Add a constness field to ast::TraitRef

This is used for both the `?const` syntax in bounds as well as the `impl
const Trait` syntax. I also considered handling these separately by
adding a variant of `TraitBoundModifier` and a field to
`ItemKind::Impl`, but this approach was less intrusive.

src/librustc_expand/build.rs

@@ -110,7 +110,7 @@ impl<'a> ExtCtxt<'a> {
     }
 
     pub fn trait_ref(&self, path: ast::Path) -> ast::TraitRef {
-        ast::TraitRef { path, ref_id: ast::DUMMY_NODE_ID }
+        ast::TraitRef { path, constness: None, ref_id: ast::DUMMY_NODE_ID }
     }
 
     pub fn poly_trait_ref(&self, span: Span, path: ast::Path) -> ast::PolyTraitRef {

src/libsyntax/ast.rs

@@ -1033,7 +1033,7 @@ impl Expr {
     pub fn to_bound(&self) -> Option<GenericBound> {
         match &self.kind {
             ExprKind::Path(None, path) => Some(GenericBound::Trait(
-                PolyTraitRef::new(Vec::new(), path.clone(), self.span),
+                PolyTraitRef::new(Vec::new(), path.clone(), None, self.span),
                 TraitBoundModifier::None,
             )),
             _ => None,
@@ -2376,6 +2376,15 @@ pub enum AttrKind {
 pub struct TraitRef {
     pub path: Path,
     pub ref_id: NodeId,
+
+    /// The `const` modifier, if any, that appears before this trait.
+    ///
+    /// |                | `constness`                 |
+    /// |----------------|-----------------------------|
+    /// | `Trait`        | `None`                      |
+    /// | `const Trait`  | `Some(Constness::Const)`    |
+    /// | `?const Trait` | `Some(Constness::NotConst)` |
+    pub constness: Option<Constness>,
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
@@ -2390,10 +2399,15 @@ pub struct PolyTraitRef {
 }
 
 impl PolyTraitRef {
-    pub fn new(generic_params: Vec<GenericParam>, path: Path, span: Span) -> Self {
+    pub fn new(
+        generic_params: Vec<GenericParam>,
+        path: Path,
+        constness: Option<Constness>,
+        span: Span,
+    ) -> Self {
         PolyTraitRef {
             bound_generic_params: generic_params,
-            trait_ref: TraitRef { path, ref_id: DUMMY_NODE_ID },
+            trait_ref: TraitRef { path, constness, ref_id: DUMMY_NODE_ID },
             span,
         }
     }

src/libsyntax/mut_visit.rs

@@ -838,7 +838,8 @@ pub fn noop_visit_variant_data<T: MutVisitor>(vdata: &mut VariantData, vis: &mut
     }
 }
 
-pub fn noop_visit_trait_ref<T: MutVisitor>(TraitRef { path, ref_id }: &mut TraitRef, vis: &mut T) {
+pub fn noop_visit_trait_ref<T: MutVisitor>(tr: &mut TraitRef, vis: &mut T) {
+    let TraitRef { path, ref_id, constness: _ } = tr;
     vis.visit_path(path);
     vis.visit_id(ref_id);
 }