desugaring-based box placement-in (take-4 branch)

src/doc/trpl/lang-items.md

@@ -11,69 +11,111 @@ it exists. The marker is the attribute `#[lang = "..."]` and there are
 various different values of `...`, i.e. various different 'lang
 items'.
 
-For example, `Box` pointers require two lang items, one for allocation
-and one for deallocation. A freestanding program that uses the `Box`
-sugar for dynamic allocations via `malloc` and `free`:
+For example, some traits in the standard library have special
+treatment. One is `Copy`: there is a `copy` lang-item attached to the
+`Copy` trait, and the Rust compiler treats this specially in two ways:
 
-```rust
-#![feature(lang_items, box_syntax, start, no_std, libc)]
-#![no_std]
+ 1. If `x` has type that implements `Copy`, then `x` can be freely
+    copied by the assignment operator:
+    ```rust,ignore
+    let y = x;
+    let z = x;
+    ```
 
-extern crate libc;
+ 2. If a type tries to implement `Copy`, the Rust compiler will
+    ensure that duplicating a value of that type will not cause any
+    noncopyable type to be duplicated.
 
-extern {
-    fn abort() -> !;
-}
+    For example, this code will be rejected:
 
-#[lang = "owned_box"]
-pub struct Box<T>(*mut T);
+    ```rust,ignore
+    #[derive(Clone)]
+    struct ThisTypeIsNotCopy(Box<i32>);
 
-#[lang = "exchange_malloc"]
-unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
-    let p = libc::malloc(size as libc::size_t) as *mut u8;
+    #[derive(Clone)]
+    struct TryToMakeThisCopy { okay: i32, not_okay: ThisTypeIsNotCopy }
 
-    // malloc failed
-    if p as usize == 0 {
-        abort();
-    }
+    // This attempt to implement `Copy` will fail.
+    impl Copy for TryToMakeThisCopy { }
+    ```
 
-    p
-}
-#[lang = "exchange_free"]
-unsafe fn deallocate(ptr: *mut u8, _size: usize, _align: usize) {
-    libc::free(ptr as *mut libc::c_void)
+The above two properties are both special qualities of the `Copy`
+trait that other traits do not share, and thus they are associated
+with whatever trait is given the `copy` lang item.
+
+Here is a freestanding program that provides its own definition of the
+`copy` lang item, that is slightly different than the definition in
+the Rust standard library:
+
+```
+#![feature(lang_items, intrinsics, start, no_std)]
+#![no_std]
+
+#[lang = "copy"]
+pub trait MyCopy {
+    // Empty.
 }
 
-#[start]
-fn main(argc: isize, argv: *const *const u8) -> isize {
-    let x = box 1;
+struct C(i32, i32);
+impl MyCopy for C { }
 
-    0
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -> isize {
+    let mut x = C(3, 4);
+    let mut y = x;
+    let mut z = x;
+    x.0 = 5;
+    y.0 = 6;
+    z.0 = 7;
+
+    #[link(name="c")]
+    extern { fn printf(f: *const u8, ...); }
+
+    let template = b"x: C(%d, %d) y: C(%d, %d), z: C(%d, %d)\n\0";
+    unsafe { printf(template as *const u8, x.0, x.1, y.0, y.1, z.0, z.1); }
+    return 0;
 }
 
+// Again, these functions and traits are used by the compiler, and are
+// normally provided by libstd.
+
 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
 #[lang = "eh_personality"] extern fn eh_personality() {}
 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
+
+#[lang="sized"] pub trait Sized: PhantomFn {}
+#[lang="phantom_fn"] pub trait PhantomFn {}
+```
+
+This compiles successfully. When we run the above code, it prints:
+```text
+x: C(5, 4) y: C(6, 4), z: C(7, 4)
 ```
+So we can freely copy instances of `C`, since it implements `MyCopy`.
 
-Note the use of `abort`: the `exchange_malloc` lang item is assumed to
-return a valid pointer, and so needs to do the check internally.
+A potentially interesting detail about the above program is that it
+differs from the Rust standard library in more than just the name
+`MyCopy`. The `std::marker::Copy` extends `std::clone::Clone`,
+ensuring that every type that implements `Copy` has a `clone` method.
+The `MyCopy` trait does *not* extend `Clone`; these values have no
+`clone` methods.
 
 Other features provided by lang items include:
 
 - overloadable operators via traits: the traits corresponding to the
   `==`, `<`, dereferencing (`*`) and `+` (etc.) operators are all
   marked with lang items; those specific four are `eq`, `ord`,
   `deref`, and `add` respectively.
-- stack unwinding and general failure; the `eh_personality`, `fail`
-  and `fail_bounds_checks` lang items.
+- stack unwinding and general failure; the `eh_personality`, `panic`
+  `panic_fmt`, and `panic_bounds_check` lang items.
 - the traits in `std::marker` used to indicate types of
   various kinds; lang items `send`, `sync` and `copy`.
 - the marker types and variance indicators found in
   `std::marker`; lang items `covariant_type`,
   `contravariant_lifetime`, etc.
+- matching with string literal patterns; the `str_eq` lang item.
 
 Lang items are loaded lazily by the compiler; e.g. if one never uses
-`Box` then there is no need to define functions for `exchange_malloc`
-and `exchange_free`. `rustc` will emit an error when an item is needed
-but not found in the current crate or any that it depends on.
+array indexing (`a[i]`) then there is no need to define a function for
+`panic_bounds_check`. `rustc` will emit an error when an item is
+needed but not found in the current crate or any that it depends on.

src/liballoc/boxed.rs

@@ -55,43 +55,149 @@
 
 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};
 
-/// A value that represents the heap. This is the default place that the `box`
-/// keyword allocates into when no place is supplied.
+// FIXME (#22181): Put in the new placement-in syntax once that lands.
+
+/// A value that represents the heap. This is the place that the `box`
+/// keyword allocates into.
 ///
 /// The following two examples are equivalent:
 ///
 /// ```
-/// # #![feature(box_heap)]
+/// # #![feature(box_heap, core)]
 /// #![feature(box_syntax)]
+/// #![feature(placement_in_syntax)]
 /// use std::boxed::HEAP;
 ///
 /// fn main() {
-///     let foo = box(HEAP) 5;
-///     let foo = box 5;
+///     let foo = box (HEAP) { 5 };
+///     let foo: Box<_> = box 5;
 /// }
 /// ```
 #[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> {
+    // FIXME: what about unsized T?
+    #[inline]
+    fn pointer(&mut self) -> *mut T { self.ptr as *mut T }
+}
+
+#[inline]
+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> {
+    #[inline]
+    fn make_place() -> IntermediateBox<T> { make_place() }
+}
+
+impl<T> InPlace<T> for IntermediateBox<T> {
+    type Owner = Box<T>;
+    #[inline]
+    unsafe fn finalize(self) -> Box<T> { finalize(self) }
+}
+
+impl<T> Boxed for Box<T> {
+    type Data = T;
+    type Place = IntermediateBox<T>;
+    #[inline]
+    unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> { finalize(b) }
+}
+
+impl<T> Placer<T> for ExchangeHeapSingleton {
+    type Place = IntermediateBox<T>;
+
+    #[inline]
+    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 +305,7 @@ 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 +70,9 @@
        test(no_crate_inject))]
 #![no_std]
 
+// SNAP ba0e1cd
+#![allow(unused_features)] // for placement_in_syntax
+
 #![feature(allocator)]
 #![feature(box_syntax)]
 #![feature(coerce_unsized)]
@@ -84,6 +87,7 @@
 #![feature(optin_builtin_traits)]
 #![feature(raw)]
 #![feature(staged_api)]
+#![feature(placement_in_syntax)]
 #![feature(unboxed_closures)]
 #![feature(unique)]
 #![feature(unsafe_no_drop_flag, filling_drop)]

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 ba0e1cd
+    #[cfg(stage0)]
     /// Moves a value to an uninitialized memory location.
     ///
     /// Drop glue is not run on the destination.