librustc: Implement associated types behind a feature gate.

The implementation essentially desugars during type collection and AST
type conversion time into the parameter scheme we have now. Only fully
qualified names--e.g. `<T as Foo>::Bar`--are supported.

src/doc/rust.md

@@ -2557,6 +2557,8 @@ The currently implemented features of the reference compiler are:
 
 * `tuple_indexing` - Allows use of tuple indexing (expressions like `expr.0`)
 
+* `associated_types` - Allows type aliases in traits. Experimental.
+
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about #[feature] directives which enabled
 the new feature (because the directive is no longer necessary). However, if

src/librustc/lint/builtin.rs

@@ -844,6 +844,17 @@ fn method_context(cx: &Context, m: &ast::Method) -> MethodContext {
                         }
                     }
                 }
+                ty::TypeTraitItem(typedef) => {
+                    match typedef.container {
+                        ty::TraitContainer(..) => TraitDefaultImpl,
+                        ty::ImplContainer(cid) => {
+                            match ty::impl_trait_ref(cx.tcx, cid) {
+                                Some(..) => TraitImpl,
+                                None => PlainImpl
+                            }
+                        }
+                    }
+                }
             }
         }
     }
@@ -1511,13 +1522,9 @@ impl LintPass for Stability {
                                 method_num: index,
                                 ..
                             }) => {
-                                match ty::trait_item(cx.tcx,
-                                                     trait_ref.def_id,
-                                                     index) {
-                                    ty::MethodTraitItem(method) => {
-                                        method.def_id
-                                    }
-                                }
+                                ty::trait_item(cx.tcx,
+                                               trait_ref.def_id,
+                                               index).def_id()
                             }
                         }
                     }

src/librustc/lint/context.rs

@@ -25,6 +25,7 @@
 //! for all lint attributes.
 
 use middle::privacy::ExportedItems;
+use middle::subst;
 use middle::ty;
 use middle::typeck::astconv::AstConv;
 use middle::typeck::infer;
@@ -491,6 +492,26 @@ impl<'a, 'tcx> AstConv<'tcx> for Context<'a, 'tcx>{
     fn ty_infer(&self, _span: Span) -> ty::t {
         infer::new_infer_ctxt(self.tcx).next_ty_var()
     }
+
+    fn associated_types_of_trait_are_valid(&self, _: ty::t, _: ast::DefId)
+                                           -> bool {
+        // FIXME(pcwalton): This is wrong.
+        true
+    }
+
+    fn associated_type_binding(&self,
+                               _: Span,
+                               _: Option<ty::t>,
+                               trait_id: ast::DefId,
+                               associated_type_id: ast::DefId)
+                               -> ty::t {
+        // FIXME(pcwalton): This is wrong.
+        let trait_def = self.get_trait_def(trait_id);
+        let index = ty::associated_type_parameter_index(self.tcx,
+                                                        &*trait_def,
+                                                        associated_type_id);
+        ty::mk_param(self.tcx, subst::TypeSpace, index, associated_type_id)
+    }
 }
 
 impl<'a, 'tcx, 'v> Visitor<'v> for Context<'a, 'tcx> {

src/librustc/metadata/csearch.rs

@@ -349,3 +349,9 @@ pub fn get_stability(cstore: &cstore::CStore,
     let cdata = cstore.get_crate_data(def.krate);
     decoder::get_stability(&*cdata, def.node)
 }
+
+pub fn is_associated_type(cstore: &cstore::CStore, def: ast::DefId) -> bool {
+    let cdata = cstore.get_crate_data(def.krate);
+    decoder::is_associated_type(&*cdata, def.node)
+}
+

src/librustc/metadata/decoder.rs

@@ -23,7 +23,7 @@ use metadata::tydecode::{parse_ty_data, parse_region_data, parse_def_id,
                          parse_bare_fn_ty_data, parse_trait_ref_data};
 use middle::def;
 use middle::lang_items;
-use middle::resolve::TraitItemKind;
+use middle::resolve::{TraitItemKind, TypeTraitItemKind};
 use middle::subst;
 use middle::ty::{ImplContainer, TraitContainer};
 use middle::ty;
@@ -167,6 +167,8 @@ fn item_visibility(item: rbml::Doc) -> ast::Visibility {
 }
 
 fn item_sort(item: rbml::Doc) -> char {
+    // NB(pcwalton): The default of 'r' here is relied upon in
+    // `is_associated_type` below.
     let mut ret = 'r';
     reader::tagged_docs(item, tag_item_trait_item_sort, |doc| {
         ret = doc.as_str_slice().as_bytes()[0] as char;
@@ -714,6 +716,7 @@ pub fn get_impl_items(cdata: Cmd, impl_id: ast::NodeId)
         let def_id = item_def_id(doc, cdata);
         match item_sort(doc) {
             'r' | 'p' => impl_items.push(ty::MethodTraitItemId(def_id)),
+            't' => impl_items.push(ty::TypeTraitItemId(def_id)),
             _ => fail!("unknown impl item sort"),
         }
         true
@@ -733,6 +736,7 @@ pub fn get_trait_item_name_and_kind(intr: Rc<IdentInterner>,
             let explicit_self = get_explicit_self(doc);
             (name, TraitItemKind::from_explicit_self_category(explicit_self))
         }
+        't' => (name, TypeTraitItemKind),
         c => {
             fail!("get_trait_item_name_and_kind(): unknown trait item kind \
                    in metadata: `{}`", c)
@@ -758,13 +762,13 @@ pub fn get_impl_or_trait_item(intr: Rc<IdentInterner>,
     };
 
     let name = item_name(&*intr, method_doc);
+    let vis = item_visibility(method_doc);
 
     match item_sort(method_doc) {
         'r' | 'p' => {
             let generics = doc_generics(method_doc, tcx, cdata,
                                         tag_method_ty_generics);
             let fty = doc_method_fty(method_doc, tcx, cdata);
-            let vis = item_visibility(method_doc);
             let explicit_self = get_explicit_self(method_doc);
             let provided_source = get_provided_source(method_doc, cdata);
 
@@ -777,6 +781,14 @@ pub fn get_impl_or_trait_item(intr: Rc<IdentInterner>,
                                                         container,
                                                         provided_source)))
         }
+        't' => {
+            ty::TypeTraitItem(Rc::new(ty::AssociatedType {
+                ident: name,
+                vis: vis,
+                def_id: def_id,
+                container: container,
+            }))
+        }
         _ => fail!("unknown impl/trait item sort"),
     }
 }
@@ -790,6 +802,7 @@ pub fn get_trait_item_def_ids(cdata: Cmd, id: ast::NodeId)
         let def_id = item_def_id(mth, cdata);
         match item_sort(mth) {
             'r' | 'p' => result.push(ty::MethodTraitItemId(def_id)),
+            't' => result.push(ty::TypeTraitItemId(def_id)),
             _ => fail!("unknown trait item sort"),
         }
         true
@@ -827,6 +840,7 @@ pub fn get_provided_trait_methods(intr: Rc<IdentInterner>,
                 ty::MethodTraitItem(ref method) => {
                     result.push((*method).clone())
                 }
+                ty::TypeTraitItem(_) => {}
             }
         }
         true
@@ -1394,3 +1408,12 @@ fn doc_generics(base_doc: rbml::Doc,
 
     ty::Generics { types: types, regions: regions }
 }
+
+pub fn is_associated_type(cdata: Cmd, id: ast::NodeId) -> bool {
+    let items = reader::get_doc(rbml::Doc::new(cdata.data()), tag_items);
+    match maybe_find_item(id, items) {
+        None => false,
+        Some(item) => item_sort(item) == 't',
+    }
+}
+