Auto merge of #27215 - pnkfelix:fsk-placer-take-5-just-in, r=nikomatsakis

Macro desugaring of `in PLACE { BLOCK }` into "simpler" expressions following the in-development "Placer" protocol.

Includes Placer API that one can override to integrate support for `in` into one's own type.  (See [RFC 809].)

[RFC 809]: https://github.com/rust-lang/rfcs/blob/master/text/0809-box-and-in-for-stdlib.md

Part of #22181

Replaced PR #26180.

Turns on the `in PLACE { BLOCK }` syntax, while leaving in support for the old `box (PLACE) EXPR` syntax (since we need to support that at least until we have a snapshot with support for `in PLACE { BLOCK }`.

(Note that we are not 100% committed to the `in PLACE { BLOCK }` syntax.  In particular I still want to play around with some other alternatives.  Still, I want to get the fundamental framework for the protocol landed so we can play with implementing it for non `Box` types.)

----

Also, this PR leaves out support for desugaring-based `box EXPR`.  We will hopefully land that in the future, but for the short term there are type-inference issues injected by that change that we want to resolve separately.

28 files changed, 899 insertions, 47 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index d9653cecc73..acf22094233 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -55,13 +55,16 @@
 
 use core::prelude::*;
 
+use heap;
+
 use core::any::Any;
 use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{self, Hash};
-use core::marker::Unsize;
+use core::marker::{self, Unsize};
 use core::mem;
 use core::ops::{CoerceUnsized, Deref, DerefMut};
+use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
 use core::ptr::Unique;
 use core::raw::{TraitObject};
 
@@ -72,7 +75,7 @@ use core::raw::{TraitObject};
 ///
 /// ```
 /// # #![feature(box_heap)]
-/// #![feature(box_syntax)]
+/// #![feature(box_syntax, placement_in_syntax)]
 /// use std::boxed::HEAP;
 ///
 /// fn main() {
@@ -83,7 +86,12 @@ use core::raw::{TraitObject};
 #[lang = "exchange_heap"]
 #[unstable(feature = "box_heap",
            reason = "may be renamed; uncertain about custom allocator design")]
-pub const HEAP: () = ();
+pub const HEAP: ExchangeHeapSingleton =
+    ExchangeHeapSingleton { _force_singleton: () };
+
+/// This the singleton type used solely for `boxed::HEAP`.
+#[derive(Copy, Clone)]
+pub struct ExchangeHeapSingleton { _force_singleton: () }
 
 /// A pointer type for heap allocation.
 ///
@@ -91,7 +99,97 @@ pub const HEAP: () = ();
 #[lang = "owned_box"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[fundamental]
-pub struct Box<T>(Unique<T>);
+pub struct Box<T: ?Sized>(Unique<T>);
+
+/// `IntermediateBox` represents uninitialized backing storage for `Box`.
+///
+/// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
+/// introducing a separate `IntermediateBox<T>`; but then you hit
+/// issues when you e.g. attempt to destructure an instance of `Box`,
+/// since it is a lang item and so it gets special handling by the
+/// compiler.  Easier just to make this parallel type for now.
+///
+/// FIXME (pnkfelix): Currently the `box` protocol only supports
+/// creating instances of sized types. This IntermediateBox is
+/// designed to be forward-compatible with a future protocol that
+/// supports creating instances of unsized types; that is why the type
+/// parameter has the `?Sized` generalization marker, and is also why
+/// this carries an explicit size. However, it probably does not need
+/// to carry the explicit alignment; that is just a work-around for
+/// the fact that the `align_of` intrinsic currently requires the
+/// input type to be Sized (which I do not think is strictly
+/// necessary).
+#[unstable(feature = "placement_in", reason = "placement box design is still being worked out.")]
+pub struct IntermediateBox<T: ?Sized>{
+    ptr: *mut u8,
+    size: usize,
+    align: usize,
+    marker: marker::PhantomData<*mut T>,
+}
+
+impl<T> Place<T> for IntermediateBox<T> {
+    fn pointer(&mut self) -> *mut T {
+        unsafe { ::core::mem::transmute(self.ptr) }
+    }
+}
+
+unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
+    let p = b.ptr as *mut T;
+    mem::forget(b);
+    mem::transmute(p)
+}
+
+fn make_place<T>() -> IntermediateBox<T> {
+    let size = mem::size_of::<T>();
+    let align = mem::align_of::<T>();
+
+    let p = if size == 0 {
+        heap::EMPTY as *mut u8
+    } else {
+        let p = unsafe {
+            heap::allocate(size, align)
+        };
+        if p.is_null() {
+            panic!("Box make_place allocation failure.");
+        }
+        p
+    };
+
+    IntermediateBox { ptr: p, size: size, align: align, marker: marker::PhantomData }
+}
+
+impl<T> BoxPlace<T> for IntermediateBox<T> {
+    fn make_place() -> IntermediateBox<T> { make_place() }
+}
+
+impl<T> InPlace<T> for IntermediateBox<T> {
+    type Owner = Box<T>;
+    unsafe fn finalize(self) -> Box<T> { finalize(self) }
+}
+
+impl<T> Boxed for Box<T> {
+    type Data = T;
+    type Place = IntermediateBox<T>;
+    unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> { finalize(b) }
+}
+
+impl<T> Placer<T> for ExchangeHeapSingleton {
+    type Place = IntermediateBox<T>;
+
+    fn make_place(self) -> IntermediateBox<T> {
+        make_place()
+    }
+}
+
+impl<T: ?Sized> Drop for IntermediateBox<T> {
+    fn drop(&mut self) {
+        if self.size > 0 {
+            unsafe {
+                heap::deallocate(self.ptr, self.size, self.align)
+            }
+        }
+    }
+}
 
 impl<T> Box<T> {
     /// Allocates memory on the heap and then moves `x` into it.
@@ -199,8 +297,7 @@ impl<T: Clone> Clone for Box<T> {
     /// let y = x.clone();
     /// ```
     #[inline]
-    fn clone(&self) -> Box<T> { box {(**self).clone()} }
-
+    fn clone(&self) -> Box<T> { box (HEAP) {(**self).clone()} }
     /// Copies `source`'s contents into `self` without creating a new allocation.
     ///
     /// # Examples
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index ead0b4259a9..f66495c4057 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -70,6 +70,8 @@
        test(no_crate_inject))]
 #![no_std]
 
+// SNAP d4432b3
+#![allow(unused_features)] // until feature(placement_in_syntax) is in snap
 #![feature(allocator)]
 #![feature(box_syntax)]
 #![feature(coerce_unsized)]
@@ -82,6 +84,8 @@
 #![feature(no_std)]
 #![feature(nonzero)]
 #![feature(optin_builtin_traits)]
+#![feature(placement_in_syntax)]
+#![feature(placement_new_protocol)]
 #![feature(raw)]
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index ef022179772..1d466895f2c 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -184,6 +184,14 @@ extern "rust-intrinsic" {
     /// elements.
     pub fn size_of<T>() -> usize;
 
+    #[cfg(not(stage0))]
+    /// Moves a value to an uninitialized memory location.
+    ///
+    /// Drop glue is not run on the destination.
+    pub fn move_val_init<T>(dst: *mut T, src: T);
+
+    // SNAP d4432b3
+    #[cfg(stage0)]
     /// Moves a value to an uninitialized memory location.
     ///
     /// Drop glue is not run on the destination.
diff --git a/src/libcore/ops.rs b/src/libcore/ops.rs
index c2a9b8c8308..2ea42011a5c 100644
--- a/src/libcore/ops.rs
+++ b/src/libcore/ops.rs
@@ -1266,3 +1266,120 @@ impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {}
 
 // *const T -> *const U
 impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
+
+/// Both `in (PLACE) EXPR` and `box EXPR` desugar into expressions
+/// that allocate an intermediate "place" that holds uninitialized
+/// state.  The desugaring evaluates EXPR, and writes the result at
+/// the address returned by the `pointer` method of this trait.
+///
+/// A `Place` can be thought of as a special representation for a
+/// hypothetical `&uninit` reference (which Rust cannot currently
+/// express directly). That is, it represents a pointer to
+/// uninitialized storage.
+///
+/// The client is responsible for two steps: First, initializing the
+/// payload (it can access its address via `pointer`). Second,
+/// converting the agent to an instance of the owning pointer, via the
+/// appropriate `finalize` method (see the `InPlace`.
+///
+/// If evaluating EXPR fails, then the destructor for the
+/// implementation of Place to clean up any intermediate state
+/// (e.g. deallocate box storage, pop a stack, etc).
+#[unstable(feature = "placement_new_protocol")]
+pub trait Place<Data: ?Sized> {
+    /// Returns the address where the input value will be written.
+    /// Note that the data at this address is generally uninitialized,
+    /// and thus one should use `ptr::write` for initializing it.
+    fn pointer(&mut self) -> *mut Data;
+}
+
+/// Interface to implementations of  `in (PLACE) EXPR`.
+///
+/// `in (PLACE) EXPR` effectively desugars into:
+///
+/// ```rust,ignore
+/// let p = PLACE;
+/// let mut place = Placer::make_place(p);
+/// let raw_place = Place::pointer(&mut place);
+/// let value = EXPR;
+/// unsafe {
+///     std::ptr::write(raw_place, value);
+///     InPlace::finalize(place)
+/// }
+/// ```
+///
+/// The type of `in (PLACE) EXPR` is derived from the type of `PLACE`;
+/// if the type of `PLACE` is `P`, then the final type of the whole
+/// expression is `P::Place::Owner` (see the `InPlace` and `Boxed`
+/// traits).
+///
+/// Values for types implementing this trait usually are transient
+/// intermediate values (e.g. the return value of `Vec::emplace_back`)
+/// or `Copy`, since the `make_place` method takes `self` by value.
+#[unstable(feature = "placement_new_protocol")]
+pub trait Placer<Data: ?Sized> {
+    /// `Place` is the intermedate agent guarding the
+    /// uninitialized state for `Data`.
+    type Place: InPlace<Data>;
+
+    /// Creates a fresh place from `self`.
+    fn make_place(self) -> Self::Place;
+}
+
+/// Specialization of `Place` trait supporting `in (PLACE) EXPR`.
+#[unstable(feature = "placement_new_protocol")]
+pub trait InPlace<Data: ?Sized>: Place<Data> {
+    /// `Owner` is the type of the end value of `in (PLACE) EXPR`
+    ///
+    /// Note that when `in (PLACE) EXPR` is solely used for
+    /// side-effecting an existing data-structure,
+    /// e.g. `Vec::emplace_back`, then `Owner` need not carry any
+    /// information at all (e.g. it can be the unit type `()` in that
+    /// case).
+    type Owner;
+
+    /// Converts self into the final value, shifting
+    /// deallocation/cleanup responsibilities (if any remain), over to
+    /// the returned instance of `Owner` and forgetting self.
+    unsafe fn finalize(self) -> Self::Owner;
+}
+
+/// Core trait for the `box EXPR` form.
+///
+/// `box EXPR` effectively desugars into:
+///
+/// ```rust,ignore
+/// let mut place = BoxPlace::make_place();
+/// let raw_place = Place::pointer(&mut place);
+/// let value = EXPR;
+/// unsafe {
+///     ::std::ptr::write(raw_place, value);
+///     Boxed::finalize(place)
+/// }
+/// ```
+///
+/// The type of `box EXPR` is supplied from its surrounding
+/// context; in the above expansion, the result type `T` is used
+/// to determine which implementation of `Boxed` to use, and that
+/// `<T as Boxed>` in turn dictates determines which
+/// implementation of `BoxPlace` to use, namely:
+/// `<<T as Boxed>::Place as BoxPlace>`.
+#[unstable(feature = "placement_new_protocol")]
+pub trait Boxed {
+    /// The kind of data that is stored in this kind of box.
+    type Data;  /* (`Data` unused b/c cannot yet express below bound.) */
+    /// The place that will negotiate the storage of the data.
+    type Place: BoxPlace<Self::Data>;
+
+    /// Converts filled place into final owning value, shifting
+    /// deallocation/cleanup responsibilities (if any remain), over to
+    /// returned instance of `Self` and forgetting `filled`.
+    unsafe fn finalize(filled: Self::Place) -> Self;
+}
+
+/// Specialization of `Place` trait supporting `box EXPR`.
+#[unstable(feature = "placement_new_protocol")]
+pub trait BoxPlace<Data: ?Sized> : Place<Data> {
+    /// Creates a globally fresh place.
+    fn make_place() -> Self;
+}
diff --git a/src/librustc/middle/effect.rs b/src/librustc/middle/effect.rs
index b2a064bf86c..3fe1e2f5e83 100644
--- a/src/librustc/middle/effect.rs
+++ b/src/librustc/middle/effect.rs
@@ -10,7 +10,7 @@
 
 //! Enforces the Rust effect system. Currently there is just one effect,
 //! `unsafe`.
-use self::UnsafeContext::*;
+use self::RootUnsafeContext::*;
 
 use middle::def;
 use middle::ty::{self, Ty};
@@ -21,8 +21,20 @@ use syntax::codemap::Span;
 use syntax::visit;
 use syntax::visit::Visitor;
 
+#[derive(Copy, Clone)]
+struct UnsafeContext {
+    push_unsafe_count: usize,
+    root: RootUnsafeContext,
+}
+
+impl UnsafeContext {
+    fn new(root: RootUnsafeContext) -> UnsafeContext {
+        UnsafeContext { root: root, push_unsafe_count: 0 }
+    }
+}
+
 #[derive(Copy, Clone, PartialEq)]
-enum UnsafeContext {
+enum RootUnsafeContext {
     SafeContext,
     UnsafeFn,
     UnsafeBlock(ast::NodeId),
@@ -44,7 +56,8 @@ struct EffectCheckVisitor<'a, 'tcx: 'a> {
 
 impl<'a, 'tcx> EffectCheckVisitor<'a, 'tcx> {
     fn require_unsafe(&mut self, span: Span, description: &str) {
-        match self.unsafe_context {
+        if self.unsafe_context.push_unsafe_count > 0 { return; }
+        match self.unsafe_context.root {
             SafeContext => {
                 // Report an error.
                 span_err!(self.tcx.sess, span, E0133,
@@ -75,9 +88,9 @@ impl<'a, 'tcx, 'v> Visitor<'v> for EffectCheckVisitor<'a, 'tcx> {
 
         let old_unsafe_context = self.unsafe_context;
         if is_unsafe_fn {
-            self.unsafe_context = UnsafeFn
+            self.unsafe_context = UnsafeContext::new(UnsafeFn)
         } else if is_item_fn {
-            self.unsafe_context = SafeContext
+            self.unsafe_context = UnsafeContext::new(SafeContext)
         }
 
         visit::walk_fn(self, fn_kind, fn_decl, block, span);
@@ -105,10 +118,18 @@ impl<'a, 'tcx, 'v> Visitor<'v> for EffectCheckVisitor<'a, 'tcx> {
                 // external blocks (e.g. `unsafe { println("") }`,
                 // expands to `unsafe { ... unsafe { ... } }` where
                 // the inner one is compiler generated).
-                if self.unsafe_context == SafeContext || source == ast::CompilerGenerated {
-                    self.unsafe_context = UnsafeBlock(block.id)
+                if self.unsafe_context.root == SafeContext || source == ast::CompilerGenerated {
+                    self.unsafe_context.root = UnsafeBlock(block.id)
                 }
             }
+            ast::PushUnsafeBlock(..) => {
+                self.unsafe_context.push_unsafe_count =
+                    self.unsafe_context.push_unsafe_count.checked_add(1).unwrap();
+            }
+            ast::PopUnsafeBlock(..) => {
+                self.unsafe_context.push_unsafe_count =
+                    self.unsafe_context.push_unsafe_count.checked_sub(1).unwrap();
+            }
         }
 
         visit::walk_block(self, block);
@@ -162,7 +183,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for EffectCheckVisitor<'a, 'tcx> {
 pub fn check_crate(tcx: &ty::ctxt) {
     let mut visitor = EffectCheckVisitor {
         tcx: tcx,
-        unsafe_context: SafeContext,
+        unsafe_context: UnsafeContext::new(SafeContext),
     };
 
     visit::walk_crate(&mut visitor, tcx.map.krate());
diff --git a/src/librustc/middle/expr_use_visitor.rs b/src/librustc/middle/expr_use_visitor.rs
index 54fc2daff2b..f27a96545dd 100644
--- a/src/librustc/middle/expr_use_visitor.rs
+++ b/src/librustc/middle/expr_use_visitor.rs
@@ -555,6 +555,11 @@ impl<'d,'t,'a,'tcx> ExprUseVisitor<'d,'t,'a,'tcx> {
                     None => {}
                 }
                 self.consume_expr(&**base);
+                if place.is_some() {
+                    self.tcx().sess.span_bug(
+                        expr.span,
+                        "box with explicit place remains after expansion");
+                }
             }
 
             ast::ExprMac(..) => {
diff --git a/src/librustc/middle/stability.rs b/src/librustc/middle/stability.rs
index 30553d62719..6d3bc7fb68c 100644
--- a/src/librustc/middle/stability.rs
+++ b/src/librustc/middle/stability.rs
@@ -543,9 +543,19 @@ pub fn check_pat(tcx: &ty::ctxt, pat: &ast::Pat,
 
 fn maybe_do_stability_check(tcx: &ty::ctxt, id: ast::DefId, span: Span,
                             cb: &mut FnMut(ast::DefId, Span, &Option<&Stability>)) {
-    if !is_staged_api(tcx, id) { return  }
-    if is_internal(tcx, span) { return }
+    if !is_staged_api(tcx, id) {
+        debug!("maybe_do_stability_check: \
+                skipping id={:?} since it is not staged_api", id);
+        return;
+    }
+    if is_internal(tcx, span) {
+        debug!("maybe_do_stability_check: \
+                skipping span={:?} since it is internal", span);
+        return;
+    }
     let ref stability = lookup(tcx, id);
+    debug!("maybe_do_stability_check: \
+            inspecting id={:?} span={:?} of stability={:?}", id, span, stability);
     cb(id, span, stability);
 }
 
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index d77d48eea48..9486e4953f8 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -232,12 +232,13 @@ impl<'tcx> Expectation<'tcx> {
 pub struct UnsafetyState {
     pub def: ast::NodeId,
     pub unsafety: ast::Unsafety,
+    pub unsafe_push_count: u32,
     from_fn: bool
 }
 
 impl UnsafetyState {
     pub fn function(unsafety: ast::Unsafety, def: ast::NodeId) -> UnsafetyState {
-        UnsafetyState { def: def, unsafety: unsafety, from_fn: true }
+        UnsafetyState { def: def, unsafety: unsafety, unsafe_push_count: 0, from_fn: true }
     }
 
     pub fn recurse(&mut self, blk: &ast::Block) -> UnsafetyState {
@@ -249,13 +250,20 @@ impl UnsafetyState {
             ast::Unsafety::Unsafe if self.from_fn => *self,
 
             unsafety => {
-                let (unsafety, def) = match blk.rules {
-                    ast::UnsafeBlock(..) => (ast::Unsafety::Unsafe, blk.id),
-                    ast::DefaultBlock => (unsafety, self.def),
+                let (unsafety, def, count) = match blk.rules {
+                    ast::PushUnsafeBlock(..) =>
+                        (unsafety, blk.id, self.unsafe_push_count.checked_add(1).unwrap()),
+                    ast::PopUnsafeBlock(..) =>
+                        (unsafety, blk.id, self.unsafe_push_count.checked_sub(1).unwrap()),
+                    ast::UnsafeBlock(..) =>
+                        (ast::Unsafety::Unsafe, blk.id, self.unsafe_push_count),
+                    ast::DefaultBlock =>
+                        (unsafety, self.def, self.unsafe_push_count),
                 };
                 UnsafetyState{ def: def,
-                             unsafety: unsafety,
-                             from_fn: false }
+                               unsafety: unsafety,
+                               unsafe_push_count: count,
+                               from_fn: false }
             }
         }
     }
@@ -4884,9 +4892,7 @@ pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &ast::ForeignItem) {
             "move_val_init" => {
                 (1,
                  vec!(
-                    tcx.mk_mut_ref(tcx.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1),
-                                                                  ty::BrAnon(0))),
-                                    param(ccx, 0)),
+                    tcx.mk_mut_ptr(param(ccx, 0)),
                     param(ccx, 0)
                   ),
                tcx.mk_nil())
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 82bc1314ad5..907eb3ed401 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -203,6 +203,7 @@
        test(no_crate_inject, attr(deny(warnings))),
        test(attr(allow(dead_code, deprecated, unused_variables, unused_mut))))]
 
+#![cfg_attr(stage0, allow(unused_features))]
 #![feature(alloc)]
 #![feature(allow_internal_unstable)]
 #![feature(associated_consts)]
@@ -234,6 +235,7 @@
 #![feature(no_std)]
 #![feature(oom)]
 #![feature(optin_builtin_traits)]
+#![feature(placement_in_syntax)]
 #![feature(rand)]
 #![feature(raw)]
 #![feature(reflect_marker)]
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index a0059d33bed..fba9db401db 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -810,6 +810,8 @@ pub type SpannedIdent = Spanned<Ident>;
 pub enum BlockCheckMode {
     DefaultBlock,
     UnsafeBlock(UnsafeSource),
+    PushUnsafeBlock(UnsafeSource),
+    PopUnsafeBlock(UnsafeSource),
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
diff --git a/src/libsyntax/codemap.rs b/src/libsyntax/codemap.rs
index e6bc3218897..17e6b2c2e12 100644
--- a/src/libsyntax/codemap.rs
+++ b/src/libsyntax/codemap.rs
@@ -980,6 +980,10 @@ impl CodeMap {
                             mac_span.lo <= span.lo && span.hi <= mac_span.hi
                         });
 
+                    debug!("span_allows_unstable: span: {:?} call_site: {:?} callee: {:?}",
+                           (span.lo, span.hi),
+                           (info.call_site.lo, info.call_site.hi),
+                           info.callee.span.map(|x| (x.lo, x.hi)));
                     debug!("span_allows_unstable: from this expansion? {}, allows unstable? {}",
                            span_comes_from_this_expansion,
                            info.callee.allow_internal_unstable);
diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 499562edc0c..409ae86db35 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -591,6 +591,12 @@ fn initial_syntax_expander_table<'feat>(ecfg: &expand::ExpansionConfig<'feat>)
     syntax_expanders.insert(intern("cfg"),
                             builtin_normal_expander(
                                     ext::cfg::expand_cfg));
+    syntax_expanders.insert(intern("push_unsafe"),
+                            builtin_normal_expander(
+                                ext::pushpop_safe::expand_push_unsafe));
+    syntax_expanders.insert(intern("pop_unsafe"),
+                            builtin_normal_expander(
+                                ext::pushpop_safe::expand_pop_unsafe));
     syntax_expanders.insert(intern("trace_macros"),
                             builtin_normal_expander(
                                     ext::trace_macros::expand_trace_macros));
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 53befc092da..faa1e5b2f51 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -33,6 +33,16 @@ use visit;
 use visit::Visitor;
 use std_inject;
 
+// Given suffix ["b","c","d"], returns path `::std::b::c::d` when
+// `fld.cx.use_std`, and `::core::b::c::d` otherwise.
+fn mk_core_path(fld: &mut MacroExpander,
+                span: Span,
+                suffix: &[&'static str]) -> ast::Path {
+    let mut idents = vec![fld.cx.ident_of_std("core")];
+    for s in suffix.iter() { idents.push(fld.cx.ident_of(*s)); }
+    fld.cx.path_global(span, idents)
+}
+
 pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
     fn push_compiler_expansion(fld: &mut MacroExpander, span: Span, expansion_desc: &str) {
         fld.cx.bt_push(ExpnInfo {
@@ -40,13 +50,26 @@ pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
             callee: NameAndSpan {
                 name: expansion_desc.to_string(),
                 format: CompilerExpansion,
+
+                // This does *not* mean code generated after
+                // `push_compiler_expansion` is automatically exempt
+                // from stability lints; must also tag such code with
+                // an appropriate span from `fld.cx.backtrace()`.
                 allow_internal_unstable: true,
+
                 span: None,
             },
         });
     }
 
-    e.and_then(|ast::Expr {id, node, span}| match node {
+    // Sets the expn_id so that we can use unstable methods.
+    fn allow_unstable(fld: &mut MacroExpander, span: Span) -> Span {
+        Span { expn_id: fld.cx.backtrace(), ..span }
+    }
+
+    let expr_span = e.span;
+    return e.and_then(|ast::Expr {id, node, span}| match node {
+
         // expr_mac should really be expr_ext or something; it's the
         // entry-point for all syntax extensions.
         ast::ExprMac(mac) => {
@@ -71,6 +94,118 @@ pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
             })
         }
 
+        // Desugar ExprBox: `in (PLACE) EXPR`
+        ast::ExprBox(Some(placer), value_expr) => {
+            // to:
+            //
+            // let p = PLACE;
+            // let mut place = Placer::make_place(p);
+            // let raw_place = Place::pointer(&mut place);
+            // push_unsafe!({
+            //     std::intrinsics::move_val_init(raw_place, pop_unsafe!( EXPR ));
+            //     InPlace::finalize(place)
+            // })
+
+            // Ensure feature-gate is enabled
+            feature_gate::check_for_placement_in(
+                fld.cx.ecfg.features,
+                &fld.cx.parse_sess.span_diagnostic,
+                expr_span);
+
+            push_compiler_expansion(fld, expr_span, "placement-in expansion");
+
+            let value_span = value_expr.span;
+            let placer_span = placer.span;
+
+            let placer_expr = fld.fold_expr(placer);
+            let value_expr = fld.fold_expr(value_expr);
+
+            let placer_ident = token::gensym_ident("placer");
+            let agent_ident = token::gensym_ident("place");
+            let p_ptr_ident = token::gensym_ident("p_ptr");
+
+            let placer = fld.cx.expr_ident(span, placer_ident);
+            let agent = fld.cx.expr_ident(span, agent_ident);
+            let p_ptr = fld.cx.expr_ident(span, p_ptr_ident);
+
+            let make_place = ["ops", "Placer", "make_place"];
+            let place_pointer = ["ops", "Place", "pointer"];
+            let move_val_init = ["intrinsics", "move_val_init"];
+            let inplace_finalize = ["ops", "InPlace", "finalize"];
+
+            let make_call = |fld: &mut MacroExpander, p, args| {
+                // We feed in the `expr_span` because codemap's span_allows_unstable
+                // allows the call_site span to inherit the `allow_internal_unstable`
+                // setting.
+                let span_unstable = allow_unstable(fld, expr_span);
+                let path = mk_core_path(fld, span_unstable, p);
+                let path = fld.cx.expr_path(path);
+                let expr_span_unstable = allow_unstable(fld, span);
+                fld.cx.expr_call(expr_span_unstable, path, args)
+            };
+
+            let stmt_let = |fld: &mut MacroExpander, bind, expr| {
+                fld.cx.stmt_let(placer_span, false, bind, expr)
+            };
+            let stmt_let_mut = |fld: &mut MacroExpander, bind, expr| {
+                fld.cx.stmt_let(placer_span, true, bind, expr)
+            };
+
+            // let placer = <placer_expr> ;
+            let s1 = stmt_let(fld, placer_ident, placer_expr);
+
+            // let mut place = Placer::make_place(placer);
+            let s2 = {
+                let call = make_call(fld, &make_place, vec![placer]);
+                stmt_let_mut(fld, agent_ident, call)
+            };
+
+            // let p_ptr = Place::pointer(&mut place);
+            let s3 = {
+                let args = vec![fld.cx.expr_mut_addr_of(placer_span, agent.clone())];
+                let call = make_call(fld, &place_pointer, args);
+                stmt_let(fld, p_ptr_ident, call)
+            };
+
+            // pop_unsafe!(EXPR));
+            let pop_unsafe_expr = pop_unsafe_expr(fld.cx, value_expr, value_span);
+
+            // push_unsafe!({
+            //     ptr::write(p_ptr, pop_unsafe!(<value_expr>));
+            //     InPlace::finalize(place)
+            // })
+            let expr = {
+                let call_move_val_init = StmtSemi(make_call(
+                    fld, &move_val_init, vec![p_ptr, pop_unsafe_expr]), ast::DUMMY_NODE_ID);
+                let call_move_val_init = codemap::respan(value_span, call_move_val_init);
+
+                let call = make_call(fld, &inplace_finalize, vec![agent]);
+                Some(push_unsafe_expr(fld.cx, vec![P(call_move_val_init)], call, span))
+            };
+
+            let block = fld.cx.block_all(span, vec![s1, s2, s3], expr);
+            let result = fld.cx.expr_block(block);
+            fld.cx.bt_pop();
+            result
+        }
+
+        // Issue #22181:
+        // Eventually a desugaring for `box EXPR`
+        // (similar to the desugaring above for `in PLACE BLOCK`)
+        // should go here, desugaring
+        //
+        // to:
+        //
+        // let mut place = BoxPlace::make_place();
+        // let raw_place = Place::pointer(&mut place);
+        // let value = $value;
+        // unsafe {
+        //     ::std::ptr::write(raw_place, value);
+        //     Boxed::finalize(place)
+        // }
+        //
+        // But for now there are type-inference issues doing that.
+
         ast::ExprWhile(cond, body, opt_ident) => {
             let cond = fld.fold_expr(cond);
             let (body, opt_ident) = expand_loop_block(body, opt_ident, fld);
@@ -360,7 +495,26 @@ pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
                 span: span
             }, fld))
         }
-    })
+    });
+
+    fn push_unsafe_expr(cx: &mut ExtCtxt, stmts: Vec<P<ast::Stmt>>,
+                        expr: P<ast::Expr>, span: Span)
+                        -> P<ast::Expr> {
+        let rules = ast::PushUnsafeBlock(ast::CompilerGenerated);
+        cx.expr_block(P(ast::Block {
+            rules: rules, span: span, id: ast::DUMMY_NODE_ID,
+            stmts: stmts, expr: Some(expr),
+        }))
+    }
+
+    fn pop_unsafe_expr(cx: &mut ExtCtxt, expr: P<ast::Expr>, span: Span)
+                       -> P<ast::Expr> {
+        let rules = ast::PopUnsafeBlock(ast::CompilerGenerated);
+        cx.expr_block(P(ast::Block {
+            rules: rules, span: span, id: ast::DUMMY_NODE_ID,
+            stmts: vec![], expr: Some(expr),
+        }))
+    }
 }
 
 /// Expand a (not-ident-style) macro invocation. Returns the result
@@ -1504,6 +1658,7 @@ impl<'feat> ExpansionConfig<'feat> {
         fn enable_trace_macros = allow_trace_macros,
         fn enable_allow_internal_unstable = allow_internal_unstable,
         fn enable_custom_derive = allow_custom_derive,
+        fn enable_pushpop_unsafe = allow_pushpop_unsafe,
     }
 }
 
diff --git a/src/libsyntax/ext/pushpop_safe.rs b/src/libsyntax/ext/pushpop_safe.rs
new file mode 100644
index 00000000000..a67d550d3cd
--- /dev/null
+++ b/src/libsyntax/ext/pushpop_safe.rs
@@ -0,0 +1,94 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+/*
+ * The compiler code necessary to support the `push_unsafe!` and
+ * `pop_unsafe!` macros.
+ *
+ * This is a hack to allow a kind of "safety hygiene", where a macro
+ * can generate code with an interior expression that inherits the
+ * safety of some outer context.
+ *
+ * For example, in:
+ *
+ * ```rust
+ * fn foo() { push_unsafe!( { EXPR_1; pop_unsafe!( EXPR_2 ) } ) }
+ * ```
+ *
+ * the `EXPR_1` is considered to be in an `unsafe` context,
+ * but `EXPR_2` is considered to be in a "safe" (i.e. checked) context.
+ *
+ * For comparison, in:
+ *
+ * ```rust
+ * fn foo() { unsafe { push_unsafe!( { EXPR_1; pop_unsafe!( EXPR_2 ) } ) } }
+ * ```
+ *
+ * both `EXPR_1` and `EXPR_2` are considered to be in `unsafe`
+ * contexts.
+ *
+ */
+
+use ast;
+use codemap::Span;
+use ext::base::*;
+use ext::base;
+use ext::build::AstBuilder;
+use feature_gate;
+use ptr::P;
+
+enum PushPop { Push, Pop }
+
+pub fn expand_push_unsafe<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
+                               -> Box<base::MacResult+'cx> {
+    expand_pushpop_unsafe(cx, sp, tts, PushPop::Push)
+}
+
+pub fn expand_pop_unsafe<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
+                               -> Box<base::MacResult+'cx> {
+    expand_pushpop_unsafe(cx, sp, tts, PushPop::Pop)
+}
+
+fn expand_pushpop_unsafe<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree],
+                                  pp: PushPop) -> Box<base::MacResult+'cx> {
+    feature_gate::check_for_pushpop_syntax(
+        cx.ecfg.features, &cx.parse_sess.span_diagnostic, sp);
+
+    let mut exprs = match get_exprs_from_tts(cx, sp, tts) {
+        Some(exprs) => exprs.into_iter(),
+        None => return DummyResult::expr(sp),
+    };
+
+    let expr = match (exprs.next(), exprs.next()) {
+        (Some(expr), None) => expr,
+        _ => {
+            let msg = match pp {
+                PushPop::Push => "push_unsafe! takes 1 arguments",
+                PushPop::Pop => "pop_unsafe! takes 1 arguments",
+            };
+            cx.span_err(sp, msg);
+            return DummyResult::expr(sp);
+        }
+    };
+
+    let source = ast::UnsafeSource::CompilerGenerated;
+    let check_mode = match pp {
+        PushPop::Push => ast::BlockCheckMode::PushUnsafeBlock(source),
+        PushPop::Pop => ast::BlockCheckMode::PopUnsafeBlock(source),
+    };
+
+    MacEager::expr(cx.expr_block(P(ast::Block {
+        stmts: vec![],
+        expr: Some(expr),
+        id: ast::DUMMY_NODE_ID,
+        rules: check_mode,
+        span: sp
+    })))
+}
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index ab8cf9ae6b6..8c6855036f6 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -80,6 +80,8 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
     ("visible_private_types", "1.0.0", Active),
     ("slicing_syntax", "1.0.0", Accepted),
     ("box_syntax", "1.0.0", Active),
+    ("placement_in_syntax", "1.0.0", Active),
+    ("pushpop_unsafe", "1.2.0", Active),
     ("on_unimplemented", "1.0.0", Active),
     ("simd_ffi", "1.0.0", Active),
     ("allocator", "1.0.0", Active),
@@ -325,6 +327,9 @@ pub struct Features {
     pub allow_trace_macros: bool,
     pub allow_internal_unstable: bool,
     pub allow_custom_derive: bool,
+    pub allow_placement_in: bool,
+    pub allow_box: bool,
+    pub allow_pushpop_unsafe: bool,
     pub simd_ffi: bool,
     pub unmarked_api: bool,
     pub negate_unsigned: bool,
@@ -348,6 +353,9 @@ impl Features {
             allow_trace_macros: false,
             allow_internal_unstable: false,
             allow_custom_derive: false,
+            allow_placement_in: false,
+            allow_box: false,
+            allow_pushpop_unsafe: false,
             simd_ffi: false,
             unmarked_api: false,
             negate_unsigned: false,
@@ -358,6 +366,36 @@ impl Features {
     }
 }
 
+const EXPLAIN_BOX_SYNTAX: &'static str =
+    "box expression syntax is experimental; you can call `Box::new` instead.";
+
+const EXPLAIN_PLACEMENT_IN: &'static str =
+    "placement-in expression syntax is experimental and subject to change.";
+
+const EXPLAIN_PUSHPOP_UNSAFE: &'static str =
+    "push/pop_unsafe macros are experimental and subject to change.";
+
+pub fn check_for_box_syntax(f: Option<&Features>, diag: &SpanHandler, span: Span) {
+    if let Some(&Features { allow_box: true, .. }) = f {
+        return;
+    }
+    emit_feature_err(diag, "box_syntax", span, EXPLAIN_BOX_SYNTAX);
+}
+
+pub fn check_for_placement_in(f: Option<&Features>, diag: &SpanHandler, span: Span) {
+    if let Some(&Features { allow_placement_in: true, .. }) = f {
+        return;
+    }
+    emit_feature_err(diag, "placement_in_syntax", span, EXPLAIN_PLACEMENT_IN);
+}
+
+pub fn check_for_pushpop_syntax(f: Option<&Features>, diag: &SpanHandler, span: Span) {
+    if let Some(&Features { allow_pushpop_unsafe: true, .. }) = f {
+        return;
+    }
+    emit_feature_err(diag, "pushpop_unsafe", span, EXPLAIN_PUSHPOP_UNSAFE);
+}
+
 struct Context<'a> {
     features: Vec<&'static str>,
     span_handler: &'a SpanHandler,
@@ -366,6 +404,11 @@ struct Context<'a> {
 }
 
 impl<'a> Context<'a> {
+    fn enable_feature(&mut self, feature: &'static str) {
+        debug!("enabling feature: {}", feature);
+        self.features.push(feature);
+    }
+
     fn gate_feature(&self, feature: &str, span: Span, explain: &str) {
         let has_feature = self.has_feature(feature);
         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", feature, span, has_feature);
@@ -488,6 +531,26 @@ impl<'a, 'v> Visitor<'v> for MacroVisitor<'a> {
     fn visit_attribute(&mut self, attr: &'v ast::Attribute) {
         self.context.check_attribute(attr, true);
     }
+
+    fn visit_expr(&mut self, e: &ast::Expr) {
+        // Issue 22181: overloaded-`box` and placement-`in` are
+        // implemented via a desugaring expansion, so their feature
+        // gates go into MacroVisitor since that works pre-expansion.
+        //
+        // Issue 22234: we also check during expansion as well.
+        // But we keep these checks as a pre-expansion check to catch
+        // uses in e.g. conditionalized code.
+
+        if let ast::ExprBox(None, _) = e.node {
+            self.context.gate_feature("box_syntax", e.span, EXPLAIN_BOX_SYNTAX);
+        }
+
+        if let ast::ExprBox(Some(_), _) = e.node {
+            self.context.gate_feature("placement_in_syntax", e.span, EXPLAIN_PLACEMENT_IN);
+        }
+
+        visit::walk_expr(self, e);
+    }
 }
 
 struct PostExpansionVisitor<'a> {
@@ -754,7 +817,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
                     match KNOWN_FEATURES.iter()
                                         .find(|& &(n, _, _)| name == n) {
                         Some(&(name, _, Active)) => {
-                            cx.features.push(name);
+                            cx.enable_feature(name);
                         }
                         Some(&(_, _, Removed)) => {
                             span_handler.span_err(mi.span, "feature has been removed");
@@ -787,6 +850,9 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
         allow_trace_macros: cx.has_feature("trace_macros"),
         allow_internal_unstable: cx.has_feature("allow_internal_unstable"),
         allow_custom_derive: cx.has_feature("custom_derive"),
+        allow_placement_in: cx.has_feature("placement_in_syntax"),
+        allow_box: cx.has_feature("box_syntax"),
+        allow_pushpop_unsafe: cx.has_feature("pushpop_unsafe"),
         simd_ffi: cx.has_feature("simd_ffi"),
         unmarked_api: cx.has_feature("unmarked_api"),
         negate_unsigned: cx.has_feature("negate_unsigned"),
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index d93af5da13c..5424c0b214a 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -120,6 +120,7 @@ pub mod ext {
     pub mod log_syntax;
     pub mod mtwt;
     pub mod quote;
+    pub mod pushpop_safe;
     pub mod source_util;
     pub mod trace_macros;
 
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 4adfa3c3119..c2a2259a80a 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2612,18 +2612,43 @@ impl<'a> Parser<'a> {
             ex = ExprAddrOf(m, e);
           }
           token::Ident(_, _) => {
-            if !self.check_keyword(keywords::Box) {
+            if !self.check_keyword(keywords::Box) && !self.check_keyword(keywords::In) {
                 return self.parse_dot_or_call_expr();
             }
 
             let lo = self.span.lo;
-            let box_hi = self.span.hi;
+            let keyword_hi = self.span.hi;
 
+            let is_in = self.token.is_keyword(keywords::In);
             try!(self.bump());
 
-            // Check for a place: `box(PLACE) EXPR`.
+            if is_in {
+              let place = try!(self.parse_expr_res(Restrictions::RESTRICTION_NO_STRUCT_LITERAL));
+              let blk = try!(self.parse_block());
+              hi = blk.span.hi;
+              let blk_expr = self.mk_expr(blk.span.lo, blk.span.hi, ExprBlock(blk));
+              ex = ExprBox(Some(place), blk_expr);
+              return Ok(self.mk_expr(lo, hi, ex));
+            }
+
+            // FIXME (#22181) Remove `box (PLACE) EXPR` support
+            // entirely after next release (enabling `(box (EXPR))`),
+            // since it will be replaced by `in PLACE { EXPR }`, ...
+            //
+            // ... but for now: check for a place: `box(PLACE) EXPR`.
+
             if try!(self.eat(&token::OpenDelim(token::Paren)) ){
-                // Support `box() EXPR` as the default.
+                // SNAP d4432b3
+                // Enable this warning after snapshot ...
+                //
+                // let box_span = mk_sp(lo, self.last_span.hi);
+                // self.span_warn(
+                //     box_span,
+                //     "deprecated syntax; use the `in` keyword now \
+                //            (e.g. change `box (<expr>) <expr>` to \
+                //                         `in <expr> { <expr> }`)");
+
+                // Continue supporting `box () EXPR` (temporarily)
                 if !try!(self.eat(&token::CloseDelim(token::Paren)) ){
                     let place = try!(self.parse_expr_nopanic());
                     try!(self.expect(&token::CloseDelim(token::Paren)));
@@ -2634,10 +2659,15 @@ impl<'a> Parser<'a> {
                         self.span_err(span,
                                       &format!("expected expression, found `{}`",
                                               this_token_to_string));
-                        let box_span = mk_sp(lo, box_hi);
+
+                        // Spanning just keyword avoids constructing
+                        // printout of arg expression (which starts
+                        // with parenthesis, as established above).
+
+                        let box_span = mk_sp(lo, keyword_hi);
                         self.span_suggestion(box_span,
-                                             "try using `box()` instead:",
-                                             "box()".to_string());
+                                             "try using `box ()` instead:",
+                                             format!("box ()"));
                         self.abort_if_errors();
                     }
                     let subexpression = try!(self.parse_prefix_expr());
@@ -2650,6 +2680,7 @@ impl<'a> Parser<'a> {
             // Otherwise, we use the unique pointer default.
             let subexpression = try!(self.parse_prefix_expr());
             hi = subexpression.span.hi;
+
             // FIXME (pnkfelix): After working out kinks with box
             // desugaring, should be `ExprBox(None, subexpression)`
             // instead.
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 6693eed6ace..448857389da 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1434,8 +1434,8 @@ impl<'a> State<'a> {
                                       attrs: &[ast::Attribute],
                                       close_box: bool) -> io::Result<()> {
         match blk.rules {
-            ast::UnsafeBlock(..) => try!(self.word_space("unsafe")),
-            ast::DefaultBlock => ()
+            ast::UnsafeBlock(..) | ast::PushUnsafeBlock(..) => try!(self.word_space("unsafe")),
+            ast::DefaultBlock    | ast::PopUnsafeBlock(..) => ()
         }
         try!(self.maybe_print_comment(blk.span.lo));
         try!(self.ann.pre(self, NodeBlock(blk)));
diff --git a/src/test/compile-fail/feature-gate-box-expr.rs b/src/test/compile-fail/feature-gate-box-expr.rs
index 8f8b035f4a9..9640b2b475b 100644
--- a/src/test/compile-fail/feature-gate-box-expr.rs
+++ b/src/test/compile-fail/feature-gate-box-expr.rs
@@ -8,15 +8,18 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main() {
-    use std::boxed::HEAP;
+// Check that `box EXPR` is feature-gated.
+//
+// See also feature-gate-placement-expr.rs
+//
+// (Note that the two tests are separated since the checks appear to
+// be performed at distinct phases, with an abort_if_errors call
+// separating them.)
 
+fn main() {
     let x = box 'c'; //~ ERROR box expression syntax is experimental
     println!("x: {}", x);
 
     let x = box () 'c'; //~ ERROR box expression syntax is experimental
     println!("x: {}", x);
-
-    let x = box (HEAP) 'c'; //~ ERROR box expression syntax is experimental
-    println!("x: {}", x);
 }
diff --git a/src/test/compile-fail/feature-gate-placement-expr.rs b/src/test/compile-fail/feature-gate-placement-expr.rs
new file mode 100644
index 00000000000..64a1d49f960
--- /dev/null
+++ b/src/test/compile-fail/feature-gate-placement-expr.rs
@@ -0,0 +1,27 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that `in PLACE { EXPR }` is feature-gated.
+//
+// See also feature-gate-box-expr.rs
+//
+// (Note that the two tests are separated since the checks appear to
+// be performed at distinct phases, with an abort_if_errors call
+// separating them.)
+
+fn main() {
+    use std::boxed::HEAP;
+
+    let x = box (HEAP) 'c'; //~ ERROR placement-in expression syntax is experimental
+    println!("x: {}", x);
+
+    let x = in HEAP { 'c' }; //~ ERROR placement-in expression syntax is experimental
+    println!("x: {}", x);
+}
diff --git a/src/test/compile-fail/feature-gate-pushpop-unsafe.rs b/src/test/compile-fail/feature-gate-pushpop-unsafe.rs
new file mode 100644
index 00000000000..e317b4c7d4d
--- /dev/null
+++ b/src/test/compile-fail/feature-gate-pushpop-unsafe.rs
@@ -0,0 +1,14 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let c = push_unsafe!('c'); //~ ERROR push/pop_unsafe macros are experimental
+    let c = pop_unsafe!('c'); //~ ERROR push/pop_unsafe macros are experimental
+}
diff --git a/src/test/compile-fail/issue-14084.rs b/src/test/compile-fail/issue-14084.rs
index 003c6644f7f..8e566d7aafa 100644
--- a/src/test/compile-fail/issue-14084.rs
+++ b/src/test/compile-fail/issue-14084.rs
@@ -9,8 +9,10 @@
 // except according to those terms.
 
 #![feature(box_syntax)]
+#![feature(placement_in_syntax)]
 
 fn main() {
     box ( () ) 0;
-    //~^ ERROR: only the exchange heap is currently supported
+    //~^ ERROR: the trait `core::ops::Placer<_>` is not implemented
+    //~| ERROR: the trait `core::ops::Placer<_>` is not implemented
 }
diff --git a/src/test/compile-fail/pushpop-unsafe-rejects.rs b/src/test/compile-fail/pushpop-unsafe-rejects.rs
new file mode 100644
index 00000000000..72c065ae714
--- /dev/null
+++ b/src/test/compile-fail/pushpop-unsafe-rejects.rs
@@ -0,0 +1,74 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Basic sanity check for `push_unsafe!(EXPR)` and
+// `pop_unsafe!(EXPR)`: we can call unsafe code when there are a
+// positive number of pushes in the stack, or if we are within a
+// normal `unsafe` block, but otherwise cannot.
+
+#![feature(pushpop_unsafe)]
+
+static mut X: i32 = 0;
+
+unsafe fn f() { X += 1; return; }
+fn g() { unsafe { X += 1_000; } return; }
+
+fn main() {
+    push_unsafe!( {
+        f(); pop_unsafe!({
+            f() //~ ERROR: call to unsafe function
+        })
+    } );
+
+    push_unsafe!({
+        f();
+        pop_unsafe!({
+            g();
+            f(); //~ ERROR: call to unsafe function
+        })
+    } );
+
+    push_unsafe!({
+        g(); pop_unsafe!({
+            unsafe {
+                f();
+            }
+            f(); //~ ERROR: call to unsafe function
+        })
+    });
+
+
+    // Note: For implementation simplicity the compiler just
+    // ICE's if you underflow the push_unsafe stack.
+    //
+    // Thus all of the following cases cause an ICE.
+    //
+    // (The "ERROR" notes are from an earlier version
+    //  that used saturated arithmetic rather than checked
+    //  arithmetic.)
+
+    //    pop_unsafe!{ g() };
+    //
+    //    push_unsafe!({
+    //        pop_unsafe!(pop_unsafe!{ g() })
+    //    });
+    //
+    //    push_unsafe!({
+    //        g();
+    //        pop_unsafe!(pop_unsafe!({
+    //            f() // ERROR: call to unsafe function
+    //        }))
+    //    });
+    //
+    //    pop_unsafe!({
+    //        f(); // ERROR: call to unsafe function
+    //    })
+
+}
diff --git a/src/test/parse-fail/parenthesized-box-expr-message.rs b/src/test/parse-fail/parenthesized-box-expr-message.rs
index 09d32a71dea..4c32d2f041e 100644
--- a/src/test/parse-fail/parenthesized-box-expr-message.rs
+++ b/src/test/parse-fail/parenthesized-box-expr-message.rs
@@ -12,7 +12,7 @@
 
 fn main() {
     box (1 + 1)
-    //~^ HELP try using `box()` instead:
-    //~| SUGGESTION box() (1 + 1)
+    //~^ HELP try using `box ()` instead:
+    //~| SUGGESTION box () (1 + 1)
     ; //~ ERROR expected expression, found `;`
 }
diff --git a/src/test/run-pass/intrinsic-move-val.rs b/src/test/run-pass/intrinsic-move-val.rs
index 2e75f2dccd1..84af4fd0a08 100644
--- a/src/test/run-pass/intrinsic-move-val.rs
+++ b/src/test/run-pass/intrinsic-move-val.rs
@@ -20,7 +20,7 @@ use std::mem::{self, transmute};
 mod rusti {
     extern "rust-intrinsic" {
         pub fn init<T>() -> T;
-        pub fn move_val_init<T>(dst: &mut T, src: T);
+        pub fn move_val_init<T>(dst: *mut T, src: T);
     }
 }
 
diff --git a/src/test/run-pass/new-box-syntax.rs b/src/test/run-pass/new-box-syntax.rs
index b5a54a90ae7..da57e8682ca 100644
--- a/src/test/run-pass/new-box-syntax.rs
+++ b/src/test/run-pass/new-box-syntax.rs
@@ -13,8 +13,13 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/ */
 
-#![allow(warnings)]
+#![allow(dead_code, unused_variables)]
 #![feature(box_syntax, box_heap)]
+#![feature(placement_in_syntax)]
+
+// during check-pretty, the expanded code needs to opt into these
+// features
+#![feature(placement_new_protocol, core_intrinsics)]
 
 // Tests that the new `box` syntax works with unique pointers.
 
@@ -30,4 +35,9 @@ pub fn main() {
     let y: Box<isize> = box 2;
     let b: Box<isize> = box()(1 + 2);
     let c = box()(3 + 4);
+
+    let s: Box<Structure> = box Structure {
+        x: 3,
+        y: 4,
+    };
 }
diff --git a/src/test/run-pass/placement-in-syntax.rs b/src/test/run-pass/placement-in-syntax.rs
new file mode 100644
index 00000000000..7bda9ae2524
--- /dev/null
+++ b/src/test/run-pass/placement-in-syntax.rs
@@ -0,0 +1,37 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(dead_code, unused_variables)]
+#![feature(box_heap)]
+#![feature(placement_in_syntax)]
+
+// Tests that the new `in` syntax works with unique pointers.
+//
+// Compare with new-box-syntax.rs
+
+use std::boxed::{Box, HEAP};
+
+struct Structure {
+    x: isize,
+    y: isize,
+}
+
+pub fn main() {
+    let x: Box<isize> = in HEAP { 2 };
+    let b: Box<isize> = in HEAP { 1 + 2 };
+    let c = in HEAP { 3 + 4 };
+
+    let s: Box<Structure> = in HEAP {
+        Structure {
+            x: 3,
+            y: 4,
+        }
+    };
+}
diff --git a/src/test/run-pass/pushpop-unsafe-okay.rs b/src/test/run-pass/pushpop-unsafe-okay.rs
new file mode 100644
index 00000000000..fc402d41368
--- /dev/null
+++ b/src/test/run-pass/pushpop-unsafe-okay.rs
@@ -0,0 +1,56 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Basic sanity check for `push_unsafe!(EXPR)` and
+// `pop_unsafe!(EXPR)`: we can call unsafe code when there are a
+// positive number of pushes in the stack, or if we are within a
+// normal `unsafe` block, but otherwise cannot.
+
+// ignore-pretty because the `push_unsafe!` and `pop_unsafe!` macros
+// are not integrated with the pretty-printer.
+
+#![feature(pushpop_unsafe)]
+
+static mut X: i32 = 0;
+
+unsafe fn f() { X += 1; return; }
+fn g() { unsafe { X += 1_000; } return; }
+
+fn check_reset_x(x: i32) -> bool {
+    #![allow(unused_parens)] // dont you judge my style choices!
+    unsafe {
+        let ret = (x == X);
+        X = 0;
+        ret
+    }
+}
+
+fn main() {
+    // double-check test infrastructure
+    assert!(check_reset_x(0));
+    unsafe { f(); }
+    assert!(check_reset_x(1));
+    assert!(check_reset_x(0));
+    { g(); }
+    assert!(check_reset_x(1000));
+    assert!(check_reset_x(0));
+    unsafe { f(); g(); g(); }
+    assert!(check_reset_x(2001));
+
+    push_unsafe!( { f(); pop_unsafe!( g() ) } );
+    assert!(check_reset_x(1_001));
+    push_unsafe!( { g(); pop_unsafe!( unsafe { f(); f(); } ) } );
+    assert!(check_reset_x(1_002));
+
+    unsafe { push_unsafe!( { f(); pop_unsafe!( { f(); f(); } ) } ); }
+    assert!(check_reset_x(3));
+    push_unsafe!( { f(); push_unsafe!( { pop_unsafe!( { f(); f(); f(); } ) } ); } );
+    assert!(check_reset_x(4));
+}