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, Show};
43 use serialize::{Encodable, Decodable, Encoder, Decoder};
45 /// An owned smart pointer.
50 #[allow(non_snake_case)]
51 /// Construct a `P<T>` from a `T` value.
52 pub fn P<T: 'static>(value: T) -> P<T> {
58 impl<T: 'static> P<T> {
59 /// Move out of the pointer.
60 /// Intended for chaining transformations not covered by `map`.
61 pub fn and_then<U, F>(self, f: F) -> U where
67 /// Transform the inner value, consuming `self` and producing a new `P<T>`.
68 pub fn map<F>(mut self, f: F) -> P<T> where
72 let p = &mut *self.ptr;
73 // FIXME(#5016) this shouldn't need to zero to be safe.
74 ptr::write(p, f(ptr::read_and_zero(p)));
80 impl<T> Deref for P<T> {
83 fn deref<'a>(&'a self) -> &'a T {
88 impl<T: 'static + Clone> Clone for P<T> {
89 fn clone(&self) -> P<T> {
94 impl<T: PartialEq> PartialEq for P<T> {
95 fn eq(&self, other: &P<T>) -> bool {
100 impl<T: Eq> Eq for P<T> {}
102 impl<T: Show> Show for P<T> {
103 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
108 impl<S, T: Hash<S>> Hash<S> for P<T> {
109 fn hash(&self, state: &mut S) {
110 (**self).hash(state);
114 impl<E, D: Decoder<E>, T: 'static + Decodable<D, E>> Decodable<D, E> for P<T> {
115 fn decode(d: &mut D) -> Result<P<T>, E> {
116 Decodable::decode(d).map(P)
120 impl<E, S: Encoder<E>, T: Encodable<S, E>> Encodable<S, E> for P<T> {
121 fn encode(&self, s: &mut S) -> Result<(), E> {