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coercion: use a generalization of the target to specialize the source…
… to coerce instead of propagating the wrong type.
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| // Copyright 2015 The Rust Project Developers. See the COPYRIGHT | ||
| // file at the top-level directory of this distribution and at | ||
| // http://rust-lang.org/COPYRIGHT. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
| // option. This file may not be copied, modified, or distributed | ||
| // except according to those terms. | ||
|
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| use std::rc::Rc; | ||
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| mod protocol { | ||
| use std::ptr; | ||
|
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| pub trait BoxPlace<T>: Place<T> { | ||
| fn make() -> Self; | ||
| } | ||
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| pub trait Place<T> { | ||
| // NOTE(eddyb) The use of `&mut T` here is to force | ||
| // the LLVM `noalias` and `dereferenceable(sizeof(T))` | ||
| // attributes, which are required for eliding the copy | ||
| // and producing actual in-place initialization via RVO. | ||
| // Neither of those attributes are present on `*mut T`, | ||
| // but `&mut T` is not a great choice either, the proper | ||
| // way might be to add those attributes to `Unique<T>`. | ||
| unsafe fn pointer(&mut self) -> &mut T; | ||
| } | ||
|
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| pub trait Boxed<T>: Sized { | ||
| type P: Place<T>; | ||
| fn write_and_fin(mut place: Self::P, | ||
| value: T) | ||
| -> Self { | ||
| unsafe { | ||
| ptr::write(place.pointer(), value); | ||
| Self::fin(place) | ||
| } | ||
| } | ||
| unsafe fn fin(filled: Self::P) -> Self; | ||
| } | ||
| } | ||
|
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| macro_rules! box_ { | ||
| ($x:expr) => { | ||
| ::protocol::Boxed::write_and_fin(::protocol::BoxPlace::make(), $x) | ||
| } | ||
| } | ||
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| // Hacky implementations of the box protocol for Box<T> and Rc<T>. | ||
| // They pass mem::uninitialized() to Box::new, and Rc::new, respectively, | ||
| // to allocate memory and will leak the allocation in case of unwinding. | ||
| mod boxed { | ||
| use std::mem; | ||
| use protocol; | ||
|
|
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| pub struct Place<T> { | ||
| ptr: *mut T | ||
| } | ||
|
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| impl<T> protocol::BoxPlace<T> for Place<T> { | ||
| fn make() -> Place<T> { | ||
| unsafe { | ||
| Place { | ||
| ptr: mem::transmute(Box::new(mem::uninitialized::<T>())) | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
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| impl<T> protocol::Place<T> for Place<T> { | ||
| unsafe fn pointer(&mut self) -> &mut T { &mut *self.ptr } | ||
| } | ||
|
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| impl<T> protocol::Boxed<T> for Box<T> { | ||
| type P = Place<T>; | ||
| unsafe fn fin(place: Place<T>) -> Box<T> { | ||
| mem::transmute(place.ptr) | ||
| } | ||
| } | ||
| } | ||
|
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| mod rc { | ||
| use std::mem; | ||
| use std::rc::Rc; | ||
| use protocol; | ||
|
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| pub struct Place<T> { | ||
| rc_ptr: *mut (), | ||
| data_ptr: *mut T | ||
| } | ||
|
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| impl<T> protocol::BoxPlace<T> for Place<T> { | ||
| fn make() -> Place<T> { | ||
| unsafe { | ||
| let rc = Rc::new(mem::uninitialized::<T>()); | ||
| Place { | ||
| data_ptr: &*rc as *const _ as *mut _, | ||
| rc_ptr: mem::transmute(rc) | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
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| impl<T> protocol::Place<T> for Place<T> { | ||
| unsafe fn pointer(&mut self) -> &mut T { | ||
| &mut *self.data_ptr | ||
| } | ||
| } | ||
|
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| impl<T> protocol::Boxed<T> for Rc<T> { | ||
| type P = Place<T>; | ||
| unsafe fn fin(place: Place<T>) -> Rc<T> { | ||
| mem::transmute(place.rc_ptr) | ||
| } | ||
| } | ||
| } | ||
|
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| fn main() { | ||
| let v = vec![1, 2, 3]; | ||
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| let bx: Box<_> = box_!(|| &v); | ||
| let rc: Rc<_> = box_!(|| &v); | ||
|
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| assert_eq!(bx(), &v); | ||
| assert_eq!(rc(), &v); | ||
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| let bx_trait: Box<Fn() -> _> = box_!(|| &v); | ||
| let rc_trait: Rc<Fn() -> _> = box_!(|| &v); | ||
|
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| assert_eq!(bx(), &v); | ||
| assert_eq!(rc(), &v); | ||
| } |