Auto merge of rust-lang#14436 - lowr:patch/normalize-assoc-type-in-path-expr, r=HKalbasi

Normalize associated types in paths in expressions

Part of rust-lang#14393

When we resolve paths in expressions (either path expressions or paths in struct expressions), there's a need of projection normalization, which `TyLoweringContext` cannot do on its own. We've been properly applying normalization for paths in struct expressions without type anchor, but not for others:

```rust
enum E {
    S { v: i32 }
    Empty,
}

impl Foo for Bar {
    type Assoc = E;
    fn foo() {
        let _ = Self::Assoc::S { v: 42 };   // path in struct expr without type anchor; we already support this
        let _ = <Self>::Assoc::S { v: 42 }; // path in struct expr with type anchor; resolves with this PR
        let _ = Self::Assoc::Empty;         // path expr; resolves with this PR
    }
}
```

With this PR we correctly resolve the whole path, but we need some more tweaks in HIR and/or IDE layers to properly resolve a qualifier (prefix) of such paths and provide IDE features that are pointed out in rust-lang#14393 to be currently broken.

Author: bors

crates/hir-def/src/resolver.rs

@@ -79,7 +79,7 @@ enum Scope {
     ExprScope(ExprScope),
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum TypeNs {
     SelfType(ImplId),
     GenericParam(TypeParamId),

crates/hir-ty/src/infer.rs

@@ -16,7 +16,7 @@
 use std::sync::Arc;
 use std::{convert::identity, ops::Index};
 
-use chalk_ir::{cast::Cast, ConstValue, DebruijnIndex, Mutability, Safety, Scalar, TypeFlags};
+use chalk_ir::{cast::Cast, DebruijnIndex, Mutability, Safety, Scalar, TypeFlags};
 use either::Either;
 use hir_def::{
     body::Body,
@@ -37,10 +37,10 @@ use rustc_hash::{FxHashMap, FxHashSet};
 use stdx::{always, never};
 
 use crate::{
-    db::HirDatabase, fold_tys, fold_tys_and_consts, infer::coerce::CoerceMany,
-    lower::ImplTraitLoweringMode, static_lifetime, to_assoc_type_id, AliasEq, AliasTy, Const,
-    DomainGoal, GenericArg, Goal, ImplTraitId, InEnvironment, Interner, ProjectionTy, RpitId,
-    Substitution, TraitRef, Ty, TyBuilder, TyExt, TyKind,
+    db::HirDatabase, fold_tys, infer::coerce::CoerceMany, lower::ImplTraitLoweringMode,
+    static_lifetime, to_assoc_type_id, AliasEq, AliasTy, DomainGoal, GenericArg, Goal, ImplTraitId,
+    InEnvironment, Interner, ProjectionTy, RpitId, Substitution, TraitRef, Ty, TyBuilder, TyExt,
+    TyKind,
 };
 
 // This lint has a false positive here. See the link below for details.
@@ -744,43 +744,13 @@ impl<'a> InferenceContext<'a> {
         self.result.standard_types.unknown.clone()
     }
 
-    /// Replaces ConstScalar::Unknown by a new type var, so we can maybe still infer it.
-    fn insert_const_vars_shallow(&mut self, c: Const) -> Const {
-        let data = c.data(Interner);
-        match &data.value {
-            ConstValue::Concrete(cc) => match cc.interned {
-                crate::ConstScalar::Unknown => self.table.new_const_var(data.ty.clone()),
-                _ => c,
-            },
-            _ => c,
-        }
-    }
-
     /// Replaces `Ty::Error` by a new type var, so we can maybe still infer it.
     fn insert_type_vars_shallow(&mut self, ty: Ty) -> Ty {
-        match ty.kind(Interner) {
-            TyKind::Error => self.table.new_type_var(),
-            TyKind::InferenceVar(..) => {
-                let ty_resolved = self.resolve_ty_shallow(&ty);
-                if ty_resolved.is_unknown() {
-                    self.table.new_type_var()
-                } else {
-                    ty
-                }
-            }
-            _ => ty,
-        }
+        self.table.insert_type_vars_shallow(ty)
     }
 
     fn insert_type_vars(&mut self, ty: Ty) -> Ty {
-        fold_tys_and_consts(
-            ty,
-            |x, _| match x {
-                Either::Left(ty) => Either::Left(self.insert_type_vars_shallow(ty)),
-                Either::Right(c) => Either::Right(self.insert_const_vars_shallow(c)),
-            },
-            DebruijnIndex::INNERMOST,
-        )
+        self.table.insert_type_vars(ty)
     }
 
     fn push_obligation(&mut self, o: DomainGoal) {
@@ -909,8 +879,6 @@ impl<'a> InferenceContext<'a> {
             None => return (self.err_ty(), None),
         };
         let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
-        // FIXME: this should resolve assoc items as well, see this example:
-        // https://play.rust-lang.org/?gist=087992e9e22495446c01c0d4e2d69521
         let (resolution, unresolved) = if value_ns {
             match self.resolver.resolve_path_in_value_ns(self.db.upcast(), path) {
                 Some(ResolveValueResult::ValueNs(value)) => match value {
@@ -964,8 +932,68 @@ impl<'a> InferenceContext<'a> {
             TypeNs::SelfType(impl_id) => {
                 let generics = crate::utils::generics(self.db.upcast(), impl_id.into());
                 let substs = generics.placeholder_subst(self.db);
-                let ty = self.db.impl_self_ty(impl_id).substitute(Interner, &substs);
-                self.resolve_variant_on_alias(ty, unresolved, mod_path)
+                let mut ty = self.db.impl_self_ty(impl_id).substitute(Interner, &substs);
+
+                let Some(mut remaining_idx) = unresolved else {
+                    return self.resolve_variant_on_alias(ty, None, mod_path);
+                };
+
+                let mut remaining_segments = path.segments().skip(remaining_idx);
+
+                // We need to try resolving unresolved segments one by one because each may resolve
+                // to a projection, which `TyLoweringContext` cannot handle on its own.
+                while !remaining_segments.is_empty() {
+                    let resolved_segment = path.segments().get(remaining_idx - 1).unwrap();
+                    let current_segment = remaining_segments.take(1);
+
+                    // If we can resolve to an enum variant, it takes priority over associated type
+                    // of the same name.
+                    if let Some((AdtId::EnumId(id), _)) = ty.as_adt() {
+                        let enum_data = self.db.enum_data(id);
+                        let name = current_segment.first().unwrap().name;
+                        if let Some(local_id) = enum_data.variant(name) {
+                            let variant = EnumVariantId { parent: id, local_id };
+                            return if remaining_segments.len() == 1 {
+                                (ty, Some(variant.into()))
+                            } else {
+                                // We still have unresolved paths, but enum variants never have
+                                // associated types!
+                                (self.err_ty(), None)
+                            };
+                        }
+                    }
+
+                    // `lower_partly_resolved_path()` returns `None` as type namespace unless
+                    // `remaining_segments` is empty, which is never the case here. We don't know
+                    // which namespace the new `ty` is in until normalized anyway.
+                    (ty, _) = ctx.lower_partly_resolved_path(
+                        resolution,
+                        resolved_segment,
+                        current_segment,
+                        false,
+                    );
+
+                    ty = self.table.insert_type_vars(ty);
+                    ty = self.table.normalize_associated_types_in(ty);
+                    ty = self.table.resolve_ty_shallow(&ty);
+                    if ty.is_unknown() {
+                        return (self.err_ty(), None);
+                    }
+
+                    // FIXME(inherent_associated_types): update `resolution` based on `ty` here.
+                    remaining_idx += 1;
+                    remaining_segments = remaining_segments.skip(1);
+                }
+
+                let variant = ty.as_adt().and_then(|(id, _)| match id {
+                    AdtId::StructId(s) => Some(VariantId::StructId(s)),
+                    AdtId::UnionId(u) => Some(VariantId::UnionId(u)),
+                    AdtId::EnumId(_) => {
+                        // FIXME Error E0071, expected struct, variant or union type, found enum `Foo`
+                        None
+                    }
+                });
+                (ty, variant)
             }
             TypeNs::TypeAliasId(it) => {
                 let container = it.lookup(self.db.upcast()).container;

crates/hir-ty/src/infer/path.rs

@@ -30,11 +30,18 @@ impl<'a> InferenceContext<'a> {
 
     fn resolve_value_path(&mut self, path: &Path, id: ExprOrPatId) -> Option<Ty> {
         let (value, self_subst) = if let Some(type_ref) = path.type_anchor() {
-            let Some(last) = path.segments().last() else { return None };
-            let ty = self.make_ty(type_ref);
-            let remaining_segments_for_ty = path.segments().take(path.segments().len() - 1);
+            let last = path.segments().last()?;
+
+            // Don't use `self.make_ty()` here as we need `orig_ns`.
             let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver);
-            let (ty, _) = ctx.lower_ty_relative_path(ty, None, remaining_segments_for_ty);
+            let (ty, orig_ns) = ctx.lower_ty_ext(type_ref);
+            let ty = self.table.insert_type_vars(ty);
+            let ty = self.table.normalize_associated_types_in(ty);
+
+            let remaining_segments_for_ty = path.segments().take(path.segments().len() - 1);
+            let (ty, _) = ctx.lower_ty_relative_path(ty, orig_ns, remaining_segments_for_ty);
+            let ty = self.table.insert_type_vars(ty);
+            let ty = self.table.normalize_associated_types_in(ty);
             self.resolve_ty_assoc_item(ty, last.name, id).map(|(it, substs)| (it, Some(substs)))?
         } else {
             // FIXME: report error, unresolved first path segment
@@ -169,7 +176,7 @@ impl<'a> InferenceContext<'a> {
     ) -> Option<(ValueNs, Substitution)> {
         let trait_ = trait_ref.hir_trait_id();
         let item =
-            self.db.trait_data(trait_).items.iter().map(|(_name, id)| (*id)).find_map(|item| {
+            self.db.trait_data(trait_).items.iter().map(|(_name, id)| *id).find_map(|item| {
                 match item {
                     AssocItemId::FunctionId(func) => {
                         if segment.name == &self.db.function_data(func).name {
@@ -288,7 +295,7 @@ impl<'a> InferenceContext<'a> {
         name: &Name,
         id: ExprOrPatId,
     ) -> Option<(ValueNs, Substitution)> {
-        let ty = self.resolve_ty_shallow(ty);
+        let ty = self.resolve_ty_shallow(&ty);
         let (enum_id, subst) = match ty.as_adt() {
             Some((AdtId::EnumId(e), subst)) => (e, subst),
             _ => return None,

crates/hir-ty/src/infer/unify.rs

@@ -7,16 +7,18 @@ use chalk_ir::{
     IntTy, TyVariableKind, UniverseIndex,
 };
 use chalk_solve::infer::ParameterEnaVariableExt;
+use either::Either;
 use ena::unify::UnifyKey;
 use hir_expand::name;
 use stdx::never;
 
 use super::{InferOk, InferResult, InferenceContext, TypeError};
 use crate::{
-    db::HirDatabase, fold_tys, static_lifetime, to_chalk_trait_id, traits::FnTrait, AliasEq,
-    AliasTy, BoundVar, Canonical, Const, DebruijnIndex, GenericArg, GenericArgData, Goal, Guidance,
-    InEnvironment, InferenceVar, Interner, Lifetime, ParamKind, ProjectionTy, ProjectionTyExt,
-    Scalar, Solution, Substitution, TraitEnvironment, Ty, TyBuilder, TyExt, TyKind, VariableKind,
+    db::HirDatabase, fold_tys, fold_tys_and_consts, static_lifetime, to_chalk_trait_id,
+    traits::FnTrait, AliasEq, AliasTy, BoundVar, Canonical, Const, ConstValue, DebruijnIndex,
+    GenericArg, GenericArgData, Goal, Guidance, InEnvironment, InferenceVar, Interner, Lifetime,
+    ParamKind, ProjectionTy, ProjectionTyExt, Scalar, Solution, Substitution, TraitEnvironment, Ty,
+    TyBuilder, TyExt, TyKind, VariableKind,
 };
 
 impl<'a> InferenceContext<'a> {
@@ -717,6 +719,45 @@ impl<'a> InferenceTable<'a> {
             None
         }
     }
+
+    pub(super) fn insert_type_vars(&mut self, ty: Ty) -> Ty {
+        fold_tys_and_consts(
+            ty,
+            |x, _| match x {
+                Either::Left(ty) => Either::Left(self.insert_type_vars_shallow(ty)),
+                Either::Right(c) => Either::Right(self.insert_const_vars_shallow(c)),
+            },
+            DebruijnIndex::INNERMOST,
+        )
+    }
+
+    /// Replaces `Ty::Error` by a new type var, so we can maybe still infer it.
+    pub(super) fn insert_type_vars_shallow(&mut self, ty: Ty) -> Ty {
+        match ty.kind(Interner) {
+            TyKind::Error => self.new_type_var(),
+            TyKind::InferenceVar(..) => {
+                let ty_resolved = self.resolve_ty_shallow(&ty);
+                if ty_resolved.is_unknown() {
+                    self.new_type_var()
+                } else {
+                    ty
+                }
+            }
+            _ => ty,
+        }
+    }
+
+    /// Replaces ConstScalar::Unknown by a new type var, so we can maybe still infer it.
+    pub(super) fn insert_const_vars_shallow(&mut self, c: Const) -> Const {
+        let data = c.data(Interner);
+        match &data.value {
+            ConstValue::Concrete(cc) => match cc.interned {
+                crate::ConstScalar::Unknown => self.new_const_var(data.ty.clone()),
+                _ => c,
+            },
+            _ => c,
+        }
+    }
 }
 
 impl<'a> fmt::Debug for InferenceTable<'a> {

crates/hir-ty/src/lower.rs

@@ -103,7 +103,7 @@ impl ImplTraitLoweringState {
 #[derive(Debug)]
 pub struct TyLoweringContext<'a> {
     pub db: &'a dyn HirDatabase,
-    pub resolver: &'a Resolver,
+    resolver: &'a Resolver,
     in_binders: DebruijnIndex,
     /// Note: Conceptually, it's thinkable that we could be in a location where
     /// some type params should be represented as placeholders, and others

crates/hir-ty/src/tests/traits.rs

@@ -4183,3 +4183,106 @@ fn test() {
 "#,
     );
 }
+
+#[test]
+fn associated_type_in_struct_expr_path() {
+    // FIXME: All annotation should be resolvable.
+    // For lines marked as unstable, see rust-lang/rust#86935.
+    // FIXME: Remove the comments once stablized.
+    check_types(
+        r#"
+trait Trait {
+    type Assoc;
+    fn f();
+}
+
+struct S { x: u32 }
+
+impl Trait for () {
+    type Assoc = S;
+
+    fn f() {
+        let x = 42;
+        let a = Self::Assoc { x };
+      //    ^ S
+        let a = <Self>::Assoc { x }; // unstable
+      //    ^ {unknown}
+
+        // should be `Copy` but we don't track ownership anyway.
+        let value = S { x };
+        if let Self::Assoc { x } = value {}
+      //                     ^ u32
+        if let <Self>::Assoc { x } = value {} // unstable
+      //                       ^ {unknown}
+    }
+}
+    "#,
+    );
+}
+
+#[test]
+fn associted_type_in_struct_expr_path_enum() {
+    // FIXME: All annotation should be resolvable.
+    // For lines marked as unstable, see rust-lang/rust#86935.
+    // FIXME: Remove the comments once stablized.
+    check_types(
+        r#"
+trait Trait {
+    type Assoc;
+    fn f();
+}
+
+enum E {
+    Unit,
+    Struct { x: u32 },
+}
+
+impl Trait for () {
+    type Assoc = E;
+
+    fn f() {
+        let a = Self::Assoc::Unit;
+      //    ^ E
+        let a = <Self>::Assoc::Unit;
+      //    ^ E
+        let a = <Self::Assoc>::Unit;
+      //    ^ E
+        let a = <<Self>::Assoc>::Unit;
+      //    ^ E
+
+        // should be `Copy` but we don't track ownership anyway.
+        let value = E::Unit;
+        if let Self::Assoc::Unit = value {}
+      //       ^^^^^^^^^^^^^^^^^ E
+        if let <Self>::Assoc::Unit = value {}
+      //       ^^^^^^^^^^^^^^^^^^^ E
+        if let <Self::Assoc>::Unit = value {}
+      //       ^^^^^^^^^^^^^^^^^^^ E
+        if let <<Self>::Assoc>::Unit = value {}
+      //       ^^^^^^^^^^^^^^^^^^^^^ E
+
+        let x = 42;
+        let a = Self::Assoc::Struct { x };
+      //    ^ E
+        let a = <Self>::Assoc::Struct { x }; // unstable
+      //    ^ {unknown}
+        let a = <Self::Assoc>::Struct { x }; // unstable
+      //    ^ {unknown}
+        let a = <<Self>::Assoc>::Struct { x }; // unstable
+      //    ^ {unknown}
+
+        // should be `Copy` but we don't track ownership anyway.
+        let value = E::Struct { x: 42 };
+        if let Self::Assoc::Struct { x } = value {}
+      //                             ^ u32
+        if let <Self>::Assoc::Struct { x } = value {} // unstable
+      //                               ^ {unknown}
+        if let <Self::Assoc>::Struct { x } = value {} // unstable
+      //                               ^ {unknown}
+        if let <<Self>::Assoc>::Struct { x } = value {} // unstable
+      //                                 ^ {unknown}
+    }
+}
+    "#,
+    );
+}