diff --git a/src/librustc/lint/builtin.rs b/src/librustc/lint/builtin.rs
index 3edcd0b515641..35c29f646e4a0 100644
--- a/src/librustc/lint/builtin.rs
+++ b/src/librustc/lint/builtin.rs
@@ -1782,9 +1782,9 @@ impl LintPass for Stability {
         if self.is_internal(cx, item.span) { return }
 
         match item.node {
-            ast::ItemTrait(_, _, _, ref supertraits, _) => {
+            ast::ItemTrait(_, _, ref supertraits, _) => {
                 for t in supertraits.iter() {
-                    if let ast::TraitTyParamBound(ref t) = *t {
+                    if let ast::TraitTyParamBound(ref t, _) = *t {
                         let id = ty::trait_ref_to_def_id(cx.tcx, &t.trait_ref);
                         self.lint(cx, id, t.trait_ref.path.span);
                     }
diff --git a/src/librustc/metadata/encoder.rs b/src/librustc/metadata/encoder.rs
index 6782b3a74813e..2a670b308b6b2 100644
--- a/src/librustc/metadata/encoder.rs
+++ b/src/librustc/metadata/encoder.rs
@@ -1308,7 +1308,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
             }
         }
       }
-      ast::ItemTrait(_, _, _, _, ref ms) => {
+      ast::ItemTrait(_, _, _, ref ms) => {
         add_to_index(item, rbml_w, index);
         rbml_w.start_tag(tag_items_data_item);
         encode_def_id(rbml_w, def_id);
diff --git a/src/librustc/middle/infer/error_reporting.rs b/src/librustc/middle/infer/error_reporting.rs
index 2f62858071f6a..3d3eac7b6ed9f 100644
--- a/src/librustc/middle/infer/error_reporting.rs
+++ b/src/librustc/middle/infer/error_reporting.rs
@@ -1043,7 +1043,6 @@ impl<'a, 'tcx> Rebuilder<'a, 'tcx> {
                 ident: ty_param.ident,
                 id: ty_param.id,
                 bounds: bounds,
-                unbound: ty_param.unbound.clone(),
                 default: ty_param.default.clone(),
                 span: ty_param.span,
             }
@@ -1063,7 +1062,7 @@ impl<'a, 'tcx> Rebuilder<'a, 'tcx> {
                     // be passing down a map.
                     ast::RegionTyParamBound(lt)
                 }
-                &ast::TraitTyParamBound(ref poly_tr) => {
+                &ast::TraitTyParamBound(ref poly_tr, modifier) => {
                     let tr = &poly_tr.trait_ref;
                     let last_seg = tr.path.segments.last().unwrap();
                     let mut insert = Vec::new();
@@ -1087,7 +1086,7 @@ impl<'a, 'tcx> Rebuilder<'a, 'tcx> {
                             path: new_path,
                             ref_id: tr.ref_id,
                         }
-                    })
+                    }, modifier)
                 }
             }
         })
diff --git a/src/librustc/middle/privacy.rs b/src/librustc/middle/privacy.rs
index 6f63ae166fe41..730da26eda3dc 100644
--- a/src/librustc/middle/privacy.rs
+++ b/src/librustc/middle/privacy.rs
@@ -122,7 +122,7 @@ impl<'v> Visitor<'v> for ParentVisitor {
             // method to the root. In this case, if the trait is private, then
             // parent all the methods to the trait to indicate that they're
             // private.
-            ast::ItemTrait(_, _, _, _, ref methods) if item.vis != ast::Public => {
+            ast::ItemTrait(_, _, _, ref methods) if item.vis != ast::Public => {
                 for m in methods.iter() {
                     match *m {
                         ast::ProvidedMethod(ref m) => {
@@ -328,7 +328,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for EmbargoVisitor<'a, 'tcx> {
 
             // Default methods on traits are all public so long as the trait
             // is public
-            ast::ItemTrait(_, _, _, _, ref methods) if public_first => {
+            ast::ItemTrait(_, _, _, ref methods) if public_first => {
                 for method in methods.iter() {
                     match *method {
                         ast::ProvidedMethod(ref m) => {
@@ -1178,7 +1178,7 @@ impl<'a, 'tcx> SanePrivacyVisitor<'a, 'tcx> {
                 }
             }
 
-            ast::ItemTrait(_, _, _, _, ref methods) => {
+            ast::ItemTrait(_, _, _, ref methods) => {
                 for m in methods.iter() {
                     match *m {
                         ast::ProvidedMethod(ref m) => {
@@ -1242,7 +1242,7 @@ impl<'a, 'tcx> SanePrivacyVisitor<'a, 'tcx> {
 
             ast::ItemStruct(ref def, _) => check_struct(&**def),
 
-            ast::ItemTrait(_, _, _, _, ref methods) => {
+            ast::ItemTrait(_, _, _, ref methods) => {
                 for m in methods.iter() {
                     match *m {
                         ast::RequiredMethod(..) => {}
@@ -1306,7 +1306,7 @@ impl<'a, 'tcx> VisiblePrivateTypesVisitor<'a, 'tcx> {
 
     fn check_ty_param_bound(&self,
                             ty_param_bound: &ast::TyParamBound) {
-        if let ast::TraitTyParamBound(ref trait_ref) = *ty_param_bound {
+        if let ast::TraitTyParamBound(ref trait_ref, _) = *ty_param_bound {
             if !self.tcx.sess.features.borrow().visible_private_types &&
                 self.path_is_private_type(trait_ref.trait_ref.ref_id) {
                     let span = trait_ref.trait_ref.path.span;
@@ -1349,7 +1349,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for VisiblePrivateTypesVisitor<'a, 'tcx> {
             // namespace (the contents have their own privacies).
             ast::ItemForeignMod(_) => {}
 
-            ast::ItemTrait(_, _, _, ref bounds, _) => {
+            ast::ItemTrait(_, _, ref bounds, _) => {
                 if !self.trait_is_public(item.id) {
                     return
                 }
diff --git a/src/librustc/middle/resolve_lifetime.rs b/src/librustc/middle/resolve_lifetime.rs
index 22dea3be1d4b6..4feafff3b9211 100644
--- a/src/librustc/middle/resolve_lifetime.rs
+++ b/src/librustc/middle/resolve_lifetime.rs
@@ -105,7 +105,7 @@ impl<'a, 'v> Visitor<'v> for LifetimeContext<'a> {
                 ast::ItemTy(_, ref generics) |
                 ast::ItemEnum(_, ref generics) |
                 ast::ItemStruct(_, ref generics) |
-                ast::ItemTrait(_, ref generics, _, _, _) |
+                ast::ItemTrait(_, ref generics, _, _) |
                 ast::ItemImpl(_, ref generics, _, _, _) => {
                     // These kinds of items have only early bound lifetime parameters.
                     let lifetimes = &generics.lifetimes;
@@ -232,7 +232,9 @@ impl<'a, 'v> Visitor<'v> for LifetimeContext<'a> {
         }
     }
 
-    fn visit_poly_trait_ref(&mut self, trait_ref: &ast::PolyTraitRef) {
+    fn visit_poly_trait_ref(&mut self, trait_ref:
+                            &ast::PolyTraitRef,
+                            _modifier: &ast::TraitBoundModifier) {
         debug!("visit_poly_trait_ref trait_ref={}", trait_ref);
 
         self.with(LateScope(&trait_ref.bound_lifetimes, self.scope), |old_scope, this| {
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 232646f64a7d1..617be82701a09 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -4740,7 +4740,7 @@ pub fn provided_trait_methods<'tcx>(cx: &ctxt<'tcx>, id: ast::DefId)
         match cx.map.find(id.node) {
             Some(ast_map::NodeItem(item)) => {
                 match item.node {
-                    ItemTrait(_, _, _, _, ref ms) => {
+                    ItemTrait(_, _, _, ref ms) => {
                         let (_, p) =
                             ast_util::split_trait_methods(ms[]);
                         p.iter()
diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index c168709eec5b9..8e3eb4bf853bb 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -1493,7 +1493,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
 
             ItemImpl(_, _, Some(_), _, _) => parent,
 
-            ItemTrait(_, _, _, _, ref items) => {
+            ItemTrait(_, _, _, ref items) => {
                 let name_bindings =
                     self.add_child(name,
                                    parent.clone(),
@@ -4093,7 +4093,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                                             impl_items[]);
             }
 
-            ItemTrait(_, ref generics, ref unbound, ref bounds, ref trait_items) => {
+            ItemTrait(_, ref generics, ref bounds, ref trait_items) => {
                 // Create a new rib for the self type.
                 let mut self_type_rib = Rib::new(ItemRibKind);
 
@@ -4114,13 +4114,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                     this.resolve_type_parameter_bounds(item.id, bounds,
                                                        TraitDerivation);
 
-                    match *unbound {
-                        Some(ref tpb) => {
-                            this.resolve_trait_reference(item.id, tpb, TraitDerivation);
-                        }
-                        None => {}
-                    }
-
                     for trait_item in (*trait_items).iter() {
                         // Create a new rib for the trait_item-specific type
                         // parameters.
@@ -4368,12 +4361,6 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             self.resolve_type_parameter_bound(type_parameter.id, bound,
                                               TraitBoundingTypeParameter);
         }
-        match &type_parameter.unbound {
-            &Some(ref unbound) =>
-                self.resolve_trait_reference(
-                    type_parameter.id, unbound, TraitBoundingTypeParameter),
-            &None => {}
-        }
         match type_parameter.default {
             Some(ref ty) => self.resolve_type(&**ty),
             None => {}
@@ -4395,7 +4382,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                                     type_parameter_bound: &TyParamBound,
                                     reference_type: TraitReferenceType) {
         match *type_parameter_bound {
-            TraitTyParamBound(ref tref) => {
+            TraitTyParamBound(ref tref, _) => {
                 self.resolve_poly_trait_reference(id, tref, reference_type)
             }
             RegionTyParamBound(..) => {}
diff --git a/src/librustc_trans/save/mod.rs b/src/librustc_trans/save/mod.rs
index f491bc84b62c4..08f5dee717a9c 100644
--- a/src/librustc_trans/save/mod.rs
+++ b/src/librustc_trans/save/mod.rs
@@ -710,7 +710,7 @@ impl <'l, 'tcx> DxrVisitor<'l, 'tcx> {
         // super-traits
         for super_bound in trait_refs.iter() {
             let trait_ref = match *super_bound {
-                ast::TraitTyParamBound(ref trait_ref) => {
+                ast::TraitTyParamBound(ref trait_ref, _) => {
                     trait_ref
                 }
                 ast::RegionTyParamBound(..) => {
@@ -1052,7 +1052,7 @@ impl<'l, 'tcx, 'v> Visitor<'v> for DxrVisitor<'l, 'tcx> {
                                   &**typ,
                                   impl_items)
             }
-            ast::ItemTrait(_, ref generics, _, ref trait_refs, ref methods) =>
+            ast::ItemTrait(_, ref generics, ref trait_refs, ref methods) =>
                 self.process_trait(item, generics, trait_refs, methods),
             ast::ItemMod(ref m) => self.process_mod(item, m),
             ast::ItemTy(ref ty, ref ty_params) => {
@@ -1076,7 +1076,7 @@ impl<'l, 'tcx, 'v> Visitor<'v> for DxrVisitor<'l, 'tcx> {
     fn visit_generics(&mut self, generics: &ast::Generics) {
         for param in generics.ty_params.iter() {
             for bound in param.bounds.iter() {
-                if let ast::TraitTyParamBound(ref trait_ref) = *bound {
+                if let ast::TraitTyParamBound(ref trait_ref, _) = *bound {
                     self.process_trait_ref(&trait_ref.trait_ref, None);
                 }
             }
diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index 81e6ec4df5627..c8a084584b5e2 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -1626,7 +1626,7 @@ pub fn partition_bounds<'a>(tcx: &ty::ctxt,
     let mut trait_def_ids = DefIdMap::new();
     for ast_bound in ast_bounds.iter() {
         match *ast_bound {
-            ast::TraitTyParamBound(ref b) => {
+            ast::TraitTyParamBound(ref b, ast::TraitBoundModifier::None) => {
                 match ::lookup_def_tcx(tcx, b.trait_ref.path.span, b.trait_ref.ref_id) {
                     def::DefTrait(trait_did) => {
                         match trait_def_ids.get(&trait_did) {
@@ -1664,6 +1664,7 @@ pub fn partition_bounds<'a>(tcx: &ty::ctxt,
                 }
                 trait_bounds.push(b);
             }
+            ast::TraitTyParamBound(_, ast::TraitBoundModifier::Maybe) => {}
             ast::RegionTyParamBound(ref l) => {
                 region_bounds.push(l);
             }
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index fb23ad8e340ce..c8c8211f29253 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -673,7 +673,7 @@ pub fn check_item(ccx: &CrateCtxt, it: &ast::Item) {
         }
 
       }
-      ast::ItemTrait(_, _, _, _, ref trait_methods) => {
+      ast::ItemTrait(_, _, _, ref trait_methods) => {
         let trait_def = ty::lookup_trait_def(ccx.tcx, local_def(it.id));
         for trait_method in trait_methods.iter() {
             match *trait_method {
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index 793c5f5b4a058..8d07f1e435503 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -259,7 +259,7 @@ fn collect_trait_methods<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                                    trait_def: &ty::TraitDef<'tcx>) {
     let tcx = ccx.tcx;
     if let ast_map::NodeItem(item) = tcx.map.get(trait_id) {
-        if let ast::ItemTrait(_, _, _, _, ref trait_items) = item.node {
+        if let ast::ItemTrait(_, _, _, ref trait_items) = item.node {
             // For each method, construct a suitable ty::Method and
             // store it into the `tcx.impl_or_trait_items` table:
             for trait_item in trait_items.iter() {
@@ -627,11 +627,6 @@ pub fn ensure_no_ty_param_bounds(ccx: &CrateCtxt,
                 ast::RegionTyParamBound(..) => { }
             }
         }
-
-        match ty_param.unbound {
-            Some(_) => { warn = true; }
-            None => { }
-        }
     }
 
     if warn {
@@ -1146,7 +1141,7 @@ pub fn convert(ccx: &CrateCtxt, it: &ast::Item) {
                                                AllowEqConstraints::DontAllow);
             }
         },
-        ast::ItemTrait(_, _, _, _, ref trait_methods) => {
+        ast::ItemTrait(_, _, _, ref trait_methods) => {
             let trait_def = trait_def_of_item(ccx, it);
 
             debug!("trait_def: ident={} trait_def={}",
@@ -1338,13 +1333,12 @@ pub fn trait_def_of_item<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
         return def.clone();
     }
 
-    let (unsafety, generics, unbound, bounds, items) = match it.node {
+    let (unsafety, generics, bounds, items) = match it.node {
         ast::ItemTrait(unsafety,
                        ref generics,
-                       ref unbound,
                        ref supertraits,
                        ref items) => {
-            (unsafety, generics, unbound, supertraits, items.as_slice())
+            (unsafety, generics, supertraits, items.as_slice())
         }
         ref s => {
             tcx.sess.span_bug(
@@ -1367,7 +1361,6 @@ pub fn trait_def_of_item<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                                 token::SELF_KEYWORD_NAME,
                                 self_param_ty,
                                 bounds.as_slice(),
-                                unbound,
                                 it.span);
 
     let substs = mk_item_substs(ccx, &ty_generics);
@@ -1683,29 +1676,37 @@ fn ty_generics_for_fn_or_method<'tcx,AC>(
                 create_type_parameters_for_associated_types)
 }
 
-// Add the Sized bound, unless the type parameter is marked as `Sized?`.
+// Add the Sized bound, unless the type parameter is marked as `?Sized`.
 fn add_unsized_bound<'tcx,AC>(this: &AC,
-                              unbound: &Option<ast::TraitRef>,
                               bounds: &mut ty::BuiltinBounds,
-                              desc: &str,
+                              ast_bounds: &[ast::TyParamBound],
                               span: Span)
                               where AC: AstConv<'tcx> {
+    // Try to find an unbound in bounds.
+    let mut unbound = None;
+    for ab in ast_bounds.iter() {
+        if let &ast::TraitTyParamBound(ref ptr, ast::TraitBoundModifier::Maybe) = ab  {
+            if unbound.is_none() {
+                assert!(ptr.bound_lifetimes.is_empty());
+                unbound = Some(ptr.trait_ref.clone());
+            } else {
+                this.tcx().sess.span_err(span, "type parameter has more than one relaxed default \
+                                                bound, only one is supported");
+            }
+        }
+    }
+
     let kind_id = this.tcx().lang_items.require(SizedTraitLangItem);
     match unbound {
-        &Some(ref tpb) => {
+        Some(ref tpb) => {
             // FIXME(#8559) currently requires the unbound to be built-in.
             let trait_def_id = ty::trait_ref_to_def_id(this.tcx(), tpb);
             match kind_id {
                 Ok(kind_id) if trait_def_id != kind_id => {
                     this.tcx().sess.span_warn(span,
-                                              format!("default bound relaxed \
-                                                       for a {}, but this \
-                                                       does nothing because \
-                                                       the given bound is not \
-                                                       a default. \
-                                                       Only `Sized?` is \
-                                                       supported",
-                                                      desc)[]);
+                                              "default bound relaxed for a type parameter, but \
+                                               this does nothing because the given bound is not \
+                                               a default. Only `?Sized` is supported");
                     ty::try_add_builtin_trait(this.tcx(),
                                               kind_id,
                                               bounds);
@@ -1717,7 +1718,7 @@ fn add_unsized_bound<'tcx,AC>(this: &AC,
             ty::try_add_builtin_trait(this.tcx(), kind_id.unwrap(), bounds);
         }
         // No lang item for Sized, so we can't add it as a bound.
-        &None => {}
+        None => {}
     }
 }
 
@@ -1807,7 +1808,7 @@ fn ty_generics<'tcx,AC>(this: &AC,
 
                 for bound in bound_pred.bounds.iter() {
                     match bound {
-                        &ast::TyParamBound::TraitTyParamBound(ref poly_trait_ref) => {
+                        &ast::TyParamBound::TraitTyParamBound(ref poly_trait_ref, _) => {
                             let trait_ref = astconv::instantiate_poly_trait_ref(
                                 this,
                                 &ExplicitRscope,
@@ -1880,7 +1881,7 @@ fn ty_generics<'tcx,AC>(this: &AC,
             for bound in param.bounds.iter() {
                 // In the above example, `ast_trait_ref` is `Iterator`.
                 let ast_trait_ref = match *bound {
-                    ast::TraitTyParamBound(ref r) => r,
+                    ast::TraitTyParamBound(ref r, _) => r,
                     ast::RegionTyParamBound(..) => { continue; }
                 };
 
@@ -1978,7 +1979,6 @@ fn get_or_create_type_parameter_def<'tcx,AC>(this: &AC,
                                 param.ident.name,
                                 param_ty,
                                 param.bounds[],
-                                &param.unbound,
                                 param.span);
     let default = match param.default {
         None => None,
@@ -2023,7 +2023,6 @@ fn compute_bounds<'tcx,AC>(this: &AC,
                            name_of_bounded_thing: ast::Name,
                            param_ty: ty::ParamTy,
                            ast_bounds: &[ast::TyParamBound],
-                           unbound: &Option<ast::TraitRef>,
                            span: Span)
                            -> ty::ParamBounds<'tcx>
                            where AC: AstConv<'tcx> {
@@ -2032,11 +2031,9 @@ fn compute_bounds<'tcx,AC>(this: &AC,
                                              param_ty,
                                              ast_bounds);
 
-
     add_unsized_bound(this,
-                      unbound,
                       &mut param_bounds.builtin_bounds,
-                      "type parameter",
+                      ast_bounds,
                       span);
 
     check_bounds_compatible(this.tcx(),
diff --git a/src/librustc_typeck/variance.rs b/src/librustc_typeck/variance.rs
index bdd8c7c3da7c0..30869186ba5e9 100644
--- a/src/librustc_typeck/variance.rs
+++ b/src/librustc_typeck/variance.rs
@@ -353,7 +353,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for TermsContext<'a, 'tcx> {
         match item.node {
             ast::ItemEnum(_, ref generics) |
             ast::ItemStruct(_, ref generics) |
-            ast::ItemTrait(_, ref generics, _, _, _) => {
+            ast::ItemTrait(_, ref generics, _, _) => {
                 for (i, p) in generics.lifetimes.iter().enumerate() {
                     let id = p.lifetime.id;
                     self.add_inferred(item.id, RegionParam, TypeSpace, i, id);
diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index cdc51bb801c58..3eda39f54a997 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -163,13 +163,12 @@ pub fn build_external_trait(cx: &DocContext, tcx: &ty::ctxt,
         }
     });
     let trait_def = ty::lookup_trait_def(tcx, did);
-    let (bounds, default_unbound) = trait_def.bounds.clean(cx);
+    let bounds = trait_def.bounds.clean(cx);
     clean::Trait {
         unsafety: def.unsafety,
         generics: (&def.generics, subst::TypeSpace).clean(cx),
         items: items.collect(),
         bounds: bounds,
-        default_unbound: default_unbound
     }
 }
 
@@ -328,7 +327,7 @@ fn build_impl(cx: &DocContext, tcx: &ty::ctxt,
             derived: clean::detect_derived(attrs.as_slice()),
             trait_: associated_trait.clean(cx).map(|bound| {
                 match bound {
-                    clean::TraitBound(polyt) => polyt.trait_,
+                    clean::TraitBound(polyt, _) => polyt.trait_,
                     clean::RegionBound(..) => unreachable!(),
                 }
             }),
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 0308194527e95..b7845f23be298 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -458,8 +458,6 @@ pub struct TyParam {
     pub did: ast::DefId,
     pub bounds: Vec<TyParamBound>,
     pub default: Option<Type>,
-    /// An optional default bound on the parameter which is unbound, like `Sized?`
-    pub default_unbound: Option<Type>
 }
 
 impl Clean<TyParam> for ast::TyParam {
@@ -469,7 +467,6 @@ impl Clean<TyParam> for ast::TyParam {
             did: ast::DefId { krate: ast::LOCAL_CRATE, node: self.id },
             bounds: self.bounds.clean(cx),
             default: self.default.clean(cx),
-            default_unbound: self.unbound.clean(cx)
         }
     }
 }
@@ -478,13 +475,12 @@ impl<'tcx> Clean<TyParam> for ty::TypeParameterDef<'tcx> {
     fn clean(&self, cx: &DocContext) -> TyParam {
         cx.external_typarams.borrow_mut().as_mut().unwrap()
           .insert(self.def_id, self.name.clean(cx));
-        let (bounds, default_unbound) = self.bounds.clean(cx);
+        let bounds = self.bounds.clean(cx);
         TyParam {
             name: self.name.clean(cx),
             did: self.def_id,
             bounds: bounds,
             default: self.default.clean(cx),
-            default_unbound: default_unbound
         }
     }
 }
@@ -492,14 +488,14 @@ impl<'tcx> Clean<TyParam> for ty::TypeParameterDef<'tcx> {
 #[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq)]
 pub enum TyParamBound {
     RegionBound(Lifetime),
-    TraitBound(PolyTrait)
+    TraitBound(PolyTrait, ast::TraitBoundModifier)
 }
 
 impl Clean<TyParamBound> for ast::TyParamBound {
     fn clean(&self, cx: &DocContext) -> TyParamBound {
         match *self {
             ast::RegionTyParamBound(lt) => RegionBound(lt.clean(cx)),
-            ast::TraitTyParamBound(ref t) => TraitBound(t.clean(cx)),
+            ast::TraitTyParamBound(ref t, modifier) => TraitBound(t.clean(cx), modifier),
         }
     }
 }
@@ -600,7 +596,7 @@ impl Clean<TyParamBound> for ty::BuiltinBound {
                 did: did,
             },
             lifetimes: vec![]
-        })
+        }, ast::TraitBoundModifier::None)
     }
 }
 
@@ -648,37 +644,20 @@ impl<'tcx> Clean<TyParamBound> for ty::TraitRef<'tcx> {
         TraitBound(PolyTrait {
             trait_: ResolvedPath { path: path, typarams: None, did: self.def_id, },
             lifetimes: late_bounds
-        })
+        }, ast::TraitBoundModifier::None)
     }
 }
 
-// Returns (bounds, default_unbound)
-impl<'tcx> Clean<(Vec<TyParamBound>, Option<Type>)> for ty::ParamBounds<'tcx> {
-    fn clean(&self, cx: &DocContext) -> (Vec<TyParamBound>, Option<Type>) {
+impl<'tcx> Clean<Vec<TyParamBound>> for ty::ParamBounds<'tcx> {
+    fn clean(&self, cx: &DocContext) -> Vec<TyParamBound> {
         let mut v = Vec::new();
-        let mut has_sized_bound = false;
-        for b in self.builtin_bounds.iter() {
-            if b != ty::BoundSized {
-                v.push(b.clean(cx));
-            } else {
-                has_sized_bound = true;
-            }
-        }
         for t in self.trait_bounds.iter() {
             v.push(t.clean(cx));
         }
         for r in self.region_bounds.iter().filter_map(|r| r.clean(cx)) {
             v.push(RegionBound(r));
         }
-        if has_sized_bound {
-            (v, None)
-        } else {
-            let ty = match ty::BoundSized.clean(cx) {
-                TraitBound(polyt) => polyt.trait_,
-                _ => unreachable!()
-            };
-            (v, Some(ty))
-        }
+        v
     }
 }
 
@@ -689,7 +668,7 @@ impl<'tcx> Clean<Option<Vec<TyParamBound>>> for subst::Substs<'tcx> {
         v.extend(self.types.iter().map(|t| TraitBound(PolyTrait {
             trait_: t.clean(cx),
             lifetimes: vec![]
-        })));
+        }, ast::TraitBoundModifier::None)));
         if v.len() > 0 {Some(v)} else {None}
     }
 }
@@ -1047,8 +1026,6 @@ pub struct Trait {
     pub items: Vec<TraitMethod>,
     pub generics: Generics,
     pub bounds: Vec<TyParamBound>,
-    /// An optional default bound not required for `Self`, like `Sized?`
-    pub default_unbound: Option<Type>
 }
 
 impl Clean<Item> for doctree::Trait {
@@ -1065,7 +1042,6 @@ impl Clean<Item> for doctree::Trait {
                 items: self.items.clean(cx),
                 generics: self.generics.clean(cx),
                 bounds: self.bounds.clean(cx),
-                default_unbound: self.default_unbound.clean(cx)
             }),
         }
     }
@@ -2412,7 +2388,6 @@ impl Clean<Item> for ty::AssociatedType {
                 },
                 bounds: vec![],
                 default: None,
-                default_unbound: None
             }),
             visibility: None,
             def_id: self.def_id,
diff --git a/src/librustdoc/doctree.rs b/src/librustdoc/doctree.rs
index 7f7c055062aaa..251ce5aefeb71 100644
--- a/src/librustdoc/doctree.rs
+++ b/src/librustdoc/doctree.rs
@@ -178,7 +178,6 @@ pub struct Trait {
     pub whence: Span,
     pub vis: ast::Visibility,
     pub stab: Option<attr::Stability>,
-    pub default_unbound: Option<ast::TraitRef> // FIXME(tomjakubowski)
 }
 
 pub struct Impl {
diff --git a/src/librustdoc/html/format.rs b/src/librustdoc/html/format.rs
index 25c4f4e01b620..f20a74f937b75 100644
--- a/src/librustdoc/html/format.rs
+++ b/src/librustdoc/html/format.rs
@@ -97,9 +97,6 @@ impl fmt::Show for clean::Generics {
                 if i > 0 {
                     try!(f.write(", ".as_bytes()))
                 }
-                if let Some(ref unbound) = tp.default_unbound {
-                    try!(write!(f, "{}? ", unbound));
-                };
                 try!(f.write(tp.name.as_bytes()));
 
                 if tp.bounds.len() > 0 {
@@ -182,8 +179,12 @@ impl fmt::Show for clean::TyParamBound {
             clean::RegionBound(ref lt) => {
                 write!(f, "{}", *lt)
             }
-            clean::TraitBound(ref ty) => {
-                write!(f, "{}", *ty)
+            clean::TraitBound(ref ty, modifier) => {
+                let modifier_str = match modifier {
+                    ast::TraitBoundModifier::None => "",
+                    ast::TraitBoundModifier::Maybe => "?",
+                };
+                write!(f, "{}{}", modifier_str, *ty)
             }
         }
     }
@@ -458,12 +459,15 @@ impl fmt::Show for clean::Type {
                            for bound in decl.bounds.iter() {
                                 match *bound {
                                     clean::RegionBound(..) => {}
-                                    clean::TraitBound(ref t) => {
+                                    clean::TraitBound(ref t, modifier) => {
                                         if ret.len() == 0 {
                                             ret.push_str(": ");
                                         } else {
                                             ret.push_str(" + ");
                                         }
+                                        if modifier == ast::TraitBoundModifier::Maybe {
+                                            ret.push_str("?");
+                                        }
                                         ret.push_str(format!("{}",
                                                              *t).as_slice());
                                     }
diff --git a/src/librustdoc/html/render.rs b/src/librustdoc/html/render.rs
index 304dbe201e8fd..bfb03cb2589c2 100644
--- a/src/librustdoc/html/render.rs
+++ b/src/librustdoc/html/render.rs
@@ -1679,9 +1679,6 @@ fn item_function(w: &mut fmt::Formatter, it: &clean::Item,
 fn item_trait(w: &mut fmt::Formatter, cx: &Context, it: &clean::Item,
               t: &clean::Trait) -> fmt::Result {
     let mut bounds = String::new();
-    if let Some(ref ty) = t.default_unbound {
-        bounds.push_str(format!(" for {}?", ty).as_slice());
-    }
     if t.bounds.len() > 0 {
         if bounds.len() > 0 {
             bounds.push(' ');
diff --git a/src/librustdoc/visit_ast.rs b/src/librustdoc/visit_ast.rs
index 4374ce5deef4e..e71711aa8d6e7 100644
--- a/src/librustdoc/visit_ast.rs
+++ b/src/librustdoc/visit_ast.rs
@@ -322,7 +322,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
                 };
                 om.constants.push(s);
             },
-            ast::ItemTrait(unsafety, ref gen, ref def_ub, ref b, ref items) => {
+            ast::ItemTrait(unsafety, ref gen, ref b, ref items) => {
                 let t = Trait {
                     unsafety: unsafety,
                     name: name,
@@ -334,7 +334,6 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
                     whence: item.span,
                     vis: item.vis,
                     stab: self.stability(item.id),
-                    default_unbound: def_ub.clone()
                 };
                 om.traits.push(t);
             },
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index debcbcd2154bd..f253943943dcc 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -371,10 +371,18 @@ pub const DUMMY_NODE_ID: NodeId = -1;
 /// detects Copy, Send and Sync.
 #[deriving(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Show)]
 pub enum TyParamBound {
-    TraitTyParamBound(PolyTraitRef),
+    TraitTyParamBound(PolyTraitRef, TraitBoundModifier),
     RegionTyParamBound(Lifetime)
 }
 
+/// A modifier on a bound, currently this is only used for `?Sized`, where the
+/// modifier is `Maybe`. Negative bounds should also be handled here.
+#[deriving(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Show)]
+pub enum TraitBoundModifier {
+    None,
+    Maybe,
+}
+
 pub type TyParamBounds = OwnedSlice<TyParamBound>;
 
 #[deriving(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Show)]
@@ -382,7 +390,6 @@ pub struct TyParam {
     pub ident: Ident,
     pub id: NodeId,
     pub bounds: TyParamBounds,
-    pub unbound: Option<TraitRef>,
     pub default: Option<P<Ty>>,
     pub span: Span
 }
@@ -1488,7 +1495,7 @@ pub struct PolyTraitRef {
     pub bound_lifetimes: Vec<LifetimeDef>,
 
     /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`
-    pub trait_ref: TraitRef
+    pub trait_ref: TraitRef,
 }
 
 #[deriving(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Show, Copy)]
@@ -1577,8 +1584,6 @@ pub enum Item_ {
     /// Represents a Trait Declaration
     ItemTrait(Unsafety,
               Generics,
-              Option<TraitRef>, // (optional) default bound not required for Self.
-                                // Currently, only Sized makes sense here.
               TyParamBounds,
               Vec<TraitItem>),
     ItemImpl(Unsafety,
diff --git a/src/libsyntax/ast_map/mod.rs b/src/libsyntax/ast_map/mod.rs
index 9b42a8f754069..5a4f5731be50d 100644
--- a/src/libsyntax/ast_map/mod.rs
+++ b/src/libsyntax/ast_map/mod.rs
@@ -780,9 +780,9 @@ impl<'ast> Visitor<'ast> for NodeCollector<'ast> {
                     None => {}
                 }
             }
-            ItemTrait(_, _, _, ref bounds, ref trait_items) => {
+            ItemTrait(_, _, ref bounds, ref trait_items) => {
                 for b in bounds.iter() {
-                    if let TraitTyParamBound(ref t) = *b {
+                    if let TraitTyParamBound(ref t, TraitBoundModifier::None) = *b {
                         self.insert(t.trait_ref.ref_id, NodeItem(i));
                     }
                 }
diff --git a/src/libsyntax/config.rs b/src/libsyntax/config.rs
index d2185a00876d2..94a3784291d0b 100644
--- a/src/libsyntax/config.rs
+++ b/src/libsyntax/config.rs
@@ -139,11 +139,11 @@ fn fold_item_underscore<F>(cx: &mut Context<F>, item: ast::Item_) -> ast::Item_
                                        .collect();
             ast::ItemImpl(u, a, b, c, impl_items)
         }
-        ast::ItemTrait(u, a, b, c, methods) => {
+        ast::ItemTrait(u, a, b, methods) => {
             let methods = methods.into_iter()
                                  .filter(|m| trait_method_in_cfg(cx, m))
                                  .collect();
-            ast::ItemTrait(u, a, b, c, methods)
+            ast::ItemTrait(u, a, b, methods)
         }
         ast::ItemStruct(def, generics) => {
             ast::ItemStruct(fold_struct(cx, def), generics)
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 77165168746b7..239af18890909 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -68,7 +68,6 @@ pub trait AstBuilder {
                span: Span,
                id: ast::Ident,
                bounds: OwnedSlice<ast::TyParamBound>,
-               unbound: Option<ast::TraitRef>,
                default: Option<P<ast::Ty>>) -> ast::TyParam;
 
     fn trait_ref(&self, path: ast::Path) -> ast::TraitRef;
@@ -414,13 +413,11 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
                span: Span,
                id: ast::Ident,
                bounds: OwnedSlice<ast::TyParamBound>,
-               unbound: Option<ast::TraitRef>,
                default: Option<P<ast::Ty>>) -> ast::TyParam {
         ast::TyParam {
             ident: id,
             id: ast::DUMMY_NODE_ID,
             bounds: bounds,
-            unbound: unbound,
             default: default,
             span: span
         }
@@ -455,7 +452,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
     }
 
     fn typarambound(&self, path: ast::Path) -> ast::TyParamBound {
-        ast::TraitTyParamBound(self.poly_trait_ref(path))
+        ast::TraitTyParamBound(self.poly_trait_ref(path), ast::TraitBoundModifier::None)
     }
 
     fn lifetime(&self, span: Span, name: ast::Name) -> ast::Lifetime {
diff --git a/src/libsyntax/ext/deriving/decodable.rs b/src/libsyntax/ext/deriving/decodable.rs
index 57dfbc0c6e8bc..3c8d74c14ee63 100644
--- a/src/libsyntax/ext/deriving/decodable.rs
+++ b/src/libsyntax/ext/deriving/decodable.rs
@@ -58,10 +58,10 @@ fn expand_deriving_decodable_imp<F>(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds {
             lifetimes: Vec::new(),
-            bounds: vec!(("__D", None, vec!(Path::new_(
+            bounds: vec!(("__D", vec!(Path::new_(
                             vec!(krate, "Decoder"), None,
                             vec!(box Literal(Path::new_local("__E"))), true))),
-                         ("__E", None, vec!()))
+                         ("__E", vec!()))
         },
         methods: vec!(
             MethodDef {
diff --git a/src/libsyntax/ext/deriving/encodable.rs b/src/libsyntax/ext/deriving/encodable.rs
index 8bd3df6232ce5..5829f34bccc5d 100644
--- a/src/libsyntax/ext/deriving/encodable.rs
+++ b/src/libsyntax/ext/deriving/encodable.rs
@@ -134,10 +134,10 @@ fn expand_deriving_encodable_imp<F>(cx: &mut ExtCtxt,
         additional_bounds: Vec::new(),
         generics: LifetimeBounds {
             lifetimes: Vec::new(),
-            bounds: vec!(("__S", None, vec!(Path::new_(
+            bounds: vec!(("__S", vec!(Path::new_(
                             vec!(krate, "Encoder"), None,
                             vec!(box Literal(Path::new_local("__E"))), true))),
-                         ("__E", None, vec!()))
+                         ("__E", vec!()))
         },
         methods: vec!(
             MethodDef {
diff --git a/src/libsyntax/ext/deriving/generic/mod.rs b/src/libsyntax/ext/deriving/generic/mod.rs
index cf0201294ae54..e4e31139d82f4 100644
--- a/src/libsyntax/ext/deriving/generic/mod.rs
+++ b/src/libsyntax/ext/deriving/generic/mod.rs
@@ -417,7 +417,6 @@ impl<'a> TraitDef<'a> {
             cx.typaram(self.span,
                        ty_param.ident,
                        OwnedSlice::from_vec(bounds),
-                       ty_param.unbound.clone(),
                        None)
         }));
 
diff --git a/src/libsyntax/ext/deriving/generic/ty.rs b/src/libsyntax/ext/deriving/generic/ty.rs
index 56d11c2377fa4..95bdd8b9ffd2f 100644
--- a/src/libsyntax/ext/deriving/generic/ty.rs
+++ b/src/libsyntax/ext/deriving/generic/ty.rs
@@ -189,15 +189,19 @@ impl<'a> Ty<'a> {
 }
 
 
-fn mk_ty_param(cx: &ExtCtxt, span: Span, name: &str,
-               bounds: &[Path], unbound: Option<ast::TraitRef>,
-               self_ident: Ident, self_generics: &Generics) -> ast::TyParam {
+fn mk_ty_param(cx: &ExtCtxt,
+               span: Span,
+               name: &str,
+               bounds: &[Path],
+               self_ident: Ident,
+               self_generics: &Generics)
+               -> ast::TyParam {
     let bounds =
         bounds.iter().map(|b| {
             let path = b.to_path(cx, span, self_ident, self_generics);
             cx.typarambound(path)
         }).collect();
-    cx.typaram(span, cx.ident_of(name), bounds, unbound, None)
+    cx.typaram(span, cx.ident_of(name), bounds, None)
 }
 
 fn mk_generics(lifetimes: Vec<ast::LifetimeDef>, ty_params: Vec<ast::TyParam>)
@@ -216,7 +220,7 @@ fn mk_generics(lifetimes: Vec<ast::LifetimeDef>, ty_params: Vec<ast::TyParam>)
 #[deriving(Clone)]
 pub struct LifetimeBounds<'a> {
     pub lifetimes: Vec<(&'a str, Vec<&'a str>)>,
-    pub bounds: Vec<(&'a str, Option<ast::TraitRef>, Vec<Path<'a>>)>,
+    pub bounds: Vec<(&'a str, Vec<Path<'a>>)>,
 }
 
 impl<'a> LifetimeBounds<'a> {
@@ -239,12 +243,11 @@ impl<'a> LifetimeBounds<'a> {
         }).collect();
         let ty_params = self.bounds.iter().map(|t| {
             match t {
-                &(ref name, ref unbound, ref bounds) => {
+                &(ref name, ref bounds) => {
                     mk_ty_param(cx,
                                 span,
                                 *name,
                                 bounds.as_slice(),
-                                unbound.clone(),
                                 self_ty,
                                 self_generics)
                 }
diff --git a/src/libsyntax/ext/deriving/hash.rs b/src/libsyntax/ext/deriving/hash.rs
index 4e59124a1294f..72e3b45dc91b7 100644
--- a/src/libsyntax/ext/deriving/hash.rs
+++ b/src/libsyntax/ext/deriving/hash.rs
@@ -30,7 +30,7 @@ pub fn expand_deriving_hash<F>(cx: &mut ExtCtxt,
                     vec!(box Literal(Path::new_local("__S"))), true),
          LifetimeBounds {
              lifetimes: Vec::new(),
-             bounds: vec!(("__S", None,
+             bounds: vec!(("__S",
                            vec!(Path::new(vec!("std", "hash", "Writer"))))),
          },
          Path::new_local("__S"))
diff --git a/src/libsyntax/ext/deriving/rand.rs b/src/libsyntax/ext/deriving/rand.rs
index 4f6e4d1fb3c10..1ddf5b2a5c31e 100644
--- a/src/libsyntax/ext/deriving/rand.rs
+++ b/src/libsyntax/ext/deriving/rand.rs
@@ -36,7 +36,6 @@ pub fn expand_deriving_rand<F>(cx: &mut ExtCtxt,
                 generics: LifetimeBounds {
                     lifetimes: Vec::new(),
                     bounds: vec!(("R",
-                                  None,
                                   vec!( Path::new(vec!("std", "rand", "Rng")) )))
                 },
                 explicit_self: None,
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 11a9fdee0b9b0..4f0169e31f229 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -736,18 +736,17 @@ pub fn noop_fold_ty_param_bound<T>(tpb: TyParamBound, fld: &mut T)
                                    -> TyParamBound
                                    where T: Folder {
     match tpb {
-        TraitTyParamBound(ty) => TraitTyParamBound(fld.fold_poly_trait_ref(ty)),
+        TraitTyParamBound(ty, modifier) => TraitTyParamBound(fld.fold_poly_trait_ref(ty), modifier),
         RegionTyParamBound(lifetime) => RegionTyParamBound(fld.fold_lifetime(lifetime)),
     }
 }
 
 pub fn noop_fold_ty_param<T: Folder>(tp: TyParam, fld: &mut T) -> TyParam {
-    let TyParam {id, ident, bounds, unbound, default, span} = tp;
+    let TyParam {id, ident, bounds, default, span} = tp;
     TyParam {
         id: fld.new_id(id),
         ident: ident,
         bounds: fld.fold_bounds(bounds),
-        unbound: unbound.map(|x| fld.fold_trait_ref(x)),
         default: default.map(|x| fld.fold_ty(x)),
         span: span
     }
@@ -1043,7 +1042,7 @@ pub fn noop_fold_item_underscore<T: Folder>(i: Item_, folder: &mut T) -> Item_ {
                      folder.fold_ty(ty),
                      new_impl_items)
         }
-        ItemTrait(unsafety, generics, unbound, bounds, methods) => {
+        ItemTrait(unsafety, generics, bounds, methods) => {
             let bounds = folder.fold_bounds(bounds);
             let methods = methods.into_iter().flat_map(|method| {
                 let r = match method {
@@ -1073,7 +1072,6 @@ pub fn noop_fold_item_underscore<T: Folder>(i: Item_, folder: &mut T) -> Item_ {
             }).collect();
             ItemTrait(unsafety,
                       folder.fold_generics(generics),
-                      unbound,
                       bounds,
                       methods)
         }
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 2f43661eebeba..fd64511cc041a 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -15,7 +15,7 @@ use self::ItemOrViewItem::*;
 
 use abi;
 use ast::{AssociatedType, BareFnTy, ClosureTy};
-use ast::{RegionTyParamBound, TraitTyParamBound};
+use ast::{RegionTyParamBound, TraitTyParamBound, TraitBoundModifier};
 use ast::{ProvidedMethod, Public, Unsafety};
 use ast::{Mod, BiAdd, Arg, Arm, Attribute, BindByRef, BindByValue};
 use ast::{BiBitAnd, BiBitOr, BiBitXor, BiRem, Block};
@@ -117,6 +117,13 @@ pub enum PathParsingMode {
     LifetimeAndTypesWithColons,
 }
 
+/// How to parse a bound, whether to allow bound modifiers such as `?`.
+#[deriving(Copy, PartialEq)]
+pub enum BoundParsingMode {
+    Bare,
+    Modified,
+}
+
 enum ItemOrViewItem {
     /// Indicates a failure to parse any kind of item. The attributes are
     /// returned.
@@ -1087,12 +1094,12 @@ impl<'a> Parser<'a> {
             let poly_trait_ref = ast::PolyTraitRef { bound_lifetimes: lifetime_defs,
                                                      trait_ref: trait_ref };
             let other_bounds = if self.eat(&token::BinOp(token::Plus)) {
-                self.parse_ty_param_bounds()
+                self.parse_ty_param_bounds(BoundParsingMode::Bare)
             } else {
                 OwnedSlice::empty()
             };
             let all_bounds =
-                Some(TraitTyParamBound(poly_trait_ref)).into_iter()
+                Some(TraitTyParamBound(poly_trait_ref, TraitBoundModifier::None)).into_iter()
                 .chain(other_bounds.into_vec().into_iter())
                 .collect();
             ast::TyPolyTraitRef(all_bounds)
@@ -1165,7 +1172,7 @@ impl<'a> Parser<'a> {
         // To be helpful, parse the proc as ever
         let _ = self.parse_legacy_lifetime_defs(lifetime_defs);
         let _ = self.parse_fn_args(false, false);
-        let _ = self.parse_colon_then_ty_param_bounds();
+        let _ = self.parse_colon_then_ty_param_bounds(BoundParsingMode::Bare);
         let _ = self.parse_ret_ty();
 
         self.obsolete(proc_span, ObsoleteProcType);
@@ -1255,7 +1262,7 @@ impl<'a> Parser<'a> {
             inputs
         };
 
-        let bounds = self.parse_colon_then_ty_param_bounds();
+        let bounds = self.parse_colon_then_ty_param_bounds(BoundParsingMode::Bare);
 
         let output = self.parse_ret_ty();
         let decl = P(FnDecl {
@@ -1481,7 +1488,7 @@ impl<'a> Parser<'a> {
             return lhs;
         }
 
-        let bounds = self.parse_ty_param_bounds();
+        let bounds = self.parse_ty_param_bounds(BoundParsingMode::Bare);
 
         // In type grammar, `+` is treated like a binary operator,
         // and hence both L and R side are required.
@@ -4029,13 +4036,14 @@ impl<'a> Parser<'a> {
 
     // Parses a sequence of bounds if a `:` is found,
     // otherwise returns empty list.
-    fn parse_colon_then_ty_param_bounds(&mut self)
+    fn parse_colon_then_ty_param_bounds(&mut self,
+                                        mode: BoundParsingMode)
                                         -> OwnedSlice<TyParamBound>
     {
         if !self.eat(&token::Colon) {
             OwnedSlice::empty()
         } else {
-            self.parse_ty_param_bounds()
+            self.parse_ty_param_bounds(mode)
         }
     }
 
@@ -4043,14 +4051,20 @@ impl<'a> Parser<'a> {
     // where   boundseq  = ( polybound + boundseq ) | polybound
     // and     polybound = ( 'for' '<' 'region '>' )? bound
     // and     bound     = 'region | trait_ref
-    // NB: The None/Some distinction is important for issue #7264.
-    fn parse_ty_param_bounds(&mut self)
+    fn parse_ty_param_bounds(&mut self,
+                             mode: BoundParsingMode)
                              -> OwnedSlice<TyParamBound>
     {
         let mut result = vec!();
         loop {
+            let question_span = self.span;
+            let ate_question = self.eat(&token::Question);
             match self.token {
                 token::Lifetime(lifetime) => {
+                    if ate_question {
+                        self.span_err(question_span,
+                                      "`?` may only modify trait bounds, not lifetime bounds");
+                    }
                     result.push(RegionTyParamBound(ast::Lifetime {
                         id: ast::DUMMY_NODE_ID,
                         span: self.span,
@@ -4060,7 +4074,18 @@ impl<'a> Parser<'a> {
                 }
                 token::ModSep | token::Ident(..) => {
                     let poly_trait_ref = self.parse_poly_trait_ref();
-                    result.push(TraitTyParamBound(poly_trait_ref))
+                    let modifier = if ate_question {
+                        if mode == BoundParsingMode::Modified {
+                            TraitBoundModifier::Maybe
+                        } else {
+                            self.span_err(question_span,
+                                          "unexpected `?`");
+                            TraitBoundModifier::None
+                        }
+                    } else {
+                        TraitBoundModifier::None
+                    };
+                    result.push(TraitTyParamBound(poly_trait_ref, modifier))
                 }
                 _ => break,
             }
@@ -4089,13 +4114,14 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// Matches typaram = (unbound`?`)? IDENT optbounds ( EQ ty )?
+    /// Matches typaram = (unbound `?`)? IDENT (`?` unbound)? optbounds ( EQ ty )?
     fn parse_ty_param(&mut self) -> TyParam {
         // This is a bit hacky. Currently we are only interested in a single
         // unbound, and it may only be `Sized`. To avoid backtracking and other
         // complications, we parse an ident, then check for `?`. If we find it,
         // we use the ident as the unbound, otherwise, we use it as the name of
-        // type param.
+        // type param. Even worse, for now, we need to check for `?` before or
+        // after the bound.
         let mut span = self.span;
         let mut ident = self.parse_ident();
         let mut unbound = None;
@@ -4106,7 +4132,14 @@ impl<'a> Parser<'a> {
             ident = self.parse_ident();
         }
 
-        let bounds = self.parse_colon_then_ty_param_bounds();
+        let mut bounds = self.parse_colon_then_ty_param_bounds(BoundParsingMode::Modified);
+        if let Some(unbound) = unbound {
+            let mut bounds_as_vec = bounds.into_vec();
+            bounds_as_vec.push(TraitTyParamBound(PolyTraitRef { bound_lifetimes: vec![],
+                                                                trait_ref: unbound },
+                                                 TraitBoundModifier::Maybe));
+            bounds = OwnedSlice::from_vec(bounds_as_vec);
+        };
 
         let default = if self.check(&token::Eq) {
             self.bump();
@@ -4118,7 +4151,6 @@ impl<'a> Parser<'a> {
             ident: ident,
             id: ast::DUMMY_NODE_ID,
             bounds: bounds,
-            unbound: unbound,
             default: default,
             span: span,
         }
@@ -4260,7 +4292,7 @@ impl<'a> Parser<'a> {
                     let bounded_ty = self.parse_ty();
 
                     if self.eat(&token::Colon) {
-                        let bounds = self.parse_ty_param_bounds();
+                        let bounds = self.parse_ty_param_bounds(BoundParsingMode::Bare);
                         let hi = self.span.hi;
                         let span = mk_sp(lo, hi);
 
@@ -4747,15 +4779,23 @@ impl<'a> Parser<'a> {
     fn parse_item_trait(&mut self, unsafety: Unsafety) -> ItemInfo {
         let ident = self.parse_ident();
         let mut tps = self.parse_generics();
-        let sized = self.parse_for_sized();
+        let unbound = self.parse_for_sized();
 
         // Parse supertrait bounds.
-        let bounds = self.parse_colon_then_ty_param_bounds();
+        let mut bounds = self.parse_colon_then_ty_param_bounds(BoundParsingMode::Bare);
+
+        if let Some(unbound) = unbound {
+            let mut bounds_as_vec = bounds.into_vec();
+            bounds_as_vec.push(TraitTyParamBound(PolyTraitRef { bound_lifetimes: vec![],
+                                                                trait_ref: unbound },
+                                                 TraitBoundModifier::Maybe));
+            bounds = OwnedSlice::from_vec(bounds_as_vec);
+        };
 
         self.parse_where_clause(&mut tps);
 
         let meths = self.parse_trait_items();
-        (ident, ItemTrait(unsafety, tps, sized, bounds, meths), None)
+        (ident, ItemTrait(unsafety, tps, bounds, meths), None)
     }
 
     fn parse_impl_items(&mut self) -> (Vec<ImplItem>, Vec<Attribute>) {
@@ -4974,12 +5014,25 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_for_sized(&mut self) -> Option<ast::TraitRef> {
+        // FIXME, this should really use TraitBoundModifier, but it will get
+        // re-jigged shortly in any case, so leaving the hacky version for now.
         if self.eat_keyword(keywords::For) {
             let span = self.span;
+            let mut ate_question = false;
+            if self.eat(&token::Question) {
+                ate_question = true;
+            }
             let ident = self.parse_ident();
-            if !self.eat(&token::Question) {
+            if self.eat(&token::Question) {
+                if ate_question {
+                    self.span_err(span,
+                        "unexpected `?`");
+                }
+                ate_question = true;
+            }
+            if !ate_question {
                 self.span_err(span,
-                    "expected 'Sized?' after `for` in trait item");
+                    "expected `?Sized` after `for` in trait item");
                 return None;
             }
             let tref = Parser::trait_ref_from_ident(ident, span);
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index accffbc35babe..02a03285d3b86 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -13,7 +13,7 @@ pub use self::AnnNode::*;
 use abi;
 use ast::{mod, FnUnboxedClosureKind, FnMutUnboxedClosureKind};
 use ast::{FnOnceUnboxedClosureKind};
-use ast::{MethodImplItem, RegionTyParamBound, TraitTyParamBound};
+use ast::{MethodImplItem, RegionTyParamBound, TraitTyParamBound, TraitBoundModifier};
 use ast::{RequiredMethod, ProvidedMethod, TypeImplItem, TypeTraitItem};
 use ast::{UnboxedClosureKind};
 use ast_util;
@@ -958,20 +958,25 @@ impl<'a> State<'a> {
                 }
                 try!(self.bclose(item.span));
             }
-            ast::ItemTrait(unsafety, ref generics, ref unbound, ref bounds, ref methods) => {
+            ast::ItemTrait(unsafety, ref generics, ref bounds, ref methods) => {
                 try!(self.head(""));
                 try!(self.print_visibility(item.vis));
                 try!(self.print_unsafety(unsafety));
                 try!(self.word_nbsp("trait"));
                 try!(self.print_ident(item.ident));
                 try!(self.print_generics(generics));
-                if let &Some(ref tref) = unbound {
-                    try!(space(&mut self.s));
-                    try!(self.word_space("for"));
-                    try!(self.print_trait_ref(tref));
-                    try!(word(&mut self.s, "?"));
+                let bounds: Vec<_> = bounds.iter().map(|b| b.clone()).collect();
+                let mut real_bounds = Vec::with_capacity(bounds.len());
+                for b in bounds.into_iter() {
+                    if let TraitTyParamBound(ref ptr, ast::TraitBoundModifier::Maybe) = b {
+                        try!(space(&mut self.s));
+                        try!(self.word_space("for ?"));
+                        try!(self.print_trait_ref(&ptr.trait_ref));
+                    } else {
+                        real_bounds.push(b);
+                    }
                 }
-                try!(self.print_bounds(":", bounds[]));
+                try!(self.print_bounds(":", real_bounds[]));
                 try!(self.print_where_clause(generics));
                 try!(word(&mut self.s, " "));
                 try!(self.bopen());
@@ -2345,7 +2350,11 @@ impl<'a> State<'a> {
                 }
 
                 try!(match *bound {
-                    TraitTyParamBound(ref tref) => {
+                    TraitTyParamBound(ref tref, TraitBoundModifier::None) => {
+                        self.print_poly_trait_ref(tref)
+                    }
+                    TraitTyParamBound(ref tref, TraitBoundModifier::Maybe) => {
+                        try!(word(&mut self.s, "?"));
                         self.print_poly_trait_ref(tref)
                     }
                     RegionTyParamBound(ref lt) => {
@@ -2412,10 +2421,6 @@ impl<'a> State<'a> {
     }
 
     pub fn print_ty_param(&mut self, param: &ast::TyParam) -> IoResult<()> {
-        if let Some(ref tref) = param.unbound {
-            try!(self.print_trait_ref(tref));
-            try!(self.word_space("?"));
-        }
         try!(self.print_ident(param.ident));
         try!(self.print_bounds(":", param.bounds[]));
         match param.default {
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 22cfea862517e..1cd21ccac7a0e 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -85,8 +85,8 @@ pub trait Visitor<'v> {
     fn visit_ty_param_bound(&mut self, bounds: &'v TyParamBound) {
         walk_ty_param_bound(self, bounds)
     }
-    fn visit_poly_trait_ref(&mut self, t: &'v PolyTraitRef) {
-        walk_poly_trait_ref(self, t)
+    fn visit_poly_trait_ref(&mut self, t: &'v PolyTraitRef, m: &'v TraitBoundModifier) {
+        walk_poly_trait_ref(self, t, m)
     }
     fn visit_struct_def(&mut self, s: &'v StructDef, _: Ident, _: &'v Generics, _: NodeId) {
         walk_struct_def(self, s)
@@ -244,7 +244,8 @@ pub fn walk_explicit_self<'v, V: Visitor<'v>>(visitor: &mut V,
 /// Like with walk_method_helper this doesn't correspond to a method
 /// in Visitor, and so it gets a _helper suffix.
 pub fn walk_poly_trait_ref<'v, V>(visitor: &mut V,
-                                         trait_ref: &'v PolyTraitRef)
+                                  trait_ref: &'v PolyTraitRef,
+                                  _modifier: &'v TraitBoundModifier)
     where V: Visitor<'v>
 {
     walk_lifetime_decls_helper(visitor, &trait_ref.bound_lifetimes);
@@ -324,7 +325,7 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item) {
                                      generics,
                                      item.id)
         }
-        ItemTrait(_, ref generics, _, ref bounds, ref methods) => {
+        ItemTrait(_, ref generics, ref bounds, ref methods) => {
             visitor.visit_generics(generics);
             walk_ty_param_bounds_helper(visitor, bounds);
             for method in methods.iter() {
@@ -558,8 +559,8 @@ pub fn walk_ty_param_bounds_helper<'v, V: Visitor<'v>>(visitor: &mut V,
 pub fn walk_ty_param_bound<'v, V: Visitor<'v>>(visitor: &mut V,
                                                bound: &'v TyParamBound) {
     match *bound {
-        TraitTyParamBound(ref typ) => {
-            visitor.visit_poly_trait_ref(typ);
+        TraitTyParamBound(ref typ, ref modifier) => {
+            visitor.visit_poly_trait_ref(typ, modifier);
         }
         RegionTyParamBound(ref lifetime) => {
             visitor.visit_lifetime_bound(lifetime);
diff --git a/src/test/run-pass/dst-raw.rs b/src/test/run-pass/dst-raw.rs
index 19af9fd7ea7b4..d3d2e3581aaf7 100644
--- a/src/test/run-pass/dst-raw.rs
+++ b/src/test/run-pass/dst-raw.rs
@@ -23,7 +23,7 @@ impl Trait for A {
     }
 }
 
-struct Foo<Sized? T> {
+struct Foo<T: ?Sized> {
     f: T
 }
 
diff --git a/src/test/run-pass/dst-struct-sole.rs b/src/test/run-pass/dst-struct-sole.rs
index 26cb27cc65392..47547bb7e5a27 100644
--- a/src/test/run-pass/dst-struct-sole.rs
+++ b/src/test/run-pass/dst-struct-sole.rs
@@ -10,7 +10,7 @@
 
 // As dst-struct.rs, but the unsized field is the only field in the struct.
 
-struct Fat<Sized? T> {
+struct Fat<T: ?Sized> {
     ptr: T
 }
 
diff --git a/src/test/run-pass/dst-struct.rs b/src/test/run-pass/dst-struct.rs
index 3644ca81d5659..fb536904ac80f 100644
--- a/src/test/run-pass/dst-struct.rs
+++ b/src/test/run-pass/dst-struct.rs
@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-struct Fat<Sized? T> {
+struct Fat<T: ?Sized> {
     f1: int,
     f2: &'static str,
     ptr: T
diff --git a/src/test/run-pass/dst-trait.rs b/src/test/run-pass/dst-trait.rs
index 907c7810736ba..abe55d78ac69a 100644
--- a/src/test/run-pass/dst-trait.rs
+++ b/src/test/run-pass/dst-trait.rs
@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-struct Fat<Sized? T> {
+struct Fat<T: ?Sized> {
     f1: int,
     f2: &'static str,
     ptr: T
diff --git a/src/test/run-pass/issue-15155.rs b/src/test/run-pass/issue-15155.rs
index 053a4916e229a..3a63e63355cd0 100644
--- a/src/test/run-pass/issue-15155.rs
+++ b/src/test/run-pass/issue-15155.rs
@@ -22,7 +22,7 @@ struct IndirectBlah { x: Box<IndirectTraitWithSend> }
 impl TraitWithSend for IndirectBlah {}
 impl IndirectTraitWithSend for IndirectBlah {}
 
-fn test_trait<Sized? T: Send>() { println!("got here!") }
+fn test_trait<T: Send + ?Sized>() { println!("got here!") }
 
 fn main() {
     test_trait::<TraitWithSend>();
diff --git a/src/test/run-pass/issue-17361.rs b/src/test/run-pass/issue-17361.rs
index fa38dcc198673..092bcf661a76a 100644
--- a/src/test/run-pass/issue-17361.rs
+++ b/src/test/run-pass/issue-17361.rs
@@ -11,6 +11,6 @@
 // Test that astconv doesn't forget about mutability of &mut str
 
 fn main() {
-    fn foo<Sized? T>(_: &mut T) {}
+    fn foo<T: ?Sized>(_: &mut T) {}
     let _f: fn(&mut str) = foo;
 }
diff --git a/src/test/run-pass/method-recursive-blanket-impl.rs b/src/test/run-pass/method-recursive-blanket-impl.rs
index b45faca2de602..4e4fb75b428cc 100644
--- a/src/test/run-pass/method-recursive-blanket-impl.rs
+++ b/src/test/run-pass/method-recursive-blanket-impl.rs
@@ -16,7 +16,7 @@
 use std::kinds::Sized;
 
 // Note: this must be generic for the problem to show up
-trait Foo<A> for Sized? {
+trait Foo<A> for ?Sized {
     fn foo(&self);
 }
 
diff --git a/src/test/run-pass/unsized.rs b/src/test/run-pass/unsized.rs
index 141d6c88dd961..07b9fac66554e 100644
--- a/src/test/run-pass/unsized.rs
+++ b/src/test/run-pass/unsized.rs
@@ -10,21 +10,22 @@
 //
 // ignore-lexer-test FIXME #15879
 
-// Test syntax checks for `Sized?` syntax.
+// Test syntax checks for `?Sized` syntax.
 
-trait T1 for Sized? {}
-pub trait T2 for Sized? {}
-trait T3<X: T1> for Sized?: T2 {}
-trait T4<Sized? X> {}
-trait T5<Sized? X, Y> {}
-trait T6<Y, Sized? X> {}
-trait T7<Sized? X, Sized? Y> {}
-trait T8<Sized? X: T2> {}
-struct S1<Sized? X>;
-enum E<Sized? X> {}
-impl <Sized? X> T1 for S1<X> {}
-fn f<Sized? X>() {}
-type TT<Sized? T> = T;
+trait T1 for ?Sized {}
+pub trait T2 for ?Sized {}
+trait T3<X: T1> for ?Sized: T2 {}
+trait T4<X: ?Sized> {}
+trait T5<X: ?Sized, Y> {}
+trait T6<Y, X: ?Sized> {}
+trait T7<X: ?Sized, Y: ?Sized> {}
+trait T8<X: ?Sized+T2> {}
+trait T9<X: T2 + ?Sized> {}
+struct S1<X: ?Sized>;
+enum E<X: ?Sized> {}
+impl <X: ?Sized> T1 for S1<X> {}
+fn f<X: ?Sized>() {}
+type TT<T: ?Sized> = T;
 
 pub fn main() {
 }
diff --git a/src/test/run-pass/unsized2.rs b/src/test/run-pass/unsized2.rs
index d28d47c0cfb85..8d2c99d4414c9 100644
--- a/src/test/run-pass/unsized2.rs
+++ b/src/test/run-pass/unsized2.rs
@@ -13,7 +13,7 @@
 // Test sized-ness checking in substitution.
 
 // Unbounded.
-fn f1<Sized? X>(x: &X) {
+fn f1<X: ?Sized>(x: &X) {
     f1::<X>(x);
 }
 fn f2<X>(x: &X) {
@@ -22,8 +22,8 @@ fn f2<X>(x: &X) {
 }
 
 // Bounded.
-trait T for Sized? {}
-fn f3<Sized? X: T>(x: &X) {
+trait T for ?Sized {}
+fn f3<X: T+?Sized>(x: &X) {
     f3::<X>(x);
 }
 fn f4<X: T>(x: &X) {
@@ -32,7 +32,7 @@ fn f4<X: T>(x: &X) {
 }
 
 // Self type.
-trait T2 for Sized? {
+trait T2 for ?Sized {
     fn f() -> Box<Self>;
 }
 struct S;
@@ -41,14 +41,14 @@ impl T2 for S {
         box S
     }
 }
-fn f5<Sized? X: T2>(x: &X) {
+fn f5<X: ?Sized+T2>(x: &X) {
     let _: Box<X> = T2::f();
 }
 fn f6<X: T2>(x: &X) {
     let _: Box<X> = T2::f();
 }
 
-trait T3 for Sized? {
+trait T3 for ?Sized {
     fn f() -> Box<Self>;
 }
 impl T3 for S {
@@ -56,7 +56,7 @@ impl T3 for S {
         box S
     }
 }
-fn f7<Sized? X: T3>(x: &X) {
+fn f7<X: ?Sized+T3>(x: &X) {
     // This is valid, but the unsized bound on X is irrelevant because any type
     // which implements T3 must have statically known size.
     let _: Box<X> = T3::f();
@@ -66,7 +66,7 @@ trait T4<X> {
     fn m1(x: &T4<X>);
     fn m2(x: &T5<X>);
 }
-trait T5<Sized? X> {
+trait T5<X: ?Sized> {
     // not an error (for now)
     fn m1(x: &T4<X>);
     fn m2(x: &T5<X>);
@@ -76,21 +76,21 @@ trait T6<X: T> {
     fn m1(x: &T4<X>);
     fn m2(x: &T5<X>);
 }
-trait T7<Sized? X: T> {
+trait T7<X: ?Sized+T> {
     // not an error (for now)
     fn m1(x: &T4<X>);
     fn m2(x: &T5<X>);
 }
 
 // The last field in a struct or variant may be unsized
-struct S2<Sized? X> {
+struct S2<X: ?Sized> {
     f: X,
 }
-struct S3<Sized? X> {
+struct S3<X: ?Sized> {
     f1: int,
     f2: X,
 }
-enum E<Sized? X> {
+enum E<X: ?Sized> {
     V1(X),
     V2{x: X},
     V3(int, X),