Merge #3050

3050: Refactor type parameters, implement argument position impl trait r=matklad a=flodiebold

I wanted to implement APIT by lowering to type parameters because we need to do that anyway for correctness and don't need Chalk support for it; this grew into some more wide-ranging refactoring of how type parameters are handled :sweat_smile: 

 - use Ty::Bound instead of Ty::Param to represent polymorphism, and explicitly
   count binders. This gets us closer to Chalk's way of doing things, and means
   that we now only use Param as a placeholder for an unknown type, e.g. within
   a generic function. I.e. we're never using Param in a situation where we want
   to substitute it, and the method to do that is gone; `subst` now always works
   on bound variables. (This changes how the types of generic functions print; 
   previously, you'd get something like `fn identity<i32>(T) -> T`, but now we
   display the substituted signature `fn identity<i32>(i32) -> i32`, which I think 
   makes more sense.)
 - once we do this, it's more natural to represent `Param` by a globally unique
   ID; the use of indices was mostly to make substituting easier. This also
   means we fix the bug where `Param` loses its name when going through Chalk.
 - I would actually like to rename `Param` to `Placeholder` to better reflect its use and
   get closer to Chalk, but I'll leave that to a follow-up.
 - introduce a context for type lowering, to allow lowering `impl Trait` to
   different things depending on where we are. And since we have that, we can
   also lower type parameters directly to variables instead of placeholders.
   Also, we'll be able to use this later to collect diagnostics.
 - implement argument position impl trait by lowering it to type parameters.
   I've realized that this is necessary to correctly implement it; e.g. consider
   `fn foo(impl Display) -> impl Something`. It's observable that the return
   type of e.g. `foo(1u32)` unifies with itself, but doesn't unify with e.g.
   `foo(1i32)`; so the return type needs to be parameterized by the argument
   type.

   
This fixes a few bugs as well:
 - type parameters 'losing' their name when they go through Chalk, as mentioned
   above (i.e. getting `[missing name]` somewhere)
 - impl trait not being considered as implementing the super traits (very
   noticeable for the `db` in RA)
 - the fact that argument impl trait was only turned into variables when the
   function got called caused type mismatches when the function was used as a
   value (fixes a few type mismatches in RA)

The one thing I'm not so happy with here is how we're lowering `impl Trait` types to variables; since `TypeRef`s don't have an identity currently, we just count how many of them we have seen while going through the function signature. That's quite fragile though, since we have to do it while desugaring generics and while lowering the type signature, and in the exact same order in both cases. We could consider either giving only `TypeRef::ImplTrait` a local id, or maybe just giving all `TypeRef`s an identity after all (we talked about this before)...

Follow-up tasks:
 - handle return position impl trait; we basically need to create a variable and some trait obligations for that variable
 - rename `Param` to `Placeholder`

Co-authored-by: Florian Diebold <[email protected]>
Co-authored-by: Florian Diebold <[email protected]>

4 files changed, 46 insertions, 63 deletions

diff --git a/crates/ra_hir_ty/src/infer/coerce.rs b/crates/ra_hir_ty/src/infer/coerce.rs
index 83c0c2c3f..f68a1439f 100644
--- a/crates/ra_hir_ty/src/infer/coerce.rs
+++ b/crates/ra_hir_ty/src/infer/coerce.rs
@@ -57,8 +57,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 let trait_ref = db.impl_trait(impl_id)?;
 
                 // `CoerseUnsized` has one generic parameter for the target type.
-                let cur_from_ty = trait_ref.substs.0.get(0)?;
-                let cur_to_ty = trait_ref.substs.0.get(1)?;
+                let cur_from_ty = trait_ref.value.substs.0.get(0)?;
+                let cur_to_ty = trait_ref.value.substs.0.get(1)?;
 
                 match (&cur_from_ty, cur_to_ty) {
                     (ty_app!(ctor1, st1), ty_app!(ctor2, st2)) => {
@@ -66,9 +66,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                         // This works for smart-pointer-like coercion, which covers all impls from std.
                         st1.iter().zip(st2.iter()).enumerate().find_map(|(i, (ty1, ty2))| {
                             match (ty1, ty2) {
-                                (Ty::Param { idx: p1, .. }, Ty::Param { idx: p2, .. })
-                                    if p1 != p2 =>
-                                {
+                                (Ty::Bound(idx1), Ty::Bound(idx2)) if idx1 != idx2 => {
                                     Some(((*ctor1, *ctor2), i))
                                 }
                                 _ => None,
@@ -256,8 +254,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 let unsize_generic_index = {
                     let mut index = None;
                     let mut multiple_param = false;
-                    field_tys[last_field_id].walk(&mut |ty| match ty {
-                        &Ty::Param { idx, .. } => {
+                    field_tys[last_field_id].value.walk(&mut |ty| match ty {
+                        &Ty::Bound(idx) => {
                             if index.is_none() {
                                 index = Some(idx);
                             } else if Some(idx) != index {
@@ -276,10 +274,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 // Check other fields do not involve it.
                 let mut multiple_used = false;
                 fields.for_each(|(field_id, _data)| {
-                    field_tys[field_id].walk(&mut |ty| match ty {
-                        &Ty::Param { idx, .. } if idx == unsize_generic_index => {
-                            multiple_used = true
-                        }
+                    field_tys[field_id].value.walk(&mut |ty| match ty {
+                        &Ty::Bound(idx) if idx == unsize_generic_index => multiple_used = true,
                         _ => {}
                     })
                 });
diff --git a/crates/ra_hir_ty/src/infer/expr.rs b/crates/ra_hir_ty/src/infer/expr.rs
index 31259a01d..3c9c02d03 100644
--- a/crates/ra_hir_ty/src/infer/expr.rs
+++ b/crates/ra_hir_ty/src/infer/expr.rs
@@ -10,7 +10,7 @@ use hir_def::{
     resolver::resolver_for_expr,
     AdtId, AssocContainerId, Lookup, StructFieldId,
 };
-use hir_expand::name::{name, Name};
+use hir_expand::name::Name;
 use ra_syntax::ast::RangeOp;
 
 use crate::{
@@ -19,8 +19,8 @@ use crate::{
     method_resolution, op,
     traits::InEnvironment,
     utils::{generics, variant_data, Generics},
-    ApplicationTy, CallableDef, InferTy, IntTy, Mutability, Obligation, Substs, TraitRef, Ty,
-    TypeCtor, TypeWalk, Uncertain,
+    ApplicationTy, Binders, CallableDef, InferTy, IntTy, Mutability, Obligation, Substs, TraitRef,
+    Ty, TypeCtor, Uncertain,
 };
 
 use super::{BindingMode, Expectation, InferenceContext, InferenceDiagnostic, TypeMismatch};
@@ -236,8 +236,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                         self.result.record_field_resolutions.insert(field.expr, field_def);
                     }
                     let field_ty = field_def
-                        .map_or(Ty::Unknown, |it| field_types[it.local_id].clone())
-                        .subst(&substs);
+                        .map_or(Ty::Unknown, |it| field_types[it.local_id].clone().subst(&substs));
                     self.infer_expr_coerce(field.expr, &Expectation::has_type(field_ty));
                 }
                 if let Some(expr) = spread {
@@ -588,10 +587,10 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.write_method_resolution(tgt_expr, func);
                 (ty, self.db.value_ty(func.into()), Some(generics(self.db, func.into())))
             }
-            None => (receiver_ty, Ty::Unknown, None),
+            None => (receiver_ty, Binders::new(0, Ty::Unknown), None),
         };
         let substs = self.substs_for_method_call(def_generics, generic_args, &derefed_receiver_ty);
-        let method_ty = method_ty.apply_substs(substs);
+        let method_ty = method_ty.subst(&substs);
         let method_ty = self.insert_type_vars(method_ty);
         self.register_obligations_for_call(&method_ty);
         let (expected_receiver_ty, param_tys, ret_ty) = match method_ty.callable_sig(self.db) {
@@ -635,7 +634,6 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     continue;
                 }
 
-                let param_ty = self.insert_vars_for_impl_trait(param_ty);
                 let param_ty = self.normalize_associated_types_in(param_ty);
                 self.infer_expr_coerce(arg, &Expectation::has_type(param_ty.clone()));
             }
@@ -648,13 +646,15 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         generic_args: Option<&GenericArgs>,
         receiver_ty: &Ty,
     ) -> Substs {
-        let (total_len, _parent_len, child_len) =
-            def_generics.as_ref().map_or((0, 0, 0), |g| g.len_split());
+        let (parent_params, self_params, type_params, impl_trait_params) =
+            def_generics.as_ref().map_or((0, 0, 0, 0), |g| g.provenance_split());
+        assert_eq!(self_params, 0); // method shouldn't have another Self param
+        let total_len = parent_params + type_params + impl_trait_params;
         let mut substs = Vec::with_capacity(total_len);
         // Parent arguments are unknown, except for the receiver type
         if let Some(parent_generics) = def_generics.as_ref().map(|p| p.iter_parent()) {
             for (_id, param) in parent_generics {
-                if param.name == name![Self] {
+                if param.provenance == hir_def::generics::TypeParamProvenance::TraitSelf {
                     substs.push(receiver_ty.clone());
                 } else {
                     substs.push(Ty::Unknown);
@@ -664,7 +664,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         // handle provided type arguments
         if let Some(generic_args) = generic_args {
             // if args are provided, it should be all of them, but we can't rely on that
-            for arg in generic_args.args.iter().take(child_len) {
+            for arg in generic_args.args.iter().take(type_params) {
                 match arg {
                     GenericArg::Type(type_ref) => {
                         let ty = self.make_ty(type_ref);
diff --git a/crates/ra_hir_ty/src/infer/pat.rs b/crates/ra_hir_ty/src/infer/pat.rs
index a14662884..e7283f24c 100644
--- a/crates/ra_hir_ty/src/infer/pat.rs
+++ b/crates/ra_hir_ty/src/infer/pat.rs
@@ -12,7 +12,7 @@ use hir_expand::name::Name;
 use test_utils::tested_by;
 
 use super::{BindingMode, InferenceContext};
-use crate::{db::HirDatabase, utils::variant_data, Substs, Ty, TypeCtor, TypeWalk};
+use crate::{db::HirDatabase, utils::variant_data, Substs, Ty, TypeCtor};
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     fn infer_tuple_struct_pat(
@@ -34,8 +34,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             let expected_ty = var_data
                 .as_ref()
                 .and_then(|d| d.field(&Name::new_tuple_field(i)))
-                .map_or(Ty::Unknown, |field| field_tys[field].clone())
-                .subst(&substs);
+                .map_or(Ty::Unknown, |field| field_tys[field].clone().subst(&substs));
             let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat, &expected_ty, default_bm);
         }
@@ -65,7 +64,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         for subpat in subpats {
             let matching_field = var_data.as_ref().and_then(|it| it.field(&subpat.name));
             let expected_ty =
-                matching_field.map_or(Ty::Unknown, |field| field_tys[field].clone()).subst(&substs);
+                matching_field.map_or(Ty::Unknown, |field| field_tys[field].clone().subst(&substs));
             let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat.pat, &expected_ty, default_bm);
         }
diff --git a/crates/ra_hir_ty/src/infer/path.rs b/crates/ra_hir_ty/src/infer/path.rs
index 2c1d4831d..686ce7a21 100644
--- a/crates/ra_hir_ty/src/infer/path.rs
+++ b/crates/ra_hir_ty/src/infer/path.rs
@@ -9,9 +9,9 @@ use hir_def::{
 };
 use hir_expand::name::Name;
 
-use crate::{db::HirDatabase, method_resolution, Substs, Ty, TypeWalk, ValueTyDefId};
+use crate::{db::HirDatabase, method_resolution, Substs, Ty, ValueTyDefId};
 
-use super::{ExprOrPatId, InferenceContext, TraitEnvironment, TraitRef};
+use super::{ExprOrPatId, InferenceContext, TraitRef};
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     pub(super) fn infer_path(
@@ -39,7 +39,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             }
             let ty = self.make_ty(type_ref);
             let remaining_segments_for_ty = path.segments().take(path.segments().len() - 1);
-            let ty = Ty::from_type_relative_path(self.db, resolver, ty, remaining_segments_for_ty);
+            let ctx = crate::lower::TyLoweringContext::new(self.db, &resolver);
+            let ty = Ty::from_type_relative_path(&ctx, ty, remaining_segments_for_ty);
             self.resolve_ty_assoc_item(
                 ty,
                 &path.segments().last().expect("path had at least one segment").name,
@@ -69,12 +70,16 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             ValueNs::EnumVariantId(it) => it.into(),
         };
 
-        let mut ty = self.db.value_ty(typable);
-        if let Some(self_subst) = self_subst {
-            ty = ty.subst(&self_subst);
-        }
-        let substs = Ty::substs_from_path(self.db, &self.resolver, path, typable);
-        let ty = ty.subst(&substs);
+        let ty = self.db.value_ty(typable);
+        // self_subst is just for the parent
+        let parent_substs = self_subst.unwrap_or_else(Substs::empty);
+        let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
+        let substs = Ty::substs_from_path(&ctx, path, typable);
+        let full_substs = Substs::builder(substs.len())
+            .use_parent_substs(&parent_substs)
+            .fill(substs.0[parent_substs.len()..].iter().cloned())
+            .build();
+        let ty = ty.subst(&full_substs);
         Some(ty)
     }
 
@@ -98,13 +103,9 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             (TypeNs::TraitId(trait_), true) => {
                 let segment =
                     remaining_segments.last().expect("there should be at least one segment here");
-                let trait_ref = TraitRef::from_resolved_path(
-                    self.db,
-                    &self.resolver,
-                    trait_.into(),
-                    resolved_segment,
-                    None,
-                );
+                let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
+                let trait_ref =
+                    TraitRef::from_resolved_path(&ctx, trait_.into(), resolved_segment, None);
                 self.resolve_trait_assoc_item(trait_ref, segment, id)
             }
             (def, _) => {
@@ -114,9 +115,9 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 // as Iterator>::Item::default`)
                 let remaining_segments_for_ty =
                     remaining_segments.take(remaining_segments.len() - 1);
+                let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
                 let ty = Ty::from_partly_resolved_hir_path(
-                    self.db,
-                    &self.resolver,
+                    &ctx,
                     def,
                     resolved_segment,
                     remaining_segments_for_ty,
@@ -173,13 +174,9 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             AssocItemId::ConstId(c) => ValueNs::ConstId(c),
             AssocItemId::TypeAliasId(_) => unreachable!(),
         };
-        let substs = Substs::build_for_def(self.db, item)
-            .use_parent_substs(&trait_ref.substs)
-            .fill_with_params()
-            .build();
 
         self.write_assoc_resolution(id, item);
-        Some((def, Some(substs)))
+        Some((def, Some(trait_ref.substs)))
     }
 
     fn resolve_ty_assoc_item(
@@ -193,14 +190,13 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         }
 
         let canonical_ty = self.canonicalizer().canonicalize_ty(ty.clone());
-        let env = TraitEnvironment::lower(self.db, &self.resolver);
         let krate = self.resolver.krate()?;
         let traits_in_scope = self.resolver.traits_in_scope(self.db);
 
         method_resolution::iterate_method_candidates(
             &canonical_ty.value,
             self.db,
-            env,
+            self.trait_env.clone(),
             krate,
             &traits_in_scope,
             Some(name),
@@ -219,12 +215,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                             .fill(iter::repeat_with(|| self.table.new_type_var()))
                             .build();
                         let impl_self_ty = self.db.impl_self_ty(impl_id).subst(&impl_substs);
-                        let substs = Substs::build_for_def(self.db, item)
-                            .use_parent_substs(&impl_substs)
-                            .fill_with_params()
-                            .build();
                         self.unify(&impl_self_ty, &ty);
-                        Some(substs)
+                        Some(impl_substs)
                     }
                     AssocContainerId::TraitId(trait_) => {
                         // we're picking this method
@@ -232,15 +224,11 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                             .push(ty.clone())
                             .fill(std::iter::repeat_with(|| self.table.new_type_var()))
                             .build();
-                        let substs = Substs::build_for_def(self.db, item)
-                            .use_parent_substs(&trait_substs)
-                            .fill_with_params()
-                            .build();
                         self.obligations.push(super::Obligation::Trait(TraitRef {
                             trait_,
-                            substs: trait_substs,
+                            substs: trait_substs.clone(),
                         }));
-                        Some(substs)
+                        Some(trait_substs)
                     }
                     AssocContainerId::ContainerId(_) => None,
                 };