Added support for trait aliases as object types.

Author: Alexander Regueiro

src/librustc/hir/lowering.rs

@@ -4875,23 +4875,24 @@ impl<'a> LoweringContext<'a> {
         let node = match qpath {
             hir::QPath::Resolved(None, path) => {
                 // Turn trait object paths into `TyKind::TraitObject` instead.
-                if let Def::Trait(_) = path.def {
-                    let principal = hir::PolyTraitRef {
-                        bound_generic_params: hir::HirVec::new(),
-                        trait_ref: hir::TraitRef {
-                            path: path.and_then(|path| path),
-                            ref_id: id.node_id,
-                            hir_ref_id: id.hir_id,
-                        },
-                        span,
-                    };
+                match path.def {
+                    Def::Trait(_) | Def::TraitAlias(_) => {
+                        let principal = hir::PolyTraitRef {
+                            bound_generic_params: hir::HirVec::new(),
+                            trait_ref: hir::TraitRef {
+                                path: path.and_then(|path| path),
+                                ref_id: id.node_id,
+                                hir_ref_id: id.hir_id,
+                            },
+                            span,
+                        };
 
-                    // The original ID is taken by the `PolyTraitRef`,
-                    // so the `Ty` itself needs a different one.
-                    id = self.next_id();
-                    hir::TyKind::TraitObject(hir_vec![principal], self.elided_dyn_bound(span))
-                } else {
-                    hir::TyKind::Path(hir::QPath::Resolved(None, path))
+                        // The original ID is taken by the `PolyTraitRef`,
+                        // so the `Ty` itself needs a different one.
+                        id = self.next_id();
+                        hir::TyKind::TraitObject(hir_vec![principal], self.elided_dyn_bound(span))
+                    }
+                    _ => hir::TyKind::Path(hir::QPath::Resolved(None, path)),
                 }
             }
             _ => hir::TyKind::Path(qpath),

src/librustc_resolve/lib.rs

@@ -538,9 +538,9 @@ impl<'a> PathSource<'a> {
         match self {
             PathSource::Type => match def {
                 Def::Struct(..) | Def::Union(..) | Def::Enum(..) |
-                Def::Trait(..) | Def::TyAlias(..) | Def::AssociatedTy(..) |
-                Def::PrimTy(..) | Def::TyParam(..) | Def::SelfTy(..) |
-                Def::Existential(..) |
+                Def::Trait(..) | Def::TraitAlias(..) | Def::TyAlias(..) |
+                Def::AssociatedTy(..) | Def::PrimTy(..) | Def::TyParam(..) |
+                Def::SelfTy(..) | Def::Existential(..) |
                 Def::ForeignTy(..) => true,
                 _ => false,
             },
@@ -3122,7 +3122,10 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
                         return (err, candidates);
                     }
                     (Def::TyAlias(..), PathSource::Trait(_)) => {
-                        err.span_label(span, "type aliases cannot be used for traits");
+                        err.span_label(span, "type aliases cannot be used as traits");
+                        if nightly_options::is_nightly_build() {
+                            err.note("did you mean to use a trait alias?");
+                        }
                         return (err, candidates);
                     }
                     (Def::Mod(..), PathSource::Expr(Some(parent))) => match parent.node {

src/librustc_typeck/astconv.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! Conversion from AST representation of types to the ty.rs
+//! Conversion from AST representation of types to the `ty.rs`
 //! representation.  The main routine here is `ast_ty_to_ty()`: each use
 //! is parameterized by an instance of `AstConv`.
 
@@ -181,7 +181,6 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         item_segment: &hir::PathSegment)
         -> &'tcx Substs<'tcx>
     {
-
         let (substs, assoc_bindings) = item_segment.with_generic_args(|generic_args| {
             self.create_substs_for_ast_path(
                 span,
@@ -948,8 +947,8 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         )
     }
 
-    /// Transform a PolyTraitRef into a PolyExistentialTraitRef by
-    /// removing the dummy Self type (TRAIT_OBJECT_DUMMY_SELF).
+    /// Transform a `PolyTraitRef` into a `PolyExistentialTraitRef` by
+    /// removing the dummy `Self` type (`TRAIT_OBJECT_DUMMY_SELF`).
     fn trait_ref_to_existential(&self, trait_ref: ty::TraitRef<'tcx>)
                                 -> ty::ExistentialTraitRef<'tcx> {
         assert_eq!(trait_ref.self_ty().sty, TRAIT_OBJECT_DUMMY_SELF);
@@ -1347,7 +1346,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         err.span_label(span, "associated type not allowed here").emit();
     }
 
-    // Check a type Path and convert it to a Ty.
+    // Check a type `Path` and convert it to a `Ty`.
     pub fn def_to_ty(&self,
                      opt_self_ty: Option<Ty<'tcx>>,
                      path: &hir::Path,
@@ -1442,8 +1441,8 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
     /// Parses the programmer's textual representation of a type into our
     /// internal notion of a type.
     pub fn ast_ty_to_ty(&self, ast_ty: &hir::Ty) -> Ty<'tcx> {
-        debug!("ast_ty_to_ty(id={:?}, ast_ty={:?})",
-               ast_ty.id, ast_ty);
+        debug!("ast_ty_to_ty(id={:?}, ast_ty={:?} ty_ty={:?})",
+               ast_ty.id, ast_ty, ast_ty.node);
 
         let tcx = self.tcx();
 

src/libsyntax/ast.rs

@@ -288,9 +288,9 @@ pub enum TraitBoundModifier {
 }
 
 /// The AST represents all type param bounds as types.
-/// typeck::collect::compute_bounds matches these against
-/// the "special" built-in traits (see middle::lang_items) and
-/// detects Copy, Send and Sync.
+/// `typeck::collect::compute_bounds` matches these against
+/// the "special" built-in traits (see `middle::lang_items`) and
+/// detects `Copy`, `Send` and `Sync`.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub enum GenericBound {
     Trait(PolyTraitRef, TraitBoundModifier),

src/libsyntax/ext/expand.rs

@@ -252,7 +252,7 @@ impl Invocation {
 
 pub struct MacroExpander<'a, 'b:'a> {
     pub cx: &'a mut ExtCtxt<'b>,
-    monotonic: bool, // c.f. `cx.monotonic_expander()`
+    monotonic: bool, // cf. `cx.monotonic_expander()`
 }
 
 impl<'a, 'b> MacroExpander<'a, 'b> {

src/libsyntax/parse/parser.rs

@@ -1532,7 +1532,7 @@ impl<'a> Parser<'a> {
                             if maybe_bounds && bounds.len() == 1 && !trailing_plus => {
                         let path = match bounds[0] {
                             GenericBound::Trait(ref pt, ..) => pt.trait_ref.path.clone(),
-                            _ => self.bug("unexpected lifetime bound"),
+                            GenericBound::Outlives(..) => self.bug("unexpected lifetime bound"),
                         };
                         self.parse_remaining_bounds(Vec::new(), path, lo, true)?
                     }

src/test/run-pass/traits/trait-alias.rs

@@ -10,13 +10,19 @@
 
 #![feature(trait_alias)]
 
-trait Foo = std::fmt::Debug;
+type Foo = std::fmt::Debug;
+type Bar = Foo;
 
 fn foo<T: Foo>(v: &T) {
     println!("{:?}", v);
 }
 
 pub fn main() {
     foo(&12345);
-    let foo: &Foo = &0i32;
+
+    let bar1: &Bar = &54321;
+    println!("{:?}", bar1);
+
+    let bar2 = Box::new(42) as Box<dyn Foo>;
+    println!("{:?}", bar2);
 }