
 
@@ -931,7 +997,20 @@ pub fn callable_item_sig(db: &dyn HirDatabase, def: CallableDefId) -> PolyFnSig
 
 pub fn associated_type_shorthand_candidates<R>(
     db: &dyn HirDatabase,
+    def: GenericDefId,
+    res: TypeNs,
+    cb: impl FnMut(&Name, &TraitRef, TypeAliasId) -> Option<R>,
+) -> Option<R> {
+    named_associated_type_shorthand_candidates(db, def, res, None, cb)
+}
+
+fn named_associated_type_shorthand_candidates<R>(
+    db: &dyn HirDatabase,
+    // If the type parameter is defined in an impl and we're in a method, there
+    // might be additional where clauses to consider
+    def: GenericDefId,
     res: TypeNs,
+    assoc_name: Option<Name>,
     mut cb: impl FnMut(&Name, &TraitRef, TypeAliasId) -> Option<R>,
 ) -> Option<R> {
     let mut search = |t| {
@@ -956,12 +1035,12 @@ pub fn associated_type_shorthand_candidates<R>(
             db.impl_trait(impl_id)?.into_value_and_skipped_binders().0,
         ),
         TypeNs::GenericParam(param_id) => {
-            let predicates = db.generic_predicates_for_param(param_id);
+            let predicates = db.generic_predicates_for_param(def, param_id.into(), assoc_name);
             let res = predicates.iter().find_map(|pred| match pred.skip_binders().skip_binders() {
                 // FIXME: how to correctly handle higher-ranked bounds here?
                 WhereClause::Implemented(tr) => search(
                     tr.clone()
-                        .shifted_out_to(&Interner, DebruijnIndex::ONE)
+                        .shifted_out_to(Interner, DebruijnIndex::ONE)
                         .expect("FIXME unexpected higher-ranked trait bound"),
                 ),
                 _ => None,
@@ -970,11 +1049,9 @@ pub fn associated_type_shorthand_candidates<R>(
                 return res;
             }
             // Handle `Self::Type` referring to own associated type in trait definitions
-            if let GenericDefId::TraitId(trait_id) = param_id.parent {
+            if let GenericDefId::TraitId(trait_id) = param_id.parent() {
                 let generics = generics(db.upcast(), trait_id.into());
-                if generics.params.types[param_id.local_id].provenance
-                    == TypeParamProvenance::TraitSelf
-                {
+                if generics.params.type_or_consts[param_id.local_id()].is_trait_self() {
                     let trait_ref = TyBuilder::trait_ref(db, trait_id)
                         .fill_with_bound_vars(DebruijnIndex::INNERMOST, 0)
                         .build();
@@ -1001,9 +1078,9 @@ pub(crate) fn field_types_query(
     let generics = generics(db.upcast(), def);
     let mut res = ArenaMap::default();
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
     for (field_id, field_data) in var_data.fields().iter() {
-        res.insert(field_id, make_binders(&generics, ctx.lower_ty(&field_data.type_ref)))
+        res.insert(field_id, make_binders(db, &generics, ctx.lower_ty(&field_data.type_ref)))
     }
     Arc::new(res)
 }
@@ -1018,33 +1095,80 @@ pub(crate) fn field_types_query(
 /// these are fine: `T: Foo<U::Item>, U: Foo<()>`.
 pub(crate) fn generic_predicates_for_param_query(
     db: &dyn HirDatabase,
-    param_id: TypeParamId,
+    def: GenericDefId,
+    param_id: TypeOrConstParamId,
+    assoc_name: Option<Name>,
 ) -> Arc<[Binders<QuantifiedWhereClause>]> {
-    let resolver = param_id.parent.resolver(db.upcast());
+    let resolver = def.resolver(db.upcast());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
-    let generics = generics(db.upcast(), param_id.parent);
-    resolver
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
+    let generics = generics(db.upcast(), def);
+    let mut predicates: Vec<_> = resolver
         .where_predicates_in_scope()
         // we have to filter out all other predicates *first*, before attempting to lower them
         .filter(|pred| match pred {
-            WherePredicate::ForLifetime { target, .. }
-            | WherePredicate::TypeBound { target, .. } => match target {
-                WherePredicateTypeTarget::TypeRef(type_ref) => {
-                    ctx.lower_ty_only_param(type_ref) == Some(param_id)
+            WherePredicate::ForLifetime { target, bound, .. }
+            | WherePredicate::TypeBound { target, bound, .. } => {
+                match target {
+                    WherePredicateTypeTarget::TypeRef(type_ref) => {
+                        if ctx.lower_ty_only_param(type_ref) != Some(param_id) {
+                            return false;
+                        }
+                    }
+                    WherePredicateTypeTarget::TypeOrConstParam(local_id) => {
+                        if *local_id != param_id.local_id {
+                            return false;
+                        }
+                    }
+                };
+
+                match &**bound {
+                    TypeBound::ForLifetime(_, path) | TypeBound::Path(path, _) => {
+                        // Only lower the bound if the trait could possibly define the associated
+                        // type we're looking for.
+
+                        let assoc_name = match &assoc_name {
+                            Some(it) => it,
+                            None => return true,
+                        };
+                        let tr = match resolver
+                            .resolve_path_in_type_ns_fully(db.upcast(), path.mod_path())
+                        {
+                            Some(TypeNs::TraitId(tr)) => tr,
+                            _ => return false,
+                        };
+
+                        all_super_traits(db.upcast(), tr).iter().any(|tr| {
+                            db.trait_data(*tr).items.iter().any(|(name, item)| {
+                                matches!(item, AssocItemId::TypeAliasId(_)) && name == assoc_name
+                            })
+                        })
+                    }
+                    TypeBound::Lifetime(_) | TypeBound::Error => false,
                 }
-                WherePredicateTypeTarget::TypeParam(local_id) => *local_id == param_id.local_id,
-            },
+            }
             WherePredicate::Lifetime { .. } => false,
         })
-        .flat_map(|pred| ctx.lower_where_predicate(pred, true).map(|p| make_binders(&generics, p)))
-        .collect()
+        .flat_map(|pred| {
+            ctx.lower_where_predicate(pred, true).map(|p| make_binders(db, &generics, p))
+        })
+        .collect();
+
+    let subst = generics.bound_vars_subst(db, DebruijnIndex::INNERMOST);
+    let explicitly_unsized_tys = ctx.unsized_types.into_inner();
+    let implicitly_sized_predicates =
+        implicitly_sized_clauses(db, param_id.parent, &explicitly_unsized_tys, &subst, &resolver)
+            .map(|p| make_binders(db, &generics, crate::wrap_empty_binders(p)));
+    predicates.extend(implicitly_sized_predicates);
+    predicates.into()
 }
 
 pub(crate) fn generic_predicates_for_param_recover(
     _db: &dyn HirDatabase,
     _cycle: &[String],
-    _param_id: &TypeParamId,
+    _def: &GenericDefId,
+    _param_id: &TypeOrConstParamId,
+    _assoc_name: &Option<Name>,
 ) -> Arc<[Binders<QuantifiedWhereClause>]> {
     Arc::new([])
 }
@@ -1054,22 +1178,21 @@ pub(crate) fn trait_environment_query(
     def: GenericDefId,
 ) -> Arc<TraitEnvironment> {
     let resolver = def.resolver(db.upcast());
-    let ctx = TyLoweringContext::new(db, &resolver)
-        .with_type_param_mode(TypeParamLoweringMode::Placeholder);
+    let ctx =
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Placeholder);
     let mut traits_in_scope = Vec::new();
     let mut clauses = Vec::new();
     for pred in resolver.where_predicates_in_scope() {
         for pred in ctx.lower_where_predicate(pred, false) {
             if let WhereClause::Implemented(tr) = &pred.skip_binders() {
-                traits_in_scope
-                    .push((tr.self_type_parameter(&Interner).clone(), tr.hir_trait_id()));
+                traits_in_scope.push((tr.self_type_parameter(Interner).clone(), tr.hir_trait_id()));
             }
-            let program_clause: chalk_ir::ProgramClause<Interner> = pred.cast(&Interner);
-            clauses.push(program_clause.into_from_env_clause(&Interner));
+            let program_clause: chalk_ir::ProgramClause<Interner> = pred.cast(Interner);
+            clauses.push(program_clause.into_from_env_clause(Interner));
         }
     }
 
-    let container: Option<AssocContainerId> = match def {
+    let container: Option<ItemContainerId> = match def {
         // FIXME: is there a function for this?
         GenericDefId::FunctionId(f) => Some(f.lookup(db.upcast()).container),
         GenericDefId::AdtId(_) => None,
@@ -1079,30 +1202,30 @@ pub(crate) fn trait_environment_query(
         GenericDefId::EnumVariantId(_) => None,
         GenericDefId::ConstId(c) => Some(c.lookup(db.upcast()).container),
     };
-    if let Some(AssocContainerId::TraitId(trait_id)) = container {
+    if let Some(ItemContainerId::TraitId(trait_id)) = container {
         // add `Self: Trait<T1, T2, ...>` to the environment in trait
         // function default implementations (and speculative code
         // inside consts or type aliases)
         cov_mark::hit!(trait_self_implements_self);
-        let substs = TyBuilder::type_params_subst(db, trait_id);
+        let substs = TyBuilder::placeholder_subst(db, trait_id);
         let trait_ref = TraitRef { trait_id: to_chalk_trait_id(trait_id), substitution: substs };
         let pred = WhereClause::Implemented(trait_ref);
-        let program_clause: chalk_ir::ProgramClause<Interner> = pred.cast(&Interner);
-        clauses.push(program_clause.into_from_env_clause(&Interner));
+        let program_clause: chalk_ir::ProgramClause<Interner> = pred.cast(Interner);
+        clauses.push(program_clause.into_from_env_clause(Interner));
     }
 
-    let subst = generics(db.upcast(), def).type_params_subst(db);
+    let subst = generics(db.upcast(), def).placeholder_subst(db);
     let explicitly_unsized_tys = ctx.unsized_types.into_inner();
     let implicitly_sized_clauses =
         implicitly_sized_clauses(db, def, &explicitly_unsized_tys, &subst, &resolver).map(|pred| {
-            let program_clause: chalk_ir::ProgramClause<Interner> = pred.cast(&Interner);
-            program_clause.into_from_env_clause(&Interner)
+            let program_clause: chalk_ir::ProgramClause<Interner> = pred.cast(Interner);
+            program_clause.into_from_env_clause(Interner)
         });
     clauses.extend(implicitly_sized_clauses);
 
     let krate = def.module(db.upcast()).krate();
 
-    let env = chalk_ir::Environment::new(&Interner).add_clauses(&Interner, clauses);
+    let env = chalk_ir::Environment::new(Interner).add_clauses(Interner, clauses);
 
     Arc::new(TraitEnvironment { krate, traits_from_clauses: traits_in_scope, env })
 }
@@ -1114,19 +1237,21 @@ pub(crate) fn generic_predicates_query(
 ) -> Arc<[Binders<QuantifiedWhereClause>]> {
     let resolver = def.resolver(db.upcast());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
     let generics = generics(db.upcast(), def);
 
     let mut predicates = resolver
         .where_predicates_in_scope()
-        .flat_map(|pred| ctx.lower_where_predicate(pred, false).map(|p| make_binders(&generics, p)))
+        .flat_map(|pred| {
+            ctx.lower_where_predicate(pred, false).map(|p| make_binders(db, &generics, p))
+        })
         .collect::<Vec<_>>();
 
-    let subst = generics.bound_vars_subst(DebruijnIndex::INNERMOST);
+    let subst = generics.bound_vars_subst(db, DebruijnIndex::INNERMOST);
     let explicitly_unsized_tys = ctx.unsized_types.into_inner();
     let implicitly_sized_predicates =
         implicitly_sized_clauses(db, def, &explicitly_unsized_tys, &subst, &resolver)
-            .map(|p| make_binders(&generics, crate::wrap_empty_binders(p)));
+            .map(|p| make_binders(db, &generics, crate::wrap_empty_binders(p)));
     predicates.extend(implicitly_sized_predicates);
     predicates.into()
 }
@@ -1141,21 +1266,21 @@ fn implicitly_sized_clauses<'a>(
     resolver: &Resolver,
 ) -> impl Iterator<Item = WhereClause> + 'a {
     let is_trait_def = matches!(def, GenericDefId::TraitId(..));
-    let generic_args = &substitution.as_slice(&Interner)[is_trait_def as usize..];
+    let generic_args = &substitution.as_slice(Interner)[is_trait_def as usize..];
     let sized_trait = resolver
         .krate()
-        .and_then(|krate| db.lang_item(krate, "sized".into()))
+        .and_then(|krate| db.lang_item(krate, SmolStr::new_inline("sized")))
         .and_then(|lang_item| lang_item.as_trait().map(to_chalk_trait_id));
 
     sized_trait.into_iter().flat_map(move |sized_trait| {
         let implicitly_sized_tys = generic_args
             .iter()
-            .filter_map(|generic_arg| generic_arg.ty(&Interner))
+            .filter_map(|generic_arg| generic_arg.ty(Interner))
             .filter(move |&self_ty| !explicitly_unsized_tys.contains(self_ty));
         implicitly_sized_tys.map(move |self_ty| {
             WhereClause::Implemented(TraitRef {
                 trait_id: sized_trait,
-                substitution: Substitution::from1(&Interner, self_ty.clone()),
+                substitution: Substitution::from1(Interner, self_ty.clone()),
             })
         })
     })
@@ -1165,32 +1290,36 @@ fn implicitly_sized_clauses<'a>(
 pub(crate) fn generic_defaults_query(
     db: &dyn HirDatabase,
     def: GenericDefId,
-) -> Arc<[Binders<Ty>]> {
+) -> Arc<[Binders<chalk_ir::GenericArg<Interner>>]> {
     let resolver = def.resolver(db.upcast());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
     let generic_params = generics(db.upcast(), def);
 
     let defaults = generic_params
         .iter()
         .enumerate()
-        .map(|(idx, (_, p))| {
+        .map(|(idx, (id, p))| {
+            let p = match p {
+                TypeOrConstParamData::TypeParamData(p) => p,
+                TypeOrConstParamData::ConstParamData(_) => {
+                    // FIXME: implement const generic defaults
+                    let val = unknown_const_as_generic(
+                        db.const_param_ty(ConstParamId::from_unchecked(id)),
+                    );
+                    return crate::make_binders_with_count(db, idx, &generic_params, val);
+                }
+            };
             let mut ty =
-                p.default.as_ref().map_or(TyKind::Error.intern(&Interner), |t| ctx.lower_ty(t));
+                p.default.as_ref().map_or(TyKind::Error.intern(Interner), |t| ctx.lower_ty(t));
 
             // Each default can only refer to previous parameters.
-            ty = crate::fold_free_vars(ty, |bound, binders| {
-                if bound.index >= idx && bound.debruijn == DebruijnIndex::INNERMOST {
-                    // type variable default referring to parameter coming
-                    // after it. This is forbidden (FIXME: report
-                    // diagnostic)
-                    TyKind::Error.intern(&Interner)
-                } else {
-                    bound.shifted_in_from(binders).to_ty(&Interner)
-                }
-            });
-
-            crate::make_only_type_binders(idx, ty)
+            // type variable default referring to parameter coming
+            // after it. This is forbidden (FIXME: report
+            // diagnostic)
+            ty = fallback_bound_vars(ty, idx);
+            let val = GenericArgData::Ty(ty).intern(Interner);
+            crate::make_binders_with_count(db, idx, &generic_params, val)
         })
         .collect();
 
@@ -1201,17 +1330,21 @@ pub(crate) fn generic_defaults_recover(
     db: &dyn HirDatabase,
     _cycle: &[String],
     def: &GenericDefId,
-) -> Arc<[Binders<Ty>]> {
+) -> Arc<[Binders<crate::GenericArg>]> {
     let generic_params = generics(db.upcast(), *def);
-
+    // FIXME: this code is not covered in tests.
     // we still need one default per parameter
     let defaults = generic_params
-        .iter()
+        .iter_id()
         .enumerate()
-        .map(|(idx, _)| {
-            let ty = TyKind::Error.intern(&Interner);
-
-            crate::make_only_type_binders(idx, ty)
+        .map(|(count, id)| {
+            let val = match id {
+                itertools::Either::Left(_) => {
+                    GenericArgData::Ty(TyKind::Error.intern(Interner)).intern(Interner)
+                }
+                itertools::Either::Right(id) => unknown_const_as_generic(db.const_param_ty(id)),
+            };
+            crate::make_binders_with_count(db, count, &generic_params, val)
         })
         .collect();
 
@@ -1223,24 +1356,29 @@ fn fn_sig_for_fn(db: &dyn HirDatabase, def: FunctionId) -> PolyFnSig {
     let resolver = def.resolver(db.upcast());
     let ctx_params = TyLoweringContext::new(db, &resolver)
         .with_impl_trait_mode(ImplTraitLoweringMode::Variable)
-        .with_type_param_mode(TypeParamLoweringMode::Variable);
-    let params = data.params.iter().map(|tr| ctx_params.lower_ty(tr)).collect::<Vec<_>>();
+        .with_type_param_mode(ParamLoweringMode::Variable);
+    let params = data.params.iter().map(|(_, tr)| ctx_params.lower_ty(tr)).collect::<Vec<_>>();
     let ctx_ret = TyLoweringContext::new(db, &resolver)
         .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
-        .with_type_param_mode(TypeParamLoweringMode::Variable);
+        .with_type_param_mode(ParamLoweringMode::Variable);
     let ret = ctx_ret.lower_ty(&data.ret_type);
     let generics = generics(db.upcast(), def.into());
-    make_binders(&generics, CallableSig::from_params_and_return(params, ret, data.is_varargs()))
+    let mut sig = CallableSig::from_params_and_return(params, ret, data.is_varargs());
+    if !data.legacy_const_generics_indices.is_empty() {
+        sig.set_legacy_const_generics_indices(&data.legacy_const_generics_indices);
+    }
+    make_binders(db, &generics, sig)
 }
 
 /// Build the declared type of a function. This should not need to look at the
 /// function body.
 fn type_for_fn(db: &dyn HirDatabase, def: FunctionId) -> Binders<Ty> {
     let generics = generics(db.upcast(), def.into());
-    let substs = generics.bound_vars_subst(DebruijnIndex::INNERMOST);
+    let substs = generics.bound_vars_subst(db, DebruijnIndex::INNERMOST);
     make_binders(
+        db,
         &generics,
-        TyKind::FnDef(CallableDefId::FunctionId(def).to_chalk(db), substs).intern(&Interner),
+        TyKind::FnDef(CallableDefId::FunctionId(def).to_chalk(db), substs).intern(Interner),
     )
 }
 
@@ -1250,9 +1388,9 @@ fn type_for_const(db: &dyn HirDatabase, def: ConstId) -> Binders<Ty> {
     let generics = generics(db.upcast(), def.into());
     let resolver = def.resolver(db.upcast());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
 
-    make_binders(&generics, ctx.lower_ty(&data.type_ref))
+    make_binders(db, &generics, ctx.lower_ty(&data.type_ref))
 }
 
 /// Build the declared type of a static.
@@ -1261,7 +1399,7 @@ fn type_for_static(db: &dyn HirDatabase, def: StaticId) -> Binders<Ty> {
     let resolver = def.resolver(db.upcast());
     let ctx = TyLoweringContext::new(db, &resolver);
 
-    Binders::empty(&Interner, ctx.lower_ty(&data.type_ref))
+    Binders::empty(Interner, ctx.lower_ty(&data.type_ref))
 }
 
 fn fn_sig_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> PolyFnSig {
@@ -1269,7 +1407,7 @@ fn fn_sig_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> PolyFnS
     let fields = struct_data.variant_data.fields();
     let resolver = def.resolver(db.upcast());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
     let params = fields.iter().map(|(_, field)| ctx.lower_ty(&field.type_ref)).collect::<Vec<_>>();
     let (ret, binders) = type_for_adt(db, def.into()).into_value_and_skipped_binders();
     Binders::new(binders, CallableSig::from_params_and_return(params, ret, false))
@@ -1282,10 +1420,11 @@ fn type_for_struct_constructor(db: &dyn HirDatabase, def: StructId) -> Binders<T
         return type_for_adt(db, def.into());
     }
     let generics = generics(db.upcast(), def.into());
-    let substs = generics.bound_vars_subst(DebruijnIndex::INNERMOST);
+    let substs = generics.bound_vars_subst(db, DebruijnIndex::INNERMOST);
     make_binders(
+        db,
         &generics,
-        TyKind::FnDef(CallableDefId::StructId(def).to_chalk(db), substs).intern(&Interner),
+        TyKind::FnDef(CallableDefId::StructId(def).to_chalk(db), substs).intern(Interner),
     )
 }
 
@@ -1295,7 +1434,7 @@ fn fn_sig_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId)
     let fields = var_data.variant_data.fields();
     let resolver = def.parent.resolver(db.upcast());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
     let params = fields.iter().map(|(_, field)| ctx.lower_ty(&field.type_ref)).collect::<Vec<_>>();
     let (ret, binders) = type_for_adt(db, def.parent.into()).into_value_and_skipped_binders();
     Binders::new(binders, CallableSig::from_params_and_return(params, ret, false))
@@ -1309,31 +1448,32 @@ fn type_for_enum_variant_constructor(db: &dyn HirDatabase, def: EnumVariantId) -
         return type_for_adt(db, def.parent.into());
     }
     let generics = generics(db.upcast(), def.parent.into());
-    let substs = generics.bound_vars_subst(DebruijnIndex::INNERMOST);
+    let substs = generics.bound_vars_subst(db, DebruijnIndex::INNERMOST);
     make_binders(
+        db,
         &generics,
-        TyKind::FnDef(CallableDefId::EnumVariantId(def).to_chalk(db), substs).intern(&Interner),
+        TyKind::FnDef(CallableDefId::EnumVariantId(def).to_chalk(db), substs).intern(Interner),
     )
 }
 
 fn type_for_adt(db: &dyn HirDatabase, adt: AdtId) -> Binders<Ty> {
     let generics = generics(db.upcast(), adt.into());
-    let b = TyBuilder::adt(db, adt);
-    let ty = b.fill_with_bound_vars(DebruijnIndex::INNERMOST, 0).build();
-    make_binders(&generics, ty)
+    let subst = generics.bound_vars_subst(db, DebruijnIndex::INNERMOST);
+    let ty = TyKind::Adt(crate::AdtId(adt), subst).intern(Interner);
+    make_binders(db, &generics, ty)
 }
 
 fn type_for_type_alias(db: &dyn HirDatabase, t: TypeAliasId) -> Binders<Ty> {
     let generics = generics(db.upcast(), t.into());
     let resolver = t.resolver(db.upcast());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
     if db.type_alias_data(t).is_extern {
-        Binders::empty(&Interner, TyKind::Foreign(crate::to_foreign_def_id(t)).intern(&Interner))
+        Binders::empty(Interner, TyKind::Foreign(crate::to_foreign_def_id(t)).intern(Interner))
     } else {
         let type_ref = &db.type_alias_data(t).type_ref;
         let inner = ctx.lower_ty(type_ref.as_deref().unwrap_or(&TypeRef::Error));
-        make_binders(&generics, inner)
+        make_binders(db, &generics, inner)
     }
 }
 
@@ -1392,7 +1532,7 @@ pub enum ValueTyDefId {
 /// namespace.
 pub(crate) fn ty_query(db: &dyn HirDatabase, def: TyDefId) -> Binders<Ty> {
     match def {
-        TyDefId::BuiltinType(it) => Binders::empty(&Interner, TyBuilder::builtin(it)),
+        TyDefId::BuiltinType(it) => Binders::empty(Interner, TyBuilder::builtin(it)),
         TyDefId::AdtId(it) => type_for_adt(db, it),
         TyDefId::TypeAliasId(it) => type_for_type_alias(db, it),
     }
@@ -1400,13 +1540,11 @@ pub(crate) fn ty_query(db: &dyn HirDatabase, def: TyDefId) -> Binders<Ty> {
 
 pub(crate) fn ty_recover(db: &dyn HirDatabase, _cycle: &[String], def: &TyDefId) -> Binders<Ty> {
     let generics = match *def {
-        TyDefId::BuiltinType(_) => {
-            return Binders::empty(&Interner, TyKind::Error.intern(&Interner))
-        }
+        TyDefId::BuiltinType(_) => return Binders::empty(Interner, TyKind::Error.intern(Interner)),
         TyDefId::AdtId(it) => generics(db.upcast(), it.into()),
         TyDefId::TypeAliasId(it) => generics(db.upcast(), it.into()),
     };
-    make_binders(&generics, TyKind::Error.intern(&Interner))
+    make_binders(db, &generics, TyKind::Error.intern(Interner))
 }
 
 pub(crate) fn value_ty_query(db: &dyn HirDatabase, def: ValueTyDefId) -> Binders<Ty> {
@@ -1421,21 +1559,32 @@ pub(crate) fn value_ty_query(db: &dyn HirDatabase, def: ValueTyDefId) -> Binders
 }
 
 pub(crate) fn impl_self_ty_query(db: &dyn HirDatabase, impl_id: ImplId) -> Binders<Ty> {
+    let impl_loc = impl_id.lookup(db.upcast());
     let impl_data = db.impl_data(impl_id);
     let resolver = impl_id.resolver(db.upcast());
+    let _cx = stdx::panic_context::enter(format!(
+        "impl_self_ty_query({:?} -> {:?} -> {:?})",
+        impl_id, impl_loc, impl_data
+    ));
     let generics = generics(db.upcast(), impl_id.into());
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
-    make_binders(&generics, ctx.lower_ty(&impl_data.self_ty))
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
+    make_binders(db, &generics, ctx.lower_ty(&impl_data.self_ty))
 }
 
+// returns None if def is a type arg
 pub(crate) fn const_param_ty_query(db: &dyn HirDatabase, def: ConstParamId) -> Ty {
-    let parent_data = db.generic_params(def.parent);
-    let data = &parent_data.consts[def.local_id];
-    let resolver = def.parent.resolver(db.upcast());
+    let parent_data = db.generic_params(def.parent());
+    let data = &parent_data.type_or_consts[def.local_id()];
+    let resolver = def.parent().resolver(db.upcast());
     let ctx = TyLoweringContext::new(db, &resolver);
-
-    ctx.lower_ty(&data.ty)
+    match data {
+        TypeOrConstParamData::TypeParamData(_) => {
+            never!();
+            Ty::new(Interner, TyKind::Error)
+        }
+        TypeOrConstParamData::ConstParamData(d) => ctx.lower_ty(&d.ty),
+    }
 }
 
 pub(crate) fn impl_self_ty_recover(
@@ -1444,16 +1593,19 @@ pub(crate) fn impl_self_ty_recover(
     impl_id: &ImplId,
 ) -> Binders<Ty> {
     let generics = generics(db.upcast(), (*impl_id).into());
-    make_binders(&generics, TyKind::Error.intern(&Interner))
+    make_binders(db, &generics, TyKind::Error.intern(Interner))
 }
 
 pub(crate) fn impl_trait_query(db: &dyn HirDatabase, impl_id: ImplId) -> Option<Binders<TraitRef>> {
+    let impl_loc = impl_id.lookup(db.upcast());
     let impl_data = db.impl_data(impl_id);
     let resolver = impl_id.resolver(db.upcast());
-    let _cx =
-        stdx::panic_context::enter(format!("impl_trait_query({:?} -> {:?})", impl_id, impl_data));
+    let _cx = stdx::panic_context::enter(format!(
+        "impl_trait_query({:?} -> {:?} -> {:?})",
+        impl_id, impl_loc, impl_data
+    ));
     let ctx =
-        TyLoweringContext::new(db, &resolver).with_type_param_mode(TypeParamLoweringMode::Variable);
+        TyLoweringContext::new(db, &resolver).with_type_param_mode(ParamLoweringMode::Variable);
     let (self_ty, binders) = db.impl_self_ty(impl_id).into_value_and_skipped_binders();
     let target_trait = impl_data.target_trait.as_ref()?;
     Some(Binders::new(binders, ctx.lower_trait_ref(target_trait, Some(self_ty))?))
@@ -1468,7 +1620,7 @@ pub(crate) fn return_type_impl_traits(
     let resolver = def.resolver(db.upcast());
     let ctx_ret = TyLoweringContext::new(db, &resolver)
         .with_impl_trait_mode(ImplTraitLoweringMode::Opaque)
-        .with_type_param_mode(TypeParamLoweringMode::Variable);
+        .with_type_param_mode(ParamLoweringMode::Variable);
     let _ret = (&ctx_ret).lower_ty(&data.ret_type);
     let generics = generics(db.upcast(), def.into());
     let return_type_impl_traits =
@@ -1476,7 +1628,7 @@ pub(crate) fn return_type_impl_traits(
     if return_type_impl_traits.impl_traits.is_empty() {
         None
     } else {
-        Some(Arc::new(make_binders(&generics, return_type_impl_traits)))
+        Some(Arc::new(make_binders(db, &generics, return_type_impl_traits)))
     }
 }
 
@@ -1487,6 +1639,65 @@ pub(crate) fn lower_to_chalk_mutability(m: hir_def::type_ref::Mutability) -> Mut
     }
 }
 
-fn make_binders<T: HasInterner<Interner = Interner>>(generics: &Generics, value: T) -> Binders<T> {
-    crate::make_only_type_binders(generics.len(), value)
+pub(crate) fn generic_arg_to_chalk<'a, T>(
+    db: &dyn HirDatabase,
+    kind_id: Either<TypeParamId, ConstParamId>,
+    arg: &'a GenericArg,
+    this: &mut T,
+    for_type: impl FnOnce(&mut T, &TypeRef) -> Ty + 'a,
+    for_const: impl FnOnce(&mut T, &ConstScalarOrPath) -> Const + 'a,
+) -> Option<crate::GenericArg> {
+    let kind = match kind_id {
+        Either::Left(_) => ParamKind::Type,
+        Either::Right(id) => {
+            let ty = db.const_param_ty(id);
+            ParamKind::Const(ty)
+        }
+    };
+    Some(match (arg, kind) {
+        (GenericArg::Type(type_ref), ParamKind::Type) => {
+            let ty = for_type(this, type_ref);
+            GenericArgData::Ty(ty).intern(Interner)
+        }
+        (GenericArg::Const(c), ParamKind::Const(_)) => {
+            GenericArgData::Const(for_const(this, c)).intern(Interner)
+        }
+        (GenericArg::Const(_), ParamKind::Type) => {
+            GenericArgData::Ty(TyKind::Error.intern(Interner)).intern(Interner)
+        }
+        (GenericArg::Type(t), ParamKind::Const(ty)) => {
+            // We want to recover simple idents, which parser detects them
+            // as types. Maybe here is not the best place to do it, but
+            // it works.
+            if let TypeRef::Path(p) = t {
+                let p = p.mod_path();
+                if p.kind == PathKind::Plain {
+                    if let [n] = p.segments() {
+                        let c = ConstScalarOrPath::Path(n.clone());
+                        return Some(GenericArgData::Const(for_const(this, &c)).intern(Interner));
+                    }
+                }
+            }
+            unknown_const_as_generic(ty)
+        }
+        (GenericArg::Lifetime(_), _) => return None,
+    })
+}
+
+pub(crate) fn const_or_path_to_chalk(
+    db: &dyn HirDatabase,
+    resolver: &Resolver,
+    value: &ConstScalarOrPath,
+    mode: ParamLoweringMode,
+    args: impl FnOnce() -> Generics,
+    debruijn: DebruijnIndex,
+) -> Const {
+    match value {
+        ConstScalarOrPath::Scalar(s) => usize_const(s.as_usize()),
+        ConstScalarOrPath::Path(n) => {
+            let path = ModPath::from_segments(PathKind::Plain, Some(n.clone()));
+            path_to_const(db, resolver, &path, mode, args, debruijn)
+                .unwrap_or_else(|| unknown_const_usize())
+        }
+    }
 }