Lower fully qualified associated type paths

I.e. `<T as Trait>::Foo`.

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/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/lower.rs

@@ -8,7 +8,7 @@
 use std::iter;
 use std::sync::Arc;
 
-use super::{FnSig, GenericPredicate, Substs, TraitRef, Ty, TypeCtor};
+use super::{FnSig, GenericPredicate, ProjectionTy, Substs, TraitRef, Ty, TypeCtor};
 use crate::{
     adt::VariantDef,
     generics::HasGenericParams,
@@ -64,7 +64,8 @@ impl Ty {
 
     pub(crate) fn from_hir_path(db: &impl HirDatabase, resolver: &Resolver, path: &Path) -> Self {
         // Resolve the path (in type namespace)
-        let resolution = resolver.resolve_path_without_assoc_items(db, path).take_types();
+        let (resolution, remaining_index) = resolver.resolve_path_segments(db, path).into_inner();
+        let resolution = resolution.take_types();
 
         let def = match resolution {
             Some(Resolution::Def(def)) => def,
@@ -73,6 +74,10 @@ impl Ty {
                 panic!("path resolved to local binding in type ns");
             }
             Some(Resolution::GenericParam(idx)) => {
+                if remaining_index.is_some() {
+                    // e.g. T::Item
+                    return Ty::Unknown;
+                }
                 return Ty::Param {
                     idx,
                     // FIXME: maybe return name in resolution?
@@ -83,18 +88,59 @@ impl Ty {
                 };
             }
             Some(Resolution::SelfType(impl_block)) => {
+                if remaining_index.is_some() {
+                    // e.g. Self::Item
+                    return Ty::Unknown;
+                }
                 return impl_block.target_ty(db);
             }
-            None => return Ty::Unknown,
+            None => {
+                // path did not resolve
+                return Ty::Unknown;
+            }
         };
 
-        let typable: TypableDef = match def.into() {
+        if let ModuleDef::Trait(trait_) = def {
-            None => return Ty::Unknown,
+            let segment = match remaining_index {
-            Some(it) => it,
+                None => path.segments.last().expect("resolved path has at least one element"),
-        };
+                Some(i) => &path.segments[i - 1],
-        let ty = db.type_for_def(typable, Namespace::Types);
+            };
-        let substs = Ty::substs_from_path(db, resolver, path, typable);
+            let trait_ref = TraitRef::from_resolved_path(db, resolver, trait_, segment, None);
-        ty.subst(&substs)
+            if let Some(remaining_index) = remaining_index {
+                if remaining_index == path.segments.len() - 1 {
+                    let segment = &path.segments[remaining_index];
+                    let associated_ty =
+                        match trait_ref.trait_.associated_type_by_name(db, segment.name.clone()) {
+                            Some(t) => t,
+                            None => {
+                                // associated type not found
+                                return Ty::Unknown;
+                            }
+                        };
+                    eprintln!(
+                        "assoc ty: {:?}, parameters: {:?}",
+                        associated_ty.name(db),
+                        trait_ref.substs
+                    );
+                    // FIXME handle type parameters on the segment
+                    Ty::Projection(ProjectionTy { associated_ty, parameters: trait_ref.substs })
+                } else {
+                    // FIXME more than one segment remaining, is this possible?
+                    Ty::Unknown
+                }
+            } else {
+                // FIXME dyn Trait without the dyn
+                Ty::Unknown
+            }
+        } else {
+            let typable: TypableDef = match def.into() {
+                None => return Ty::Unknown,
+                Some(it) => it,
+            };
+            let ty = db.type_for_def(typable, Namespace::Types);
+            let substs = Ty::substs_from_path(db, resolver, path, typable);
+            ty.subst(&substs)
+        }
     }
 
     pub(super) fn substs_from_path_segment(
@@ -219,14 +265,25 @@ impl TraitRef {
             Resolution::Def(ModuleDef::Trait(tr)) => tr,
             _ => return None,
         };
-        let mut substs = Self::substs_from_path(db, resolver, path, resolved);
+        let segment = path.segments.last().expect("path should have at least one segment");
+        Some(TraitRef::from_resolved_path(db, resolver, resolved, segment, explicit_self_ty))
+    }
+
+    fn from_resolved_path(
+        db: &impl HirDatabase,
+        resolver: &Resolver,
+        resolved: Trait,
+        segment: &PathSegment,
+        explicit_self_ty: Option<Ty>,
+    ) -> Self {
+        let mut substs = TraitRef::substs_from_path(db, resolver, segment, resolved);
         if let Some(self_ty) = explicit_self_ty {
             // FIXME this could be nicer
             let mut substs_vec = substs.0.to_vec();
             substs_vec[0] = self_ty;
             substs.0 = substs_vec.into();
         }
-        Some(TraitRef { trait_: resolved, substs })
+        TraitRef { trait_: resolved, substs }
     }
 
     pub(crate) fn from_hir(
@@ -245,11 +302,12 @@ impl TraitRef {
     fn substs_from_path(
         db: &impl HirDatabase,
         resolver: &Resolver,
-        path: &Path,
+        segment: &PathSegment,
         resolved: Trait,
     ) -> Substs {
-        let segment = path.segments.last().expect("path should have at least one segment");
+        let has_self_param =
-        substs_from_path_segment(db, resolver, segment, Some(resolved.into()), true)
+            segment.args_and_bindings.as_ref().map(|a| a.has_self_type).unwrap_or(false);
+        substs_from_path_segment(db, resolver, segment, Some(resolved.into()), !has_self_param)
     }
 
     pub(crate) fn for_trait(db: &impl HirDatabase, trait_: Trait) -> TraitRef {

crates/ra_hir/src/ty/tests.rs

@@ -2515,10 +2515,10 @@ fn test<T: Iterable>() {
         @r###"
    ⋮
    ⋮[108; 227) '{     ...ter; }': ()
-   ⋮[118; 119) 'x': i32
+   ⋮[118; 119) 'x': <S as Iterable>::Item
-   ⋮[145; 146) '1': i32
+   ⋮[145; 146) '1': <S as Iterable>::Item
-   ⋮[156; 157) 'y': {unknown}
+   ⋮[156; 157) 'y': <T as Iterable>::Item
-   ⋮[183; 192) 'no_matter': {unknown}
+   ⋮[183; 192) 'no_matter': <T as Iterable>::Item
    ⋮[202; 203) 'z': {unknown}
    ⋮[215; 224) 'no_matter': {unknown}
     "###
@@ -2552,9 +2552,9 @@ fn test() {
    ⋮[205; 209) 'foo1': fn foo1<S>(T) -> {unknown}
    ⋮[205; 212) 'foo1(S)': {unknown}
    ⋮[210; 211) 'S': S
-   ⋮[222; 223) 'y': {unknown}
+   ⋮[222; 223) 'y': <S as Iterable>::Item
-   ⋮[226; 230) 'foo2': fn foo2<S>(T) -> {unknown}
+   ⋮[226; 230) 'foo2': fn foo2<S>(T) -> <T as Iterable>::Item
-   ⋮[226; 233) 'foo2(S)': {unknown}
+   ⋮[226; 233) 'foo2(S)': <S as Iterable>::Item
    ⋮[231; 232) 'S': S
     "###
     );

crates/ra_syntax/src/ast/extensions.rs

@@ -91,6 +91,7 @@ impl ast::Attr {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum PathSegmentKind {
     Name(ast::NameRef),
+    Type { type_ref: Option<ast::TypeRef>, trait_ref: Option<ast::PathType> },
     SelfKw,
     SuperKw,
     CrateKw,
@@ -112,6 +113,15 @@ impl ast::PathSegment {
                 T![self] => PathSegmentKind::SelfKw,
                 T![super] => PathSegmentKind::SuperKw,
                 T![crate] => PathSegmentKind::CrateKw,
+                T![<] => {
+                    // <T> or <T as Trait>
+                    // T is any TypeRef, Trait has to be a PathType
+                    let mut type_refs =
+                        self.syntax().children().filter(|node| ast::TypeRef::can_cast(node.kind()));
+                    let type_ref = type_refs.next().and_then(ast::TypeRef::cast);
+                    let trait_ref = type_refs.next().and_then(ast::PathType::cast);
+                    PathSegmentKind::Type { type_ref, trait_ref }
+                }
                 _ => return None,
             }
         };