src/libsyntax/ptr.rs

   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.
   4 //
   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.
  10
  11 //! The AST pointer
  12 //!
  13 //! Provides `P<T>`, a frozen owned smart pointer, as a replacement for `@T` in
  14 //! the AST.
  15 //!
  16 //! # Motivations and benefits
  17 //!
  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>`.
  25 //!
  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".
  29 //!
  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).
  33 //!
  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.
  38
  39 use std::fmt::{self, Display, Debug};
  40 use std::hash::{Hash, Hasher};
  41 use std::ops::Deref;
  42 use std::ptr;
  43
  44 use serialize::{Encodable, Decodable, Encoder, Decoder};
  45
  46 /// An owned smart pointer.
  47 pub struct P<T> {
  48     ptr: Box<T>
  49 }
  50
  51 #[allow(non_snake_case)]
  52 /// Construct a `P<T>` from a `T` value.
  53 pub fn P<T: 'static>(value: T) -> P<T> {
  54     P {
  55         ptr: box value
  56     }
  57 }
  58
  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
  63         F: FnOnce(T) -> U,
  64     {
  65         f(*self.ptr)
  66     }
  67
  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
  70         F: FnOnce(T) -> T,
  71     {
  72         unsafe {
  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)));
  76         }
  77         self
  78     }
  79 }
  80
  81 impl<T> Deref for P<T> {
  82     type Target = T;
  83
  84     fn deref<'a>(&'a self) -> &'a T {
  85         &*self.ptr
  86     }
  87 }
  88
  89 impl<T: 'static + Clone> Clone for P<T> {
  90     fn clone(&self) -> P<T> {
  91         P((**self).clone())
  92     }
  93 }
  94
  95 impl<T: PartialEq> PartialEq for P<T> {
  96     fn eq(&self, other: &P<T>) -> bool {
  97         **self == **other
  98     }
  99 }
 100
 101 impl<T: Eq> Eq for P<T> {}
 102
 103 impl<T: Debug> Debug for P<T> {
 104     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 105         Debug::fmt(&**self, f)
 106     }
 107 }
 108 impl<T: Display> Display for P<T> {
 109     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 110         Display::fmt(&**self, f)
 111     }
 112 }
 113
 114 #[stable(feature = "rust1", since = "1.0.0")]
 115 impl<T> fmt::Pointer for P<T> {
 116     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 117         fmt::Pointer::fmt(&self.ptr, f)
 118     }
 119 }
 120
 121 impl<T: Hash> Hash for P<T> {
 122     fn hash<H: Hasher>(&self, state: &mut H) {
 123         (**self).hash(state);
 124     }
 125 }
 126
 127 impl<T: 'static + Decodable> Decodable for P<T> {
 128     fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {
 129         Decodable::decode(d).map(P)
 130     }
 131 }
 132
 133 impl<T: Encodable> Encodable for P<T> {
 134     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
 135         (**self).encode(s)
 136     }
 137 }