1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
13 //! Provides `P<T>`, a frozen owned smart pointer, as a replacement for `@T` in
16 //! # Motivations and benefits
18 //! * **Identity**: sharing AST nodes is problematic for the various analysis
19 //! passes (e.g. one may be able to bypass the borrow checker with a shared
20 //! `ExprAddrOf` node taking a mutable borrow). The only reason `@T` in the
21 //! AST hasn't caused issues is because of inefficient folding passes which
22 //! would always deduplicate any such shared nodes. Even if the AST were to
23 //! switch to an arena, this would still hold, i.e. it couldn't use `&'a T`,
24 //! but rather a wrapper like `P<'a, T>`.
26 //! * **Immutability**: `P<T>` disallows mutating its inner `T`, unlike `Box<T>`
27 //! (unless it contains an `Unsafe` interior, but that may be denied later).
28 //! This mainly prevents mistakes, but can also enforces a kind of "purity".
30 //! * **Efficiency**: folding can reuse allocation space for `P<T>` and `Vec<T>`,
31 //! the latter even when the input and output types differ (as it would be the
32 //! case with arenas or a GADT AST using type parameters to toggle features).
34 //! * **Maintainability**: `P<T>` provides a fixed interface - `Deref`,
35 //! `and_then` and `map` - which can remain fully functional even if the
36 //! implementation changes (using a special thread-local heap, for example).
37 //! Moreover, a switch to, e.g. `P<'a, T>` would be easy and mostly automated.
39 use std
::fmt
::{self, Display, Debug}
;
40 use std
::hash
::{Hash, Hasher}
;
44 use serialize
::{Encodable, Decodable, Encoder, Decoder}
;
46 /// An owned smart pointer.
51 #[allow(non_snake_case)]
52 /// Construct a `P<T>` from a `T` value.
53 pub fn P
<T
: '
static>(value
: T
) -> P
<T
> {
59 impl<T
: '
static> P
<T
> {
60 /// Move out of the pointer.
61 /// Intended for chaining transformations not covered by `map`.
62 pub fn and_then
<U
, F
>(self, f
: F
) -> U
where
68 /// Transform the inner value, consuming `self` and producing a new `P<T>`.
69 pub fn map
<F
>(mut self, f
: F
) -> P
<T
> where
73 let p
= &mut *self.ptr
;
74 // FIXME(#5016) this shouldn't need to drop-fill to be safe.
75 ptr
::write(p
, f(ptr
::read_and_drop(p
)));
81 impl<T
> Deref
for P
<T
> {
84 fn deref
<'a
>(&'a
self) -> &'a T
{
89 impl<T
: '
static + Clone
> Clone
for P
<T
> {
90 fn clone(&self) -> P
<T
> {
95 impl<T
: PartialEq
> PartialEq
for P
<T
> {
96 fn eq(&self, other
: &P
<T
>) -> bool
{
101 impl<T
: Eq
> Eq
for P
<T
> {}
103 impl<T
: Debug
> Debug
for P
<T
> {
104 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
105 Debug
::fmt(&**self, f
)
108 impl<T
: Display
> Display
for P
<T
> {
109 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
110 Display
::fmt(&**self, f
)
114 impl<T
: Hash
> Hash
for P
<T
> {
115 fn hash
<H
: Hasher
>(&self, state
: &mut H
) {
116 (**self).hash(state
);
120 impl<T
: '
static + Decodable
> Decodable
for P
<T
> {
121 fn decode
<D
: Decoder
>(d
: &mut D
) -> Result
<P
<T
>, D
::Error
> {
122 Decodable
::decode(d
).map(P
)
126 impl<T
: Encodable
> Encodable
for P
<T
> {
127 fn encode
<S
: Encoder
>(&self, s
: &mut S
) -> Result
<(), S
::Error
> {