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

…akis

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.

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() {
@@ -83,15 +86,110 @@ 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.
 ///
 /// See the [module-level documentation](../../std/boxed/index.html) for more.
 #[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

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)]

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.

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;
+}

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());

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(..) => {