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.

2 files changed, 107 insertions, 6 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)]