Merge #1677

1677: Associated types r=flodiebold a=flodiebold

This implements basic support for (fully qualified) associated type projections:
 - handle fully qualified paths like `<T as Trait>::AssocType` (basically desugaring to something like `Trait<Self=T>::AssocType`)
 - lower these to a new `Ty::Projection` enum variant
 - also introduce `Ty::UnselectedProjection` for cases like `T::AssocType` where the trait from which the type comes isn't specified, but these aren't handled further so far
 - in inference, normalize these projections using Chalk: basically, when encountering a type e.g. from a type annotation or signature, we replace these `Ty::Projection`s by type variables and add obligations to normalize the associated type

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

Author: bors[bot]

crates/ra_assists/src/auto_import.rs

@@ -71,6 +71,7 @@ fn compare_path_segment(a: &SmolStr, b: &ast::PathSegment) -> bool {
             ast::PathSegmentKind::SelfKw => a == "self",
             ast::PathSegmentKind::SuperKw => a == "super",
             ast::PathSegmentKind::CrateKw => a == "crate",
+            ast::PathSegmentKind::Type { .. } => false, // not allowed in imports
         }
     } else {
         false

crates/ra_hir/src/code_model.rs

@@ -838,6 +838,10 @@ impl TypeAlias {
         self.id.module(db)
     }
 
+    pub fn krate(self, db: &impl DefDatabase) -> Option<Crate> {
+        self.module(db).krate(db)
+    }
+
     /// The containing impl block, if this is a method.
     pub fn impl_block(self, db: &impl DefDatabase) -> Option<ImplBlock> {
         let module_impls = db.impls_in_module(self.module(db));

crates/ra_hir/src/path.rs

@@ -25,6 +25,12 @@ pub struct PathSegment {
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct GenericArgs {
     pub args: Vec<GenericArg>,
+    /// This specifies whether the args contain a Self type as the first
+    /// element. This is the case for path segments like `<T as Trait>`, where
+    /// `T` is actually a type parameter for the path `Trait` specifying the
+    /// Self type. Otherwise, when we have a path `Trait<X, Y>`, the Self type
+    /// is left out.
+    pub has_self_type: bool,
     // someday also bindings
 }
 
@@ -74,6 +80,28 @@ impl Path {
                     let segment = PathSegment { name: name.as_name(), args_and_bindings: args };
                     segments.push(segment);
                 }
+                ast::PathSegmentKind::Type { type_ref, trait_ref } => {
+                    assert!(path.qualifier().is_none()); // this can only occur at the first segment
+
+                    // FIXME: handle <T> syntax (type segments without trait)
+
+                    // <T as Trait<A>>::Foo desugars to Trait<Self=T, A>::Foo
+                    let path = Path::from_ast(trait_ref?.path()?)?;
+                    kind = path.kind;
+                    let mut prefix_segments = path.segments;
+                    prefix_segments.reverse();
+                    segments.extend(prefix_segments);
+                    // Insert the type reference (T in the above example) as Self parameter for the trait
+                    let self_type = TypeRef::from_ast(type_ref?);
+                    let mut last_segment = segments.last_mut()?;
+                    if last_segment.args_and_bindings.is_none() {
+                        last_segment.args_and_bindings = Some(Arc::new(GenericArgs::empty()));
+                    };
+                    let args = last_segment.args_and_bindings.as_mut().unwrap();
+                    let mut args_inner = Arc::make_mut(args);
+                    args_inner.has_self_type = true;
+                    args_inner.args.insert(0, GenericArg::Type(self_type));
+                }
                 ast::PathSegmentKind::CrateKw => {
                     kind = PathKind::Crate;
                     break;
@@ -144,11 +172,15 @@ impl GenericArgs {
         }
         // lifetimes and assoc type args ignored for now
         if !args.is_empty() {
-            Some(GenericArgs { args })
+            Some(GenericArgs { args, has_self_type: false })
         } else {
             None
         }
     }
+
+    pub(crate) fn empty() -> GenericArgs {
+        GenericArgs { args: Vec::new(), has_self_type: false }
+    }
 }
 
 impl From<Name> for Path {
@@ -236,6 +268,10 @@ fn convert_path(prefix: Option<Path>, path: ast::Path) -> Option<Path> {
             }
             Path { kind: PathKind::Super, segments: Vec::new() }
         }
+        ast::PathSegmentKind::Type { .. } => {
+            // not allowed in imports
+            return None;
+        }
     };
     Some(res)
 }

crates/ra_hir/src/ty.rs

@@ -94,6 +94,12 @@ pub enum TypeCtor {
 
     /// A tuple type.  For example, `(i32, bool)`.
     Tuple { cardinality: u16 },
+
+    /// Represents an associated item like `Iterator::Item`.  This is used
+    /// when we have tried to normalize a projection like `T::Item` but
+    /// couldn't find a better representation.  In that case, we generate
+    /// an **application type** like `(Iterator::Item)<T>`.
+    AssociatedType(TypeAlias),
 }
 
 /// A nominal type with (maybe 0) type parameters. This might be a primitive
@@ -114,6 +120,12 @@ pub struct ProjectionTy {
     pub parameters: Substs,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub struct UnselectedProjectionTy {
+    pub type_name: Name,
+    pub parameters: Substs,
+}
+
 /// A type.
 ///
 /// See also the `TyKind` enum in rustc (librustc/ty/sty.rs), which represents
@@ -127,6 +139,18 @@ pub enum Ty {
     /// several other things.
     Apply(ApplicationTy),
 
+    /// A "projection" type corresponds to an (unnormalized)
+    /// projection like `<P0 as Trait<P1..Pn>>::Foo`. Note that the
+    /// trait and all its parameters are fully known.
+    Projection(ProjectionTy),
+
+    /// This is a variant of a projection in which the trait is
+    /// **not** known.  It corresponds to a case where people write
+    /// `T::Item` without specifying the trait. We would then try to
+    /// figure out the trait by looking at all the traits that are in
+    /// scope.
+    UnselectedProjection(UnselectedProjectionTy),
+
     /// A type parameter; for example, `T` in `fn f<T>(x: T) {}
     Param {
         /// The index of the parameter (starting with parameters from the
@@ -352,6 +376,16 @@ impl Ty {
                     t.walk(f);
                 }
             }
+            Ty::Projection(p_ty) => {
+                for t in p_ty.parameters.iter() {
+                    t.walk(f);
+                }
+            }
+            Ty::UnselectedProjection(p_ty) => {
+                for t in p_ty.parameters.iter() {
+                    t.walk(f);
+                }
+            }
             Ty::Param { .. } | Ty::Bound(_) | Ty::Infer(_) | Ty::Unknown => {}
         }
         f(self);
@@ -362,6 +396,12 @@ impl Ty {
             Ty::Apply(a_ty) => {
                 a_ty.parameters.walk_mut(f);
             }
+            Ty::Projection(p_ty) => {
+                p_ty.parameters.walk_mut(f);
+            }
+            Ty::UnselectedProjection(p_ty) => {
+                p_ty.parameters.walk_mut(f);
+            }
             Ty::Param { .. } | Ty::Bound(_) | Ty::Infer(_) | Ty::Unknown => {}
         }
         f(self);
@@ -572,15 +612,61 @@ impl HirDisplay for ApplicationTy {
                     write!(f, ">")?;
                 }
             }
+            TypeCtor::AssociatedType(type_alias) => {
+                let trait_name = type_alias
+                    .parent_trait(f.db)
+                    .and_then(|t| t.name(f.db))
+                    .unwrap_or_else(Name::missing);
+                let name = type_alias.name(f.db);
+                write!(f, "{}::{}", trait_name, name)?;
+                if self.parameters.len() > 0 {
+                    write!(f, "<")?;
+                    f.write_joined(&*self.parameters.0, ", ")?;
+                    write!(f, ">")?;
+                }
+            }
         }
         Ok(())
     }
 }
 
+impl HirDisplay for ProjectionTy {
+    fn hir_fmt(&self, f: &mut HirFormatter<impl HirDatabase>) -> fmt::Result {
+        let trait_name = self
+            .associated_ty
+            .parent_trait(f.db)
+            .and_then(|t| t.name(f.db))
+            .unwrap_or_else(Name::missing);
+        write!(f, "<{} as {}", self.parameters[0].display(f.db), trait_name,)?;
+        if self.parameters.len() > 1 {
+            write!(f, "<")?;
+            f.write_joined(&self.parameters[1..], ", ")?;
+            write!(f, ">")?;
+        }
+        write!(f, ">::{}", self.associated_ty.name(f.db))?;
+        Ok(())
+    }
+}
+
+impl HirDisplay for UnselectedProjectionTy {
+    fn hir_fmt(&self, f: &mut HirFormatter<impl HirDatabase>) -> fmt::Result {
+        write!(f, "{}", self.parameters[0].display(f.db))?;
+        if self.parameters.len() > 1 {
+            write!(f, "<")?;
+            f.write_joined(&self.parameters[1..], ", ")?;
+            write!(f, ">")?;
+        }
+        write!(f, "::{}", self.type_name)?;
+        Ok(())
+    }
+}
+
 impl HirDisplay for Ty {
     fn hir_fmt(&self, f: &mut HirFormatter<impl HirDatabase>) -> fmt::Result {
         match self {
             Ty::Apply(a_ty) => a_ty.hir_fmt(f)?,
+            Ty::Projection(p_ty) => p_ty.hir_fmt(f)?,
+            Ty::UnselectedProjection(p_ty) => p_ty.hir_fmt(f)?,
             Ty::Param { name, .. } => write!(f, "{}", name)?,
             Ty::Bound(idx) => write!(f, "?{}", idx)?,
             Ty::Unknown => write!(f, "{{unknown}}")?,
@@ -606,3 +692,17 @@ impl HirDisplay for TraitRef {
         Ok(())
     }
 }
+
+impl HirDisplay for Obligation {
+    fn hir_fmt(&self, f: &mut HirFormatter<impl HirDatabase>) -> fmt::Result {
+        match self {
+            Obligation::Trait(tr) => write!(f, "Implements({})", tr.display(f.db)),
+            Obligation::Projection(proj) => write!(
+                f,
+                "Normalize({} => {})",
+                proj.projection_ty.display(f.db),
+                proj.ty.display(f.db)
+            ),
+        }
+    }
+}

crates/ra_hir/src/ty/infer.rs

@@ -245,7 +245,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             &self.resolver,
             type_ref,
         );
-        self.insert_type_vars(ty)
+        let ty = self.insert_type_vars(ty);
+        self.normalize_associated_types_in(ty)
     }
 
     fn unify_substs(&mut self, substs1: &Substs, substs2: &Substs, depth: usize) -> bool {
@@ -411,6 +412,32 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         ty
     }
 
+    /// Recurses through the given type, normalizing associated types mentioned
+    /// in it by replacing them by type variables and registering obligations to
+    /// resolve later. This should be done once for every type we get from some
+    /// type annotation (e.g. from a let type annotation, field type or function
+    /// call). `make_ty` handles this already, but e.g. for field types we need
+    /// to do it as well.
+    fn normalize_associated_types_in(&mut self, ty: Ty) -> Ty {
+        let ty = self.resolve_ty_as_possible(&mut vec![], ty);
+        ty.fold(&mut |ty| match ty {
+            Ty::Projection(proj_ty) => self.normalize_projection_ty(proj_ty),
+            Ty::UnselectedProjection(proj_ty) => {
+                // FIXME use Chalk's unselected projection support
+                Ty::UnselectedProjection(proj_ty)
+            }
+            _ => ty,
+        })
+    }
+
+    fn normalize_projection_ty(&mut self, proj_ty: ProjectionTy) -> Ty {
+        let var = self.new_type_var();
+        let predicate = ProjectionPredicate { projection_ty: proj_ty.clone(), ty: var.clone() };
+        let obligation = Obligation::Projection(predicate);
+        self.obligations.push(obligation);
+        var
+    }
+
     /// Resolves the type completely; type variables without known type are
     /// replaced by Ty::Unknown.
     fn resolve_ty_completely(&mut self, tv_stack: &mut Vec<TypeVarId>, ty: Ty) -> Ty {
@@ -549,6 +576,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 let substs = Ty::substs_from_path(self.db, &self.resolver, path, typable);
                 let ty = ty.subst(&substs);
                 let ty = self.insert_type_vars(ty);
+                let ty = self.normalize_associated_types_in(ty);
                 Some(ty)
             }
             Resolution::LocalBinding(pat) => {
@@ -670,6 +698,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 .and_then(|d| d.field(self.db, &Name::tuple_field_name(i)))
                 .map_or(Ty::Unknown, |field| field.ty(self.db))
                 .subst(&substs);
+            let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat, &expected_ty, default_bm);
         }
 
@@ -697,6 +726,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             let matching_field = def.and_then(|it| it.field(self.db, &subpat.name));
             let expected_ty =
                 matching_field.map_or(Ty::Unknown, |field| field.ty(self.db)).subst(&substs);
+            let expected_ty = self.normalize_associated_types_in(expected_ty);
             self.infer_pat(subpat.pat, &expected_ty, default_bm);
         }
 
@@ -927,9 +957,11 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         self.unify(&expected_receiver_ty, &actual_receiver_ty);
 
         let param_iter = param_tys.into_iter().chain(repeat(Ty::Unknown));
-        for (arg, param) in args.iter().zip(param_iter) {
-            self.infer_expr(*arg, &Expectation::has_type(param));
+        for (arg, param_ty) in args.iter().zip(param_iter) {
+            let param_ty = self.normalize_associated_types_in(param_ty);
+            self.infer_expr(*arg, &Expectation::has_type(param_ty));
         }
+        let ret_ty = self.normalize_associated_types_in(ret_ty);
         ret_ty
     }
 
@@ -1020,9 +1052,11 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 };
                 self.register_obligations_for_call(&callee_ty);
                 let param_iter = param_tys.into_iter().chain(repeat(Ty::Unknown));
-                for (arg, param) in args.iter().zip(param_iter) {
-                    self.infer_expr(*arg, &Expectation::has_type(param));
+                for (arg, param_ty) in args.iter().zip(param_iter) {
+                    let param_ty = self.normalize_associated_types_in(param_ty);
+                    self.infer_expr(*arg, &Expectation::has_type(param_ty));
                 }
+                let ret_ty = self.normalize_associated_types_in(ret_ty);
                 ret_ty
             }
             Expr::MethodCall { receiver, args, method_name, generic_args } => self
@@ -1120,7 +1154,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     _ => None,
                 })
                 .unwrap_or(Ty::Unknown);
-                self.insert_type_vars(ty)
+                let ty = self.insert_type_vars(ty);
+                self.normalize_associated_types_in(ty)
             }
             Expr::Await { expr } => {
                 let inner_ty = self.infer_expr(*expr, &Expectation::none());