work around fallout from these changes in rustc

src/librustc/infer/canonical.rs

@@ -40,6 +40,7 @@ use traits::{Obligation, ObligationCause, PredicateObligation};
 use ty::{self, CanonicalVar, Lift, Region, Slice, Ty, TyCtxt, TypeFlags};
 use ty::subst::{Kind, UnpackedKind};
 use ty::fold::{TypeFoldable, TypeFolder};
+use util::captures::Captures;
 use util::common::CellUsizeExt;
 
 use rustc_data_structures::indexed_vec::IndexVec;
@@ -382,7 +383,7 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         unsubstituted_region_constraints: &'a QueryRegionConstraints<'tcx>,
         result_subst: &'a CanonicalVarValues<'tcx>,
-    ) -> impl Iterator<Item = PredicateObligation<'tcx>> + 'a {
+    ) -> impl Iterator<Item = PredicateObligation<'tcx>> + Captures<'gcx> + 'a {
         let QueryRegionConstraints {
             region_outlives,
             ty_outlives,

src/librustc/lib.rs

@@ -157,6 +157,7 @@ pub mod traits;
 pub mod ty;
 
 pub mod util {
+    pub mod captures;
     pub mod common;
     pub mod ppaux;
     pub mod nodemap;

src/librustc/traits/specialize/specialization_graph.rs

@@ -19,6 +19,7 @@ use ty::{self, TyCtxt, TypeFoldable};
 use ty::fast_reject::{self, SimplifiedType};
 use rustc_data_structures::sync::Lrc;
 use syntax::ast::Name;
+use util::captures::Captures;
 use util::nodemap::{DefIdMap, FxHashMap};
 
 /// A per-trait graph of impls in specialization order. At the moment, this
@@ -313,9 +314,10 @@ impl<'a, 'gcx, 'tcx> Node {
     }
 
     /// Iterate over the items defined directly by the given (impl or trait) node.
-    #[inline] // FIXME(#35870) Avoid closures being unexported due to impl Trait.
-    pub fn items(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>)
-                 -> impl Iterator<Item = ty::AssociatedItem> + 'a {
+    pub fn items(
+        &self,
+        tcx: TyCtxt<'a, 'gcx, 'tcx>,
+    ) -> impl Iterator<Item = ty::AssociatedItem> + 'a {
         tcx.associated_items(self.def_id())
     }
 
@@ -367,9 +369,13 @@ impl<'a, 'gcx, 'tcx> Ancestors {
     /// Search the items from the given ancestors, returning each definition
     /// with the given name and the given kind.
     #[inline] // FIXME(#35870) Avoid closures being unexported due to impl Trait.
-    pub fn defs(self, tcx: TyCtxt<'a, 'gcx, 'tcx>, trait_item_name: Name,
-                trait_item_kind: ty::AssociatedKind, trait_def_id: DefId)
-                -> impl Iterator<Item = NodeItem<ty::AssociatedItem>> + 'a {
+    pub fn defs(
+        self,
+        tcx: TyCtxt<'a, 'gcx, 'tcx>,
+        trait_item_name: Name,
+        trait_item_kind: ty::AssociatedKind,
+        trait_def_id: DefId,
+    ) -> impl Iterator<Item = NodeItem<ty::AssociatedItem>> + Captures<'gcx> + Captures<'tcx> + 'a {
         self.flat_map(move |node| {
             node.items(tcx).filter(move |impl_item| {
                 impl_item.kind == trait_item_kind &&

src/librustc/ty/mod.rs

@@ -34,6 +34,7 @@ use ty;
 use ty::subst::{Subst, Substs};
 use ty::util::{IntTypeExt, Discr};
 use ty::walk::TypeWalker;
+use util::captures::Captures;
 use util::nodemap::{NodeSet, DefIdMap, FxHashMap};
 
 use serialize::{self, Encodable, Encoder};
@@ -1942,8 +1943,10 @@ impl<'a, 'gcx, 'tcx> AdtDef {
     }
 
     #[inline]
-    pub fn discriminants(&'a self, tcx: TyCtxt<'a, 'gcx, 'tcx>)
-                         -> impl Iterator<Item=Discr<'tcx>> + 'a {
+    pub fn discriminants(
+        &'a self,
+        tcx: TyCtxt<'a, 'gcx, 'tcx>,
+    ) -> impl Iterator<Item=Discr<'tcx>> + Captures<'gcx> + 'a {
         let repr_type = self.repr.discr_type();
         let initial = repr_type.initial_discriminant(tcx.global_tcx());
         let mut prev_discr = None::<Discr<'tcx>>;
@@ -2290,7 +2293,9 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     /// Returns an iterator of the def-ids for all body-owners in this
     /// crate. If you would prefer to iterate over the bodies
     /// themselves, you can do `self.hir.krate().body_ids.iter()`.
-    pub fn body_owners(self) -> impl Iterator<Item = DefId> + 'a {
+    pub fn body_owners(
+        self,
+    ) -> impl Iterator<Item = DefId> + Captures<'tcx> + Captures<'gcx> + 'a {
         self.hir.krate()
                 .body_ids
                 .iter()
@@ -2394,11 +2399,13 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
-    #[inline] // FIXME(#35870) Avoid closures being unexported due to impl Trait.
-    pub fn associated_items(self, def_id: DefId)
-                            -> impl Iterator<Item = ty::AssociatedItem> + 'a {
+    pub fn associated_items(
+        self,
+        def_id: DefId,
+    ) -> impl Iterator<Item = ty::AssociatedItem> + 'a {
         let def_ids = self.associated_item_def_ids(def_id);
-        (0..def_ids.len()).map(move |i| self.associated_item(def_ids[i]))
+        Box::new((0..def_ids.len()).map(move |i| self.associated_item(def_ids[i])))
+            as Box<dyn Iterator<Item = ty::AssociatedItem> + 'a>
     }
 
     /// Returns true if the impls are the same polarity and are implementing

src/librustc/ty/sty.rs

@@ -18,6 +18,7 @@ use rustc_data_structures::indexed_vec::Idx;
 use ty::subst::{Substs, Subst, Kind, UnpackedKind};
 use ty::{self, AdtDef, TypeFlags, Ty, TyCtxt, TypeFoldable};
 use ty::{Slice, TyS};
+use util::captures::Captures;
 
 use std::iter;
 use std::cmp::Ordering;
@@ -384,9 +385,11 @@ impl<'a, 'gcx, 'tcx> ClosureSubsts<'tcx> {
     /// This returns the types of the MIR locals which had to be stored across suspension points.
     /// It is calculated in rustc_mir::transform::generator::StateTransform.
     /// All the types here must be in the tuple in GeneratorInterior.
-    pub fn state_tys(self, def_id: DefId, tcx: TyCtxt<'a, 'gcx, 'tcx>) ->
-        impl Iterator<Item=Ty<'tcx>> + 'a
-    {
+    pub fn state_tys(
+        self,
+        def_id: DefId,
+        tcx: TyCtxt<'a, 'gcx, 'tcx>,
+    ) -> impl Iterator<Item=Ty<'tcx>> + Captures<'gcx> + 'a {
         let state = tcx.generator_layout(def_id).fields.iter();
         state.map(move |d| d.ty.subst(tcx, self.substs))
     }
@@ -403,7 +406,7 @@ impl<'a, 'gcx, 'tcx> ClosureSubsts<'tcx> {
     /// This is the types of all the fields stored in a generator.
     /// It includes the upvars, state types and the state discriminant which is u32.
     pub fn field_tys(self, def_id: DefId, tcx: TyCtxt<'a, 'gcx, 'tcx>) ->
-        impl Iterator<Item=Ty<'tcx>> + 'a
+        impl Iterator<Item=Ty<'tcx>> + Captures<'gcx> + 'a
     {
         self.pre_transforms_tys(def_id, tcx).chain(self.state_tys(def_id, tcx))
     }

src/librustc/util/captures.rs

@@ -0,0 +1,18 @@
+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+/// "Signaling" trait used in impl trait to tag lifetimes that you may
+/// need to capture but don't really need for other reasons.
+/// Basically a workaround; see [this comment] for details.
+///
+/// [this comment]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
+pub trait Captures<'a> { }
+
+impl<'a, T: ?Sized> Captures<'a> for T { }

src/librustc_metadata/decoder.rs

@@ -29,6 +29,7 @@ use rustc::session::Session;
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc::ty::codec::TyDecoder;
 use rustc::mir::Mir;
+use rustc::util::captures::Captures;
 use rustc::util::nodemap::FxHashMap;
 
 use std::collections::BTreeMap;
@@ -146,7 +147,10 @@ impl<'a, 'tcx: 'a, T: Decodable> Lazy<T> {
 }
 
 impl<'a, 'tcx: 'a, T: Decodable> LazySeq<T> {
-    pub fn decode<M: Metadata<'a, 'tcx>>(self, meta: M) -> impl Iterator<Item = T> + 'a {
+    pub fn decode<M: Metadata<'a, 'tcx>>(
+        self,
+        meta: M,
+    ) -> impl Iterator<Item = T> + Captures<'tcx> + 'a {
         let mut dcx = meta.decoder(self.position);
         dcx.lazy_state = LazyState::NodeStart(self.position);
         (0..self.len).map(move |_| T::decode(&mut dcx).unwrap())

src/librustc_typeck/collect.rs

@@ -35,8 +35,9 @@ use rustc::ty::{ToPredicate, ReprOptions};
 use rustc::ty::{self, AdtKind, ToPolyTraitRef, Ty, TyCtxt};
 use rustc::ty::maps::Providers;
 use rustc::ty::util::IntTypeExt;
-use rustc::util::nodemap::{FxHashSet, FxHashMap};
 use rustc::ty::util::Discr;
+use rustc::util::captures::Captures;
+use rustc::util::nodemap::{FxHashSet, FxHashMap};
 
 use syntax::{abi, ast};
 use syntax::ast::MetaItemKind;
@@ -1281,7 +1282,7 @@ fn is_unsized<'gcx: 'tcx, 'tcx>(astconv: &AstConv<'gcx, 'tcx>,
 fn early_bound_lifetimes_from_generics<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     ast_generics: &'a hir::Generics)
-    -> impl Iterator<Item=&'a hir::LifetimeDef>
+    -> impl Iterator<Item=&'a hir::LifetimeDef> + Captures<'tcx>
 {
     ast_generics
         .lifetimes()