diff --git a/src/libcore/benches/fmt.rs b/src/libcore/benches/fmt.rs
new file mode 100644
index 0000000000000..92f10c760c6d2
--- /dev/null
+++ b/src/libcore/benches/fmt.rs
@@ -0,0 +1,110 @@
+use std::io::{self, Write as IoWrite};
+use std::fmt::{self, Write as FmtWrite};
+use test::Bencher;
+
+#[bench]
+fn write_vec_value(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        for _ in 0..1000 {
+            mem.write_all("abc".as_bytes()).unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_ref(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            wr.write_all("abc".as_bytes()).unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_macro1(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            write!(wr, "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_macro2(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            write!(wr, "{}", "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_vec_macro_debug(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = Vec::new();
+        let wr = &mut mem as &mut dyn io::Write;
+        for _ in 0..1000 {
+            write!(wr, "{:?}", "☃").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_value(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        for _ in 0..1000 {
+            mem.write_str("abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_ref(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        let wr = &mut mem as &mut dyn fmt::Write;
+        for _ in 0..1000 {
+            wr.write_str("abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_macro1(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        for _ in 0..1000 {
+            write!(mem, "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_macro2(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        let wr = &mut mem as &mut dyn fmt::Write;
+        for _ in 0..1000 {
+            write!(wr, "{}", "abc").unwrap();
+        }
+    });
+}
+
+#[bench]
+fn write_str_macro_debug(bh: &mut Bencher) {
+    bh.iter(|| {
+        let mut mem = String::new();
+        let wr = &mut mem as &mut dyn fmt::Write;
+        for _ in 0..1000 {
+            write!(wr, "{:?}", "☃").unwrap();
+        }
+    });
+}
diff --git a/src/libcore/benches/lib.rs b/src/libcore/benches/lib.rs
index 5b4971c81dd92..48572af611a5b 100644
--- a/src/libcore/benches/lib.rs
+++ b/src/libcore/benches/lib.rs
@@ -11,3 +11,4 @@ mod iter;
 mod num;
 mod ops;
 mod slice;
+mod fmt;
diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index 214b5d3a84f24..fe9a8e1f6fd6e 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -997,28 +997,30 @@ pub fn write(output: &mut dyn Write, args: Arguments) -> Result {
         curarg: args.args.iter(),
     };
 
-    let mut pieces = args.pieces.iter();
+    let mut idx = 0;
 
     match args.fmt {
         None => {
             // We can use default formatting parameters for all arguments.
-            for (arg, piece) in args.args.iter().zip(pieces.by_ref()) {
+            for (arg, piece) in args.args.iter().zip(args.pieces.iter()) {
                 formatter.buf.write_str(*piece)?;
                 (arg.formatter)(arg.value, &mut formatter)?;
+                idx += 1;
             }
         }
         Some(fmt) => {
             // Every spec has a corresponding argument that is preceded by
             // a string piece.
-            for (arg, piece) in fmt.iter().zip(pieces.by_ref()) {
+            for (arg, piece) in fmt.iter().zip(args.pieces.iter()) {
                 formatter.buf.write_str(*piece)?;
                 formatter.run(arg)?;
+                idx += 1;
             }
         }
     }
 
     // There can be only one trailing string piece left.
-    if let Some(piece) = pieces.next() {
+    if let Some(piece) = args.pieces.get(idx) {
         formatter.buf.write_str(*piece)?;
     }
 
diff --git a/src/libcore/iter/iterator.rs b/src/libcore/iter/iterator.rs
index 640af74817282..0ad29afbadeac 100644
--- a/src/libcore/iter/iterator.rs
+++ b/src/libcore/iter/iterator.rs
@@ -2358,7 +2358,7 @@ pub trait Iterator {
     ///
     /// ```
     /// fn factorial(n: u32) -> u32 {
-    ///     (1..).take_while(|&i| i <= n).product()
+    ///     (1..=n).product()
     /// }
     /// assert_eq!(factorial(0), 1);
     /// assert_eq!(factorial(1), 1);
diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 1ef5428a789cf..974906b682d21 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -329,6 +329,8 @@ pub use self::sources::{RepeatWith, repeat_with};
 pub use self::sources::{Empty, empty};
 #[stable(feature = "iter_once", since = "1.2.0")]
 pub use self::sources::{Once, once};
+#[unstable(feature = "iter_once_with", issue = "57581")]
+pub use self::sources::{OnceWith, once_with};
 #[unstable(feature = "iter_unfold", issue = "55977")]
 pub use self::sources::{Unfold, unfold, Successors, successors};
 
diff --git a/src/libcore/iter/sources.rs b/src/libcore/iter/sources.rs
index 2a39089a8a229..2590fa6023a53 100644
--- a/src/libcore/iter/sources.rs
+++ b/src/libcore/iter/sources.rs
@@ -377,6 +377,119 @@ pub fn once<T>(value: T) -> Once<T> {
     Once { inner: Some(value).into_iter() }
 }
 
+/// An iterator that repeats elements of type `A` endlessly by
+/// applying the provided closure `F: FnMut() -> A`.
+///
+/// This `struct` is created by the [`once_with`] function.
+/// See its documentation for more.
+///
+/// [`once_with`]: fn.once_with.html
+#[derive(Copy, Clone, Debug)]
+#[unstable(feature = "iter_once_with", issue = "57581")]
+pub struct OnceWith<F> {
+    gen: Option<F>,
+}
+
+#[unstable(feature = "iter_once_with", issue = "57581")]
+impl<A, F: FnOnce() -> A> Iterator for OnceWith<F> {
+    type Item = A;
+
+    #[inline]
+    fn next(&mut self) -> Option<A> {
+        self.gen.take().map(|f| f())
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.gen.iter().size_hint()
+    }
+}
+
+#[unstable(feature = "iter_once_with", issue = "57581")]
+impl<A, F: FnOnce() -> A> DoubleEndedIterator for OnceWith<F> {
+    fn next_back(&mut self) -> Option<A> {
+        self.next()
+    }
+}
+
+#[unstable(feature = "iter_once_with", issue = "57581")]
+impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F> {
+    fn len(&self) -> usize {
+        self.gen.iter().len()
+    }
+}
+
+#[unstable(feature = "iter_once_with", issue = "57581")]
+impl<A, F: FnOnce() -> A> FusedIterator for OnceWith<F> {}
+
+#[unstable(feature = "iter_once_with", issue = "57581")]
+unsafe impl<A, F: FnOnce() -> A> TrustedLen for OnceWith<F> {}
+
+/// Creates an iterator that lazily generates a value exactly once by invoking
+/// the provided closure.
+///
+/// This is commonly used to adapt a single value generator into a [`chain`] of
+/// other kinds of iteration. Maybe you have an iterator that covers almost
+/// everything, but you need an extra special case. Maybe you have a function
+/// which works on iterators, but you only need to process one value.
+///
+/// Unlike [`once`], this function will lazily generate the value on request.
+///
+/// [`once`]: fn.once.html
+/// [`chain`]: trait.Iterator.html#method.chain
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// #![feature(iter_once_with)]
+///
+/// use std::iter;
+///
+/// // one is the loneliest number
+/// let mut one = iter::once_with(|| 1);
+///
+/// assert_eq!(Some(1), one.next());
+///
+/// // just one, that's all we get
+/// assert_eq!(None, one.next());
+/// ```
+///
+/// Chaining together with another iterator. Let's say that we want to iterate
+/// over each file of the `.foo` directory, but also a configuration file,
+/// `.foorc`:
+///
+/// ```no_run
+/// #![feature(iter_once_with)]
+///
+/// use std::iter;
+/// use std::fs;
+/// use std::path::PathBuf;
+///
+/// let dirs = fs::read_dir(".foo").unwrap();
+///
+/// // we need to convert from an iterator of DirEntry-s to an iterator of
+/// // PathBufs, so we use map
+/// let dirs = dirs.map(|file| file.unwrap().path());
+///
+/// // now, our iterator just for our config file
+/// let config = iter::once_with(|| PathBuf::from(".foorc"));
+///
+/// // chain the two iterators together into one big iterator
+/// let files = dirs.chain(config);
+///
+/// // this will give us all of the files in .foo as well as .foorc
+/// for f in files {
+///     println!("{:?}", f);
+/// }
+/// ```
+#[inline]
+#[unstable(feature = "iter_once_with", issue = "57581")]
+pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> {
+    OnceWith { gen: Some(gen) }
+}
+
 /// Creates a new iterator where each iteration calls the provided closure
 /// `F: FnMut(&mut St) -> Option<T>`.
 ///
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 8879112fcf0b9..33c0da8a54049 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -79,6 +79,7 @@
 #![feature(extern_types)]
 #![feature(fundamental)]
 #![feature(intrinsics)]
+#![feature(iter_once_with)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
 #![feature(never_type)]
diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index cf19851c17b35..3944bc749d029 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1,3 +1,4 @@
+use core::cell::Cell;
 use core::iter::*;
 use core::{i8, i16, isize};
 use core::usize;
@@ -1906,6 +1907,23 @@ fn test_once() {
     assert_eq!(it.next(), None);
 }
 
+#[test]
+fn test_once_with() {
+    let count = Cell::new(0);
+    let mut it = once_with(|| {
+        count.set(count.get() + 1);
+        42
+    });
+
+    assert_eq!(count.get(), 0);
+    assert_eq!(it.next(), Some(42));
+    assert_eq!(count.get(), 1);
+    assert_eq!(it.next(), None);
+    assert_eq!(count.get(), 1);
+    assert_eq!(it.next(), None);
+    assert_eq!(count.get(), 1);
+}
+
 #[test]
 fn test_empty() {
     let mut it = empty::<i32>();
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 72846daf16a6b..a9b8decfd0262 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -12,6 +12,7 @@
 #![feature(hashmap_internals)]
 #![feature(iter_copied)]
 #![feature(iter_nth_back)]
+#![feature(iter_once_with)]
 #![feature(iter_unfold)]
 #![feature(pattern)]
 #![feature(range_is_empty)]
diff --git a/src/libfmt_macros/lib.rs b/src/libfmt_macros/lib.rs
index 32ae878909f30..da440cdd72f80 100644
--- a/src/libfmt_macros/lib.rs
+++ b/src/libfmt_macros/lib.rs
@@ -72,6 +72,15 @@ pub enum Position<'a> {
     ArgumentNamed(&'a str),
 }
 
+impl Position<'_> {
+    pub fn index(&self) -> Option<usize> {
+        match self {
+            ArgumentIs(i) | ArgumentImplicitlyIs(i) => Some(*i),
+            _ => None,
+        }
+    }
+}
+
 /// Enum of alignments which are supported.
 #[derive(Copy, Clone, PartialEq)]
 pub enum Alignment {
diff --git a/src/librustc/dep_graph/dep_node.rs b/src/librustc/dep_graph/dep_node.rs
index 427fe51e6ff9c..d1067b70778ee 100644
--- a/src/librustc/dep_graph/dep_node.rs
+++ b/src/librustc/dep_graph/dep_node.rs
@@ -476,6 +476,9 @@ define_dep_nodes!( <'tcx>
     [] CheckModLoops(DefId),
     [] CheckModUnstableApiUsage(DefId),
     [] CheckModItemTypes(DefId),
+    [] CheckModPrivacy(DefId),
+    [] CheckModIntrinsics(DefId),
+    [] CheckModLiveness(DefId),
     [] CollectModItemTypes(DefId),
 
     [] Reachability,
diff --git a/src/librustc/hir/map/mod.rs b/src/librustc/hir/map/mod.rs
index 869baef1f5afc..d9ca37c937bc7 100644
--- a/src/librustc/hir/map/mod.rs
+++ b/src/librustc/hir/map/mod.rs
@@ -509,6 +509,21 @@ impl<'hir> Map<'hir> {
         &self.forest.krate.attrs
     }
 
+    pub fn get_module(&self, module: DefId) -> (&'hir Mod, Span, NodeId)
+    {
+        let node_id = self.as_local_node_id(module).unwrap();
+        self.read(node_id);
+        match self.find_entry(node_id).unwrap().node {
+            Node::Item(&Item {
+                span,
+                node: ItemKind::Mod(ref m),
+                ..
+            }) => (m, span, node_id),
+            Node::Crate => (&self.forest.krate.module, self.forest.krate.span, node_id),
+            _ => panic!("not a module")
+        }
+    }
+
     pub fn visit_item_likes_in_module<V>(&self, module: DefId, visitor: &mut V)
         where V: ItemLikeVisitor<'hir>
     {
diff --git a/src/librustc/ich/impls_syntax.rs b/src/librustc/ich/impls_syntax.rs
index 70ec72d73bc6c..de567183a3c05 100644
--- a/src/librustc/ich/impls_syntax.rs
+++ b/src/librustc/ich/impls_syntax.rs
@@ -258,7 +258,7 @@ for tokenstream::TokenTree {
             tokenstream::TokenTree::Delimited(span, delim, ref tts) => {
                 span.hash_stable(hcx, hasher);
                 std_hash::Hash::hash(&delim, hasher);
-                for sub_tt in tts.stream().trees() {
+                for sub_tt in tts.trees() {
                     sub_tt.hash_stable(hcx, hasher);
                 }
             }
diff --git a/src/librustc/middle/intrinsicck.rs b/src/librustc/middle/intrinsicck.rs
index 1716daaa107c4..a0f7954eb0c55 100644
--- a/src/librustc/middle/intrinsicck.rs
+++ b/src/librustc/middle/intrinsicck.rs
@@ -2,6 +2,7 @@ use hir::def::Def;
 use hir::def_id::DefId;
 use ty::{self, Ty, TyCtxt};
 use ty::layout::{LayoutError, Pointer, SizeSkeleton, VariantIdx};
+use ty::query::{Providers, queries};
 
 use rustc_target::spec::abi::Abi::RustIntrinsic;
 use rustc_data_structures::indexed_vec::Idx;
@@ -10,10 +11,23 @@ use hir::intravisit::{self, Visitor, NestedVisitorMap};
 use hir;
 
 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
-    let mut visitor = ItemVisitor {
-        tcx,
+    for &module in tcx.hir().krate().modules.keys() {
+        queries::check_mod_intrinsics::ensure(tcx, tcx.hir().local_def_id(module));
+    }
+}
+
+fn check_mod_intrinsics<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>, module_def_id: DefId) {
+    tcx.hir().visit_item_likes_in_module(
+        module_def_id,
+        &mut ItemVisitor { tcx }.as_deep_visitor()
+    );
+}
+
+pub fn provide(providers: &mut Providers<'_>) {
+    *providers = Providers {
+        check_mod_intrinsics,
+        ..*providers
     };
-    tcx.hir().krate().visit_all_item_likes(&mut visitor.as_deep_visitor());
 }
 
 struct ItemVisitor<'a, 'tcx: 'a> {
diff --git a/src/librustc/middle/liveness.rs b/src/librustc/middle/liveness.rs
index a78cf1a471b4b..0a1802a4e12c9 100644
--- a/src/librustc/middle/liveness.rs
+++ b/src/librustc/middle/liveness.rs
@@ -100,6 +100,7 @@ use self::VarKind::*;
 use hir::def::*;
 use hir::Node;
 use ty::{self, TyCtxt};
+use ty::query::{Providers, queries};
 use lint;
 use errors::Applicability;
 use util::nodemap::{NodeMap, HirIdMap, HirIdSet};
@@ -114,8 +115,9 @@ use syntax::ptr::P;
 use syntax::symbol::keywords;
 use syntax_pos::Span;
 
-use hir::{Expr, HirId};
 use hir;
+use hir::{Expr, HirId};
+use hir::def_id::DefId;
 use hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
 
 /// For use with `propagate_through_loop`.
@@ -179,11 +181,24 @@ impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
     fn visit_arm(&mut self, a: &'tcx hir::Arm) { visit_arm(self, a); }
 }
 
+fn check_mod_liveness<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>, module_def_id: DefId) {
+    tcx.hir().visit_item_likes_in_module(module_def_id, &mut IrMaps::new(tcx).as_deep_visitor());
+}
+
 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
-    tcx.hir().krate().visit_all_item_likes(&mut IrMaps::new(tcx).as_deep_visitor());
+    for &module in tcx.hir().krate().modules.keys() {
+        queries::check_mod_liveness::ensure(tcx, tcx.hir().local_def_id(module));
+    }
     tcx.sess.abort_if_errors();
 }
 
+pub fn provide(providers: &mut Providers<'_>) {
+    *providers = Providers {
+        check_mod_liveness,
+        ..*providers
+    };
+}
+
 impl fmt::Debug for LiveNode {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "ln({})", self.get())
diff --git a/src/librustc/ty/query/config.rs b/src/librustc/ty/query/config.rs
index ca5d1f6bd3203..c20846aebb877 100644
--- a/src/librustc/ty/query/config.rs
+++ b/src/librustc/ty/query/config.rs
@@ -109,6 +109,33 @@ impl<'tcx> QueryDescription<'tcx> for queries::check_mod_item_types<'tcx> {
     }
 }
 
+impl<'tcx> QueryDescription<'tcx> for queries::check_mod_privacy<'tcx> {
+    fn describe(
+        tcx: TyCtxt<'_, '_, '_>,
+        key: DefId,
+    ) -> Cow<'static, str> {
+        format!("checking privacy in {}", key.describe_as_module(tcx)).into()
+    }
+}
+
+impl<'tcx> QueryDescription<'tcx> for queries::check_mod_intrinsics<'tcx> {
+    fn describe(
+        tcx: TyCtxt<'_, '_, '_>,
+        key: DefId,
+    ) -> Cow<'static, str> {
+        format!("checking intrinsics in {}", key.describe_as_module(tcx)).into()
+    }
+}
+
+impl<'tcx> QueryDescription<'tcx> for queries::check_mod_liveness<'tcx> {
+    fn describe(
+        tcx: TyCtxt<'_, '_, '_>,
+        key: DefId,
+    ) -> Cow<'static, str> {
+        format!("checking liveness of variables in {}", key.describe_as_module(tcx)).into()
+    }
+}
+
 impl<'tcx> QueryDescription<'tcx> for queries::collect_mod_item_types<'tcx> {
     fn describe(
         tcx: TyCtxt<'_, '_, '_>,
diff --git a/src/librustc/ty/query/mod.rs b/src/librustc/ty/query/mod.rs
index 39d76ceed9507..88c20547a2108 100644
--- a/src/librustc/ty/query/mod.rs
+++ b/src/librustc/ty/query/mod.rs
@@ -264,6 +264,12 @@ define_queries! { <'tcx>
 
         [] fn check_mod_item_types: CheckModItemTypes(DefId) -> (),
 
+        [] fn check_mod_privacy: CheckModPrivacy(DefId) -> (),
+
+        [] fn check_mod_intrinsics: CheckModIntrinsics(DefId) -> (),
+
+        [] fn check_mod_liveness: CheckModLiveness(DefId) -> (),
+
         [] fn collect_mod_item_types: CollectModItemTypes(DefId) -> (),
 
         /// Caches CoerceUnsized kinds for impls on custom types.
diff --git a/src/librustc/ty/query/plumbing.rs b/src/librustc/ty/query/plumbing.rs
index 562cd75a75ff4..17ed89d140696 100644
--- a/src/librustc/ty/query/plumbing.rs
+++ b/src/librustc/ty/query/plumbing.rs
@@ -1281,6 +1281,9 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::CheckModLoops => { force!(check_mod_loops, def_id!()); }
         DepKind::CheckModUnstableApiUsage => { force!(check_mod_unstable_api_usage, def_id!()); }
         DepKind::CheckModItemTypes => { force!(check_mod_item_types, def_id!()); }
+        DepKind::CheckModPrivacy => { force!(check_mod_privacy, def_id!()); }
+        DepKind::CheckModIntrinsics => { force!(check_mod_intrinsics, def_id!()); }
+        DepKind::CheckModLiveness => { force!(check_mod_liveness, def_id!()); }
         DepKind::CollectModItemTypes => { force!(collect_mod_item_types, def_id!()); }
         DepKind::Reachability => { force!(reachable_set, LOCAL_CRATE); }
         DepKind::MirKeys => { force!(mir_keys, LOCAL_CRATE); }
diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index 3b7de37ae4b3f..960faabb9b00c 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -1170,6 +1170,8 @@ pub fn default_provide(providers: &mut ty::query::Providers) {
     ty::provide(providers);
     traits::provide(providers);
     stability::provide(providers);
+    middle::intrinsicck::provide(providers);
+    middle::liveness::provide(providers);
     reachable::provide(providers);
     rustc_passes::provide(providers);
     rustc_traits::provide(providers);
diff --git a/src/librustc_lint/builtin.rs b/src/librustc_lint/builtin.rs
index 72bcf8edfdd21..3cc2e8057b8e5 100644
--- a/src/librustc_lint/builtin.rs
+++ b/src/librustc_lint/builtin.rs
@@ -1540,7 +1540,7 @@ impl KeywordIdents {
                     _ => {},
                 }
                 TokenTree::Delimited(_, _, tts) => {
-                    self.check_tokens(cx, tts.stream())
+                    self.check_tokens(cx, tts)
                 },
             }
         }
diff --git a/src/librustc_privacy/lib.rs b/src/librustc_privacy/lib.rs
index 4890369e13f20..10ac1caa692ca 100644
--- a/src/librustc_privacy/lib.rs
+++ b/src/librustc_privacy/lib.rs
@@ -22,12 +22,12 @@ use rustc::lint;
 use rustc::middle::privacy::{AccessLevel, AccessLevels};
 use rustc::ty::{self, TyCtxt, Ty, TraitRef, TypeFoldable, GenericParamDefKind};
 use rustc::ty::fold::TypeVisitor;
-use rustc::ty::query::Providers;
+use rustc::ty::query::{Providers, queries};
 use rustc::ty::subst::Substs;
 use rustc::util::nodemap::NodeSet;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::sync::Lrc;
-use syntax::ast::{self, CRATE_NODE_ID, Ident};
+use syntax::ast::{self, DUMMY_NODE_ID, Ident};
 use syntax::attr;
 use syntax::symbol::keywords;
 use syntax_pos::Span;
@@ -782,6 +782,10 @@ impl<'a, 'tcx> Visitor<'tcx> for NamePrivacyVisitor<'a, 'tcx> {
         NestedVisitorMap::All(&self.tcx.hir())
     }
 
+    fn visit_mod(&mut self, _m: &'tcx hir::Mod, _s: Span, _n: ast::NodeId) {
+        // Don't visit modules inside
+    }
+
     fn visit_nested_body(&mut self, body: hir::BodyId) {
         let orig_tables = mem::replace(&mut self.tables, self.tcx.body_tables(body));
         let body = self.tcx.hir().body(body);
@@ -917,6 +921,10 @@ impl<'a, 'tcx> Visitor<'tcx> for TypePrivacyVisitor<'a, 'tcx> {
         NestedVisitorMap::All(&self.tcx.hir())
     }
 
+    fn visit_mod(&mut self, _m: &'tcx hir::Mod, _s: Span, _n: ast::NodeId) {
+        // Don't visit modules inside
+    }
+
     fn visit_nested_body(&mut self, body: hir::BodyId) {
         let orig_tables = mem::replace(&mut self.tables, self.tcx.body_tables(body));
         let orig_in_body = mem::replace(&mut self.in_body, true);
@@ -1659,6 +1667,7 @@ impl<'a, 'tcx> Visitor<'tcx> for PrivateItemsInPublicInterfacesVisitor<'a, 'tcx>
 pub fn provide(providers: &mut Providers) {
     *providers = Providers {
         privacy_access_levels,
+        check_mod_privacy,
         ..*providers
     };
 }
@@ -1667,34 +1676,43 @@ pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> Lrc<AccessLevels> {
     tcx.privacy_access_levels(LOCAL_CRATE)
 }
 
-fn privacy_access_levels<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                                   krate: CrateNum)
-                                   -> Lrc<AccessLevels> {
-    assert_eq!(krate, LOCAL_CRATE);
-
-    let krate = tcx.hir().krate();
+fn check_mod_privacy<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>, module_def_id: DefId) {
     let empty_tables = ty::TypeckTables::empty(None);
 
     // Check privacy of names not checked in previous compilation stages.
     let mut visitor = NamePrivacyVisitor {
         tcx,
         tables: &empty_tables,
-        current_item: CRATE_NODE_ID,
+        current_item: DUMMY_NODE_ID,
         empty_tables: &empty_tables,
     };
-    intravisit::walk_crate(&mut visitor, krate);
+    let (module, span, node_id) = tcx.hir().get_module(module_def_id);
+    intravisit::walk_mod(&mut visitor, module, node_id);
 
     // Check privacy of explicitly written types and traits as well as
     // inferred types of expressions and patterns.
     let mut visitor = TypePrivacyVisitor {
         tcx,
         tables: &empty_tables,
-        current_item: DefId::local(CRATE_DEF_INDEX),
+        current_item: module_def_id,
         in_body: false,
-        span: krate.span,
+        span,
         empty_tables: &empty_tables,
     };
-    intravisit::walk_crate(&mut visitor, krate);
+    intravisit::walk_mod(&mut visitor, module, node_id);
+}
+
+fn privacy_access_levels<'tcx>(
+    tcx: TyCtxt<'_, 'tcx, 'tcx>,
+    krate: CrateNum,
+) -> Lrc<AccessLevels> {
+    assert_eq!(krate, LOCAL_CRATE);
+
+    let krate = tcx.hir().krate();
+
+    for &module in tcx.hir().krate().modules.keys() {
+        queries::check_mod_privacy::ensure(tcx, tcx.hir().local_def_id(module));
+    }
 
     // Build up a set of all exported items in the AST. This is a set of all
     // items which are reachable from external crates based on visibility.
diff --git a/src/librustc_typeck/coherence/mod.rs b/src/librustc_typeck/coherence/mod.rs
index ce71be07efd42..8053ed130e91b 100644
--- a/src/librustc_typeck/coherence/mod.rs
+++ b/src/librustc_typeck/coherence/mod.rs
@@ -171,13 +171,23 @@ fn check_impl_overlap<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, node_id: ast::NodeI
         // This is something like impl Trait1 for Trait2. Illegal
         // if Trait1 is a supertrait of Trait2 or Trait2 is not object safe.
 
-        if let Some(principal_def_id) = data.principal_def_id() {
-            if !tcx.is_object_safe(principal_def_id) {
+        let component_def_ids = data.iter().flat_map(|predicate| {
+            match predicate.skip_binder() {
+                ty::ExistentialPredicate::Trait(tr) => Some(tr.def_id),
+                ty::ExistentialPredicate::AutoTrait(def_id) => Some(*def_id),
+                // An associated type projection necessarily comes with
+                // an additional `Trait` requirement.
+                ty::ExistentialPredicate::Projection(..) => None,
+            }
+        });
+
+        for component_def_id in component_def_ids {
+            if !tcx.is_object_safe(component_def_id) {
                 // This is an error, but it will be reported by wfcheck.  Ignore it here.
                 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.
             } else {
                 let mut supertrait_def_ids =
-                    traits::supertrait_def_ids(tcx, principal_def_id);
+                    traits::supertrait_def_ids(tcx, component_def_id);
                 if supertrait_def_ids.any(|d| d == trait_def_id) {
                     let sp = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
                     struct_span_err!(tcx.sess,
@@ -193,6 +203,5 @@ fn check_impl_overlap<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, node_id: ast::NodeI
                 }
             }
         }
-        // FIXME: also check auto-trait def-ids? (e.g. `impl Sync for Foo+Sync`)?
     }
 }
diff --git a/src/librustdoc/html/static/main.js b/src/librustdoc/html/static/main.js
index 9db1c68609039..ceadbd5afac5f 100644
--- a/src/librustdoc/html/static/main.js
+++ b/src/librustdoc/html/static/main.js
@@ -2431,7 +2431,7 @@ if (!DOMTokenList.prototype.remove) {
             return;
         }
         var crates_text = [];
-        if (crates.length > 1) {
+        if (Object.keys(crates).length > 1) {
             for (var crate in crates) {
                 if (crates.hasOwnProperty(crate)) {
                     crates_text.push(crate);
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index e3a8980a975c1..1e91f4adc36d7 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -15,7 +15,7 @@ use rustc_target::spec::abi::Abi;
 use source_map::{dummy_spanned, respan, Spanned};
 use symbol::{keywords, Symbol};
 use syntax_pos::{Span, DUMMY_SP};
-use tokenstream::{ThinTokenStream, TokenStream};
+use tokenstream::TokenStream;
 use ThinVec;
 
 use rustc_data_structures::fx::FxHashSet;
@@ -1216,7 +1216,7 @@ pub type Mac = Spanned<Mac_>;
 pub struct Mac_ {
     pub path: Path,
     pub delim: MacDelimiter,
-    pub tts: ThinTokenStream,
+    pub tts: TokenStream,
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Debug)]
@@ -1228,13 +1228,13 @@ pub enum MacDelimiter {
 
 impl Mac_ {
     pub fn stream(&self) -> TokenStream {
-        self.tts.stream()
+        self.tts.clone()
     }
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct MacroDef {
-    pub tokens: ThinTokenStream,
+    pub tokens: TokenStream,
     pub legacy: bool,
 }
 
diff --git a/src/libsyntax/attr/mod.rs b/src/libsyntax/attr/mod.rs
index d03563f8891aa..0f8ca5e7b9982 100644
--- a/src/libsyntax/attr/mod.rs
+++ b/src/libsyntax/attr/mod.rs
@@ -565,7 +565,7 @@ impl MetaItemKind {
             }
             Some(TokenTree::Delimited(_, delim, ref tts)) if delim == token::Paren => {
                 tokens.next();
-                tts.stream()
+                tts.clone()
             }
             _ => return Some(MetaItemKind::Word),
         };
diff --git a/src/libsyntax/ext/quote.rs b/src/libsyntax/ext/quote.rs
index c3124144009ab..c01e7f538b90d 100644
--- a/src/libsyntax/ext/quote.rs
+++ b/src/libsyntax/ext/quote.rs
@@ -748,7 +748,7 @@ fn statements_mk_tt(cx: &ExtCtxt, tt: &TokenTree, quoted: bool) -> Vec<ast::Stmt
         },
         TokenTree::Delimited(span, delim, ref tts) => {
             let mut stmts = statements_mk_tt(cx, &TokenTree::open_tt(span.open, delim), false);
-            stmts.extend(statements_mk_tts(cx, tts.stream()));
+            stmts.extend(statements_mk_tts(cx, tts.clone()));
             stmts.extend(statements_mk_tt(cx, &TokenTree::close_tt(span.close, delim), false));
             stmts
         }
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 8ac103856dcd1..a4c3b38f691ed 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -598,7 +598,7 @@ pub fn noop_fold_tt<T: Folder>(tt: TokenTree, fld: &mut T) -> TokenTree {
         TokenTree::Delimited(span, delim, tts) => TokenTree::Delimited(
             DelimSpan::from_pair(fld.new_span(span.open), fld.new_span(span.close)),
             delim,
-            fld.fold_tts(tts.stream()).into(),
+            fld.fold_tts(tts).into(),
         ),
     }
 }
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index ea205530ca5cc..ddb350faa546b 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -817,7 +817,7 @@ mod tests {
                 )
                 if name_macro_rules.name == "macro_rules"
                 && name_zip.name == "zip" => {
-                    let tts = &macro_tts.stream().trees().collect::<Vec<_>>();
+                    let tts = &macro_tts.trees().collect::<Vec<_>>();
                     match (tts.len(), tts.get(0), tts.get(1), tts.get(2)) {
                         (
                             3,
@@ -826,7 +826,7 @@ mod tests {
                             Some(&TokenTree::Delimited(_, second_delim, ref second_tts)),
                         )
                         if macro_delim == token::Paren => {
-                            let tts = &first_tts.stream().trees().collect::<Vec<_>>();
+                            let tts = &first_tts.trees().collect::<Vec<_>>();
                             match (tts.len(), tts.get(0), tts.get(1)) {
                                 (
                                     2,
@@ -836,7 +836,7 @@ mod tests {
                                 if first_delim == token::Paren && ident.name == "a" => {},
                                 _ => panic!("value 3: {:?} {:?}", first_delim, first_tts),
                             }
-                            let tts = &second_tts.stream().trees().collect::<Vec<_>>();
+                            let tts = &second_tts.trees().collect::<Vec<_>>();
                             match (tts.len(), tts.get(0), tts.get(1)) {
                                 (
                                     2,
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 823c786bded26..8073c24c7dbee 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -46,7 +46,7 @@ use print::pprust;
 use ptr::P;
 use parse::PResult;
 use ThinVec;
-use tokenstream::{self, DelimSpan, ThinTokenStream, TokenTree, TokenStream};
+use tokenstream::{self, DelimSpan, TokenTree, TokenStream, TreeAndJoint};
 use symbol::{Symbol, keywords};
 
 use std::borrow::Cow;
@@ -280,17 +280,17 @@ struct TokenCursorFrame {
 /// on the parser.
 #[derive(Clone)]
 enum LastToken {
-    Collecting(Vec<TokenStream>),
-    Was(Option<TokenStream>),
+    Collecting(Vec<TreeAndJoint>),
+    Was(Option<TreeAndJoint>),
 }
 
 impl TokenCursorFrame {
-    fn new(sp: DelimSpan, delim: DelimToken, tts: &ThinTokenStream) -> Self {
+    fn new(sp: DelimSpan, delim: DelimToken, tts: &TokenStream) -> Self {
         TokenCursorFrame {
             delim: delim,
             span: sp,
             open_delim: delim == token::NoDelim,
-            tree_cursor: tts.stream().into_trees(),
+            tree_cursor: tts.clone().into_trees(),
             close_delim: delim == token::NoDelim,
             last_token: LastToken::Was(None),
         }
@@ -2330,7 +2330,7 @@ impl<'a> Parser<'a> {
         })
     }
 
-    fn expect_delimited_token_tree(&mut self) -> PResult<'a, (MacDelimiter, ThinTokenStream)> {
+    fn expect_delimited_token_tree(&mut self) -> PResult<'a, (MacDelimiter, TokenStream)> {
         let delim = match self.token {
             token::OpenDelim(delim) => delim,
             _ => {
@@ -2350,7 +2350,7 @@ impl<'a> Parser<'a> {
             token::Brace => MacDelimiter::Brace,
             token::NoDelim => self.bug("unexpected no delimiter"),
         };
-        Ok((delim, tts.stream().into()))
+        Ok((delim, tts.into()))
     }
 
     /// At the bottom (top?) of the precedence hierarchy,
@@ -4641,7 +4641,7 @@ impl<'a> Parser<'a> {
                 let ident = self.parse_ident()?;
                 let tokens = if self.check(&token::OpenDelim(token::Brace)) {
                     match self.parse_token_tree() {
-                        TokenTree::Delimited(_, _, tts) => tts.stream(),
+                        TokenTree::Delimited(_, _, tts) => tts,
                         _ => unreachable!(),
                     }
                 } else if self.check(&token::OpenDelim(token::Paren)) {
@@ -7766,7 +7766,7 @@ impl<'a> Parser<'a> {
             &mut self.token_cursor.stack[prev].last_token
         };
 
-        // Pull our the toekns that we've collected from the call to `f` above
+        // Pull out the tokens that we've collected from the call to `f` above.
         let mut collected_tokens = match *last_token {
             LastToken::Collecting(ref mut v) => mem::replace(v, Vec::new()),
             LastToken::Was(_) => panic!("our vector went away?"),
@@ -7785,10 +7785,9 @@ impl<'a> Parser<'a> {
         // call. In that case we need to record all the tokens we collected in
         // our parent list as well. To do that we push a clone of our stream
         // onto the previous list.
-        let stream = collected_tokens.into_iter().collect::<TokenStream>();
         match prev_collecting {
             Some(mut list) => {
-                list.push(stream.clone());
+                list.extend(collected_tokens.iter().cloned());
                 list.extend(extra_token);
                 *last_token = LastToken::Collecting(list);
             }
@@ -7797,7 +7796,7 @@ impl<'a> Parser<'a> {
             }
         }
 
-        Ok((ret?, stream))
+        Ok((ret?, TokenStream::new(collected_tokens)))
     }
 
     pub fn parse_item(&mut self) -> PResult<'a, Option<P<Item>>> {
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 2ad3d3a6d6487..c53594032a00a 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -807,7 +807,7 @@ pub trait PrintState<'a> {
             TokenTree::Delimited(_, delim, tts) => {
                 self.writer().word(token_to_string(&token::OpenDelim(delim)))?;
                 self.writer().space()?;
-                self.print_tts(tts.stream())?;
+                self.print_tts(tts)?;
                 self.writer().space()?;
                 self.writer().word(token_to_string(&token::CloseDelim(delim)))
             },
diff --git a/src/libsyntax/tokenstream.rs b/src/libsyntax/tokenstream.rs
index fb72ef9c956ce..f5d2d6f18ee87 100644
--- a/src/libsyntax/tokenstream.rs
+++ b/src/libsyntax/tokenstream.rs
@@ -41,7 +41,7 @@ pub enum TokenTree {
     /// A single token
     Token(Span, token::Token),
     /// A delimited sequence of token trees
-    Delimited(DelimSpan, DelimToken, ThinTokenStream),
+    Delimited(DelimSpan, DelimToken, TokenStream),
 }
 
 impl TokenTree {
@@ -62,8 +62,7 @@ impl TokenTree {
             (&TokenTree::Token(_, ref tk), &TokenTree::Token(_, ref tk2)) => tk == tk2,
             (&TokenTree::Delimited(_, delim, ref tts),
              &TokenTree::Delimited(_, delim2, ref tts2)) => {
-                delim == delim2 &&
-                tts.stream().eq_unspanned(&tts2.stream())
+                delim == delim2 && tts.eq_unspanned(&tts2)
             }
             (_, _) => false,
         }
@@ -81,8 +80,7 @@ impl TokenTree {
             }
             (&TokenTree::Delimited(_, delim, ref tts),
              &TokenTree::Delimited(_, delim2, ref tts2)) => {
-                delim == delim2 &&
-                tts.stream().probably_equal_for_proc_macro(&tts2.stream())
+                delim == delim2 && tts.probably_equal_for_proc_macro(&tts2)
             }
             (_, _) => false,
         }
@@ -113,7 +111,7 @@ impl TokenTree {
     }
 
     pub fn joint(self) -> TokenStream {
-        TokenStream::Tree(self, Joint)
+        TokenStream::new(vec![(self, Joint)])
     }
 
     /// Returns the opening delimiter as a token tree.
@@ -143,18 +141,19 @@ impl TokenTree {
 /// The goal is for procedural macros to work with `TokenStream`s and `TokenTree`s
 /// instead of a representation of the abstract syntax tree.
 /// Today's `TokenTree`s can still contain AST via `Token::Interpolated` for back-compat.
+///
+/// The use of `Option` is an optimization that avoids the need for an
+/// allocation when the stream is empty. However, it is not guaranteed that an
+/// empty stream is represented with `None`; it may be represented as a `Some`
+/// around an empty `Vec`.
 #[derive(Clone, Debug)]
-pub enum TokenStream {
-    Empty,
-    Tree(TokenTree, IsJoint),
-    Stream(Lrc<Vec<TreeAndJoint>>),
-}
+pub struct TokenStream(Option<Lrc<Vec<TreeAndJoint>>>);
 
 pub type TreeAndJoint = (TokenTree, IsJoint);
 
 // `TokenStream` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-static_assert!(MEM_SIZE_OF_TOKEN_STREAM: mem::size_of::<TokenStream>() == 32);
+static_assert!(MEM_SIZE_OF_TOKEN_STREAM: mem::size_of::<TokenStream>() == 8);
 
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum IsJoint {
@@ -169,7 +168,7 @@ impl TokenStream {
     /// separating the two arguments with a comma for diagnostic suggestions.
     pub(crate) fn add_comma(&self) -> Option<(TokenStream, Span)> {
         // Used to suggest if a user writes `foo!(a b);`
-        if let TokenStream::Stream(ref stream) = self {
+        if let Some(ref stream) = self.0 {
             let mut suggestion = None;
             let mut iter = stream.iter().enumerate().peekable();
             while let Some((pos, ts)) = iter.next() {
@@ -201,7 +200,7 @@ impl TokenStream {
 
 impl From<TokenTree> for TokenStream {
     fn from(tree: TokenTree) -> TokenStream {
-        TokenStream::Tree(tree, NonJoint)
+        TokenStream::new(vec![(tree, NonJoint)])
     }
 }
 
@@ -233,7 +232,7 @@ impl PartialEq<TokenStream> for TokenStream {
 
 impl TokenStream {
     pub fn len(&self) -> usize {
-        if let TokenStream::Stream(ref slice) = self {
+        if let Some(ref slice) = self.0 {
             slice.len()
         } else {
             0
@@ -241,13 +240,13 @@ impl TokenStream {
     }
 
     pub fn empty() -> TokenStream {
-        TokenStream::Empty
+        TokenStream(None)
     }
 
     pub fn is_empty(&self) -> bool {
-        match self {
-            TokenStream::Empty => true,
-            _ => false,
+        match self.0 {
+            None => true,
+            Some(ref stream) => stream.is_empty(),
         }
     }
 
@@ -258,10 +257,9 @@ impl TokenStream {
             _ => {
                 let mut vec = vec![];
                 for stream in streams {
-                    match stream {
-                        TokenStream::Empty => {},
-                        TokenStream::Tree(tree, is_joint) => vec.push((tree, is_joint)),
-                        TokenStream::Stream(stream2) => vec.extend(stream2.iter().cloned()),
+                    match stream.0 {
+                        None => {},
+                        Some(stream2) => vec.extend(stream2.iter().cloned()),
                     }
                 }
                 TokenStream::new(vec)
@@ -269,22 +267,16 @@ impl TokenStream {
         }
     }
 
-    pub fn new(mut streams: Vec<TreeAndJoint>) -> TokenStream {
+    pub fn new(streams: Vec<TreeAndJoint>) -> TokenStream {
         match streams.len() {
-            0 => TokenStream::empty(),
-            1 => {
-                let (tree, is_joint) = streams.pop().unwrap();
-                TokenStream::Tree(tree, is_joint)
-            }
-            _ => TokenStream::Stream(Lrc::new(streams)),
+            0 => TokenStream(None),
+            _ => TokenStream(Some(Lrc::new(streams))),
         }
     }
 
     pub fn append_to_tree_and_joint_vec(self, vec: &mut Vec<TreeAndJoint>) {
-        match self {
-            TokenStream::Empty => {}
-            TokenStream::Tree(tree, is_joint) => vec.push((tree, is_joint)),
-            TokenStream::Stream(stream) => vec.extend(stream.iter().cloned()),
+        if let Some(stream) = self.0 {
+            vec.extend(stream.iter().cloned());
         }
     }
 
@@ -349,51 +341,36 @@ impl TokenStream {
     }
 
     pub fn map_enumerated<F: FnMut(usize, TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
-        match self {
-            TokenStream::Empty => TokenStream::Empty,
-            TokenStream::Tree(tree, is_joint) => TokenStream::Tree(f(0, tree), is_joint),
-            TokenStream::Stream(stream) => TokenStream::Stream(Lrc::new(
+        TokenStream(self.0.map(|stream| {
+            Lrc::new(
                 stream
                     .iter()
                     .enumerate()
                     .map(|(i, (tree, is_joint))| (f(i, tree.clone()), *is_joint))
-                    .collect()
-            )),
-        }
+                    .collect())
+        }))
     }
 
     pub fn map<F: FnMut(TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
-        match self {
-            TokenStream::Empty => TokenStream::Empty,
-            TokenStream::Tree(tree, is_joint) => TokenStream::Tree(f(tree), is_joint),
-            TokenStream::Stream(stream) => TokenStream::Stream(Lrc::new(
+        TokenStream(self.0.map(|stream| {
+            Lrc::new(
                 stream
                     .iter()
                     .map(|(tree, is_joint)| (f(tree.clone()), *is_joint))
-                    .collect()
-            )),
-        }
+                    .collect())
+        }))
     }
 
-    fn first_tree_and_joint(&self) -> Option<(TokenTree, IsJoint)> {
-        match self {
-            TokenStream::Empty => None,
-            TokenStream::Tree(ref tree, is_joint) => Some((tree.clone(), *is_joint)),
-            TokenStream::Stream(ref stream) => Some(stream.first().unwrap().clone())
-        }
+    fn first_tree_and_joint(&self) -> Option<TreeAndJoint> {
+        self.0.as_ref().map(|stream| {
+            stream.first().unwrap().clone()
+        })
     }
 
     fn last_tree_if_joint(&self) -> Option<TokenTree> {
-        match self {
-            TokenStream::Empty => None,
-            TokenStream::Tree(ref tree, is_joint) => {
-                if *is_joint == Joint {
-                    Some(tree.clone())
-                } else {
-                    None
-                }
-            }
-            TokenStream::Stream(ref stream) => {
+        match self.0 {
+            None => None,
+            Some(ref stream) => {
                 if let (tree, Joint) = stream.last().unwrap() {
                     Some(tree.clone())
                 } else {
@@ -422,7 +399,7 @@ impl TokenStreamBuilder {
                     self.push_all_but_last_tree(&last_stream);
                     let glued_span = last_span.to(span);
                     let glued_tt = TokenTree::Token(glued_span, glued_tok);
-                    let glued_tokenstream = TokenStream::Tree(glued_tt, is_joint);
+                    let glued_tokenstream = TokenStream::new(vec![(glued_tt, is_joint)]);
                     self.0.push(glued_tokenstream);
                     self.push_all_but_first_tree(&stream);
                     return
@@ -437,23 +414,21 @@ impl TokenStreamBuilder {
     }
 
     fn push_all_but_last_tree(&mut self, stream: &TokenStream) {
-        if let TokenStream::Stream(ref streams) = stream {
+        if let Some(ref streams) = stream.0 {
             let len = streams.len();
             match len {
                 1 => {}
-                2 => self.0.push(TokenStream::Tree(streams[0].0.clone(), streams[0].1)),
-                _ => self.0.push(TokenStream::Stream(Lrc::new(streams[0 .. len - 1].to_vec()))),
+                _ => self.0.push(TokenStream(Some(Lrc::new(streams[0 .. len - 1].to_vec())))),
             }
         }
     }
 
     fn push_all_but_first_tree(&mut self, stream: &TokenStream) {
-        if let TokenStream::Stream(ref streams) = stream {
+        if let Some(ref streams) = stream.0 {
             let len = streams.len();
             match len {
                 1 => {}
-                2 => self.0.push(TokenStream::Tree(streams[1].0.clone(), streams[1].1)),
-                _ => self.0.push(TokenStream::Stream(Lrc::new(streams[1 .. len].to_vec()))),
+                _ => self.0.push(TokenStream(Some(Lrc::new(streams[1 .. len].to_vec())))),
             }
         }
     }
@@ -479,17 +454,9 @@ impl Cursor {
     }
 
     pub fn next_with_joint(&mut self) -> Option<TreeAndJoint> {
-        match self.stream {
-            TokenStream::Empty => None,
-            TokenStream::Tree(ref tree, ref is_joint) => {
-                if self.index == 0 {
-                    self.index = 1;
-                    Some((tree.clone(), *is_joint))
-                } else {
-                    None
-                }
-            }
-            TokenStream::Stream(ref stream) => {
+        match self.stream.0 {
+            None => None,
+            Some(ref stream) => {
                 if self.index < stream.len() {
                     self.index += 1;
                     Some(stream[self.index - 1].clone())
@@ -505,63 +472,19 @@ impl Cursor {
             return;
         }
         let index = self.index;
-        let stream = mem::replace(&mut self.stream, TokenStream::Empty);
+        let stream = mem::replace(&mut self.stream, TokenStream(None));
         *self = TokenStream::from_streams(vec![stream, new_stream]).into_trees();
         self.index = index;
     }
 
     pub fn look_ahead(&self, n: usize) -> Option<TokenTree> {
-        match self.stream {
-            TokenStream::Empty => None,
-            TokenStream::Tree(ref tree, _) => {
-                if n == 0 && self.index == 0 {
-                    Some(tree.clone())
-                } else {
-                    None
-                }
-            }
-            TokenStream::Stream(ref stream) =>
-                stream[self.index ..].get(n).map(|(tree, _)| tree.clone()),
+        match self.stream.0 {
+            None => None,
+            Some(ref stream) => stream[self.index ..].get(n).map(|(tree, _)| tree.clone()),
         }
     }
 }
 
-/// The `TokenStream` type is large enough to represent a single `TokenTree` without allocation.
-/// `ThinTokenStream` is smaller, but needs to allocate to represent a single `TokenTree`.
-/// We must use `ThinTokenStream` in `TokenTree::Delimited` to avoid infinite size due to recursion.
-#[derive(Debug, Clone)]
-pub struct ThinTokenStream(Option<Lrc<Vec<TreeAndJoint>>>);
-
-impl ThinTokenStream {
-    pub fn stream(&self) -> TokenStream {
-        self.clone().into()
-    }
-}
-
-impl From<TokenStream> for ThinTokenStream {
-    fn from(stream: TokenStream) -> ThinTokenStream {
-        ThinTokenStream(match stream {
-            TokenStream::Empty => None,
-            TokenStream::Tree(tree, is_joint) => Some(Lrc::new(vec![(tree, is_joint)])),
-            TokenStream::Stream(stream) => Some(stream),
-        })
-    }
-}
-
-impl From<ThinTokenStream> for TokenStream {
-    fn from(stream: ThinTokenStream) -> TokenStream {
-        stream.0.map(TokenStream::Stream).unwrap_or_else(TokenStream::empty)
-    }
-}
-
-impl Eq for ThinTokenStream {}
-
-impl PartialEq<ThinTokenStream> for ThinTokenStream {
-    fn eq(&self, other: &ThinTokenStream) -> bool {
-        TokenStream::from(self.clone()) == TokenStream::from(other.clone())
-    }
-}
-
 impl fmt::Display for TokenStream {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.write_str(&pprust::tokens_to_string(self.clone()))
@@ -580,18 +503,6 @@ impl Decodable for TokenStream {
     }
 }
 
-impl Encodable for ThinTokenStream {
-    fn encode<E: Encoder>(&self, encoder: &mut E) -> Result<(), E::Error> {
-        TokenStream::from(self.clone()).encode(encoder)
-    }
-}
-
-impl Decodable for ThinTokenStream {
-    fn decode<D: Decoder>(decoder: &mut D) -> Result<ThinTokenStream, D::Error> {
-        TokenStream::decode(decoder).map(Into::into)
-    }
-}
-
 #[derive(Debug, Copy, Clone, PartialEq, RustcEncodable, RustcDecodable)]
 pub struct DelimSpan {
     pub open: Span,
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 156546bbba94a..8cbd47ca70fde 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -832,7 +832,7 @@ pub fn walk_attribute<'a, V: Visitor<'a>>(visitor: &mut V, attr: &'a Attribute)
 pub fn walk_tt<'a, V: Visitor<'a>>(visitor: &mut V, tt: TokenTree) {
     match tt {
         TokenTree::Token(_, tok) => visitor.visit_token(tok),
-        TokenTree::Delimited(_, _, tts) => visitor.visit_tts(tts.stream()),
+        TokenTree::Delimited(_, _, tts) => visitor.visit_tts(tts),
     }
 }
 
diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index 3e3bca7080fb6..215df4224c5f1 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -493,7 +493,10 @@ impl<'a, 'b> Context<'a, 'b> {
 
                 let fill = arg.format.fill.unwrap_or(' ');
 
-                if *arg != simple_arg || fill != ' ' {
+                let pos_simple =
+                    arg.position.index() == simple_arg.position.index();
+
+                if !pos_simple || arg.format != simple_arg.format || fill != ' ' {
                     self.all_pieces_simple = false;
                 }
 
diff --git a/src/libsyntax_ext/proc_macro_server.rs b/src/libsyntax_ext/proc_macro_server.rs
index 158cbc791ef50..7de9b9343a8fa 100644
--- a/src/libsyntax_ext/proc_macro_server.rs
+++ b/src/libsyntax_ext/proc_macro_server.rs
@@ -269,7 +269,7 @@ impl ToInternal<TokenStream> for TokenTree<Group, Punct, Ident, Literal> {
         };
 
         let tree = tokenstream::TokenTree::Token(span, token);
-        TokenStream::Tree(tree, if joint { Joint } else { NonJoint })
+        TokenStream::new(vec![(tree, if joint { Joint } else { NonJoint })])
     }
 }
 
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs
new file mode 100644
index 0000000000000..5ea69190951e1
--- /dev/null
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs
@@ -0,0 +1,29 @@
+#![feature(optin_builtin_traits)]
+
+// Test for issue #56934 - that it is impossible to redundantly
+// implement an auto-trait for a trait object type that contains it.
+
+// Negative impl variant.
+
+auto trait Marker1 {}
+auto trait Marker2 {}
+
+trait Object: Marker1 {}
+
+// A supertrait marker is illegal...
+impl !Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+// ...and also a direct component.
+impl !Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+
+// But implementing a marker if it is not present is OK.
+impl !Marker2 for dyn Object {} // OK
+
+// A non-principal trait-object type is orphan even in its crate.
+impl !Send for dyn Marker2 {} //~ ERROR E0117
+
+// And impl'ing a remote marker for a local trait object is forbidden
+// by one of these special orphan-like rules.
+impl !Send for dyn Object {} //~ ERROR E0321
+impl !Send for dyn Object + Marker2 {} //~ ERROR E0321
+
+fn main() { }
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr
new file mode 100644
index 0000000000000..c8a146cdd4456
--- /dev/null
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr
@@ -0,0 +1,37 @@
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:14:1
+   |
+LL | impl !Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
+
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:16:1
+   |
+LL | impl !Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
+
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:22:1
+   |
+LL | impl !Send for dyn Marker2 {} //~ ERROR E0117
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate
+   |
+   = note: the impl does not reference only types defined in this crate
+   = note: define and implement a trait or new type instead
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:26:1
+   |
+LL | impl !Send for dyn Object {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:27:1
+   |
+LL | impl !Send for dyn Object + Marker2 {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error: aborting due to 5 previous errors
+
+Some errors occurred: E0117, E0321, E0371.
+For more information about an error, try `rustc --explain E0117`.
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs
new file mode 100644
index 0000000000000..6b5689e8260f0
--- /dev/null
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs
@@ -0,0 +1,29 @@
+#![feature(optin_builtin_traits)]
+
+// Test for issue #56934 - that it is impossible to redundantly
+// implement an auto-trait for a trait object type that contains it.
+
+// Positive impl variant.
+
+auto trait Marker1 {}
+auto trait Marker2 {}
+
+trait Object: Marker1 {}
+
+// A supertrait marker is illegal...
+impl Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+// ...and also a direct component.
+impl Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+
+// But implementing a marker if it is not present is OK.
+impl Marker2 for dyn Object {} // OK
+
+// A non-principal trait-object type is orphan even in its crate.
+unsafe impl Send for dyn Marker2 {} //~ ERROR E0117
+
+// And impl'ing a remote marker for a local trait object is forbidden
+// by one of these special orphan-like rules.
+unsafe impl Send for dyn Object {} //~ ERROR E0321
+unsafe impl Send for dyn Object + Marker2 {} //~ ERROR E0321
+
+fn main() { }
diff --git a/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr
new file mode 100644
index 0000000000000..78ca2f5279d63
--- /dev/null
+++ b/src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr
@@ -0,0 +1,37 @@
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:14:1
+   |
+LL | impl Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
+
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:16:1
+   |
+LL | impl Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
+
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:22:1
+   |
+LL | unsafe impl Send for dyn Marker2 {} //~ ERROR E0117
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate
+   |
+   = note: the impl does not reference only types defined in this crate
+   = note: define and implement a trait or new type instead
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:26:1
+   |
+LL | unsafe impl Send for dyn Object {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:27:1
+   |
+LL | unsafe impl Send for dyn Object + Marker2 {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error: aborting due to 5 previous errors
+
+Some errors occurred: E0117, E0321, E0371.
+For more information about an error, try `rustc --explain E0117`.
diff --git a/src/test/ui/pattern/const-pat-ice.rs b/src/test/ui/pattern/const-pat-ice.rs
index 6496a2ab69f5c..865c54be1ad7b 100644
--- a/src/test/ui/pattern/const-pat-ice.rs
+++ b/src/test/ui/pattern/const-pat-ice.rs
@@ -1,4 +1,5 @@
 // failure-status: 101
+// rustc-env:RUST_BACKTRACE=0
 
 // This is a repro test for an ICE in our pattern handling of constants.