diff --git a/Cargo.lock b/Cargo.lock
index 0772dc6..0fe3138 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -201,7 +201,9 @@
 name = "build-manifest"
 version = "0.1.0"
 dependencies = [
+ "reqwest",
  "serde",
+ "serde_json",
  "toml",
 ]
 
diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index 500d576..076bcd8 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -2000,6 +2000,8 @@
     }
 
     fn run(self, builder: &Builder<'_>) {
+        // This gets called by `promote-release`
+        // (https://github.com/rust-lang/rust-central-station/tree/master/promote-release).
         let mut cmd = builder.tool_cmd(Tool::BuildManifest);
         if builder.config.dry_run {
             return;
@@ -2010,10 +2012,14 @@
         let addr = builder.config.dist_upload_addr.as_ref().unwrap_or_else(|| {
             panic!("\n\nfailed to specify `dist.upload-addr` in `config.toml`\n\n")
         });
-        let file = builder.config.dist_gpg_password_file.as_ref().unwrap_or_else(|| {
-            panic!("\n\nfailed to specify `dist.gpg-password-file` in `config.toml`\n\n")
-        });
-        let pass = t!(fs::read_to_string(&file));
+        let pass = if env::var("BUILD_MANIFEST_DISABLE_SIGNING").is_err() {
+            let file = builder.config.dist_gpg_password_file.as_ref().unwrap_or_else(|| {
+                panic!("\n\nfailed to specify `dist.gpg-password-file` in `config.toml`\n\n")
+            });
+            t!(fs::read_to_string(&file))
+        } else {
+            String::new()
+        };
 
         let today = output(Command::new("date").arg("+%Y-%m-%d"));
 
diff --git a/src/ci/azure-pipelines/steps/run.yml b/src/ci/azure-pipelines/steps/run.yml
index ac6b344..da0a899 100644
--- a/src/ci/azure-pipelines/steps/run.yml
+++ b/src/ci/azure-pipelines/steps/run.yml
@@ -147,8 +147,15 @@
     git clone --depth=1 https://github.com/rust-lang-nursery/rust-toolstate.git
     cd rust-toolstate
     python2.7 "$BUILD_SOURCESDIRECTORY/src/tools/publish_toolstate.py" "$(git rev-parse HEAD)" "$(git log --format=%s -n1 HEAD)" "" ""
+    # Only check maintainers if this build is supposed to publish toolstate.
+    # Builds that are not supposed to publish don't have the access token.
+    if [ -n "${TOOLSTATE_PUBLISH+is_set}" ]; then
+      TOOLSTATE_VALIDATE_MAINTAINERS_REPO=rust-lang/rust python2.7 "${BUILD_SOURCESDIRECTORY}/src/tools/publish_toolstate.py"
+    fi
     cd ..
     rm -rf rust-toolstate
+  env:
+    TOOLSTATE_REPO_ACCESS_TOKEN: $(TOOLSTATE_REPO_ACCESS_TOKEN)
   condition: and(succeeded(), not(variables.SKIP_JOB), eq(variables['IMAGE'], 'mingw-check'))
   displayName: Verify the publish_toolstate script works
 
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 370e5cf..9e6ed92 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -117,7 +117,7 @@
 #![feature(allocator_internals)]
 #![feature(on_unimplemented)]
 #![feature(rustc_const_unstable)]
-#![feature(const_vec_new)]
+#![cfg_attr(bootstrap, feature(const_vec_new))]
 #![feature(slice_partition_dedup)]
 #![feature(maybe_uninit_extra, maybe_uninit_slice)]
 #![feature(alloc_layout_extra)]
diff --git a/src/liballoc/raw_vec.rs b/src/liballoc/raw_vec.rs
index cf025ee..ee75fc2 100644
--- a/src/liballoc/raw_vec.rs
+++ b/src/liballoc/raw_vec.rs
@@ -113,13 +113,38 @@
 }
 
 impl<T> RawVec<T, Global> {
+    /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
+    /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
+    ///
+    /// If you change `RawVec<T>::new` or dependencies, please take care to not
+    /// introduce anything that would truly violate `min_const_fn`.
+    ///
+    /// NOTE: We could avoid this hack and check conformance with some
+    /// `#[rustc_force_min_const_fn]` attribute which requires conformance
+    /// with `min_const_fn` but does not necessarily allow calling it in
+    /// `stable(...) const fn` / user code not enabling `foo` when
+    /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
+    pub const NEW: Self = Self::new();
+
     /// Creates the biggest possible `RawVec` (on the system heap)
     /// without allocating. If `T` has positive size, then this makes a
     /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
     /// `RawVec` with capacity `usize::MAX`. Useful for implementing
     /// delayed allocation.
     pub const fn new() -> Self {
-        Self::new_in(Global)
+        // FIXME(Centril): Reintegrate this with `fn new_in` when we can.
+
+        // `!0` is `usize::MAX`. This branch should be stripped at compile time.
+        // FIXME(mark-i-m): use this line when `if`s are allowed in `const`:
+        //let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
+
+        // `Unique::empty()` doubles as "unallocated" and "zero-sized allocation".
+        RawVec {
+            ptr: Unique::empty(),
+            // FIXME(mark-i-m): use `cap` when ifs are allowed in const
+            cap: [0, !0][(mem::size_of::<T>() == 0) as usize],
+            a: Global,
+        }
     }
 
     /// Creates a `RawVec` (on the system heap) with exactly the
diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index b65f191..1166e7b 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -369,7 +369,7 @@
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_string_new")]
+    #[cfg_attr(bootstrap, rustc_const_unstable(feature = "const_string_new"))]
     pub const fn new() -> String {
         String { vec: Vec::new() }
     }
diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index c513658..405969a 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -314,10 +314,10 @@
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_vec_new")]
+    #[cfg_attr(bootstrap, rustc_const_unstable(feature = "const_vec_new"))]
     pub const fn new() -> Vec<T> {
         Vec {
-            buf: RawVec::new(),
+            buf: RawVec::NEW,
             len: 0,
         }
     }
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index c74b2fe..723855c 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -7,6 +7,7 @@
 use crate::session::search_paths::SearchPath;
 
 use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::sync::Lrc;
 
 use rustc_target::spec::{LinkerFlavor, MergeFunctions, PanicStrategy, RelroLevel};
 use rustc_target::spec::{Target, TargetTriple};
@@ -19,6 +20,7 @@
 use syntax::parse::token;
 use syntax::symbol::{sym, Symbol};
 use syntax::feature_gate::UnstableFeatures;
+use syntax::source_map::SourceMap;
 
 use errors::emitter::HumanReadableErrorType;
 use errors::{ColorConfig, FatalError, Handler};
@@ -1850,11 +1852,20 @@
     opts
 }
 
+struct NullEmitter;
+
+impl errors::emitter::Emitter for NullEmitter {
+    fn emit_diagnostic(&mut self, _: &errors::DiagnosticBuilder<'_>) {}
+}
+
 // Converts strings provided as `--cfg [cfgspec]` into a `crate_cfg`.
 pub fn parse_cfgspecs(cfgspecs: Vec<String>) -> FxHashSet<(String, Option<String>)> {
     syntax::with_default_globals(move || {
         let cfg = cfgspecs.into_iter().map(|s| {
-            let sess = ParseSess::new(FilePathMapping::empty());
+
+            let cm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
+            let handler = Handler::with_emitter(false, None, Box::new(NullEmitter));
+            let sess = ParseSess::with_span_handler(handler, cm);
             let filename = FileName::cfg_spec_source_code(&s);
             let mut parser = new_parser_from_source_str(&sess, filename, s.to_string());
 
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 17eb4a8..5ca819e 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -1938,9 +1938,15 @@
     pub vis: Visibility,
 }
 
-/// The definition of an abstract data type -- a struct or enum.
+/// The definition of a user-defined type, e.g., a `struct`, `enum`, or `union`.
 ///
 /// These are all interned (by `intern_adt_def`) into the `adt_defs` table.
+///
+/// The initialism *"Adt"* stands for an [*algebraic data type (ADT)*][adt].
+/// This is slightly wrong because `union`s are not ADTs.
+/// Moreover, Rust only allows recursive data types through indirection.
+///
+/// [adt]: https://en.wikipedia.org/wiki/Algebraic_data_type
 pub struct AdtDef {
     /// `DefId` of the struct, enum or union item.
     pub did: DefId,
diff --git a/src/librustc_asan/build.rs b/src/librustc_asan/build.rs
index cc856ba..645707c 100644
--- a/src/librustc_asan/build.rs
+++ b/src/librustc_asan/build.rs
@@ -4,6 +4,7 @@
 use cmake::Config;
 
 fn main() {
+    println!("cargo:rerun-if-env-changed=RUSTC_BUILD_SANITIZERS");
     if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
         return;
     }
diff --git a/src/librustc_codegen_llvm/lib.rs b/src/librustc_codegen_llvm/lib.rs
index 9f2c303..34e39af 100644
--- a/src/librustc_codegen_llvm/lib.rs
+++ b/src/librustc_codegen_llvm/lib.rs
@@ -226,21 +226,21 @@
                 for &(name, _) in back::write::RELOC_MODEL_ARGS.iter() {
                     println!("    {}", name);
                 }
-                println!("");
+                println!();
             }
             PrintRequest::CodeModels => {
                 println!("Available code models:");
                 for &(name, _) in back::write::CODE_GEN_MODEL_ARGS.iter(){
                     println!("    {}", name);
                 }
-                println!("");
+                println!();
             }
             PrintRequest::TlsModels => {
                 println!("Available TLS models:");
                 for &(name, _) in back::write::TLS_MODEL_ARGS.iter(){
                     println!("    {}", name);
                 }
-                println!("");
+                println!();
             }
             req => llvm_util::print(req, sess),
         }
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 005cdcf..8c5d853 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -134,8 +134,11 @@
 
 impl Callbacks for TimePassesCallbacks {
     fn config(&mut self, config: &mut interface::Config) {
+        // If a --prints=... option has been given, we don't print the "total"
+        // time because it will mess up the --prints output. See #64339.
         self.time_passes =
-            config.opts.debugging_opts.time_passes || config.opts.debugging_opts.time;
+            config.opts.prints.is_empty() &&
+            (config.opts.debugging_opts.time_passes || config.opts.debugging_opts.time);
     }
 }
 
diff --git a/src/librustc_lsan/build.rs b/src/librustc_lsan/build.rs
index d5f3e37..73720d8 100644
--- a/src/librustc_lsan/build.rs
+++ b/src/librustc_lsan/build.rs
@@ -4,6 +4,7 @@
 use cmake::Config;
 
 fn main() {
+    println!("cargo:rerun-if-env-changed=RUSTC_BUILD_SANITIZERS");
     if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
         return;
     }
diff --git a/src/librustc_mir/const_eval.rs b/src/librustc_mir/const_eval.rs
index 57ddaa4..3f53f84 100644
--- a/src/librustc_mir/const_eval.rs
+++ b/src/librustc_mir/const_eval.rs
@@ -134,9 +134,8 @@
     ecx: &mut CompileTimeEvalContext<'mir, 'tcx>,
     cid: GlobalId<'tcx>,
     body: &'mir mir::Body<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
 ) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
-    debug!("eval_body_using_ecx: {:?}, {:?}", cid, param_env);
+    debug!("eval_body_using_ecx: {:?}, {:?}", cid, ecx.param_env);
     let tcx = ecx.tcx.tcx;
     let layout = ecx.layout_of(body.return_ty().subst(tcx, cid.instance.substs))?;
     assert!(!layout.is_unsized());
@@ -162,7 +161,6 @@
         ecx,
         cid.instance.def_id(),
         ret,
-        param_env,
     )?;
 
     debug!("eval_body_using_ecx done: {:?}", *ret);
@@ -658,7 +656,7 @@
 
     let res = ecx.load_mir(cid.instance.def, cid.promoted);
     res.and_then(
-        |body| eval_body_using_ecx(&mut ecx, cid, body, key.param_env)
+        |body| eval_body_using_ecx(&mut ecx, cid, body)
     ).and_then(|place| {
         Ok(RawConst {
             alloc_id: place.ptr.assert_ptr().alloc_id,
diff --git a/src/librustc_mir/interpret/intern.rs b/src/librustc_mir/interpret/intern.rs
index 4cbbc0f..95647ce 100644
--- a/src/librustc_mir/interpret/intern.rs
+++ b/src/librustc_mir/interpret/intern.rs
@@ -3,7 +3,7 @@
 //! After a const evaluation has computed a value, before we destroy the const evaluator's session
 //! memory, we need to extract all memory allocations to the global memory pool so they stay around.
 
-use rustc::ty::{Ty, TyCtxt, ParamEnv, self};
+use rustc::ty::{Ty, self};
 use rustc::mir::interpret::{InterpResult, ErrorHandled};
 use rustc::hir;
 use rustc::hir::def_id::DefId;
@@ -11,32 +11,29 @@
 use rustc_data_structures::fx::FxHashSet;
 
 use syntax::ast::Mutability;
-use syntax_pos::Span;
 
 use super::{
-    ValueVisitor, MemoryKind, Pointer, AllocId, MPlaceTy, Scalar,
+    ValueVisitor, MemoryKind, AllocId, MPlaceTy, Scalar,
 };
 use crate::const_eval::{CompileTimeInterpreter, CompileTimeEvalContext};
 
 struct InternVisitor<'rt, 'mir, 'tcx> {
-    /// previously encountered safe references
-    ref_tracking: &'rt mut RefTracking<(MPlaceTy<'tcx>, Mutability, InternMode)>,
+    /// The ectx from which we intern.
     ecx: &'rt mut CompileTimeEvalContext<'mir, 'tcx>,
-    param_env: ParamEnv<'tcx>,
+    /// Previously encountered safe references.
+    ref_tracking: &'rt mut RefTracking<(MPlaceTy<'tcx>, Mutability, InternMode)>,
+    /// A list of all encountered allocations. After type-based interning, we traverse this list to
+    /// also intern allocations that are only referenced by a raw pointer or inside a union.
+    leftover_allocations: &'rt mut FxHashSet<AllocId>,
     /// The root node of the value that we're looking at. This field is never mutated and only used
     /// for sanity assertions that will ICE when `const_qualif` screws up.
     mode: InternMode,
     /// This field stores the mutability of the value *currently* being checked.
-    /// It is set to mutable when an `UnsafeCell` is encountered
-    /// When recursing across a reference, we don't recurse but store the
-    /// value to be checked in `ref_tracking` together with the mutability at which we are checking
-    /// the value.
-    /// When encountering an immutable reference, we treat everything as immutable that is behind
-    /// it.
+    /// When encountering a mutable reference, we determine the pointee mutability
+    /// taking into account the mutability of the context: `& &mut i32` is entirely immutable,
+    /// despite the nested mutable reference!
+    /// The field gets updated when an `UnsafeCell` is encountered.
     mutability: Mutability,
-    /// A list of all encountered relocations. After type-based interning, we traverse this list to
-    /// also intern allocations that are only referenced by a raw pointer or inside a union.
-    leftover_relocations: &'rt mut FxHashSet<AllocId>,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Hash, Eq)]
@@ -45,9 +42,10 @@
     /// `static`. In a `static mut` we start out as mutable and thus can also contain further `&mut`
     /// that will actually be treated as mutable.
     Static,
-    /// UnsafeCell is OK in the value of a constant, but not behind references in a constant
+    /// UnsafeCell is OK in the value of a constant: `const FOO = Cell::new(0)` creates
+    /// a new cell every time it is used.
     ConstBase,
-    /// `UnsafeCell` ICEs
+    /// `UnsafeCell` ICEs.
     Const,
 }
 
@@ -55,48 +53,100 @@
 /// into the memory of other constants or statics
 struct IsStaticOrFn;
 
+/// Intern an allocation without looking at its children.
+/// `mode` is the mode of the environment where we found this pointer.
+/// `mutablity` is the mutability of the place to be interned; even if that says
+/// `immutable` things might become mutable if `ty` is not frozen.
+/// `ty` can be `None` if there is no potential interior mutability
+/// to account for (e.g. for vtables).
+fn intern_shallow<'rt, 'mir, 'tcx>(
+    ecx: &'rt mut CompileTimeEvalContext<'mir, 'tcx>,
+    leftover_allocations: &'rt mut FxHashSet<AllocId>,
+    mode: InternMode,
+    alloc_id: AllocId,
+    mutability: Mutability,
+    ty: Option<Ty<'tcx>>,
+) -> InterpResult<'tcx, Option<IsStaticOrFn>> {
+    trace!(
+        "InternVisitor::intern {:?} with {:?}",
+        alloc_id, mutability,
+    );
+    // remove allocation
+    let tcx = ecx.tcx;
+    let memory = ecx.memory_mut();
+    let (kind, mut alloc) = match memory.alloc_map.remove(&alloc_id) {
+        Some(entry) => entry,
+        None => {
+            // Pointer not found in local memory map. It is either a pointer to the global
+            // map, or dangling.
+            // If the pointer is dangling (neither in local nor global memory), we leave it
+            // to validation to error. The `delay_span_bug` ensures that we don't forget such
+            // a check in validation.
+            if tcx.alloc_map.lock().get(alloc_id).is_none() {
+                tcx.sess.delay_span_bug(ecx.tcx.span, "tried to intern dangling pointer");
+            }
+            // treat dangling pointers like other statics
+            // just to stop trying to recurse into them
+            return Ok(Some(IsStaticOrFn));
+        },
+    };
+    // This match is just a canary for future changes to `MemoryKind`, which most likely need
+    // changes in this function.
+    match kind {
+        MemoryKind::Stack | MemoryKind::Vtable => {},
+    }
+    // Set allocation mutability as appropriate. This is used by LLVM to put things into
+    // read-only memory, and also by Miri when evluating other constants/statics that
+    // access this one.
+    if mode == InternMode::Static {
+        // When `ty` is `None`, we assume no interior mutability.
+        let frozen = ty.map_or(true, |ty| ty.is_freeze(
+            ecx.tcx.tcx,
+            ecx.param_env,
+            ecx.tcx.span,
+        ));
+        // For statics, allocation mutability is the combination of the place mutability and
+        // the type mutability.
+        // The entire allocation needs to be mutable if it contains an `UnsafeCell` anywhere.
+        if mutability == Mutability::Immutable && frozen {
+            alloc.mutability = Mutability::Immutable;
+        } else {
+            // Just making sure we are not "upgrading" an immutable allocation to mutable.
+            assert_eq!(alloc.mutability, Mutability::Mutable);
+        }
+    } else {
+        // We *could* be non-frozen at `ConstBase`, for constants like `Cell::new(0)`.
+        // But we still intern that as immutable as the memory cannot be changed once the
+        // initial value was computed.
+        // Constants are never mutable.
+        assert_eq!(
+            mutability, Mutability::Immutable,
+            "Something went very wrong: mutability requested for a constant"
+        );
+        alloc.mutability = Mutability::Immutable;
+    };
+    // link the alloc id to the actual allocation
+    let alloc = tcx.intern_const_alloc(alloc);
+    leftover_allocations.extend(alloc.relocations().iter().map(|&(_, ((), reloc))| reloc));
+    tcx.alloc_map.lock().set_alloc_id_memory(alloc_id, alloc);
+    Ok(None)
+}
+
 impl<'rt, 'mir, 'tcx> InternVisitor<'rt, 'mir, 'tcx> {
-    /// Intern an allocation without looking at its children
     fn intern_shallow(
         &mut self,
-        ptr: Pointer,
+        alloc_id: AllocId,
         mutability: Mutability,
+        ty: Option<Ty<'tcx>>,
     ) -> InterpResult<'tcx, Option<IsStaticOrFn>> {
-        trace!(
-            "InternVisitor::intern {:?} with {:?}",
-            ptr, mutability,
-        );
-        // remove allocation
-        let tcx = self.ecx.tcx;
-        let memory = self.ecx.memory_mut();
-        let (kind, mut alloc) = match memory.alloc_map.remove(&ptr.alloc_id) {
-            Some(entry) => entry,
-            None => {
-                // if the pointer is dangling (neither in local nor global memory), we leave it
-                // to validation to error. The `delay_span_bug` ensures that we don't forget such
-                // a check in validation.
-                if tcx.alloc_map.lock().get(ptr.alloc_id).is_none() {
-                    tcx.sess.delay_span_bug(self.ecx.tcx.span, "tried to intern dangling pointer");
-                }
-                // treat dangling pointers like other statics
-                // just to stop trying to recurse into them
-                return Ok(Some(IsStaticOrFn));
-            },
-        };
-        // This match is just a canary for future changes to `MemoryKind`, which most likely need
-        // changes in this function.
-        match kind {
-            MemoryKind::Stack | MemoryKind::Vtable => {},
-        }
-        // Ensure llvm knows to only put this into immutable memory if the value is immutable either
-        // by being behind a reference or by being part of a static or const without interior
-        // mutability
-        alloc.mutability = mutability;
-        // link the alloc id to the actual allocation
-        let alloc = tcx.intern_const_alloc(alloc);
-        self.leftover_relocations.extend(alloc.relocations().iter().map(|&(_, ((), reloc))| reloc));
-        tcx.alloc_map.lock().set_alloc_id_memory(ptr.alloc_id, alloc);
-        Ok(None)
+        intern_shallow(
+            self.ecx,
+            self.leftover_allocations,
+            self.mode,
+            alloc_id,
+            mutability,
+            ty,
+        )
     }
 }
 
@@ -119,14 +169,16 @@
     ) -> InterpResult<'tcx> {
         if let Some(def) = mplace.layout.ty.ty_adt_def() {
             if Some(def.did) == self.ecx.tcx.lang_items().unsafe_cell_type() {
-                // We are crossing over an `UnsafeCell`, we can mutate again
+                // We are crossing over an `UnsafeCell`, we can mutate again. This means that
+                // References we encounter inside here are interned as pointing to mutable
+                // allocations.
                 let old = std::mem::replace(&mut self.mutability, Mutability::Mutable);
                 assert_ne!(
                     self.mode, InternMode::Const,
                     "UnsafeCells are not allowed behind references in constants. This should have \
                     been prevented statically by const qualification. If this were allowed one \
-                    would be able to change a constant at one use site and other use sites may \
-                    arbitrarily decide to change, too.",
+                    would be able to change a constant at one use site and other use sites could \
+                    observe that mutation.",
                 );
                 let walked = self.walk_aggregate(mplace, fields);
                 self.mutability = old;
@@ -145,12 +197,13 @@
             // Handle trait object vtables
             if let Ok(meta) = value.to_meta() {
                 if let ty::Dynamic(..) =
-                    self.ecx.tcx.struct_tail_erasing_lifetimes(referenced_ty, self.param_env).sty
+                    self.ecx.tcx.struct_tail_erasing_lifetimes(
+                        referenced_ty, self.ecx.param_env).sty
                 {
                     if let Ok(vtable) = meta.unwrap().to_ptr() {
                         // explitly choose `Immutable` here, since vtables are immutable, even
                         // if the reference of the fat pointer is mutable
-                        self.intern_shallow(vtable, Mutability::Immutable)?;
+                        self.intern_shallow(vtable.alloc_id, Mutability::Immutable, None)?;
                     }
                 }
             }
@@ -177,7 +230,7 @@
                     (InternMode::Const, hir::Mutability::MutMutable) => {
                         match referenced_ty.sty {
                             ty::Array(_, n)
-                                if n.eval_usize(self.ecx.tcx.tcx, self.param_env) == 0 => {}
+                                if n.eval_usize(self.ecx.tcx.tcx, self.ecx.param_env) == 0 => {}
                             ty::Slice(_)
                                 if value.to_meta().unwrap().unwrap().to_usize(self.ecx)? == 0 => {}
                             _ => bug!("const qualif failed to prevent mutable references"),
@@ -195,21 +248,13 @@
                     (Mutability::Mutable, hir::Mutability::MutMutable) => Mutability::Mutable,
                     _ => Mutability::Immutable,
                 };
-                // Compute the mutability of the allocation
-                let intern_mutability = intern_mutability(
-                    self.ecx.tcx.tcx,
-                    self.param_env,
-                    mplace.layout.ty,
-                    self.ecx.tcx.span,
-                    mutability,
-                );
                 // Recursing behind references changes the intern mode for constants in order to
                 // cause assertions to trigger if we encounter any `UnsafeCell`s.
                 let mode = match self.mode {
                     InternMode::ConstBase => InternMode::Const,
                     other => other,
                 };
-                match self.intern_shallow(ptr, intern_mutability)? {
+                match self.intern_shallow(ptr.alloc_id, mutability, Some(mplace.layout.ty))? {
                     // No need to recurse, these are interned already and statics may have
                     // cycles, so we don't want to recurse there
                     Some(IsStaticOrFn) => {},
@@ -224,69 +269,45 @@
     }
 }
 
-/// Figure out the mutability of the allocation.
-/// Mutable if it has interior mutability *anywhere* in the type.
-fn intern_mutability<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    param_env: ParamEnv<'tcx>,
-    ty: Ty<'tcx>,
-    span: Span,
-    mutability: Mutability,
-) -> Mutability {
-    let has_interior_mutability = !ty.is_freeze(tcx, param_env, span);
-    if has_interior_mutability {
-        Mutability::Mutable
-    } else {
-        mutability
-    }
-}
-
 pub fn intern_const_alloc_recursive(
     ecx: &mut CompileTimeEvalContext<'mir, 'tcx>,
     def_id: DefId,
     ret: MPlaceTy<'tcx>,
-    // FIXME(oli-obk): can we scrap the param env? I think we can, the final value of a const eval
-    // must always be monomorphic, right?
-    param_env: ty::ParamEnv<'tcx>,
 ) -> InterpResult<'tcx> {
     let tcx = ecx.tcx;
     // this `mutability` is the mutability of the place, ignoring the type
-    let (mutability, base_intern_mode) = match tcx.static_mutability(def_id) {
+    let (base_mutability, base_intern_mode) = match tcx.static_mutability(def_id) {
         Some(hir::Mutability::MutImmutable) => (Mutability::Immutable, InternMode::Static),
-        None => (Mutability::Immutable, InternMode::ConstBase),
         // `static mut` doesn't care about interior mutability, it's mutable anyway
         Some(hir::Mutability::MutMutable) => (Mutability::Mutable, InternMode::Static),
+        // consts, promoteds. FIXME: what about array lengths, array initializers?
+        None => (Mutability::Immutable, InternMode::ConstBase),
     };
 
-    // type based interning
-    let mut ref_tracking = RefTracking::new((ret, mutability, base_intern_mode));
-    let leftover_relocations = &mut FxHashSet::default();
-
-    // This mutability is the combination of the place mutability and the type mutability. If either
-    // is mutable, `alloc_mutability` is mutable. This exists because the entire allocation needs
-    // to be mutable if it contains an `UnsafeCell` anywhere. The other `mutability` exists so that
-    // the visitor does not treat everything outside the `UnsafeCell` as mutable.
-    let alloc_mutability = intern_mutability(
-        tcx.tcx, param_env, ret.layout.ty, tcx.span, mutability,
-    );
+    // Type based interning.
+    // `ref_tracking` tracks typed references we have seen and still need to crawl for
+    // more typed information inside them.
+    // `leftover_allocations` collects *all* allocations we see, because some might not
+    // be available in a typed way. They get interned at the end.
+    let mut ref_tracking = RefTracking::new((ret, base_mutability, base_intern_mode));
+    let leftover_allocations = &mut FxHashSet::default();
 
     // start with the outermost allocation
-    InternVisitor {
-        ref_tracking: &mut ref_tracking,
+    intern_shallow(
         ecx,
-        mode: base_intern_mode,
-        leftover_relocations,
-        param_env,
-        mutability,
-    }.intern_shallow(ret.ptr.to_ptr()?, alloc_mutability)?;
+        leftover_allocations,
+        base_intern_mode,
+        ret.ptr.to_ptr()?.alloc_id,
+        base_mutability,
+        Some(ret.layout.ty)
+    )?;
 
     while let Some(((mplace, mutability, mode), _)) = ref_tracking.todo.pop() {
         let interned = InternVisitor {
             ref_tracking: &mut ref_tracking,
             ecx,
             mode,
-            leftover_relocations,
-            param_env,
+            leftover_allocations,
             mutability,
         }.visit_value(mplace);
         if let Err(error) = interned {
@@ -309,15 +330,23 @@
     // Intern the rest of the allocations as mutable. These might be inside unions, padding, raw
     // pointers, ... So we can't intern them according to their type rules
 
-    let mut todo: Vec<_> = leftover_relocations.iter().cloned().collect();
+    let mut todo: Vec<_> = leftover_allocations.iter().cloned().collect();
     while let Some(alloc_id) = todo.pop() {
-        if let Some((_, alloc)) = ecx.memory_mut().alloc_map.remove(&alloc_id) {
-            // We can't call the `intern` method here, as its logic is tailored to safe references.
-            // So we hand-roll the interning logic here again
+        if let Some((_, mut alloc)) = ecx.memory_mut().alloc_map.remove(&alloc_id) {
+            // We can't call the `intern_shallow` method here, as its logic is tailored to safe
+            // references and a `leftover_allocations` set (where we only have a todo-list here).
+            // So we hand-roll the interning logic here again.
+            if base_intern_mode != InternMode::Static {
+                // If it's not a static, it *must* be immutable.
+                // We cannot have mutable memory inside a constant.
+                // FIXME: ideally we would assert that they already are immutable, to double-
+                // check our static checks.
+                alloc.mutability = Mutability::Immutable;
+            }
             let alloc = tcx.intern_const_alloc(alloc);
             tcx.alloc_map.lock().set_alloc_id_memory(alloc_id, alloc);
             for &(_, ((), reloc)) in alloc.relocations().iter() {
-                if leftover_relocations.insert(reloc) {
+                if leftover_allocations.insert(reloc) {
                     todo.push(reloc);
                 }
             }
diff --git a/src/librustc_msan/build.rs b/src/librustc_msan/build.rs
index de1676f..a81786e 100644
--- a/src/librustc_msan/build.rs
+++ b/src/librustc_msan/build.rs
@@ -4,6 +4,7 @@
 use cmake::Config;
 
 fn main() {
+    println!("cargo:rerun-if-env-changed=RUSTC_BUILD_SANITIZERS");
     if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
         return;
     }
diff --git a/src/librustc_tsan/build.rs b/src/librustc_tsan/build.rs
index 6df9691..f9333e1 100644
--- a/src/librustc_tsan/build.rs
+++ b/src/librustc_tsan/build.rs
@@ -4,6 +4,7 @@
 use cmake::Config;
 
 fn main() {
+    println!("cargo:rerun-if-env-changed=RUSTC_BUILD_SANITIZERS");
     if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
         return;
     }
diff --git a/src/libstd/Cargo.toml b/src/libstd/Cargo.toml
index 20442ab..af1d240 100644
--- a/src/libstd/Cargo.toml
+++ b/src/libstd/Cargo.toml
@@ -25,17 +25,11 @@
 unwind = { path = "../libunwind" }
 hashbrown = { version = "0.5.0", features = ['rustc-dep-of-std'] }
 
-[dependencies.backtrace]
+[dependencies.backtrace_rs]
+package = "backtrace"
 version = "0.3.37"
-default-features = false # don't use coresymbolication on OSX
-features = [
-  "rustc-dep-of-std", # enable build support for integrating into libstd
-  "dbghelp",          # backtrace/symbolize on MSVC
-  "libbacktrace",     # symbolize on most platforms
-  "libunwind",        # backtrace on most platforms
-  "dladdr",           # symbolize on platforms w/o libbacktrace
-]
-optional = true
+default-features = false # without the libstd `backtrace` feature, stub out everything
+features = [ "rustc-dep-of-std" ] # enable build support for integrating into libstd
 
 [dev-dependencies]
 rand = "0.7"
@@ -65,6 +59,13 @@
 [features]
 default = ["std_detect_file_io", "std_detect_dlsym_getauxval"]
 
+backtrace = [
+  "backtrace_rs/dbghelp",          # backtrace/symbolize on MSVC
+  "backtrace_rs/libbacktrace",     # symbolize on most platforms
+  "backtrace_rs/libunwind",        # backtrace on most platforms
+  "backtrace_rs/dladdr",           # symbolize on platforms w/o libbacktrace
+]
+
 panic-unwind = ["panic_unwind"]
 profiler = ["profiler_builtins"]
 compiler-builtins-c = ["alloc/compiler-builtins-c"]
diff --git a/src/libstd/backtrace.rs b/src/libstd/backtrace.rs
index 5d46ef7..61c42a5 100644
--- a/src/libstd/backtrace.rs
+++ b/src/libstd/backtrace.rs
@@ -97,6 +97,7 @@
 use crate::sync::Mutex;
 use crate::sys_common::backtrace::{output_filename, lock};
 use crate::vec::Vec;
+use backtrace_rs as backtrace;
 use backtrace::BytesOrWideString;
 
 /// A captured OS thread stack backtrace.
diff --git a/src/libstd/panicking.rs b/src/libstd/panicking.rs
index db4089c..28fb402 100644
--- a/src/libstd/panicking.rs
+++ b/src/libstd/panicking.rs
@@ -17,8 +17,7 @@
 use crate::raw;
 use crate::sys::stdio::panic_output;
 use crate::sys_common::rwlock::RWLock;
-use crate::sys_common::thread_info;
-use crate::sys_common::util;
+use crate::sys_common::{thread_info, util, backtrace};
 use crate::thread;
 
 #[cfg(not(test))]
@@ -157,20 +156,18 @@
 }
 
 fn default_hook(info: &PanicInfo<'_>) {
-    #[cfg(feature = "backtrace")]
-    use crate::sys_common::{backtrace as backtrace_mod};
-
     // If this is a double panic, make sure that we print a backtrace
     // for this panic. Otherwise only print it if logging is enabled.
-    #[cfg(feature = "backtrace")]
-    let log_backtrace = {
+    let log_backtrace = if cfg!(feature = "backtrace") {
         let panics = update_panic_count(0);
 
         if panics >= 2 {
-            Some(backtrace::PrintFmt::Full)
+            Some(backtrace_rs::PrintFmt::Full)
         } else {
-            backtrace_mod::log_enabled()
+            backtrace::log_enabled()
         }
+    } else {
+        None
     };
 
     // The current implementation always returns `Some`.
@@ -190,14 +187,13 @@
         let _ = writeln!(err, "thread '{}' panicked at '{}', {}",
                          name, msg, location);
 
-        #[cfg(feature = "backtrace")]
-        {
+        if cfg!(feature = "backtrace") {
             use crate::sync::atomic::{AtomicBool, Ordering};
 
             static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 
             if let Some(format) = log_backtrace {
-                let _ = backtrace_mod::print(err, format);
+                let _ = backtrace::print(err, format);
             } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
                 let _ = writeln!(err, "note: run with `RUST_BACKTRACE=1` \
                                        environment variable to display a backtrace.");
diff --git a/src/libstd/process.rs b/src/libstd/process.rs
index c50025a..b8d57cf 100644
--- a/src/libstd/process.rs
+++ b/src/libstd/process.rs
@@ -422,7 +422,7 @@
 /// // Execute `ls` in the current directory of the program.
 /// list_dir.status().expect("process failed to execute");
 ///
-/// println!("");
+/// println!();
 ///
 /// // Change `ls` to execute in the root directory.
 /// list_dir.current_dir("/");
diff --git a/src/libstd/sys_common/backtrace.rs b/src/libstd/sys_common/backtrace.rs
index 1a78abf..01711d4 100644
--- a/src/libstd/sys_common/backtrace.rs
+++ b/src/libstd/sys_common/backtrace.rs
@@ -7,10 +7,9 @@
 use crate::borrow::Cow;
 use crate::io::prelude::*;
 use crate::path::{self, Path, PathBuf};
-use crate::sync::atomic::{self, Ordering};
 use crate::sys::mutex::Mutex;
 
-use backtrace::{BacktraceFmt, BytesOrWideString, PrintFmt};
+use backtrace_rs::{BacktraceFmt, BytesOrWideString, PrintFmt};
 
 /// Max number of frames to print.
 const MAX_NB_FRAMES: usize = 100;
@@ -74,14 +73,14 @@
     bt_fmt.add_context()?;
     let mut idx = 0;
     let mut res = Ok(());
-    backtrace::trace_unsynchronized(|frame| {
+    backtrace_rs::trace_unsynchronized(|frame| {
         if print_fmt == PrintFmt::Short && idx > MAX_NB_FRAMES {
             return false;
         }
 
         let mut hit = false;
         let mut stop = false;
-        backtrace::resolve_frame_unsynchronized(frame, |symbol| {
+        backtrace_rs::resolve_frame_unsynchronized(frame, |symbol| {
             hit = true;
             if print_fmt == PrintFmt::Short {
                 if let Some(sym) = symbol.name().and_then(|s| s.as_str()) {
@@ -130,6 +129,8 @@
 // For now logging is turned off by default, and this function checks to see
 // whether the magical environment variable is present to see if it's turned on.
 pub fn log_enabled() -> Option<PrintFmt> {
+    use crate::sync::atomic::{self, Ordering};
+
     // Setting environment variables for Fuchsia components isn't a standard
     // or easily supported workflow. For now, always display backtraces.
     if cfg!(target_os = "fuchsia") {
diff --git a/src/libstd/sys_common/mod.rs b/src/libstd/sys_common/mod.rs
index 9190a3b..cba3eca 100644
--- a/src/libstd/sys_common/mod.rs
+++ b/src/libstd/sys_common/mod.rs
@@ -41,7 +41,6 @@
 
 pub mod alloc;
 pub mod at_exit_imp;
-#[cfg(feature = "backtrace")]
 pub mod backtrace;
 pub mod condvar;
 pub mod io;
diff --git a/src/libsyntax/ext/tt/macro_rules.rs b/src/libsyntax/ext/tt/macro_rules.rs
index 46ffa52..b27e9c5 100644
--- a/src/libsyntax/ext/tt/macro_rules.rs
+++ b/src/libsyntax/ext/tt/macro_rules.rs
@@ -877,9 +877,9 @@
         // Now `last` holds the complete set of NT tokens that could
         // end the sequence before SUFFIX. Check that every one works with `suffix`.
         'each_last: for token in &last.tokens {
-            if let TokenTree::MetaVarDecl(_, ref name, ref frag_spec) = *token {
+            if let TokenTree::MetaVarDecl(_, name, frag_spec) = *token {
                 for next_token in &suffix_first.tokens {
-                    match is_in_follow(next_token, &frag_spec.as_str()) {
+                    match is_in_follow(next_token, frag_spec.name) {
                         IsInFollow::Invalid(msg, help) => {
                             sess.span_diagnostic
                                 .struct_span_err(next_token.span(), &msg)
@@ -948,7 +948,7 @@
 
 fn token_can_be_followed_by_any(tok: &quoted::TokenTree) -> bool {
     if let quoted::TokenTree::MetaVarDecl(_, _, frag_spec) = *tok {
-        frag_can_be_followed_by_any(&frag_spec.as_str())
+        frag_can_be_followed_by_any(frag_spec.name)
     } else {
         // (Non NT's can always be followed by anthing in matchers.)
         true
@@ -963,15 +963,15 @@
 /// specifier which consumes at most one token tree can be followed by
 /// a fragment specifier (indeed, these fragments can be followed by
 /// ANYTHING without fear of future compatibility hazards).
-fn frag_can_be_followed_by_any(frag: &str) -> bool {
+fn frag_can_be_followed_by_any(frag: Symbol) -> bool {
     match frag {
-        "item"     | // always terminated by `}` or `;`
-        "block"    | // exactly one token tree
-        "ident"    | // exactly one token tree
-        "literal"  | // exactly one token tree
-        "meta"     | // exactly one token tree
-        "lifetime" | // exactly one token tree
-        "tt" =>   // exactly one token tree
+        sym::item     | // always terminated by `}` or `;`
+        sym::block    | // exactly one token tree
+        sym::ident    | // exactly one token tree
+        sym::literal  | // exactly one token tree
+        sym::meta     | // exactly one token tree
+        sym::lifetime | // exactly one token tree
+        sym::tt =>   // exactly one token tree
             true,
 
         _ =>
@@ -993,7 +993,7 @@
 /// break macros that were relying on that binary operator as a
 /// separator.
 // when changing this do not forget to update doc/book/macros.md!
-fn is_in_follow(tok: &quoted::TokenTree, frag: &str) -> IsInFollow {
+fn is_in_follow(tok: &quoted::TokenTree, frag: Symbol) -> IsInFollow {
     use quoted::TokenTree;
 
     if let TokenTree::Token(Token { kind: token::CloseDelim(_), .. }) = *tok {
@@ -1002,17 +1002,17 @@
         IsInFollow::Yes
     } else {
         match frag {
-            "item" => {
+            sym::item => {
                 // since items *must* be followed by either a `;` or a `}`, we can
                 // accept anything after them
                 IsInFollow::Yes
             }
-            "block" => {
+            sym::block => {
                 // anything can follow block, the braces provide an easy boundary to
                 // maintain
                 IsInFollow::Yes
             }
-            "stmt" | "expr" => {
+            sym::stmt | sym::expr => {
                 const TOKENS: &[&str] = &["`=>`", "`,`", "`;`"];
                 match tok {
                     TokenTree::Token(token) => match token.kind {
@@ -1022,7 +1022,7 @@
                     _ => IsInFollow::No(TOKENS),
                 }
             }
-            "pat" => {
+            sym::pat => {
                 const TOKENS: &[&str] = &["`=>`", "`,`", "`=`", "`|`", "`if`", "`in`"];
                 match tok {
                     TokenTree::Token(token) => match token.kind {
@@ -1033,7 +1033,7 @@
                     _ => IsInFollow::No(TOKENS),
                 }
             }
-            "path" | "ty" => {
+            sym::path | sym::ty => {
                 const TOKENS: &[&str] = &[
                     "`{`", "`[`", "`=>`", "`,`", "`>`", "`=`", "`:`", "`;`", "`|`", "`as`",
                     "`where`",
@@ -1061,20 +1061,20 @@
                     _ => IsInFollow::No(TOKENS),
                 }
             }
-            "ident" | "lifetime" => {
+            sym::ident | sym::lifetime => {
                 // being a single token, idents and lifetimes are harmless
                 IsInFollow::Yes
             }
-            "literal" => {
+            sym::literal => {
                 // literals may be of a single token, or two tokens (negative numbers)
                 IsInFollow::Yes
             }
-            "meta" | "tt" => {
+            sym::meta | sym::tt => {
                 // being either a single token or a delimited sequence, tt is
                 // harmless
                 IsInFollow::Yes
             }
-            "vis" => {
+            sym::vis => {
                 // Explicitly disallow `priv`, on the off chance it comes back.
                 const TOKENS: &[&str] = &["`,`", "an ident", "a type"];
                 match tok {
@@ -1099,7 +1099,7 @@
                     _ => IsInFollow::No(TOKENS),
                 }
             }
-            "" => IsInFollow::Yes, // kw::Invalid
+            kw::Invalid => IsInFollow::Yes,
             _ => IsInFollow::Invalid(
                 format!("invalid fragment specifier `{}`", frag),
                 VALID_FRAGMENT_NAMES_MSG,
diff --git a/src/test/debuginfo/function-arg-initialization.rs b/src/test/debuginfo/function-arg-initialization.rs
index fef6253..8c86d2c 100644
--- a/src/test/debuginfo/function-arg-initialization.rs
+++ b/src/test/debuginfo/function-arg-initialization.rs
@@ -242,12 +242,12 @@
 
 fn binding(a: i64, b: u64, c: f64) {
     let x = 0; // #break
-    println!("")
+    println!()
 }
 
 fn assignment(mut a: u64, b: u64, c: f64) {
     a = b; // #break
-    println!("")
+    println!()
 }
 
 fn function_call(x: u64, y: u64, z: f64) {
diff --git a/src/test/ui/collections-const-new.rs b/src/test/ui/collections-const-new.rs
index e01b0df..a93f9a1 100644
--- a/src/test/ui/collections-const-new.rs
+++ b/src/test/ui/collections-const-new.rs
@@ -1,15 +1,11 @@
-// run-pass
+// check-pass
 
-#![allow(dead_code)]
 // Test several functions can be used for constants
 // 1. Vec::new()
 // 2. String::new()
 
-#![feature(const_vec_new)]
-#![feature(const_string_new)]
-
 const MY_VEC: Vec<usize> = Vec::new();
 
 const MY_STRING: String = String::new();
 
-pub fn main() {}
+fn main() {}
diff --git a/src/test/ui/conditional-compilation/cfg-arg-invalid-6.rs b/src/test/ui/conditional-compilation/cfg-arg-invalid-6.rs
new file mode 100644
index 0000000..9fa726f
--- /dev/null
+++ b/src/test/ui/conditional-compilation/cfg-arg-invalid-6.rs
@@ -0,0 +1,3 @@
+// compile-flags: --cfg a{
+// error-pattern: invalid `--cfg` argument: `a{` (expected `key` or `key="value"`)
+fn main() {}
diff --git a/src/test/ui/conditional-compilation/cfg-arg-invalid-6.stderr b/src/test/ui/conditional-compilation/cfg-arg-invalid-6.stderr
new file mode 100644
index 0000000..7d2087b
--- /dev/null
+++ b/src/test/ui/conditional-compilation/cfg-arg-invalid-6.stderr
@@ -0,0 +1,2 @@
+error: invalid `--cfg` argument: `a{` (expected `key` or `key="value"`)
+
diff --git a/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.rs b/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.rs
index 5fb9253..8b17f68 100644
--- a/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.rs
+++ b/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.rs
@@ -14,8 +14,9 @@
 impl Foo<u32> for () {
     const X: u32 = 42;
 }
+
 impl Foo<Vec<u32>> for String {
-    const X: Vec<u32> = Vec::new(); //~ ERROR not yet stable as a const fn
+    const X: Vec<u32> = Vec::new();
 }
 
 impl Bar<u32, ()> for () {}
diff --git a/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.stderr b/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.stderr
index c56ebf6..5bc7b70 100644
--- a/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.stderr
+++ b/src/test/ui/consts/miri_unleashed/feature-gate-unleash_the_miri_inside_of_you.stderr
@@ -4,13 +4,5 @@
 LL |     const F: u32 = (U::X, 42).1;
    |                    ^^^^^^^^^^ constants cannot evaluate destructors
 
-error: `std::vec::Vec::<T>::new` is not yet stable as a const fn
-  --> $DIR/feature-gate-unleash_the_miri_inside_of_you.rs:18:25
-   |
-LL |     const X: Vec<u32> = Vec::new();
-   |                         ^^^^^^^^^^
-   |
-   = help: add `#![feature(const_vec_new)]` to the crate attributes to enable
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
diff --git a/src/test/ui/consts/miri_unleashed/mutable_const.rs b/src/test/ui/consts/miri_unleashed/mutable_const.rs
new file mode 100644
index 0000000..b476e04
--- /dev/null
+++ b/src/test/ui/consts/miri_unleashed/mutable_const.rs
@@ -0,0 +1,20 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+
+#![feature(const_raw_ptr_deref)]
+#![deny(const_err)]
+
+use std::cell::UnsafeCell;
+
+// make sure we do not just intern this as mutable
+const MUTABLE_BEHIND_RAW: *mut i32 = &UnsafeCell::new(42) as *const _ as *mut _;
+
+const MUTATING_BEHIND_RAW: () = {
+    // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
+    unsafe {
+        *MUTABLE_BEHIND_RAW = 99 //~ WARN skipping const checks
+        //~^ ERROR any use of this value will cause an error
+        //~^^ tried to modify constant memory
+    }
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/miri_unleashed/mutable_const.stderr b/src/test/ui/consts/miri_unleashed/mutable_const.stderr
new file mode 100644
index 0000000..507d4823
--- /dev/null
+++ b/src/test/ui/consts/miri_unleashed/mutable_const.stderr
@@ -0,0 +1,27 @@
+warning: skipping const checks
+  --> $DIR/mutable_const.rs:14:9
+   |
+LL |         *MUTABLE_BEHIND_RAW = 99
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: any use of this value will cause an error
+  --> $DIR/mutable_const.rs:14:9
+   |
+LL | / const MUTATING_BEHIND_RAW: () = {
+LL | |     // Test that `MUTABLE_BEHIND_RAW` is actually immutable, by doing this at const time.
+LL | |     unsafe {
+LL | |         *MUTABLE_BEHIND_RAW = 99
+   | |         ^^^^^^^^^^^^^^^^^^^^^^^^ tried to modify constant memory
+...  |
+LL | |     }
+LL | | };
+   | |__-
+   |
+note: lint level defined here
+  --> $DIR/mutable_const.rs:4:9
+   |
+LL | #![deny(const_err)]
+   |         ^^^^^^^^^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/consts/miri_unleashed/mutable_references_ice.stderr b/src/test/ui/consts/miri_unleashed/mutable_references_ice.stderr
index 82569e2..28cf353 100644
--- a/src/test/ui/consts/miri_unleashed/mutable_references_ice.stderr
+++ b/src/test/ui/consts/miri_unleashed/mutable_references_ice.stderr
@@ -6,7 +6,7 @@
 
 thread 'rustc' panicked at 'assertion failed: `(left != right)`
   left: `Const`,
- right: `Const`: UnsafeCells are not allowed behind references in constants. This should have been prevented statically by const qualification. If this were allowed one would be able to change a constant at one use site and other use sites may arbitrarily decide to change, too.', src/librustc_mir/interpret/intern.rs:LL:CC
+ right: `Const`: UnsafeCells are not allowed behind references in constants. This should have been prevented statically by const qualification. If this were allowed one would be able to change a constant at one use site and other use sites could observe that mutation.', src/librustc_mir/interpret/intern.rs:LL:CC
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
 
 error: internal compiler error: unexpected panic
diff --git a/src/test/ui/drop/dropck_legal_cycles.rs b/src/test/ui/drop/dropck_legal_cycles.rs
index a4f4c26..fb13fd7 100644
--- a/src/test/ui/drop/dropck_legal_cycles.rs
+++ b/src/test/ui/drop/dropck_legal_cycles.rs
@@ -143,7 +143,7 @@
     v[0].descend_into_self(&mut c);
     assert!(!c.saw_prev_marked); // <-- different from below, b/c acyclic above
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 1: { v[0] -> v[1], v[1] -> v[0] };
     // does not exercise `v` itself
@@ -158,7 +158,7 @@
     v[0].descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 2: { v[0] -> v, v[1] -> v }
     let v: V = Named::new("v");
@@ -171,7 +171,7 @@
     v.descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 3: { hk0 -> hv0, hv0 -> hk0, hk1 -> hv1, hv1 -> hk1 };
     // does not exercise `h` itself
@@ -193,7 +193,7 @@
         assert!(c.saw_prev_marked);
     }
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 4: { h -> (hmk0,hmv0,hmk1,hmv1), {hmk0,hmv0,hmk1,hmv1} -> h }
 
@@ -216,7 +216,7 @@
         // break;
     }
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 5: { vd[0] -> vd[1], vd[1] -> vd[0] };
     // does not exercise vd itself
@@ -232,7 +232,7 @@
     vd[0].descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 6: { vd -> (vd0, vd1), {vd0, vd1} -> vd }
     let mut vd: VecDeque<VD> = VecDeque::new();
@@ -247,7 +247,7 @@
     vd[0].descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 7: { vm -> (vm0, vm1), {vm0, vm1} -> vm }
     let mut vm: HashMap<usize, VM> = HashMap::new();
@@ -262,7 +262,7 @@
     vm[&0].descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 8: { ll -> (ll0, ll1), {ll0, ll1} -> ll }
     let mut ll: LinkedList<LL> = LinkedList::new();
@@ -282,7 +282,7 @@
         // break;
     }
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 9: { bh -> (bh0, bh1), {bh0, bh1} -> bh }
     let mut bh: BinaryHeap<BH> = BinaryHeap::new();
@@ -302,7 +302,7 @@
         // break;
     }
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 10: { btm -> (btk0, btv1), {bt0, bt1} -> btm }
     let mut btm: BTreeMap<BTM, BTM> = BTreeMap::new();
@@ -323,7 +323,7 @@
         // break;
     }
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 10: { bts -> (bts0, bts1), {bts0, bts1} -> btm }
     let mut bts: BTreeSet<BTS> = BTreeSet::new();
@@ -343,7 +343,7 @@
         // break;
     }
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 11: { rc0 -> (rc1, rc2), rc1 -> (), rc2 -> rc0 }
     let (rc0, rc1, rc2): (RCRC, RCRC, RCRC);
@@ -361,7 +361,7 @@
     rc0.descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // We want to take the previous Rc case and generalize it to Arc.
     //
@@ -395,7 +395,7 @@
     arc0.descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 13: { arc0 -> (arc1, arc2), arc1 -> (), arc2 -> arc0 }, rwlocks
     let (arc0, arc1, arc2): (ARCRW, ARCRW, ARCRW);
@@ -413,7 +413,7 @@
     arc0.descend_into_self(&mut c);
     assert!(c.saw_prev_marked);
 
-    if PRINT { println!(""); }
+    if PRINT { println!(); }
 
     // Cycle 14: { arc0 -> (arc1, arc2), arc1 -> (), arc2 -> arc0 }, mutexs
     let (arc0, arc1, arc2): (ARCM, ARCM, ARCM);
diff --git a/src/test/ui/for/for-c-in-str.rs b/src/test/ui/for/for-c-in-str.rs
index 43b1a04..0fbc796 100644
--- a/src/test/ui/for/for-c-in-str.rs
+++ b/src/test/ui/for/for-c-in-str.rs
@@ -6,6 +6,6 @@
     //~| NOTE `&str` is not an iterator
     //~| HELP the trait `std::iter::Iterator` is not implemented for `&str`
     //~| NOTE required by `std::iter::IntoIterator::into_iter`
-        println!("");
+        println!();
     }
 }
diff --git a/src/tools/build-manifest/Cargo.toml b/src/tools/build-manifest/Cargo.toml
index c364479..63ae445 100644
--- a/src/tools/build-manifest/Cargo.toml
+++ b/src/tools/build-manifest/Cargo.toml
@@ -7,3 +7,5 @@
 [dependencies]
 toml = "0.5"
 serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+reqwest = "0.9"
diff --git a/src/tools/build-manifest/src/main.rs b/src/tools/build-manifest/src/main.rs
index 9ffa939..c2d642b 100644
--- a/src/tools/build-manifest/src/main.rs
+++ b/src/tools/build-manifest/src/main.rs
@@ -1,12 +1,19 @@
+//! Build a dist manifest, hash and sign everything.
+//! This gets called by `promote-release`
+//! (https://github.com/rust-lang/rust-central-station/tree/master/promote-release)
+//! via `x.py dist hash-and-sign`; the cmdline arguments are set up
+//! by rustbuild (in `src/bootstrap/dist.rs`).
+
 use toml;
 use serde::Serialize;
 
 use std::collections::BTreeMap;
 use std::env;
 use std::fs;
-use std::io::{self, Read, Write};
+use std::io::{self, Read, Write, BufRead, BufReader};
 use std::path::{PathBuf, Path};
 use std::process::{Command, Stdio};
+use std::collections::HashMap;
 
 static HOSTS: &[&str] = &[
     "aarch64-unknown-linux-gnu",
@@ -146,6 +153,9 @@
     "x86_64-pc-windows-gnu",
 ];
 
+static TOOLSTATE: &str =
+    "https://raw.githubusercontent.com/rust-lang-nursery/rust-toolstate/master/history/linux.tsv";
+
 #[derive(Serialize)]
 #[serde(rename_all = "kebab-case")]
 struct Manifest {
@@ -270,6 +280,7 @@
     // Do not ask for a passphrase while manually testing
     let mut passphrase = String::new();
     if should_sign {
+        // `x.py` passes the passphrase via stdin.
         t!(io::stdin().read_to_string(&mut passphrase));
     }
 
@@ -353,6 +364,7 @@
         self.lldb_git_commit_hash = self.git_commit_hash("lldb", "x86_64-unknown-linux-gnu");
         self.miri_git_commit_hash = self.git_commit_hash("miri", "x86_64-unknown-linux-gnu");
 
+        self.check_toolstate();
         self.digest_and_sign();
         let manifest = self.build_manifest();
         self.write_channel_files(&self.rust_release, &manifest);
@@ -362,6 +374,37 @@
         }
     }
 
+    /// If a tool does not pass its tests, don't ship it.
+    /// Right now, we do this only for Miri.
+    fn check_toolstate(&mut self) {
+        // Get the toolstate for this rust revision.
+        let rev = self.rust_git_commit_hash.as_ref().expect("failed to determine rust git hash");
+        let toolstates = reqwest::get(TOOLSTATE).expect("failed to get toolstates");
+        let toolstates = BufReader::new(toolstates);
+        let toolstate = toolstates.lines()
+            .find_map(|line| {
+                let line = line.expect("failed to read toolstate lines");
+                let mut pieces = line.splitn(2, '\t');
+                let commit = pieces.next().expect("malformed toolstate line");
+                if commit != rev {
+                    // Not the right commit.
+                    return None;
+                }
+                // Return the 2nd piece, the JSON.
+                Some(pieces.next().expect("malformed toolstate line").to_owned())
+            })
+            .expect("failed to find toolstate for rust commit");
+        let toolstate: HashMap<String, String> =
+            serde_json::from_str(&toolstate).expect("toolstate is malformed JSON");
+        // Mark some tools as missing based on toolstate.
+        if toolstate.get("miri").map(|s| &*s as &str) != Some("test-pass") {
+            println!("Miri tests are not passing, removing component");
+            self.miri_version = None;
+            self.miri_git_commit_hash = None;
+        }
+    }
+
+    /// Hash all files, compute their signatures, and collect the hashes in `self.digests`.
     fn digest_and_sign(&mut self) {
         for file in t!(self.input.read_dir()).map(|e| t!(e).path()) {
             let filename = file.file_name().unwrap().to_str().unwrap();
@@ -532,19 +575,20 @@
             .as_ref()
             .cloned()
             .map(|version| (version, true))
-            .unwrap_or_default();
+            .unwrap_or_default(); // `is_present` defaults to `false` here.
 
-        // miri needs to build std with xargo, which doesn't allow stable/beta:
-        // <https://github.com/japaric/xargo/pull/204#issuecomment-374888868>
+        // Miri is nightly-only; never ship it for other trains.
         if pkgname == "miri-preview" && self.rust_release != "nightly" {
-            is_present = false; // ignore it
+            is_present = false; // Pretend the component is entirely missing.
         }
 
         let targets = targets.iter().map(|name| {
             if is_present {
+                // The component generally exists, but it might still be missing for this target.
                 let filename = self.filename(pkgname, name);
                 let digest = match self.digests.remove(&filename) {
                     Some(digest) => digest,
+                    // This component does not exist for this target -- skip it.
                     None => return (name.to_string(), Target::unavailable()),
                 };
                 let xz_filename = filename.replace(".tar.gz", ".tar.xz");
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index 467b777..7c51de5 100644
--- a/src/tools/compiletest/src/main.rs
+++ b/src/tools/compiletest/src/main.rs
@@ -253,7 +253,7 @@
     if args.len() == 1 || args[1] == "-h" || args[1] == "--help" {
         let message = format!("Usage: {} [OPTIONS] [TESTNAME...]", argv0);
         println!("{}", opts.usage(&message));
-        println!("");
+        println!();
         panic!()
     }
 
@@ -265,7 +265,7 @@
     if matches.opt_present("h") || matches.opt_present("help") {
         let message = format!("Usage: {} [OPTIONS]  [TESTNAME...]", argv0);
         println!("{}", opts.usage(&message));
-        println!("");
+        println!();
         panic!()
     }
 
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index aff5546..baed27d 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -2593,7 +2593,7 @@
                     "  actual:   {}",
                     codegen_units_to_str(&actual_item.codegen_units)
                 );
-                println!("");
+                println!();
             }
         }
 
@@ -3526,7 +3526,7 @@
                             }
                         }
                     }
-                    println!("");
+                    println!();
                 }
             }
         }
diff --git a/src/tools/publish_toolstate.py b/src/tools/publish_toolstate.py
index 2e2505b..4060b90 100755
--- a/src/tools/publish_toolstate.py
+++ b/src/tools/publish_toolstate.py
@@ -7,6 +7,8 @@
 ## It is set as callback for `src/ci/docker/x86_64-gnu-tools/repo.sh` by the CI scripts
 ## when a new commit lands on `master` (i.e., after it passed all checks on `auto`).
 
+from __future__ import print_function
+
 import sys
 import re
 import os
@@ -20,21 +22,26 @@
     import urllib.request as urllib2
 
 # List of people to ping when the status of a tool or a book changed.
+# These should be collaborators of the rust-lang/rust repository (with at least
+# read privileges on it). CI will fail otherwise.
 MAINTAINERS = {
-    'miri': '@oli-obk @RalfJung @eddyb',
-    'clippy-driver': '@Manishearth @llogiq @mcarton @oli-obk @phansch @flip1995 @yaahc',
-    'rls': '@Xanewok',
-    'rustfmt': '@topecongiro',
-    'book': '@carols10cents @steveklabnik',
-    'nomicon': '@frewsxcv @Gankro',
-    'reference': '@steveklabnik @Havvy @matthewjasper @ehuss',
-    'rust-by-example': '@steveklabnik @marioidival @projektir',
-    'embedded-book': (
-        '@adamgreig @andre-richter @jamesmunns @korken89 '
-        '@ryankurte @thejpster @therealprof'
-    ),
-    'edition-guide': '@ehuss @Centril @steveklabnik',
-    'rustc-guide': '@mark-i-m @spastorino @amanjeev'
+    'miri': {'oli-obk', 'RalfJung', 'eddyb'},
+    'clippy-driver': {
+        'Manishearth', 'llogiq', 'mcarton', 'oli-obk', 'phansch', 'flip1995',
+        'yaahc',
+    },
+    'rls': {'Xanewok'},
+    'rustfmt': {'topecongiro'},
+    'book': {'carols10cents', 'steveklabnik'},
+    'nomicon': {'frewsxcv', 'Gankra'},
+    'reference': {'steveklabnik', 'Havvy', 'matthewjasper', 'ehuss'},
+    'rust-by-example': {'steveklabnik', 'marioidival'},
+    'embedded-book': {
+        'adamgreig', 'andre-richter', 'jamesmunns', 'korken89',
+        'ryankurte', 'thejpster', 'therealprof',
+    },
+    'edition-guide': {'ehuss', 'Centril', 'steveklabnik'},
+    'rustc-guide': {'mark-i-m', 'spastorino', 'amanjeev'},
 }
 
 REPOS = {
@@ -52,6 +59,50 @@
 }
 
 
+def validate_maintainers(repo, github_token):
+    '''Ensure all maintainers are assignable on a GitHub repo'''
+    next_link_re = re.compile(r'<([^>]+)>; rel="next"')
+
+    # Load the list of assignable people in the GitHub repo
+    assignable = []
+    url = 'https://api.github.com/repos/%s/collaborators?per_page=100' % repo
+    while url is not None:
+        response = urllib2.urlopen(urllib2.Request(url, headers={
+            'Authorization': 'token ' + github_token,
+            # Properly load nested teams.
+            'Accept': 'application/vnd.github.hellcat-preview+json',
+        }))
+        assignable.extend(user['login'] for user in json.load(response))
+        # Load the next page if available
+        url = None
+        link_header = response.headers.get('Link')
+        if link_header:
+            matches = next_link_re.match(link_header)
+            if matches is not None:
+                url = matches.group(1)
+
+    errors = False
+    for tool, maintainers in MAINTAINERS.items():
+        for maintainer in maintainers:
+            if maintainer not in assignable:
+                errors = True
+                print(
+                    "error: %s maintainer @%s is not assignable in the %s repo"
+                    % (tool, maintainer, repo),
+                )
+
+    if errors:
+        print()
+        print("  To be assignable, a person needs to be explicitly listed as a")
+        print("  collaborator in the repository settings. The simple way to")
+        print("  fix this is to ask someone with 'admin' privileges on the repo")
+        print("  to add the person or whole team as a collaborator with 'read'")
+        print("  privileges. Those privileges don't grant any extra permissions")
+        print("  so it's safe to apply them.")
+        print()
+        print("The build will fail due to this.")
+        exit(1)
+
 def read_current_status(current_commit, path):
     '''Reads build status of `current_commit` from content of `history/*.tsv`
     '''
@@ -73,13 +124,12 @@
 def issue(
     tool,
     status,
-    maintainers,
+    assignees,
     relevant_pr_number,
     relevant_pr_user,
     pr_reviewer,
 ):
     # Open an issue about the toolstate failure.
-    assignees = [x.strip() for x in maintainers.split('@') if x != '']
     if status == 'test-fail':
         status_description = 'has failing tests'
     else:
@@ -100,7 +150,7 @@
             REPOS.get(tool), relevant_pr_user, pr_reviewer
         )),
         'title': '`{}` no longer builds after {}'.format(tool, relevant_pr_number),
-        'assignees': assignees,
+        'assignees': list(assignees),
         'labels': ['T-compiler', 'I-nominated'],
     })
     print("Creating issue:\n{}".format(request))
@@ -150,18 +200,19 @@
                 old = status[os]
                 new = s.get(tool, old)
                 status[os] = new
+                maintainers = ' '.join('@'+name for name in MAINTAINERS[tool])
                 if new > old: # comparing the strings, but they are ordered appropriately!
                     # things got fixed or at least the status quo improved
                     changed = True
                     message += '🎉 {} on {}: {} → {} (cc {}, @rust-lang/infra).\n' \
-                        .format(tool, os, old, new, MAINTAINERS.get(tool))
+                        .format(tool, os, old, new, maintainers)
                 elif new < old:
                     # tests or builds are failing and were not failing before
                     changed = True
                     title = '💔 {} on {}: {} → {}' \
                         .format(tool, os, old, new)
                     message += '{} (cc {}, @rust-lang/infra).\n' \
-                        .format(title, MAINTAINERS.get(tool))
+                        .format(title, maintainers)
                     # Most tools only create issues for build failures.
                     # Other failures can be spurious.
                     if new == 'build-fail' or (tool == 'miri' and new == 'test-fail'):
@@ -200,6 +251,16 @@
 
 
 if __name__ == '__main__':
+    repo = os.environ.get('TOOLSTATE_VALIDATE_MAINTAINERS_REPO')
+    if repo:
+        github_token = os.environ.get('TOOLSTATE_REPO_ACCESS_TOKEN')
+        if github_token:
+            validate_maintainers(repo, github_token)
+        else:
+            print('skipping toolstate maintainers validation since no GitHub token is present')
+        # When validating maintainers don't run the full script.
+        exit(0)
+
     cur_commit = sys.argv[1]
     cur_datetime = datetime.datetime.utcnow().strftime('%Y-%m-%dT%H:%M:%SZ')
     cur_commit_msg = sys.argv[2]