Auto merge of #55994 - alexreg:fix-trait-alias-1, r=nikomatsakis

Disallow multiple regular traits in trait objects when trait aliases used

This ensures code like the following fails to compile.

```rust
#![feature(trait_alias)]

trait Foo = std::io::Read + std::io::Write;

fn main() {
    let foo: Box<dyn Foo>;
}
```

CC @nikomatsakis @scalexm

Author: bors

src/liballoc/slice.rs

@@ -133,12 +133,12 @@ pub use core::slice::{RChunks, RChunksMut, RChunksExact, RChunksExactMut};
 ////////////////////////////////////////////////////////////////////////////////
 
 // HACK(japaric) needed for the implementation of `vec!` macro during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use self::hack::into_vec;
 
 // HACK(japaric) needed for the implementation of `Vec::clone` during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use self::hack::to_vec;
 
@@ -383,7 +383,7 @@ impl<T> [T] {
     pub fn to_vec(&self) -> Vec<T>
         where T: Clone
     {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::to_vec(self)
     }
 
@@ -404,7 +404,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn into_vec(self: Box<Self>) -> Vec<T> {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::into_vec(self)
     }
 

src/librustc/hir/map/mod.rs

@@ -815,7 +815,7 @@ impl<'hir> Map<'hir> {
         }
     }
 
-    /// Returns the name associated with the given NodeId's AST.
+    /// Returns the name associated with the given `NodeId`'s AST.
     pub fn name(&self, id: NodeId) -> Name {
         match self.get(id) {
             Node::Item(i) => i.name,

src/librustc/hir/mod.rs

@@ -297,7 +297,7 @@ impl Lifetime {
     }
 }
 
-/// A "Path" is essentially Rust's notion of a name; for instance:
+/// A `Path` is essentially Rust's notion of a name; for instance:
 /// `std::cmp::PartialEq`. It's represented as a sequence of identifiers,
 /// along with a bunch of supporting information.
 #[derive(Clone, RustcEncodable, RustcDecodable)]
@@ -2039,12 +2039,12 @@ pub enum UseKind {
     ListStem,
 }
 
-/// TraitRef's appear in impls.
+/// `TraitRef` are references to traits in impls.
 ///
-/// resolve maps each TraitRef's ref_id to its defining trait; that's all
-/// that the ref_id is for. Note that ref_id's value is not the NodeId of the
-/// trait being referred to but just a unique NodeId that serves as a key
-/// within the DefMap.
+/// `resolve` maps each `TraitRef`'s `ref_id` to its defining trait; that's all
+/// that the `ref_id` is for. Note that `ref_id`'s value is not the `NodeId` of the
+/// trait being referred to but just a unique `NodeId` that serves as a key
+/// within the `DefMap`.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct TraitRef {
     pub path: Path,
@@ -2054,10 +2054,10 @@ pub struct TraitRef {
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct PolyTraitRef {
-    /// The `'a` in `<'a> Foo<&'a T>`
+    /// The `'a` in `<'a> Foo<&'a T>`.
     pub bound_generic_params: HirVec<GenericParam>,
 
-    /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`
+    /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`.
     pub trait_ref: TraitRef,
 
     pub span: Span,

src/librustc/infer/outlives/verify.rs

@@ -151,9 +151,9 @@ impl<'cx, 'gcx, 'tcx> VerifyBoundCx<'cx, 'gcx, 'tcx> {
     }
 
     fn recursive_type_bound(&self, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
-        let mut bounds = ty.walk_shallow()
+        let mut bounds: Vec<_> = ty.walk_shallow()
             .map(|subty| self.type_bound(subty))
-            .collect::<Vec<_>>();
+            .collect();
 
         let mut regions = smallvec![];
         ty.push_regions(&mut regions);

src/librustc/traits/mod.rs

@@ -51,8 +51,9 @@ pub use self::specialize::{OverlapError, specialization_graph, translate_substs}
 pub use self::specialize::find_associated_item;
 pub use self::engine::{TraitEngine, TraitEngineExt};
 pub use self::util::{elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs};
-pub use self::util::{supertraits, supertrait_def_ids, Supertraits, SupertraitDefIds};
-pub use self::util::transitive_bounds;
+pub use self::util::{supertraits, supertrait_def_ids, transitive_bounds,
+                     Supertraits, SupertraitDefIds};
+pub use self::util::{expand_trait_refs, TraitRefExpander};
 
 #[allow(dead_code)]
 pub mod auto_trait;
@@ -1017,7 +1018,7 @@ fn vtable_methods<'a, 'tcx>(
     )
 }
 
-impl<'tcx,O> Obligation<'tcx,O> {
+impl<'tcx, O> Obligation<'tcx,O> {
     pub fn new(cause: ObligationCause<'tcx>,
                param_env: ty::ParamEnv<'tcx>,
                predicate: O)

src/librustc/traits/select.rs

@@ -38,6 +38,7 @@ use super::{
 };
 
 use dep_graph::{DepKind, DepNodeIndex};
+use hir;
 use hir::def_id::DefId;
 use infer;
 use infer::{InferCtxt, InferOk, TypeFreshener};
@@ -47,8 +48,6 @@ use ty::fast_reject;
 use ty::relate::TypeRelation;
 use ty::subst::{Subst, Substs};
 use ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
-
-use hir;
 use rustc_data_structures::bit_set::GrowableBitSet;
 use rustc_data_structures::sync::Lock;
 use rustc_target::spec::abi::Abi;
@@ -1759,7 +1758,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             bounds
         );
 
-        let matching_bound = util::elaborate_predicates(self.tcx(), bounds.predicates)
+        let elaborated_predicates = util::elaborate_predicates(self.tcx(), bounds.predicates);
+        let matching_bound = elaborated_predicates
             .filter_to_traits()
             .find(|bound| {
                 self.probe(|this, _| {