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…sakis

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
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bors committed Apr 4, 2018
2 parents c75d5e2 + 9b5859a commit 199b7e2
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Showing 41 changed files with 39 additions and 1,392 deletions.
3 changes: 0 additions & 3 deletions src/grammar/parser-lalr.y
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Original file line number Diff line number Diff line change
Expand Up @@ -1400,7 +1400,6 @@ nonblock_expr
| BREAK lifetime { $$ = mk_node("ExprBreak", 1, $2); }
| YIELD { $$ = mk_node("ExprYield", 0); }
| YIELD expr { $$ = mk_node("ExprYield", 1, $2); }
| nonblock_expr LARROW expr { $$ = mk_node("ExprInPlace", 2, $1, $3); }
| nonblock_expr '=' expr { $$ = mk_node("ExprAssign", 2, $1, $3); }
| nonblock_expr SHLEQ expr { $$ = mk_node("ExprAssignShl", 2, $1, $3); }
| nonblock_expr SHREQ expr { $$ = mk_node("ExprAssignShr", 2, $1, $3); }
Expand Down Expand Up @@ -1463,7 +1462,6 @@ expr
| BREAK ident { $$ = mk_node("ExprBreak", 1, $2); }
| YIELD { $$ = mk_node("ExprYield", 0); }
| YIELD expr { $$ = mk_node("ExprYield", 1, $2); }
| expr LARROW expr { $$ = mk_node("ExprInPlace", 2, $1, $3); }
| expr '=' expr { $$ = mk_node("ExprAssign", 2, $1, $3); }
| expr SHLEQ expr { $$ = mk_node("ExprAssignShl", 2, $1, $3); }
| expr SHREQ expr { $$ = mk_node("ExprAssignShr", 2, $1, $3); }
Expand Down Expand Up @@ -1527,7 +1525,6 @@ expr_nostruct
| BREAK ident { $$ = mk_node("ExprBreak", 1, $2); }
| YIELD { $$ = mk_node("ExprYield", 0); }
| YIELD expr { $$ = mk_node("ExprYield", 1, $2); }
| expr_nostruct LARROW expr_nostruct { $$ = mk_node("ExprInPlace", 2, $1, $3); }
| expr_nostruct '=' expr_nostruct { $$ = mk_node("ExprAssign", 2, $1, $3); }
| expr_nostruct SHLEQ expr_nostruct { $$ = mk_node("ExprAssignShl", 2, $1, $3); }
| expr_nostruct SHREQ expr_nostruct { $$ = mk_node("ExprAssignShr", 2, $1, $3); }
Expand Down
66 changes: 1 addition & 65 deletions src/liballoc/binary_heap.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -155,7 +155,7 @@
#![allow(missing_docs)]
#![stable(feature = "rust1", since = "1.0.0")]

use core::ops::{Deref, DerefMut, Place, Placer, InPlace};
use core::ops::{Deref, DerefMut};
use core::iter::{FromIterator, FusedIterator};
use core::mem::{swap, size_of};
use core::ptr;
Expand Down Expand Up @@ -1195,67 +1195,3 @@ impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BinaryHeap<T> {
self.extend(iter.into_iter().cloned());
}
}

#[unstable(feature = "collection_placement",
reason = "placement protocol is subject to change",
issue = "30172")]
pub struct BinaryHeapPlace<'a, T: 'a>
where T: Clone + Ord {
heap: *mut BinaryHeap<T>,
place: vec::PlaceBack<'a, T>,
}

#[unstable(feature = "collection_placement",
reason = "placement protocol is subject to change",
issue = "30172")]
impl<'a, T: Clone + Ord + fmt::Debug> fmt::Debug for BinaryHeapPlace<'a, T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_tuple("BinaryHeapPlace")
.field(&self.place)
.finish()
}
}

#[unstable(feature = "collection_placement",
reason = "placement protocol is subject to change",
issue = "30172")]
impl<'a, T: 'a> Placer<T> for &'a mut BinaryHeap<T>
where T: Clone + Ord {
type Place = BinaryHeapPlace<'a, T>;

fn make_place(self) -> Self::Place {
let ptr = self as *mut BinaryHeap<T>;
let place = Placer::make_place(self.data.place_back());
BinaryHeapPlace {
heap: ptr,
place,
}
}
}

#[unstable(feature = "collection_placement",
reason = "placement protocol is subject to change",
issue = "30172")]
unsafe impl<'a, T> Place<T> for BinaryHeapPlace<'a, T>
where T: Clone + Ord {
fn pointer(&mut self) -> *mut T {
self.place.pointer()
}
}

#[unstable(feature = "collection_placement",
reason = "placement protocol is subject to change",
issue = "30172")]
impl<'a, T> InPlace<T> for BinaryHeapPlace<'a, T>
where T: Clone + Ord {
type Owner = &'a T;

unsafe fn finalize(self) -> &'a T {
self.place.finalize();

let heap: &mut BinaryHeap<T> = &mut *self.heap;
let len = heap.len();
let i = heap.sift_up(0, len - 1);
heap.data.get_unchecked(i)
}
}
151 changes: 1 addition & 150 deletions src/liballoc/boxed.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -55,55 +55,21 @@

#![stable(feature = "rust1", since = "1.0.0")]

use heap::Heap;
use raw_vec::RawVec;

use core::any::Any;
use core::borrow;
use core::cmp::Ordering;
use core::fmt;
use core::hash::{Hash, Hasher};
use core::heap::{Alloc, Layout};
use core::iter::FusedIterator;
use core::marker::{self, Unpin, Unsize};
use core::marker::{Unpin, Unsize};
use core::mem::{self, Pin};
use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
use core::ptr::{self, NonNull, Unique};
use core::convert::From;
use str::from_boxed_utf8_unchecked;

/// A value that represents the heap. This is the default place that the `box`
/// keyword allocates into when no place is supplied.
///
/// The following two examples are equivalent:
///
/// ```
/// #![feature(box_heap)]
///
/// #![feature(box_syntax, placement_in_syntax)]
/// use std::boxed::HEAP;
///
/// fn main() {
/// let foo: Box<i32> = in HEAP { 5 };
/// let foo = box 5;
/// }
/// ```
#[unstable(feature = "box_heap",
reason = "may be renamed; uncertain about custom allocator design",
issue = "27779")]
pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };

/// This the singleton type used solely for `boxed::HEAP`.
#[unstable(feature = "box_heap",
reason = "may be renamed; uncertain about custom allocator design",
issue = "27779")]
#[allow(missing_debug_implementations)]
#[derive(Copy, Clone)]
pub struct ExchangeHeapSingleton {
_force_singleton: (),
}

/// A pointer type for heap allocation.
///
/// See the [module-level documentation](../../std/boxed/index.html) for more.
Expand All @@ -112,121 +78,6 @@ pub struct ExchangeHeapSingleton {
#[stable(feature = "rust1", since = "1.0.0")]
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.",
issue = "27779")]
#[allow(missing_debug_implementations)]
pub struct IntermediateBox<T: ?Sized> {
ptr: *mut u8,
layout: Layout,
marker: marker::PhantomData<*mut T>,
}

#[unstable(feature = "placement_in",
reason = "placement box design is still being worked out.",
issue = "27779")]
unsafe impl<T> Place<T> for IntermediateBox<T> {
fn pointer(&mut self) -> *mut T {
self.ptr as *mut T
}
}

unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
let p = b.ptr as *mut T;
mem::forget(b);
Box::from_raw(p)
}

fn make_place<T>() -> IntermediateBox<T> {
let layout = Layout::new::<T>();

let p = if layout.size() == 0 {
mem::align_of::<T>() as *mut u8
} else {
unsafe {
Heap.alloc(layout.clone()).unwrap_or_else(|err| {
Heap.oom(err)
})
}
};

IntermediateBox {
ptr: p,
layout,
marker: marker::PhantomData,
}
}

#[unstable(feature = "placement_in",
reason = "placement box design is still being worked out.",
issue = "27779")]
impl<T> BoxPlace<T> for IntermediateBox<T> {
fn make_place() -> IntermediateBox<T> {
make_place()
}
}

#[unstable(feature = "placement_in",
reason = "placement box design is still being worked out.",
issue = "27779")]
impl<T> InPlace<T> for IntermediateBox<T> {
type Owner = Box<T>;
unsafe fn finalize(self) -> Box<T> {
finalize(self)
}
}

#[unstable(feature = "placement_new_protocol", issue = "27779")]
impl<T> Boxed for Box<T> {
type Data = T;
type Place = IntermediateBox<T>;
unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
finalize(b)
}
}

#[unstable(feature = "placement_in",
reason = "placement box design is still being worked out.",
issue = "27779")]
impl<T> Placer<T> for ExchangeHeapSingleton {
type Place = IntermediateBox<T>;

fn make_place(self) -> IntermediateBox<T> {
make_place()
}
}

#[unstable(feature = "placement_in",
reason = "placement box design is still being worked out.",
issue = "27779")]
impl<T: ?Sized> Drop for IntermediateBox<T> {
fn drop(&mut self) {
if self.layout.size() > 0 {
unsafe {
Heap.dealloc(self.ptr, self.layout.clone())
}
}
}
}

impl<T> Box<T> {
/// Allocates memory on the heap and then places `x` into it.
///
Expand Down
12 changes: 4 additions & 8 deletions src/liballoc/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -76,7 +76,6 @@
#![deny(missing_debug_implementations)]

#![cfg_attr(test, allow(deprecated))] // rand
#![cfg_attr(test, feature(placement_in))]
#![cfg_attr(not(test), feature(core_float))]
#![cfg_attr(not(test), feature(exact_size_is_empty))]
#![cfg_attr(not(test), feature(generator_trait))]
Expand Down Expand Up @@ -108,8 +107,6 @@
#![feature(optin_builtin_traits)]
#![feature(pattern)]
#![feature(pin)]
#![feature(placement_in_syntax)]
#![feature(placement_new_protocol)]
#![feature(ptr_internals)]
#![feature(rustc_attrs)]
#![feature(slice_get_slice)]
Expand All @@ -128,8 +125,8 @@
#![feature(pointer_methods)]
#![feature(inclusive_range_fields)]

#![cfg_attr(not(test), feature(fn_traits, placement_new_protocol, swap_with_slice, i128))]
#![cfg_attr(test, feature(test, box_heap))]
#![cfg_attr(not(test), feature(fn_traits, swap_with_slice, i128))]
#![cfg_attr(test, feature(test))]

// Allow testing this library

Expand Down Expand Up @@ -159,13 +156,12 @@ pub mod heap;

// Need to conditionally define the mod from `boxed.rs` to avoid
// duplicating the lang-items when building in test cfg; but also need
// to allow code to have `use boxed::HEAP;`
// and `use boxed::Box;` declarations.
// to allow code to have `use boxed::Box;` declarations.
#[cfg(not(test))]
pub mod boxed;
#[cfg(test)]
mod boxed {
pub use std::boxed::{Box, IntermediateBox, HEAP};
pub use std::boxed::Box;
}
#[cfg(test)]
mod boxed_test;
Expand Down
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