diff --git a/src/bootstrap/bootstrap.py b/src/bootstrap/bootstrap.py
index 5966bb65df9c8..cc5ee53affc66 100644
--- a/src/bootstrap/bootstrap.py
+++ b/src/bootstrap/bootstrap.py
@@ -314,7 +314,6 @@ def __init__(self):
         self.build_dir = os.path.join(os.getcwd(), "build")
         self.clean = False
         self.config_toml = ''
-        self.printed = False
         self.rust_root = os.path.abspath(os.path.join(__file__, '../../..'))
         self.use_locked_deps = ''
         self.use_vendored_sources = ''
@@ -336,7 +335,6 @@ def download_stage0(self):
         if self.rustc().startswith(self.bin_root()) and \
                 (not os.path.exists(self.rustc()) or
                  self.program_out_of_date(self.rustc_stamp())):
-            self.print_what_bootstrap_means()
             if os.path.exists(self.bin_root()):
                 shutil.rmtree(self.bin_root())
             filename = "rust-std-{}-{}.tar.gz".format(
@@ -354,7 +352,6 @@ def download_stage0(self):
         if self.cargo().startswith(self.bin_root()) and \
                 (not os.path.exists(self.cargo()) or
                  self.program_out_of_date(self.cargo_stamp())):
-            self.print_what_bootstrap_means()
             filename = "cargo-{}-{}.tar.gz".format(cargo_channel, self.build)
             self._download_stage0_helper(filename, "cargo")
             self.fix_executable("{}/bin/cargo".format(self.bin_root()))
@@ -555,23 +552,6 @@ def exe_suffix():
             return '.exe'
         return ''
 
-    def print_what_bootstrap_means(self):
-        """Prints more information about the build system"""
-        if hasattr(self, 'printed'):
-            return
-        self.printed = True
-        if os.path.exists(self.bootstrap_binary()):
-            return
-        if '--help' not in sys.argv or len(sys.argv) == 1:
-            return
-
-        print('info: the build system for Rust is written in Rust, so this')
-        print('      script is now going to download a stage0 rust compiler')
-        print('      and then compile the build system itself')
-        print('')
-        print('info: in the meantime you can read more about rustbuild at')
-        print('      src/bootstrap/README.md before the download finishes')
-
     def bootstrap_binary(self):
         """Return the path of the boostrap binary
 
@@ -585,7 +565,6 @@ def bootstrap_binary(self):
 
     def build_bootstrap(self):
         """Build bootstrap"""
-        self.print_what_bootstrap_means()
         build_dir = os.path.join(self.build_dir, "bootstrap")
         if self.clean and os.path.exists(build_dir):
             shutil.rmtree(build_dir)
@@ -670,8 +649,16 @@ def set_dev_environment(self):
         self._download_url = 'https://dev-static.rust-lang.org'
 
 
-def bootstrap():
+def bootstrap(help_triggered):
     """Configure, fetch, build and run the initial bootstrap"""
+
+    # If the user is asking for help, let them know that the whole download-and-build
+    # process has to happen before anything is printed out.
+    if help_triggered:
+        print("info: Downloading and building bootstrap before processing --help")
+        print("      command. See src/bootstrap/README.md for help with common")
+        print("      commands.")
+
     parser = argparse.ArgumentParser(description='Build rust')
     parser.add_argument('--config')
     parser.add_argument('--build')
@@ -708,7 +695,7 @@ def bootstrap():
             print('      and so in order to preserve your $HOME this will now')
             print('      use vendored sources by default. Note that if this')
             print('      does not work you should run a normal build first')
-            print('      before running a command like `sudo make install`')
+            print('      before running a command like `sudo ./x.py install`')
 
     if build.use_vendored_sources:
         if not os.path.exists('.cargo'):
@@ -734,7 +721,10 @@ def bootstrap():
     if 'dev' in data:
         build.set_dev_environment()
 
-    build.update_submodules()
+    # No help text depends on submodules. This check saves ~1 minute of git commands, even if
+    # all the submodules are present and downloaded!
+    if not help_triggered:
+        build.update_submodules()
 
     # Fetch/build the bootstrap
     build.build = args.build or build.build_triple()
@@ -760,7 +750,7 @@ def main():
     help_triggered = (
         '-h' in sys.argv) or ('--help' in sys.argv) or (len(sys.argv) == 1)
     try:
-        bootstrap()
+        bootstrap(help_triggered)
         if not help_triggered:
             print("Build completed successfully in {}".format(
                 format_build_time(time() - start_time)))
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index 55d9723527e6d..a791dd13f0f4b 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -312,6 +312,7 @@ fn invoke_rustdoc(builder: &Builder, compiler: Compiler, target: Interned<String
     cmd.arg("--html-after-content").arg(&footer)
         .arg("--html-before-content").arg(&version_info)
         .arg("--html-in-header").arg(&favicon)
+        .arg("--markdown-no-toc")
         .arg("--markdown-playground-url")
         .arg("https://play.rust-lang.org/")
         .arg("-o").arg(&out)
diff --git a/src/libcore/benches/iter.rs b/src/libcore/benches/iter.rs
index b284d855c4515..6c597301ac204 100644
--- a/src/libcore/benches/iter.rs
+++ b/src/libcore/benches/iter.rs
@@ -281,3 +281,32 @@ bench_sums! {
     bench_take_while_chain_ref_sum,
     (0i64..1000000).chain(1000000..).take_while(|&x| x < 1111111)
 }
+
+// Checks whether Skip<Zip<A,B>> is as fast as Zip<Skip<A>, Skip<B>>, from
+// https://users.rust-lang.org/t/performance-difference-between-iterator-zip-and-skip-order/15743
+#[bench]
+fn bench_zip_then_skip(b: &mut Bencher) {
+    let v: Vec<_> = (0..100_000).collect();
+    let t: Vec<_> = (0..100_000).collect();
+
+    b.iter(|| {
+        let s = v.iter().zip(t.iter()).skip(10000)
+            .take_while(|t| *t.0 < 10100)
+            .map(|(a, b)| *a + *b)
+            .sum::<u64>();
+        assert_eq!(s, 2009900);
+    });
+}
+#[bench]
+fn bench_skip_then_zip(b: &mut Bencher) {
+    let v: Vec<_> = (0..100_000).collect();
+    let t: Vec<_> = (0..100_000).collect();
+
+    b.iter(|| {
+        let s = v.iter().skip(10000).zip(t.iter().skip(10000))
+            .take_while(|t| *t.0 < 10100)
+            .map(|(a, b)| *a + *b)
+            .sum::<u64>();
+        assert_eq!(s, 2009900);
+    });
+}
diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 623cad754dd72..257d7d6caaaf8 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -1045,6 +1045,11 @@ impl<A, B> Iterator for Zip<A, B> where A: Iterator, B: Iterator
     fn size_hint(&self) -> (usize, Option<usize>) {
         ZipImpl::size_hint(self)
     }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        ZipImpl::nth(self, n)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1065,6 +1070,14 @@ trait ZipImpl<A, B> {
     fn new(a: A, b: B) -> Self;
     fn next(&mut self) -> Option<Self::Item>;
     fn size_hint(&self) -> (usize, Option<usize>);
+    fn nth(&mut self, n: usize) -> Option<Self::Item>;
+    fn super_nth(&mut self, mut n: usize) -> Option<Self::Item> {
+        while let Some(x) = self.next() {
+            if n == 0 { return Some(x) }
+            n -= 1;
+        }
+        None
+    }
     fn next_back(&mut self) -> Option<Self::Item>
         where A: DoubleEndedIterator + ExactSizeIterator,
               B: DoubleEndedIterator + ExactSizeIterator;
@@ -1094,6 +1107,11 @@ impl<A, B> ZipImpl<A, B> for Zip<A, B>
         })
     }
 
+    #[inline]
+    default fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        self.super_nth(n)
+    }
+
     #[inline]
     default fn next_back(&mut self) -> Option<(A::Item, B::Item)>
         where A: DoubleEndedIterator + ExactSizeIterator,
@@ -1174,6 +1192,24 @@ impl<A, B> ZipImpl<A, B> for Zip<A, B>
         (len, Some(len))
     }
 
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        let delta = cmp::min(n, self.len - self.index);
+        let end = self.index + delta;
+        while self.index < end {
+            let i = self.index;
+            self.index += 1;
+            if A::may_have_side_effect() {
+                unsafe { self.a.get_unchecked(i); }
+            }
+            if B::may_have_side_effect() {
+                unsafe { self.b.get_unchecked(i); }
+            }
+        }
+
+        self.super_nth(n - delta)
+    }
+
     #[inline]
     fn next_back(&mut self) -> Option<(A::Item, B::Item)>
         where A: DoubleEndedIterator + ExactSizeIterator,
diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index edd75f7795ed7..a962efadd64e9 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -144,6 +144,43 @@ fn test_iterator_chain_find() {
     assert_eq!(iter.next(), None);
 }
 
+#[test]
+fn test_zip_nth() {
+    let xs = [0, 1, 2, 4, 5];
+    let ys = [10, 11, 12];
+
+    let mut it = xs.iter().zip(&ys);
+    assert_eq!(it.nth(0), Some((&0, &10)));
+    assert_eq!(it.nth(1), Some((&2, &12)));
+    assert_eq!(it.nth(0), None);
+
+    let mut it = xs.iter().zip(&ys);
+    assert_eq!(it.nth(3), None);
+
+    let mut it = ys.iter().zip(&xs);
+    assert_eq!(it.nth(3), None);
+}
+
+#[test]
+fn test_zip_nth_side_effects() {
+    let mut a = Vec::new();
+    let mut b = Vec::new();
+    let value = [1, 2, 3, 4, 5, 6].iter().cloned()
+        .map(|n| {
+            a.push(n);
+            n * 10
+        })
+        .zip([2, 3, 4, 5, 6, 7, 8].iter().cloned().map(|n| {
+            b.push(n * 100);
+            n * 1000
+        }))
+        .skip(1)
+        .nth(3);
+    assert_eq!(value, Some((50, 6000)));
+    assert_eq!(a, vec![1, 2, 3, 4, 5]);
+    assert_eq!(b, vec![200, 300, 400, 500, 600]);
+}
+
 #[test]
 fn test_iterator_step_by() {
     // Identity
diff --git a/src/librustc/dep_graph/debug.rs b/src/librustc/dep_graph/debug.rs
index e22552008d5a8..e56333aba9be5 100644
--- a/src/librustc/dep_graph/debug.rs
+++ b/src/librustc/dep_graph/debug.rs
@@ -54,7 +54,7 @@ pub struct EdgeFilter {
 }
 
 impl EdgeFilter {
-    pub fn new(test: &str) -> Result<EdgeFilter, Box<Error>> {
+    pub fn new(test: &str) -> Result<EdgeFilter, Box<dyn Error>> {
         let parts: Vec<_> = test.split("->").collect();
         if parts.len() != 2 {
             Err(format!("expected a filter like `a&b -> c&d`, not `{}`", test).into())
diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index a41ac5bceed27..fa745bf165577 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -80,13 +80,13 @@ pub struct LoweringContext<'a> {
     // Use to assign ids to hir nodes that do not directly correspond to an ast node
     sess: &'a Session,
 
-    cstore: &'a CrateStore,
+    cstore: &'a dyn CrateStore,
 
     // As we walk the AST we must keep track of the current 'parent' def id (in
     // the form of a DefIndex) so that if we create a new node which introduces
     // a definition, then we can properly create the def id.
     parent_def: Option<DefIndex>,
-    resolver: &'a mut Resolver,
+    resolver: &'a mut dyn Resolver,
     name_map: FxHashMap<Ident, Name>,
 
     /// The items being lowered are collected here.
@@ -177,10 +177,10 @@ enum ImplTraitContext {
 }
 
 pub fn lower_crate(sess: &Session,
-                   cstore: &CrateStore,
+                   cstore: &dyn CrateStore,
                    dep_graph: &DepGraph,
                    krate: &Crate,
-                   resolver: &mut Resolver)
+                   resolver: &mut dyn Resolver)
                    -> hir::Crate {
     // We're constructing the HIR here; we don't care what we will
     // read, since we haven't even constructed the *input* to
diff --git a/src/librustc/hir/map/collector.rs b/src/librustc/hir/map/collector.rs
index 99b1e5783e01e..9bbda9d74479b 100644
--- a/src/librustc/hir/map/collector.rs
+++ b/src/librustc/hir/map/collector.rs
@@ -123,7 +123,7 @@ impl<'a, 'hir> NodeCollector<'a, 'hir> {
 
     pub(super) fn finalize_and_compute_crate_hash(self,
                                                   crate_disambiguator: CrateDisambiguator,
-                                                  cstore: &CrateStore,
+                                                  cstore: &dyn CrateStore,
                                                   codemap: &CodeMap,
                                                   commandline_args_hash: u64)
                                                   -> (Vec<MapEntry<'hir>>, Svh) {
diff --git a/src/librustc/hir/map/def_collector.rs b/src/librustc/hir/map/def_collector.rs
index d68b18dd2f17d..cdd6395747865 100644
--- a/src/librustc/hir/map/def_collector.rs
+++ b/src/librustc/hir/map/def_collector.rs
@@ -26,7 +26,7 @@ pub struct DefCollector<'a> {
     definitions: &'a mut Definitions,
     parent_def: Option<DefIndex>,
     expansion: Mark,
-    pub visit_macro_invoc: Option<&'a mut FnMut(MacroInvocationData)>,
+    pub visit_macro_invoc: Option<&'a mut dyn FnMut(MacroInvocationData)>,
 }
 
 pub struct MacroInvocationData {
diff --git a/src/librustc/hir/map/mod.rs b/src/librustc/hir/map/mod.rs
index 3799bdada888e..61fae4609d54f 100644
--- a/src/librustc/hir/map/mod.rs
+++ b/src/librustc/hir/map/mod.rs
@@ -19,6 +19,8 @@ use dep_graph::{DepGraph, DepNode, DepKind, DepNodeIndex};
 
 use hir::def_id::{CRATE_DEF_INDEX, DefId, LocalDefId, DefIndexAddressSpace};
 
+use middle::cstore::CrateStore;
+
 use syntax::abi::Abi;
 use syntax::ast::{self, Name, NodeId, CRATE_NODE_ID};
 use syntax::codemap::Spanned;
@@ -1136,8 +1138,9 @@ impl Named for StructField { fn name(&self) -> Name { self.name } }
 impl Named for TraitItem { fn name(&self) -> Name { self.name } }
 impl Named for ImplItem { fn name(&self) -> Name { self.name } }
 
+
 pub fn map_crate<'hir>(sess: &::session::Session,
-                       cstore: &::middle::cstore::CrateStore,
+                       cstore: &dyn CrateStore,
                        forest: &'hir mut Forest,
                        definitions: &'hir Definitions)
                        -> Map<'hir> {
diff --git a/src/librustc/hir/print.rs b/src/librustc/hir/print.rs
index 30c1ad01d1401..ed8cea3eb6563 100644
--- a/src/librustc/hir/print.rs
+++ b/src/librustc/hir/print.rs
@@ -62,7 +62,7 @@ pub trait PpAnn {
 
 pub struct NoAnn;
 impl PpAnn for NoAnn {}
-pub const NO_ANN: &'static PpAnn = &NoAnn;
+pub const NO_ANN: &'static dyn PpAnn = &NoAnn;
 
 impl PpAnn for hir::Crate {
     fn nested(&self, state: &mut State, nested: Nested) -> io::Result<()> {
@@ -83,7 +83,7 @@ pub struct State<'a> {
     literals: Peekable<vec::IntoIter<comments::Literal>>,
     cur_cmnt: usize,
     boxes: Vec<pp::Breaks>,
-    ann: &'a (PpAnn + 'a),
+    ann: &'a (dyn PpAnn + 'a),
 }
 
 impl<'a> PrintState<'a> for State<'a> {
@@ -126,9 +126,9 @@ pub fn print_crate<'a>(cm: &'a CodeMap,
                        sess: &ParseSess,
                        krate: &hir::Crate,
                        filename: FileName,
-                       input: &mut Read,
-                       out: Box<Write + 'a>,
-                       ann: &'a PpAnn,
+                       input: &mut dyn Read,
+                       out: Box<dyn Write + 'a>,
+                       ann: &'a dyn PpAnn,
                        is_expanded: bool)
                        -> io::Result<()> {
     let mut s = State::new_from_input(cm, sess, filename, input, out, ann, is_expanded);
@@ -145,9 +145,9 @@ impl<'a> State<'a> {
     pub fn new_from_input(cm: &'a CodeMap,
                           sess: &ParseSess,
                           filename: FileName,
-                          input: &mut Read,
-                          out: Box<Write + 'a>,
-                          ann: &'a PpAnn,
+                          input: &mut dyn Read,
+                          out: Box<dyn Write + 'a>,
+                          ann: &'a dyn PpAnn,
                           is_expanded: bool)
                           -> State<'a> {
         let (cmnts, lits) = comments::gather_comments_and_literals(sess, filename, input);
@@ -167,8 +167,8 @@ impl<'a> State<'a> {
     }
 
     pub fn new(cm: &'a CodeMap,
-               out: Box<Write + 'a>,
-               ann: &'a PpAnn,
+               out: Box<dyn Write + 'a>,
+               ann: &'a dyn PpAnn,
                comments: Option<Vec<comments::Comment>>,
                literals: Option<Vec<comments::Literal>>)
                -> State<'a> {
@@ -184,7 +184,7 @@ impl<'a> State<'a> {
     }
 }
 
-pub fn to_string<F>(ann: &PpAnn, f: F) -> String
+pub fn to_string<F>(ann: &dyn PpAnn, f: F) -> String
     where F: FnOnce(&mut State) -> io::Result<()>
 {
     let mut wr = Vec::new();
diff --git a/src/librustc/ich/hcx.rs b/src/librustc/ich/hcx.rs
index 67f6c0c2e49e8..4dcab6a04ee20 100644
--- a/src/librustc/ich/hcx.rs
+++ b/src/librustc/ich/hcx.rs
@@ -49,7 +49,7 @@ pub fn compute_ignored_attr_names() -> FxHashSet<Symbol> {
 pub struct StableHashingContext<'gcx> {
     sess: &'gcx Session,
     definitions: &'gcx Definitions,
-    cstore: &'gcx CrateStore,
+    cstore: &'gcx dyn CrateStore,
     body_resolver: BodyResolver<'gcx>,
     hash_spans: bool,
     hash_bodies: bool,
@@ -88,7 +88,7 @@ impl<'gcx> StableHashingContext<'gcx> {
     pub fn new(sess: &'gcx Session,
                krate: &'gcx hir::Crate,
                definitions: &'gcx Definitions,
-               cstore: &'gcx CrateStore)
+               cstore: &'gcx dyn CrateStore)
                -> Self {
         let hash_spans_initial = !sess.opts.debugging_opts.incremental_ignore_spans;
 
diff --git a/src/librustc/infer/region_constraints/mod.rs b/src/librustc/infer/region_constraints/mod.rs
index 4a75037aa5035..ed89d1d2f57a5 100644
--- a/src/librustc/infer/region_constraints/mod.rs
+++ b/src/librustc/infer/region_constraints/mod.rs
@@ -896,7 +896,7 @@ impl<'a, 'gcx, 'tcx> GenericKind<'tcx> {
 }
 
 impl<'a, 'gcx, 'tcx> VerifyBound<'tcx> {
-    fn for_each_region(&self, f: &mut FnMut(ty::Region<'tcx>)) {
+    fn for_each_region(&self, f: &mut dyn FnMut(ty::Region<'tcx>)) {
         match self {
             &VerifyBound::AnyRegion(ref rs) | &VerifyBound::AllRegions(ref rs) => for &r in rs {
                 f(r);
diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index f05ed0a460d87..d08a41010ab16 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -41,8 +41,6 @@
        html_root_url = "https://doc.rust-lang.org/nightly/")]
 #![deny(warnings)]
 
-#![cfg_attr(not(stage0), allow(bare_trait_object))]
-
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(conservative_impl_trait)]
diff --git a/src/librustc/lint/mod.rs b/src/librustc/lint/mod.rs
index a51d06c06edd3..5cc7cd5440f8c 100644
--- a/src/librustc/lint/mod.rs
+++ b/src/librustc/lint/mod.rs
@@ -280,8 +280,8 @@ pub trait EarlyLintPass: LintPass {
 }
 
 /// A lint pass boxed up as a trait object.
-pub type EarlyLintPassObject = Box<EarlyLintPass + 'static>;
-pub type LateLintPassObject = Box<for<'a, 'tcx> LateLintPass<'a, 'tcx> + 'static>;
+pub type EarlyLintPassObject = Box<dyn EarlyLintPass + 'static>;
+pub type LateLintPassObject = Box<dyn for<'a, 'tcx> LateLintPass<'a, 'tcx> + 'static>;
 
 /// Identifies a lint known to the compiler.
 #[derive(Clone, Copy, Debug)]
diff --git a/src/librustc/middle/cstore.rs b/src/librustc/middle/cstore.rs
index 9708afd204593..b0aaa76c75ed7 100644
--- a/src/librustc/middle/cstore.rs
+++ b/src/librustc/middle/cstore.rs
@@ -225,10 +225,10 @@ pub struct ExternBodyNestedBodies {
 /// (it'd break incremental compilation) and should only be called pre-HIR (e.g.
 /// during resolve)
 pub trait CrateStore {
-    fn crate_data_as_rc_any(&self, krate: CrateNum) -> Rc<Any>;
+    fn crate_data_as_rc_any(&self, krate: CrateNum) -> Rc<dyn Any>;
 
     // access to the metadata loader
-    fn metadata_loader(&self) -> &MetadataLoader;
+    fn metadata_loader(&self) -> &dyn MetadataLoader;
 
     // resolve
     fn def_key(&self, def: DefId) -> DefKey;
@@ -297,7 +297,7 @@ pub struct DummyCrateStore;
 
 #[allow(unused_variables)]
 impl CrateStore for DummyCrateStore {
-    fn crate_data_as_rc_any(&self, krate: CrateNum) -> Rc<Any>
+    fn crate_data_as_rc_any(&self, krate: CrateNum) -> Rc<dyn Any>
         { bug!("crate_data_as_rc_any") }
     // item info
     fn visibility_untracked(&self, def: DefId) -> ty::Visibility { bug!("visibility") }
@@ -351,7 +351,7 @@ impl CrateStore for DummyCrateStore {
     fn postorder_cnums_untracked(&self) -> Vec<CrateNum> { bug!("postorder_cnums_untracked") }
 
     // access to the metadata loader
-    fn metadata_loader(&self) -> &MetadataLoader { bug!("metadata_loader") }
+    fn metadata_loader(&self) -> &dyn MetadataLoader { bug!("metadata_loader") }
 }
 
 pub trait CrateLoader {
diff --git a/src/librustc/middle/dependency_format.rs b/src/librustc/middle/dependency_format.rs
index db0ecb6aa5eb1..e7055827c491f 100644
--- a/src/librustc/middle/dependency_format.rs
+++ b/src/librustc/middle/dependency_format.rs
@@ -319,7 +319,7 @@ fn attempt_static<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> Option<DependencyLis
 // also skip this step entirely.
 fn activate_injected_dep(injected: Option<CrateNum>,
                          list: &mut DependencyList,
-                         replaces_injected: &Fn(CrateNum) -> bool) {
+                         replaces_injected: &dyn Fn(CrateNum) -> bool) {
     for (i, slot) in list.iter().enumerate() {
         let cnum = CrateNum::new(i + 1);
         if !replaces_injected(cnum) {
diff --git a/src/librustc/middle/expr_use_visitor.rs b/src/librustc/middle/expr_use_visitor.rs
index 7db75a5166898..0ee25ae8116fd 100644
--- a/src/librustc/middle/expr_use_visitor.rs
+++ b/src/librustc/middle/expr_use_visitor.rs
@@ -239,7 +239,7 @@ impl OverloadedCallType {
 // This is the code that actually walks the tree.
 pub struct ExprUseVisitor<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     mc: mc::MemCategorizationContext<'a, 'gcx, 'tcx>,
-    delegate: &'a mut Delegate<'tcx>,
+    delegate: &'a mut dyn Delegate<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
 }
 
@@ -274,7 +274,7 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx, 'tcx> {
     ///   `None` means that rvalues will be given more conservative lifetimes.
     ///
     /// See also `with_infer`, which is used *during* typeck.
-    pub fn new(delegate: &'a mut (Delegate<'tcx>+'a),
+    pub fn new(delegate: &'a mut (dyn Delegate<'tcx>+'a),
                tcx: TyCtxt<'a, 'tcx, 'tcx>,
                param_env: ty::ParamEnv<'tcx>,
                region_scope_tree: &'a region::ScopeTree,
@@ -294,7 +294,7 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx, 'tcx> {
 }
 
 impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
-    pub fn with_infer(delegate: &'a mut (Delegate<'tcx>+'a),
+    pub fn with_infer(delegate: &'a mut (dyn Delegate<'tcx>+'a),
                       infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
                       param_env: ty::ParamEnv<'tcx>,
                       region_scope_tree: &'a region::ScopeTree,
diff --git a/src/librustc/middle/liveness.rs b/src/librustc/middle/liveness.rs
index 10497c95e27d0..d13b16dce8986 100644
--- a/src/librustc/middle/liveness.rs
+++ b/src/librustc/middle/liveness.rs
@@ -673,7 +673,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
     }
 
     fn write_vars<F>(&self,
-                     wr: &mut Write,
+                     wr: &mut dyn Write,
                      ln: LiveNode,
                      mut test: F)
                      -> io::Result<()> where
@@ -694,7 +694,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
     fn ln_str(&self, ln: LiveNode) -> String {
         let mut wr = Vec::new();
         {
-            let wr = &mut wr as &mut Write;
+            let wr = &mut wr as &mut dyn Write;
             write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
             self.write_vars(wr, ln, |idx| self.users[idx].reader);
             write!(wr, "  writes");
diff --git a/src/librustc/mir/interpret/error.rs b/src/librustc/mir/interpret/error.rs
index 9ebfe25c107a9..f9ea544156ce3 100644
--- a/src/librustc/mir/interpret/error.rs
+++ b/src/librustc/mir/interpret/error.rs
@@ -35,7 +35,7 @@ impl<'tcx> From<EvalErrorKind<'tcx>> for EvalError<'tcx> {
 pub enum EvalErrorKind<'tcx> {
     /// This variant is used by machines to signal their own errors that do not
     /// match an existing variant
-    MachineError(Box<Error>),
+    MachineError(Box<dyn Error>),
     FunctionPointerTyMismatch(FnSig<'tcx>, FnSig<'tcx>),
     NoMirFor(String),
     UnterminatedCString(MemoryPointer),
@@ -248,7 +248,7 @@ impl<'tcx> Error for EvalError<'tcx> {
         }
     }
 
-    fn cause(&self) -> Option<&Error> {
+    fn cause(&self) -> Option<&dyn Error> {
         use self::EvalErrorKind::*;
         match self.kind {
             MachineError(ref inner) => Some(&**inner),
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index b69f5d6c8bdd7..ca17e18beccba 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -341,7 +341,7 @@ macro_rules! hash_option {
     ($opt_name:ident, $opt_expr:expr, $sub_hashes:expr, [UNTRACKED]) => ({});
     ($opt_name:ident, $opt_expr:expr, $sub_hashes:expr, [TRACKED]) => ({
         if $sub_hashes.insert(stringify!($opt_name),
-                              $opt_expr as &dep_tracking::DepTrackingHash).is_some() {
+                              $opt_expr as &dyn dep_tracking::DepTrackingHash).is_some() {
             bug!("Duplicate key in CLI DepTrackingHash: {}", stringify!($opt_name))
         }
     });
@@ -1456,7 +1456,7 @@ pub enum OptionStability {
 }
 
 pub struct RustcOptGroup {
-    pub apply: Box<Fn(&mut getopts::Options) -> &mut getopts::Options>,
+    pub apply: Box<dyn Fn(&mut getopts::Options) -> &mut getopts::Options>,
     pub name: &'static str,
     pub stability: OptionStability,
 }
@@ -2256,7 +2256,7 @@ mod dep_tracking {
     }
 
     // This is a stable hash because BTreeMap is a sorted container
-    pub fn stable_hash(sub_hashes: BTreeMap<&'static str, &DepTrackingHash>,
+    pub fn stable_hash(sub_hashes: BTreeMap<&'static str, &dyn DepTrackingHash>,
                        hasher: &mut DefaultHasher,
                        error_format: ErrorOutputType) {
         for (key, sub_hash) in sub_hashes {
diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index 2437eaadc6d48..c0085a803dc40 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -904,7 +904,7 @@ pub fn build_session_with_codemap(sopts: config::Options,
                                   local_crate_source_file: Option<PathBuf>,
                                   registry: errors::registry::Registry,
                                   codemap: Rc<codemap::CodeMap>,
-                                  emitter_dest: Option<Box<Write + Send>>)
+                                  emitter_dest: Option<Box<dyn Write + Send>>)
                                   -> Session {
     // FIXME: This is not general enough to make the warning lint completely override
     // normal diagnostic warnings, since the warning lint can also be denied and changed
@@ -923,7 +923,7 @@ pub fn build_session_with_codemap(sopts: config::Options,
 
     let external_macro_backtrace = sopts.debugging_opts.external_macro_backtrace;
 
-    let emitter: Box<Emitter> = match (sopts.error_format, emitter_dest) {
+    let emitter: Box<dyn Emitter> = match (sopts.error_format, emitter_dest) {
         (config::ErrorOutputType::HumanReadable(color_config), None) => {
             Box::new(EmitterWriter::stderr(color_config, Some(codemap.clone()),
                      false, sopts.debugging_opts.teach)
@@ -1122,7 +1122,7 @@ pub enum IncrCompSession {
 }
 
 pub fn early_error(output: config::ErrorOutputType, msg: &str) -> ! {
-    let emitter: Box<Emitter> = match output {
+    let emitter: Box<dyn Emitter> = match output {
         config::ErrorOutputType::HumanReadable(color_config) => {
             Box::new(EmitterWriter::stderr(color_config, None, false, false))
         }
@@ -1137,7 +1137,7 @@ pub fn early_error(output: config::ErrorOutputType, msg: &str) -> ! {
 }
 
 pub fn early_warn(output: config::ErrorOutputType, msg: &str) {
-    let emitter: Box<Emitter> = match output {
+    let emitter: Box<dyn Emitter> = match output {
         config::ErrorOutputType::HumanReadable(color_config) => {
             Box::new(EmitterWriter::stderr(color_config, None, false, false))
         }
diff --git a/src/librustc/traits/error_reporting.rs b/src/librustc/traits/error_reporting.rs
index dba23c22647f8..b1d2142060699 100644
--- a/src/librustc/traits/error_reporting.rs
+++ b/src/librustc/traits/error_reporting.rs
@@ -499,7 +499,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                         item_name: ast::Name,
                                         _impl_item_def_id: DefId,
                                         trait_item_def_id: DefId,
-                                        requirement: &fmt::Display)
+                                        requirement: &dyn fmt::Display)
                                         -> DiagnosticBuilder<'tcx>
     {
         let msg = "impl has stricter requirements than trait";
diff --git a/src/librustc/traits/specialize/specialization_graph.rs b/src/librustc/traits/specialize/specialization_graph.rs
index a10169e13e60a..efc4089367486 100644
--- a/src/librustc/traits/specialize/specialization_graph.rs
+++ b/src/librustc/traits/specialize/specialization_graph.rs
@@ -190,13 +190,13 @@ impl<'a, 'gcx, 'tcx> Children {
         Ok(Inserted::BecameNewSibling(last_lint))
     }
 
-    fn iter_mut(&'a mut self) -> Box<Iterator<Item = &'a mut DefId> + 'a> {
+    fn iter_mut(&'a mut self) -> Box<dyn Iterator<Item = &'a mut DefId> + 'a> {
         let nonblanket = self.nonblanket_impls.iter_mut().flat_map(|(_, v)| v.iter_mut());
         Box::new(self.blanket_impls.iter_mut().chain(nonblanket))
     }
 
     fn filtered_mut(&'a mut self, sty: SimplifiedType)
-                    -> Box<Iterator<Item = &'a mut DefId> + 'a> {
+                    -> Box<dyn Iterator<Item = &'a mut DefId> + 'a> {
         let nonblanket = self.nonblanket_impls.entry(sty).or_insert(vec![]).iter_mut();
         Box::new(self.blanket_impls.iter_mut().chain(nonblanket))
     }
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index a39d63d05eeaf..a02115c4afb85 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -799,7 +799,7 @@ pub struct GlobalCtxt<'tcx> {
     global_arenas: &'tcx GlobalArenas<'tcx>,
     global_interners: CtxtInterners<'tcx>,
 
-    cstore: &'tcx CrateStore,
+    cstore: &'tcx dyn CrateStore,
 
     pub sess: &'tcx Session,
 
@@ -883,7 +883,7 @@ pub struct GlobalCtxt<'tcx> {
     /// This is intended to only get used during the trans phase of the compiler
     /// when satisfying the query for a particular codegen unit. Internally in
     /// the query it'll send data along this channel to get processed later.
-    pub tx_to_llvm_workers: mpsc::Sender<Box<Any + Send>>,
+    pub tx_to_llvm_workers: mpsc::Sender<Box<dyn Any + Send>>,
 
     output_filenames: Arc<OutputFilenames>,
 }
@@ -1131,7 +1131,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     /// value (types, substs, etc.) can only be used while `ty::tls` has a valid
     /// reference to the context, to allow formatting values that need it.
     pub fn create_and_enter<F, R>(s: &'tcx Session,
-                                  cstore: &'tcx CrateStore,
+                                  cstore: &'tcx dyn CrateStore,
                                   local_providers: ty::maps::Providers<'tcx>,
                                   extern_providers: ty::maps::Providers<'tcx>,
                                   arenas: &'tcx AllArenas<'tcx>,
@@ -1139,7 +1139,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                                   hir: hir_map::Map<'tcx>,
                                   on_disk_query_result_cache: maps::OnDiskCache<'tcx>,
                                   crate_name: &str,
-                                  tx: mpsc::Sender<Box<Any + Send>>,
+                                  tx: mpsc::Sender<Box<dyn Any + Send>>,
                                   output_filenames: &OutputFilenames,
                                   f: F) -> R
                                   where F: for<'b> FnOnce(TyCtxt<'b, 'tcx, 'tcx>) -> R
@@ -1312,7 +1312,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 
     // Note that this is *untracked* and should only be used within the query
     // system if the result is otherwise tracked through queries
-    pub fn crate_data_as_rc_any(self, cnum: CrateNum) -> Rc<Any> {
+    pub fn crate_data_as_rc_any(self, cnum: CrateNum) -> Rc<dyn Any> {
         self.cstore.crate_data_as_rc_any(cnum)
     }
 
diff --git a/src/librustc/ty/fold.rs b/src/librustc/ty/fold.rs
index c5b82730e488c..4cc7406af721e 100644
--- a/src/librustc/ty/fold.rs
+++ b/src/librustc/ty/fold.rs
@@ -277,7 +277,7 @@ pub struct RegionFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
     skipped_regions: &'a mut bool,
     current_depth: u32,
-    fld_r: &'a mut (FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx> + 'a),
+    fld_r: &'a mut (dyn FnMut(ty::Region<'tcx>, u32) -> ty::Region<'tcx> + 'a),
 }
 
 impl<'a, 'gcx, 'tcx> RegionFolder<'a, 'gcx, 'tcx> {
@@ -330,7 +330,7 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for RegionFolder<'a, 'gcx, 'tcx> {
 struct RegionReplacer<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
     current_depth: u32,
-    fld_r: &'a mut (FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
+    fld_r: &'a mut (dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx> + 'a),
     map: BTreeMap<ty::BoundRegion, ty::Region<'tcx>>
 }
 
diff --git a/src/librustc/ty/util.rs b/src/librustc/ty/util.rs
index 44771444c8aa8..47ad7cbcb56f7 100644
--- a/src/librustc/ty/util.rs
+++ b/src/librustc/ty/util.rs
@@ -419,7 +419,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn calculate_dtor(
         self,
         adt_did: DefId,
-        validate: &mut FnMut(Self, DefId) -> Result<(), ErrorReported>
+        validate: &mut dyn FnMut(Self, DefId) -> Result<(), ErrorReported>
     ) -> Option<ty::Destructor> {
         let drop_trait = if let Some(def_id) = self.lang_items().drop_trait() {
             def_id
diff --git a/src/librustc/util/common.rs b/src/librustc/util/common.rs
index 55e9a98e7ef13..bdb120ea59c8a 100644
--- a/src/librustc/util/common.rs
+++ b/src/librustc/util/common.rs
@@ -38,7 +38,7 @@ pub struct ErrorReported;
 thread_local!(static TIME_DEPTH: Cell<usize> = Cell::new(0));
 
 lazy_static! {
-    static ref DEFAULT_HOOK: Box<Fn(&panic::PanicInfo) + Sync + Send + 'static> = {
+    static ref DEFAULT_HOOK: Box<dyn Fn(&panic::PanicInfo) + Sync + Send + 'static> = {
         let hook = panic::take_hook();
         panic::set_hook(Box::new(panic_hook));
         hook
diff --git a/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs b/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs
index a36e26c0b7d5f..88f2b59675186 100644
--- a/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs
+++ b/src/librustc_back/target/armv7_unknown_linux_musleabihf.rs
@@ -12,13 +12,7 @@ use LinkerFlavor;
 use target::{Target, TargetOptions, TargetResult};
 
 pub fn target() -> TargetResult {
-    let mut base = super::linux_musl_base::opts();
-
-    // Most of these settings are copied from the armv7_unknown_linux_gnueabihf
-    // target.
-    base.features = "+v7,+vfp3,+neon".to_string();
-    base.cpu = "cortex-a8".to_string();
-    base.max_atomic_width = Some(64);
+    let base = super::linux_musl_base::opts();
     Ok(Target {
         // It's important we use "gnueabihf" and not "musleabihf" here. LLVM
         // uses it to determine the calling convention and float ABI, and LLVM
@@ -33,9 +27,15 @@ pub fn target() -> TargetResult {
         target_env: "musl".to_string(),
         target_vendor: "unknown".to_string(),
         linker_flavor: LinkerFlavor::Gcc,
+
+        // Most of these settings are copied from the armv7_unknown_linux_gnueabihf
+        // target.
         options: TargetOptions {
+            features: "+v7,+vfp3,+d16,+thumb2,-neon".to_string(),
+            cpu: "generic".to_string(),
+            max_atomic_width: Some(64),
             abi_blacklist: super::arm_base::abi_blacklist(),
             .. base
-        },
+        }
     })
 }
diff --git a/src/librustc_data_structures/transitive_relation.rs b/src/librustc_data_structures/transitive_relation.rs
index ba7ab0c07c66a..6d63bc4436fe8 100644
--- a/src/librustc_data_structures/transitive_relation.rs
+++ b/src/librustc_data_structures/transitive_relation.rs
@@ -10,16 +10,16 @@
 
 use bitvec::BitMatrix;
 use fx::FxHashMap;
+use sync::Lock;
 use rustc_serialize::{Encodable, Encoder, Decodable, Decoder};
 use stable_hasher::{HashStable, StableHasher, StableHasherResult};
-use std::cell::RefCell;
 use std::fmt::Debug;
 use std::hash::Hash;
 use std::mem;
 
 
 #[derive(Clone, Debug)]
-pub struct TransitiveRelation<T: Clone + Debug + Eq + Hash + Clone> {
+pub struct TransitiveRelation<T: Clone + Debug + Eq + Hash> {
     // List of elements. This is used to map from a T to a usize.
     elements: Vec<T>,
 
@@ -32,14 +32,14 @@ pub struct TransitiveRelation<T: Clone + Debug + Eq + Hash + Clone> {
 
     // This is a cached transitive closure derived from the edges.
     // Currently, we build it lazilly and just throw out any existing
-    // copy whenever a new edge is added. (The RefCell is to permit
+    // copy whenever a new edge is added. (The Lock is to permit
     // the lazy computation.) This is kind of silly, except for the
     // fact its size is tied to `self.elements.len()`, so I wanted to
     // wait before building it up to avoid reallocating as new edges
     // are added with new elements. Perhaps better would be to ask the
     // user for a batch of edges to minimize this effect, but I
     // already wrote the code this way. :P -nmatsakis
-    closure: RefCell<Option<BitMatrix>>,
+    closure: Lock<Option<BitMatrix>>,
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable, Debug)]
@@ -51,13 +51,13 @@ struct Edge {
     target: Index,
 }
 
-impl<T: Clone + Debug + Eq + Hash + Clone> TransitiveRelation<T> {
+impl<T: Clone + Debug + Eq + Hash> TransitiveRelation<T> {
     pub fn new() -> TransitiveRelation<T> {
         TransitiveRelation {
             elements: vec![],
             map: FxHashMap(),
             edges: vec![],
-            closure: RefCell::new(None),
+            closure: Lock::new(None),
         }
     }
 
@@ -72,7 +72,7 @@ impl<T: Clone + Debug + Eq + Hash + Clone> TransitiveRelation<T> {
     fn add_index(&mut self, a: T) -> Index {
         let &mut TransitiveRelation {
             ref mut elements,
-            ref closure,
+            ref mut closure,
             ref mut map,
             ..
         } = self;
@@ -82,7 +82,7 @@ impl<T: Clone + Debug + Eq + Hash + Clone> TransitiveRelation<T> {
                elements.push(a);
 
                // if we changed the dimensions, clear the cache
-               *closure.borrow_mut() = None;
+               *closure.get_mut() = None;
 
                Index(elements.len() - 1)
            })
@@ -122,7 +122,7 @@ impl<T: Clone + Debug + Eq + Hash + Clone> TransitiveRelation<T> {
             self.edges.push(edge);
 
             // added an edge, clear the cache
-            *self.closure.borrow_mut() = None;
+            *self.closure.get_mut() = None;
         }
     }
 
@@ -443,7 +443,7 @@ impl<T> Decodable for TransitiveRelation<T>
                               .enumerate()
                               .map(|(index, elem)| (elem.clone(), Index(index)))
                               .collect();
-            Ok(TransitiveRelation { elements, edges, map, closure: RefCell::new(None) })
+            Ok(TransitiveRelation { elements, edges, map, closure: Lock::new(None) })
         })
     }
 }
diff --git a/src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs b/src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs
index 948c1ac0b1362..b6d8e14b74757 100644
--- a/src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs
@@ -176,7 +176,7 @@ impl<'gcx, 'tcx> UseFinder<'gcx, 'tcx> {
         None
     }
 
-    fn def_use(&self, location: Location, thing: &MirVisitable<'tcx>) -> (bool, bool) {
+    fn def_use(&self, location: Location, thing: &dyn MirVisitable<'tcx>) -> (bool, bool) {
         let mut visitor = DefUseVisitor {
             defined: false,
             used: false,
diff --git a/src/librustc_mir/borrow_check/nll/mod.rs b/src/librustc_mir/borrow_check/nll/mod.rs
index 07e5091da9c1e..dbfb8a6d06e63 100644
--- a/src/librustc_mir/borrow_check/nll/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/mod.rs
@@ -260,10 +260,10 @@ fn dump_annotation<'a, 'gcx, 'tcx>(
 
 fn for_each_region_constraint(
     closure_region_requirements: &ClosureRegionRequirements,
-    with_msg: &mut FnMut(&str) -> io::Result<()>,
+    with_msg: &mut dyn FnMut(&str) -> io::Result<()>,
 ) -> io::Result<()> {
     for req in &closure_region_requirements.outlives_requirements {
-        let subject: &Debug = match &req.subject {
+        let subject: &dyn Debug = match &req.subject {
             ClosureOutlivesSubject::Region(subject) => subject,
             ClosureOutlivesSubject::Ty(ty) => ty,
         };
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs b/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs
index 69ecafa66aee7..631b1d0f8941d 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs
@@ -23,7 +23,7 @@ const REGION_WIDTH: usize = 8;
 
 impl<'tcx> RegionInferenceContext<'tcx> {
     /// Write out our state into the `.mir` files.
-    pub(crate) fn dump_mir(&self, out: &mut Write) -> io::Result<()> {
+    pub(crate) fn dump_mir(&self, out: &mut dyn Write) -> io::Result<()> {
         writeln!(out, "| Free Region Mapping")?;
 
         for region in self.regions() {
@@ -67,7 +67,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// inference resulted in the values that it did when debugging.
     fn for_each_constraint(
         &self,
-        with_msg: &mut FnMut(&str) -> io::Result<()>,
+        with_msg: &mut dyn FnMut(&str) -> io::Result<()>,
     ) -> io::Result<()> {
         for region in self.definitions.indices() {
             let value = self.liveness_constraints.region_value_str(region);
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/graphviz.rs b/src/librustc_mir/borrow_check/nll/region_infer/graphviz.rs
index 7923b159d80dc..db773240809c5 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/graphviz.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/graphviz.rs
@@ -20,7 +20,7 @@ use super::*;
 
 impl<'tcx> RegionInferenceContext<'tcx> {
     /// Write out the region constraint graph.
-    pub(crate) fn dump_graphviz(&self, mut w: &mut Write) -> io::Result<()> {
+    pub(crate) fn dump_graphviz(&self, mut w: &mut dyn Write) -> io::Result<()> {
         dot::render(self, &mut w)
     }
 }
diff --git a/src/librustc_mir/borrow_check/nll/type_check/mod.rs b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
index d57b8e78f18a9..a06d39d225c45 100644
--- a/src/librustc_mir/borrow_check/nll/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
@@ -127,7 +127,7 @@ fn type_check_internal<'gcx, 'tcx>(
     mir: &Mir<'tcx>,
     region_bound_pairs: &[(ty::Region<'tcx>, GenericKind<'tcx>)],
     implicit_region_bound: Option<ty::Region<'tcx>>,
-    extra: &mut FnMut(&mut TypeChecker<'_, 'gcx, 'tcx>),
+    extra: &mut dyn FnMut(&mut TypeChecker<'_, 'gcx, 'tcx>),
 ) -> MirTypeckRegionConstraints<'tcx> {
     let mut checker = TypeChecker::new(
         infcx,
@@ -231,7 +231,7 @@ impl<'a, 'b, 'gcx, 'tcx> TypeVerifier<'a, 'b, 'gcx, 'tcx> {
         self.cx.infcx.tcx
     }
 
-    fn sanitize_type(&mut self, parent: &fmt::Debug, ty: Ty<'tcx>) -> Ty<'tcx> {
+    fn sanitize_type(&mut self, parent: &dyn fmt::Debug, ty: Ty<'tcx>) -> Ty<'tcx> {
         if ty.has_escaping_regions() || ty.references_error() {
             span_mirbug_and_err!(self, parent, "bad type {:?}", ty)
         } else {
@@ -516,7 +516,7 @@ impl<'a, 'b, 'gcx, 'tcx> TypeVerifier<'a, 'b, 'gcx, 'tcx> {
 
     fn field_ty(
         &mut self,
-        parent: &fmt::Debug,
+        parent: &dyn fmt::Debug,
         base_ty: PlaceTy<'tcx>,
         field: Field,
         location: Location,
@@ -1171,7 +1171,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
     fn assert_iscleanup(
         &mut self,
         mir: &Mir<'tcx>,
-        ctxt: &fmt::Debug,
+        ctxt: &dyn fmt::Debug,
         bb: BasicBlock,
         iscleanuppad: bool,
     ) {
diff --git a/src/librustc_mir/dataflow/mod.rs b/src/librustc_mir/dataflow/mod.rs
index 9c7d9b398cc56..aa7bb6f97786c 100644
--- a/src/librustc_mir/dataflow/mod.rs
+++ b/src/librustc_mir/dataflow/mod.rs
@@ -61,7 +61,7 @@ pub(crate) struct DataflowBuilder<'a, 'tcx: 'a, BD> where BD: BitDenotation
 pub(crate) struct DebugFormatted(String);
 
 impl DebugFormatted {
-    pub fn new(input: &fmt::Debug) -> DebugFormatted {
+    pub fn new(input: &dyn fmt::Debug) -> DebugFormatted {
         DebugFormatted(format!("{:?}", input))
     }
 }
diff --git a/src/librustc_mir/interpret/const_eval.rs b/src/librustc_mir/interpret/const_eval.rs
index d3b084fde6ab8..bc555368f0f5f 100644
--- a/src/librustc_mir/interpret/const_eval.rs
+++ b/src/librustc_mir/interpret/const_eval.rs
@@ -168,7 +168,7 @@ impl Error for ConstEvalError {
         }
     }
 
-    fn cause(&self) -> Option<&Error> {
+    fn cause(&self) -> Option<&dyn Error> {
         None
     }
 }
diff --git a/src/librustc_mir/lib.rs b/src/librustc_mir/lib.rs
index 9887ac82a2ea6..8c15d1cf8b03a 100644
--- a/src/librustc_mir/lib.rs
+++ b/src/librustc_mir/lib.rs
@@ -15,7 +15,6 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 */
 
 #![deny(warnings)]
-#![cfg_attr(not(stage0), allow(bare_trait_object))]
 
 #![feature(box_patterns)]
 #![feature(box_syntax)]
diff --git a/src/librustc_mir/transform/dump_mir.rs b/src/librustc_mir/transform/dump_mir.rs
index 98753eaa5a354..a16ef2adea9be 100644
--- a/src/librustc_mir/transform/dump_mir.rs
+++ b/src/librustc_mir/transform/dump_mir.rs
@@ -49,7 +49,7 @@ impl fmt::Display for Disambiguator {
 
 
 pub fn on_mir_pass<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                             pass_num: &fmt::Display,
+                             pass_num: &dyn fmt::Display,
                              pass_name: &str,
                              source: MirSource,
                              mir: &Mir<'tcx>,
diff --git a/src/librustc_mir/transform/mod.rs b/src/librustc_mir/transform/mod.rs
index 54a7a0eafe0af..24a2790b1e79a 100644
--- a/src/librustc_mir/transform/mod.rs
+++ b/src/librustc_mir/transform/mod.rs
@@ -161,7 +161,7 @@ pub macro run_passes($tcx:ident, $mir:ident, $def_id:ident, $suite_index:expr; $
             promoted
         };
         let mut index = 0;
-        let mut run_pass = |pass: &MirPass| {
+        let mut run_pass = |pass: &dyn MirPass| {
             let run_hooks = |mir: &_, index, is_after| {
                 dump_mir::on_mir_pass($tcx, &format_args!("{:03}-{:03}", suite_index, index),
                                       &pass.name(), source, mir, is_after);
diff --git a/src/librustc_mir/util/liveness.rs b/src/librustc_mir/util/liveness.rs
index 6251b64bb279d..42ddabddd2dcd 100644
--- a/src/librustc_mir/util/liveness.rs
+++ b/src/librustc_mir/util/liveness.rs
@@ -425,12 +425,12 @@ pub fn write_mir_fn<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     src: MirSource,
     mir: &Mir<'tcx>,
-    w: &mut Write,
+    w: &mut dyn Write,
     result: &LivenessResult,
 ) -> io::Result<()> {
     write_mir_intro(tcx, src, mir, w)?;
     for block in mir.basic_blocks().indices() {
-        let print = |w: &mut Write, prefix, result: &IndexVec<BasicBlock, LocalSet>| {
+        let print = |w: &mut dyn Write, prefix, result: &IndexVec<BasicBlock, LocalSet>| {
             let live: Vec<String> = mir.local_decls
                 .indices()
                 .filter(|i| result[block].contains(i))
diff --git a/src/librustc_mir/util/pretty.rs b/src/librustc_mir/util/pretty.rs
index f326b6e9274fc..4509cace794d4 100644
--- a/src/librustc_mir/util/pretty.rs
+++ b/src/librustc_mir/util/pretty.rs
@@ -72,14 +72,14 @@ pub enum PassWhere {
 ///   or `typeck` and `bar` both appear in the name.
 pub fn dump_mir<'a, 'gcx, 'tcx, F>(
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
-    pass_num: Option<&Display>,
+    pass_num: Option<&dyn Display>,
     pass_name: &str,
-    disambiguator: &Display,
+    disambiguator: &dyn Display,
     source: MirSource,
     mir: &Mir<'tcx>,
     extra_data: F,
 ) where
-    F: FnMut(PassWhere, &mut Write) -> io::Result<()>,
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
 {
     if !dump_enabled(tcx, pass_name, source) {
         return;
@@ -127,15 +127,15 @@ pub fn dump_enabled<'a, 'gcx, 'tcx>(
 
 fn dump_matched_mir_node<'a, 'gcx, 'tcx, F>(
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
-    pass_num: Option<&Display>,
+    pass_num: Option<&dyn Display>,
     pass_name: &str,
     node_path: &str,
-    disambiguator: &Display,
+    disambiguator: &dyn Display,
     source: MirSource,
     mir: &Mir<'tcx>,
     mut extra_data: F,
 ) where
-    F: FnMut(PassWhere, &mut Write) -> io::Result<()>,
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
 {
     let _: io::Result<()> = do catch {
         let mut file = create_dump_file(tcx, "mir", pass_num, pass_name, disambiguator, source)?;
@@ -169,9 +169,9 @@ fn dump_matched_mir_node<'a, 'gcx, 'tcx, F>(
 fn dump_path(
     tcx: TyCtxt<'_, '_, '_>,
     extension: &str,
-    pass_num: Option<&Display>,
+    pass_num: Option<&dyn Display>,
     pass_name: &str,
-    disambiguator: &Display,
+    disambiguator: &dyn Display,
     source: MirSource,
 ) -> PathBuf {
     let promotion_id = match source.promoted {
@@ -217,9 +217,9 @@ fn dump_path(
 pub(crate) fn create_dump_file(
     tcx: TyCtxt<'_, '_, '_>,
     extension: &str,
-    pass_num: Option<&Display>,
+    pass_num: Option<&dyn Display>,
     pass_name: &str,
-    disambiguator: &Display,
+    disambiguator: &dyn Display,
     source: MirSource,
 ) -> io::Result<fs::File> {
     let file_path = dump_path(tcx, extension, pass_num, pass_name, disambiguator, source);
@@ -233,7 +233,7 @@ pub(crate) fn create_dump_file(
 pub fn write_mir_pretty<'a, 'gcx, 'tcx>(
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
     single: Option<DefId>,
-    w: &mut Write,
+    w: &mut dyn Write,
 ) -> io::Result<()> {
     writeln!(
         w,
@@ -274,10 +274,10 @@ pub fn write_mir_fn<'a, 'gcx, 'tcx, F>(
     src: MirSource,
     mir: &Mir<'tcx>,
     extra_data: &mut F,
-    w: &mut Write,
+    w: &mut dyn Write,
 ) -> io::Result<()>
 where
-    F: FnMut(PassWhere, &mut Write) -> io::Result<()>,
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
 {
     write_mir_intro(tcx, src, mir, w)?;
     for block in mir.basic_blocks().indices() {
@@ -298,10 +298,10 @@ pub fn write_basic_block<'cx, 'gcx, 'tcx, F>(
     block: BasicBlock,
     mir: &Mir<'tcx>,
     extra_data: &mut F,
-    w: &mut Write,
+    w: &mut dyn Write,
 ) -> io::Result<()>
 where
-    F: FnMut(PassWhere, &mut Write) -> io::Result<()>,
+    F: FnMut(PassWhere, &mut dyn Write) -> io::Result<()>,
 {
     let data = &mir[block];
 
@@ -362,7 +362,7 @@ where
 /// a statement.
 fn write_extra<'cx, 'gcx, 'tcx, F>(
     tcx: TyCtxt<'cx, 'gcx, 'tcx>,
-    write: &mut Write,
+    write: &mut dyn Write,
     mut visit_op: F,
 ) -> io::Result<()>
 where
@@ -450,7 +450,7 @@ fn write_scope_tree(
     tcx: TyCtxt,
     mir: &Mir,
     scope_tree: &FxHashMap<VisibilityScope, Vec<VisibilityScope>>,
-    w: &mut Write,
+    w: &mut dyn Write,
     parent: VisibilityScope,
     depth: usize,
 ) -> io::Result<()> {
@@ -515,7 +515,7 @@ pub fn write_mir_intro<'a, 'gcx, 'tcx>(
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
     src: MirSource,
     mir: &Mir,
-    w: &mut Write,
+    w: &mut dyn Write,
 ) -> io::Result<()> {
     write_mir_sig(tcx, src, mir, w)?;
     writeln!(w, "{{")?;
@@ -553,7 +553,7 @@ pub fn write_mir_intro<'a, 'gcx, 'tcx>(
     Ok(())
 }
 
-fn write_mir_sig(tcx: TyCtxt, src: MirSource, mir: &Mir, w: &mut Write) -> io::Result<()> {
+fn write_mir_sig(tcx: TyCtxt, src: MirSource, mir: &Mir, w: &mut dyn Write) -> io::Result<()> {
     let id = tcx.hir.as_local_node_id(src.def_id).unwrap();
     let body_owner_kind = tcx.hir.body_owner_kind(id);
     match (body_owner_kind, src.promoted) {
@@ -597,7 +597,7 @@ fn write_mir_sig(tcx: TyCtxt, src: MirSource, mir: &Mir, w: &mut Write) -> io::R
     Ok(())
 }
 
-fn write_temp_decls(mir: &Mir, w: &mut Write) -> io::Result<()> {
+fn write_temp_decls(mir: &Mir, w: &mut dyn Write) -> io::Result<()> {
     // Compiler-introduced temporary types.
     for temp in mir.temps_iter() {
         writeln!(
diff --git a/src/librustdoc/README.md b/src/librustdoc/README.md
new file mode 100644
index 0000000000000..b0a5ae3718df3
--- /dev/null
+++ b/src/librustdoc/README.md
@@ -0,0 +1,172 @@
+# The walking tour of rustdoc
+
+Rustdoc is implemented entirely within the crate `librustdoc`. After partially compiling a crate to
+get its AST (technically the HIR map) from rustc, librustdoc performs two major steps past that to
+render a set of documentation:
+
+* "Clean" the AST into a form that's more suited to creating documentation (and slightly more
+  resistant to churn in the compiler).
+* Use this cleaned AST to render a crate's documentation, one page at a time.
+
+Naturally, there's more than just this, and those descriptions simplify out lots of details, but
+that's the high-level overview.
+
+(Side note: this is a library crate! The `rustdoc` binary is crated using the project in
+`src/tools/rustdoc`. Note that literally all that does is call the `main()` that's in this crate's
+`lib.rs`, though.)
+
+## Cheat sheet
+
+* Use `x.py build --stage 1 src/libstd src/tools/rustdoc` to make a useable rustdoc you can run on
+  other projects.
+  * Add `src/libtest` to be able to use `rustdoc --test`.
+  * If you've used `rustup toolchain link local /path/to/build/$TARGET/stage1` previously, then
+    after the previous build command, `cargo +local doc` will Just Work.
+* Use `x.py doc --stage 1 src/libstd` to use this rustdoc to generate the standard library docs.
+  * The completed docs will be available in `build/$TARGET/doc/std`, though the bundle is meant to
+    be used as though you would copy out the `doc` folder to a web server, since that's where the
+    CSS/JS and landing page are.
+* Most of the HTML printing code is in `html/format.rs` and `html/render.rs`. It's in a bunch of
+  `fmt::Display` implementations and supplementary functions.
+* The types that got `Display` impls above are defined in `clean/mod.rs`, right next to the custom
+  `Clean` trait used to process them out of the rustc HIR.
+* The bits specific to using rustdoc as a test harness are in `test.rs`.
+* The Markdown renderer is loaded up in `html/markdown.rs`, including functions for extracting
+  doctests from a given block of Markdown.
+* The tests on rustdoc *output* are located in `src/test/rustdoc`, where they're handled by the test
+  runner of rustbuild and the supplementary script `src/etc/htmldocck.py`.
+* Tests on search index generation are located in `src/test/rustdoc-js`, as a series of JavaScript
+  files that encode queries on the standard library search index and expected results.
+
+## From crate to clean
+
+In `core.rs` are two central items: the `DocContext` struct, and the `run_core` function. The latter
+is where rustdoc calls out to rustc to compile a crate to the point where rustdoc can take over. The
+former is a state container used when crawling through a crate to gather its documentation.
+
+The main process of crate crawling is done in `clean/mod.rs` through several implementations of the
+`Clean` trait defined within. This is a conversion trait, which defines one method:
+
+```rust
+pub trait Clean<T> {
+    fn clean(&self, cx: &DocContext) -> T;
+}
+```
+
+`clean/mod.rs` also defines the types for the "cleaned" AST used later on to render documentation
+pages. Each usually accompanies an implementation of `Clean` that takes some AST or HIR type from
+rustc and converts it into the appropriate "cleaned" type. "Big" items like modules or associated
+items may have some extra processing in its `Clean` implementation, but for the most part these
+impls are straightforward conversions. The "entry point" to this module is the `impl Clean<Crate>
+for visit_ast::RustdocVisitor`, which is called by `run_core` above.
+
+You see, I actually lied a little earlier: There's another AST transformation that happens before
+the events in `clean/mod.rs`.  In `visit_ast.rs` is the type `RustdocVisitor`, which *actually*
+crawls a `hir::Crate` to get the first intermediate representation, defined in `doctree.rs`. This
+pass is mainly to get a few intermediate wrappers around the HIR types and to process visibility
+and inlining. This is where `#[doc(inline)]`, `#[doc(no_inline)]`, and `#[doc(hidden)]` are
+processed, as well as the logic for whether a `pub use` should get the full page or a "Reexport"
+line in the module page.
+
+The other major thing that happens in `clean/mod.rs` is the collection of doc comments and
+`#[doc=""]` attributes into a separate field of the Attributes struct, present on anything that gets
+hand-written documentation. This makes it easier to collect this documentation later in the process.
+
+The primary output of this process is a clean::Crate with a tree of Items which describe the
+publicly-documentable items in the target crate.
+
+### Hot potato
+
+Before moving on to the next major step, a few important "passes" occur over the documentation.
+These do things like combine the separate "attributes" into a single string and strip leading
+whitespace to make the document easier on the markdown parser, or drop items that are not public or
+deliberately hidden with `#[doc(hidden)]`. These are all implemented in the `passes/` directory, one
+file per pass. By default, all of these passes are run on a crate, but the ones regarding dropping
+private/hidden items can be bypassed by passing `--document-private-items` to rustdoc.
+
+(Strictly speaking, you can fine-tune the passes run and even add your own, but [we're trying to
+deprecate that][44136]. If you need finer-grain control over these passes, please let us know!)
+
+[44136]: https://github.com/rust-lang/rust/issues/44136
+
+## From clean to crate
+
+This is where the "second phase" in rustdoc begins. This phase primarily lives in the `html/`
+folder, and it all starts with `run()` in `html/render.rs`. This code is responsible for setting up
+the `Context`, `SharedContext`, and `Cache` which are used during rendering, copying out the static
+files which live in every rendered set of documentation (things like the fonts, CSS, and JavaScript
+that live in `html/static/`), creating the search index, and printing out the source code rendering,
+before beginning the process of rendering all the documentation for the crate.
+
+Several functions implemented directly on `Context` take the `clean::Crate` and set up some state
+between rendering items or recursing on a module's child items. From here the "page rendering"
+begins, via an enormous `write!()` call in `html/layout.rs`. The parts that actually generate HTML
+from the items and documentation occurs within a series of `std::fmt::Display` implementations and
+functions that pass around a `&mut std::fmt::Formatter`. The top-level implementation that writes
+out the page body is the `impl<'a> fmt::Display for Item<'a>` in `html/render.rs`, which switches
+out to one of several `item_*` functions based on the kind of `Item` being rendered.
+
+Depending on what kind of rendering code you're looking for, you'll probably find it either in
+`html/render.rs` for major items like "what sections should I print for a struct page" or
+`html/format.rs` for smaller component pieces like "how should I print a where clause as part of
+some other item".
+
+Whenever rustdoc comes across an item that should print hand-written documentation alongside, it
+calls out to `html/markdown.rs` which interfaces with the Markdown parser. This is exposed as a
+series of types that wrap a string of Markdown, and implement `fmt::Display` to emit HTML text. It
+takes special care to enable certain features like footnotes and tables and add syntax highlighting
+to Rust code blocks (via `html/highlight.rs`) before running the Markdown parser. There's also a
+function in here (`find_testable_code`) that specifically scans for Rust code blocks so the
+test-runner code can find all the doctests in the crate.
+
+### From soup to nuts
+
+(alternate title: ["An unbroken thread that stretches from those first `Cell`s to us"][video])
+
+[video]: https://www.youtube.com/watch?v=hOLAGYmUQV0
+
+It's important to note that the AST cleaning can ask the compiler for information (crucially,
+`DocContext` contains a `TyCtxt`), but page rendering cannot. The `clean::Crate` created within
+`run_core` is passed outside the compiler context before being handed to `html::render::run`. This
+means that a lot of the "supplementary data" that isn't immediately available inside an item's
+definition, like which trait is the `Deref` trait used by the language, needs to be collected during
+cleaning, stored in the `DocContext`, and passed along to the `SharedContext` during HTML rendering.
+This manifests as a bunch of shared state, context variables, and `RefCell`s.
+
+Also of note is that some items that come from "asking the compiler" don't go directly into the
+`DocContext` - for example, when loading items from a foreign crate, rustdoc will ask about trait
+implementations and generate new `Item`s for the impls based on that information. This goes directly
+into the returned `Crate` rather than roundabout through the `DocContext`. This way, these
+implementations can be collected alongside the others, right before rendering the HTML.
+
+## Other tricks up its sleeve
+
+All this describes the process for generating HTML documentation from a Rust crate, but there are
+couple other major modes that rustdoc runs in. It can also be run on a standalone Markdown file, or
+it can run doctests on Rust code or standalone Markdown files. For the former, it shortcuts straight
+to `html/markdown.rs`, optionally including a mode which inserts a Table of Contents to the output
+HTML.
+
+For the latter, rustdoc runs a similar partial-compilation to get relevant documentation in
+`test.rs`, but instead of going through the full clean and render process, it runs a much simpler
+crate walk to grab *just* the hand-written documentation. Combined with the aforementioned
+"`find_testable_code`" in `html/markdown.rs`, it builds up a collection of tests to run before
+handing them off to the libtest test runner. One notable location in `test.rs` is the function
+`make_test`, which is where hand-written doctests get transformed into something that can be
+executed.
+
+## Dotting i's and crossing t's
+
+So that's rustdoc's code in a nutshell, but there's more things in the repo that deal with it. Since
+we have the full `compiletest` suite at hand, there's a set of tests in `src/test/rustdoc` that make
+sure the final HTML is what we expect in various situations. These tests also use a supplementary
+script, `src/etc/htmldocck.py`, that allows it to look through the final HTML using XPath notation
+to get a precise look at the output. The full description of all the commands available to rustdoc
+tests is in `htmldocck.py`.
+
+In addition, there are separate tests for the search index and rustdoc's ability to query it. The
+files in `src/test/rustdoc-js` each contain a different search query and the expected results,
+broken out by search tab. These files are processed by a script in `src/tools/rustdoc-js` and the
+Node.js runtime. These tests don't have as thorough of a writeup, but a broad example that features
+results in all tabs can be found in `basic.js`. The basic idea is that you match a given `QUERY`
+with a set of `EXPECTED` results, complete with the full item path of each item.
diff --git a/src/libstd_unicode/char.rs b/src/libstd_unicode/char.rs
index 844ff7a3c1252..5dd9c62775097 100644
--- a/src/libstd_unicode/char.rs
+++ b/src/libstd_unicode/char.rs
@@ -59,7 +59,7 @@ pub use version::UnicodeVersion;
 /// [`to_lowercase`]: ../../std/primitive.char.html#method.to_lowercase
 /// [`char`]: ../../std/primitive.char.html
 #[stable(feature = "rust1", since = "1.0.0")]
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct ToLowercase(CaseMappingIter);
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -81,7 +81,7 @@ impl FusedIterator for ToLowercase {}
 /// [`to_uppercase`]: ../../std/primitive.char.html#method.to_uppercase
 /// [`char`]: ../../std/primitive.char.html
 #[stable(feature = "rust1", since = "1.0.0")]
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct ToUppercase(CaseMappingIter);
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -95,7 +95,7 @@ impl Iterator for ToUppercase {
 #[unstable(feature = "fused", issue = "35602")]
 impl FusedIterator for ToUppercase {}
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 enum CaseMappingIter {
     Three(char, char, char),
     Two(char, char),