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Auto merge of #48333 - aidanhs:aphs-no-place-for-placement, r=nikomatsakis
Remove all unstable placement features Closes #22181, #27779. Effectively makes the assortment of placement RFCs (rust-lang/rfcs#470, rust-lang/rfcs#809, rust-lang/rfcs#1228) 'unaccepted'. It leaves `box_syntax` and keeps the `<-` token as recognised by libsyntax. ------------------------ I don't know the correct process for unaccepting an unstable feature that was accepted as an RFC so...here's a PR. Let me preface this by saying I'm not particularly happy about doing this (I know it'll be unpopular), but I think it's the most honest expression of how things stand today. I've been motivated by a [post on reddit](https://www.reddit.com/r/rust/comments/7wrqk2/when_will_box_and_placementin_syntax_be_stable/) which asks when these features will be stable - the features have received little RFC-style design work since the end of 2015 (~2 years ago) and leaving them in limbo confuses people who want to know where they're up to. Without additional design work that needs to happen (see the collection of unresolved questions later in this post) they can't really get stabilised, and I think that design work would be most suited to an RFC rather than (currently mostly unused) experimental features in Rust nightly. I have my own motivations - it's very simple to 'defeat' placement in debug mode today and I don't want a placement in Rust that a) has no guarantees to work and b) has no plan for in-place serde deserialisation. There's a quote in [1]: "Ordinarily these uncertainties might lead to the RFC being postponed. [The RFC seems like a promising direction hence we will accept since it] will thus give us immediate experience with the design and help in determining the best final solution.". I propose that there have been enough additional uncertainties raised since then that the original direction is less promising and we should be think about the problem anew. (a historical note: the first mention of placement (under that name - uninit pointers were earlier) in an RFC AFAIK is [0] in late 2014 (pre-1.0). RFCs since then have built on this base - [1] is a comment in Feb 2015 accepting a more conservative design of the Place* traits - this is back when serde still required aster and seemed to break every other nightly! A lot has changed since then, perhaps placement should too) ------------------------ Concrete unresolved questions include: - making placement work in debug mode [7] - making placement work for serde/with fallible creation [5], [irlo2], [8] - trait design: - opting into not consuming the placer in `Placer::make_place` - [2] - trait proliferation - [4] (+ others in that thread) - fallible allocation - [3], [4] (+ others in that thread) - support for DSTs/unsized structs (if at all) - [1], [6] More speculative unresolved questions include: - better trait design with in the context of future language features [irlo1] (Q11), [irlo3] - interaction between custom allocators and placement [irlo3] [0] rust-lang/rfcs#470 [1] rust-lang/rfcs#809 (comment) [2] rust-lang/rfcs#1286 [3] rust-lang/rfcs#1315 [4] #27779 (comment) [5] #27779 (comment) [6] #27779 (comment) [7] #27779 (comment) [8] rust-lang/rfcs#1228 (comment) [irlo1] https://internals.rust-lang.org/t/placement-nwbi-faq-new-box-in-left-arrow/2789 [irlo2] https://internals.rust-lang.org/t/placement-nwbi-faq-new-box-in-left-arrow/2789/19 [irlo3] https://internals.rust-lang.org/t/lang-team-minutes-feature-status-report-placement-in-and-box/4646
2 parents c75d5e2 + 9b5859a commit 199b7e2

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src/grammar/parser-lalr.y

-3
Original file line numberDiff line numberDiff line change
@@ -1400,7 +1400,6 @@ nonblock_expr
14001400
| BREAK lifetime { $$ = mk_node("ExprBreak", 1, $2); }
14011401
| YIELD { $$ = mk_node("ExprYield", 0); }
14021402
| YIELD expr { $$ = mk_node("ExprYield", 1, $2); }
1403-
| nonblock_expr LARROW expr { $$ = mk_node("ExprInPlace", 2, $1, $3); }
14041403
| nonblock_expr '=' expr { $$ = mk_node("ExprAssign", 2, $1, $3); }
14051404
| nonblock_expr SHLEQ expr { $$ = mk_node("ExprAssignShl", 2, $1, $3); }
14061405
| nonblock_expr SHREQ expr { $$ = mk_node("ExprAssignShr", 2, $1, $3); }
@@ -1463,7 +1462,6 @@ expr
14631462
| BREAK ident { $$ = mk_node("ExprBreak", 1, $2); }
14641463
| YIELD { $$ = mk_node("ExprYield", 0); }
14651464
| YIELD expr { $$ = mk_node("ExprYield", 1, $2); }
1466-
| expr LARROW expr { $$ = mk_node("ExprInPlace", 2, $1, $3); }
14671465
| expr '=' expr { $$ = mk_node("ExprAssign", 2, $1, $3); }
14681466
| expr SHLEQ expr { $$ = mk_node("ExprAssignShl", 2, $1, $3); }
14691467
| expr SHREQ expr { $$ = mk_node("ExprAssignShr", 2, $1, $3); }
@@ -1527,7 +1525,6 @@ expr_nostruct
15271525
| BREAK ident { $$ = mk_node("ExprBreak", 1, $2); }
15281526
| YIELD { $$ = mk_node("ExprYield", 0); }
15291527
| YIELD expr { $$ = mk_node("ExprYield", 1, $2); }
1530-
| expr_nostruct LARROW expr_nostruct { $$ = mk_node("ExprInPlace", 2, $1, $3); }
15311528
| expr_nostruct '=' expr_nostruct { $$ = mk_node("ExprAssign", 2, $1, $3); }
15321529
| expr_nostruct SHLEQ expr_nostruct { $$ = mk_node("ExprAssignShl", 2, $1, $3); }
15331530
| expr_nostruct SHREQ expr_nostruct { $$ = mk_node("ExprAssignShr", 2, $1, $3); }

src/liballoc/binary_heap.rs

+1-65
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@@ -155,7 +155,7 @@
155155
#![allow(missing_docs)]
156156
#![stable(feature = "rust1", since = "1.0.0")]
157157

158-
use core::ops::{Deref, DerefMut, Place, Placer, InPlace};
158+
use core::ops::{Deref, DerefMut};
159159
use core::iter::{FromIterator, FusedIterator};
160160
use core::mem::{swap, size_of};
161161
use core::ptr;
@@ -1195,67 +1195,3 @@ impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BinaryHeap<T> {
11951195
self.extend(iter.into_iter().cloned());
11961196
}
11971197
}
1198-
1199-
#[unstable(feature = "collection_placement",
1200-
reason = "placement protocol is subject to change",
1201-
issue = "30172")]
1202-
pub struct BinaryHeapPlace<'a, T: 'a>
1203-
where T: Clone + Ord {
1204-
heap: *mut BinaryHeap<T>,
1205-
place: vec::PlaceBack<'a, T>,
1206-
}
1207-
1208-
#[unstable(feature = "collection_placement",
1209-
reason = "placement protocol is subject to change",
1210-
issue = "30172")]
1211-
impl<'a, T: Clone + Ord + fmt::Debug> fmt::Debug for BinaryHeapPlace<'a, T> {
1212-
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1213-
f.debug_tuple("BinaryHeapPlace")
1214-
.field(&self.place)
1215-
.finish()
1216-
}
1217-
}
1218-
1219-
#[unstable(feature = "collection_placement",
1220-
reason = "placement protocol is subject to change",
1221-
issue = "30172")]
1222-
impl<'a, T: 'a> Placer<T> for &'a mut BinaryHeap<T>
1223-
where T: Clone + Ord {
1224-
type Place = BinaryHeapPlace<'a, T>;
1225-
1226-
fn make_place(self) -> Self::Place {
1227-
let ptr = self as *mut BinaryHeap<T>;
1228-
let place = Placer::make_place(self.data.place_back());
1229-
BinaryHeapPlace {
1230-
heap: ptr,
1231-
place,
1232-
}
1233-
}
1234-
}
1235-
1236-
#[unstable(feature = "collection_placement",
1237-
reason = "placement protocol is subject to change",
1238-
issue = "30172")]
1239-
unsafe impl<'a, T> Place<T> for BinaryHeapPlace<'a, T>
1240-
where T: Clone + Ord {
1241-
fn pointer(&mut self) -> *mut T {
1242-
self.place.pointer()
1243-
}
1244-
}
1245-
1246-
#[unstable(feature = "collection_placement",
1247-
reason = "placement protocol is subject to change",
1248-
issue = "30172")]
1249-
impl<'a, T> InPlace<T> for BinaryHeapPlace<'a, T>
1250-
where T: Clone + Ord {
1251-
type Owner = &'a T;
1252-
1253-
unsafe fn finalize(self) -> &'a T {
1254-
self.place.finalize();
1255-
1256-
let heap: &mut BinaryHeap<T> = &mut *self.heap;
1257-
let len = heap.len();
1258-
let i = heap.sift_up(0, len - 1);
1259-
heap.data.get_unchecked(i)
1260-
}
1261-
}

src/liballoc/boxed.rs

+1-150
Original file line numberDiff line numberDiff line change
@@ -55,55 +55,21 @@
5555
5656
#![stable(feature = "rust1", since = "1.0.0")]
5757

58-
use heap::Heap;
5958
use raw_vec::RawVec;
6059

6160
use core::any::Any;
6261
use core::borrow;
6362
use core::cmp::Ordering;
6463
use core::fmt;
6564
use core::hash::{Hash, Hasher};
66-
use core::heap::{Alloc, Layout};
6765
use core::iter::FusedIterator;
68-
use core::marker::{self, Unpin, Unsize};
66+
use core::marker::{Unpin, Unsize};
6967
use core::mem::{self, Pin};
7068
use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
71-
use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
7269
use core::ptr::{self, NonNull, Unique};
7370
use core::convert::From;
7471
use str::from_boxed_utf8_unchecked;
7572

76-
/// A value that represents the heap. This is the default place that the `box`
77-
/// keyword allocates into when no place is supplied.
78-
///
79-
/// The following two examples are equivalent:
80-
///
81-
/// ```
82-
/// #![feature(box_heap)]
83-
///
84-
/// #![feature(box_syntax, placement_in_syntax)]
85-
/// use std::boxed::HEAP;
86-
///
87-
/// fn main() {
88-
/// let foo: Box<i32> = in HEAP { 5 };
89-
/// let foo = box 5;
90-
/// }
91-
/// ```
92-
#[unstable(feature = "box_heap",
93-
reason = "may be renamed; uncertain about custom allocator design",
94-
issue = "27779")]
95-
pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };
96-
97-
/// This the singleton type used solely for `boxed::HEAP`.
98-
#[unstable(feature = "box_heap",
99-
reason = "may be renamed; uncertain about custom allocator design",
100-
issue = "27779")]
101-
#[allow(missing_debug_implementations)]
102-
#[derive(Copy, Clone)]
103-
pub struct ExchangeHeapSingleton {
104-
_force_singleton: (),
105-
}
106-
10773
/// A pointer type for heap allocation.
10874
///
10975
/// See the [module-level documentation](../../std/boxed/index.html) for more.
@@ -112,121 +78,6 @@ pub struct ExchangeHeapSingleton {
11278
#[stable(feature = "rust1", since = "1.0.0")]
11379
pub struct Box<T: ?Sized>(Unique<T>);
11480

115-
/// `IntermediateBox` represents uninitialized backing storage for `Box`.
116-
///
117-
/// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
118-
/// introducing a separate `IntermediateBox<T>`; but then you hit
119-
/// issues when you e.g. attempt to destructure an instance of `Box`,
120-
/// since it is a lang item and so it gets special handling by the
121-
/// compiler. Easier just to make this parallel type for now.
122-
///
123-
/// FIXME (pnkfelix): Currently the `box` protocol only supports
124-
/// creating instances of sized types. This IntermediateBox is
125-
/// designed to be forward-compatible with a future protocol that
126-
/// supports creating instances of unsized types; that is why the type
127-
/// parameter has the `?Sized` generalization marker, and is also why
128-
/// this carries an explicit size. However, it probably does not need
129-
/// to carry the explicit alignment; that is just a work-around for
130-
/// the fact that the `align_of` intrinsic currently requires the
131-
/// input type to be Sized (which I do not think is strictly
132-
/// necessary).
133-
#[unstable(feature = "placement_in",
134-
reason = "placement box design is still being worked out.",
135-
issue = "27779")]
136-
#[allow(missing_debug_implementations)]
137-
pub struct IntermediateBox<T: ?Sized> {
138-
ptr: *mut u8,
139-
layout: Layout,
140-
marker: marker::PhantomData<*mut T>,
141-
}
142-
143-
#[unstable(feature = "placement_in",
144-
reason = "placement box design is still being worked out.",
145-
issue = "27779")]
146-
unsafe impl<T> Place<T> for IntermediateBox<T> {
147-
fn pointer(&mut self) -> *mut T {
148-
self.ptr as *mut T
149-
}
150-
}
151-
152-
unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
153-
let p = b.ptr as *mut T;
154-
mem::forget(b);
155-
Box::from_raw(p)
156-
}
157-
158-
fn make_place<T>() -> IntermediateBox<T> {
159-
let layout = Layout::new::<T>();
160-
161-
let p = if layout.size() == 0 {
162-
mem::align_of::<T>() as *mut u8
163-
} else {
164-
unsafe {
165-
Heap.alloc(layout.clone()).unwrap_or_else(|err| {
166-
Heap.oom(err)
167-
})
168-
}
169-
};
170-
171-
IntermediateBox {
172-
ptr: p,
173-
layout,
174-
marker: marker::PhantomData,
175-
}
176-
}
177-
178-
#[unstable(feature = "placement_in",
179-
reason = "placement box design is still being worked out.",
180-
issue = "27779")]
181-
impl<T> BoxPlace<T> for IntermediateBox<T> {
182-
fn make_place() -> IntermediateBox<T> {
183-
make_place()
184-
}
185-
}
186-
187-
#[unstable(feature = "placement_in",
188-
reason = "placement box design is still being worked out.",
189-
issue = "27779")]
190-
impl<T> InPlace<T> for IntermediateBox<T> {
191-
type Owner = Box<T>;
192-
unsafe fn finalize(self) -> Box<T> {
193-
finalize(self)
194-
}
195-
}
196-
197-
#[unstable(feature = "placement_new_protocol", issue = "27779")]
198-
impl<T> Boxed for Box<T> {
199-
type Data = T;
200-
type Place = IntermediateBox<T>;
201-
unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
202-
finalize(b)
203-
}
204-
}
205-
206-
#[unstable(feature = "placement_in",
207-
reason = "placement box design is still being worked out.",
208-
issue = "27779")]
209-
impl<T> Placer<T> for ExchangeHeapSingleton {
210-
type Place = IntermediateBox<T>;
211-
212-
fn make_place(self) -> IntermediateBox<T> {
213-
make_place()
214-
}
215-
}
216-
217-
#[unstable(feature = "placement_in",
218-
reason = "placement box design is still being worked out.",
219-
issue = "27779")]
220-
impl<T: ?Sized> Drop for IntermediateBox<T> {
221-
fn drop(&mut self) {
222-
if self.layout.size() > 0 {
223-
unsafe {
224-
Heap.dealloc(self.ptr, self.layout.clone())
225-
}
226-
}
227-
}
228-
}
229-
23081
impl<T> Box<T> {
23182
/// Allocates memory on the heap and then places `x` into it.
23283
///

src/liballoc/lib.rs

+4-8
Original file line numberDiff line numberDiff line change
@@ -76,7 +76,6 @@
7676
#![deny(missing_debug_implementations)]
7777

7878
#![cfg_attr(test, allow(deprecated))] // rand
79-
#![cfg_attr(test, feature(placement_in))]
8079
#![cfg_attr(not(test), feature(core_float))]
8180
#![cfg_attr(not(test), feature(exact_size_is_empty))]
8281
#![cfg_attr(not(test), feature(generator_trait))]
@@ -108,8 +107,6 @@
108107
#![feature(optin_builtin_traits)]
109108
#![feature(pattern)]
110109
#![feature(pin)]
111-
#![feature(placement_in_syntax)]
112-
#![feature(placement_new_protocol)]
113110
#![feature(ptr_internals)]
114111
#![feature(rustc_attrs)]
115112
#![feature(slice_get_slice)]
@@ -128,8 +125,8 @@
128125
#![feature(pointer_methods)]
129126
#![feature(inclusive_range_fields)]
130127

131-
#![cfg_attr(not(test), feature(fn_traits, placement_new_protocol, swap_with_slice, i128))]
132-
#![cfg_attr(test, feature(test, box_heap))]
128+
#![cfg_attr(not(test), feature(fn_traits, swap_with_slice, i128))]
129+
#![cfg_attr(test, feature(test))]
133130

134131
// Allow testing this library
135132

@@ -159,13 +156,12 @@ pub mod heap;
159156

160157
// Need to conditionally define the mod from `boxed.rs` to avoid
161158
// duplicating the lang-items when building in test cfg; but also need
162-
// to allow code to have `use boxed::HEAP;`
163-
// and `use boxed::Box;` declarations.
159+
// to allow code to have `use boxed::Box;` declarations.
164160
#[cfg(not(test))]
165161
pub mod boxed;
166162
#[cfg(test)]
167163
mod boxed {
168-
pub use std::boxed::{Box, IntermediateBox, HEAP};
164+
pub use std::boxed::Box;
169165
}
170166
#[cfg(test)]
171167
mod boxed_test;

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