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 //!   `ExprKind::AddrOf` 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::iter::FromIterator;
  41 use std::ops::Deref;
  42 use std::{mem, ptr, slice, vec};
  43
  44 use serialize::{Encodable, Decodable, Encoder, Decoder};
  45
  46 /// An owned smart pointer.
  47 #[derive(Hash, PartialEq, Eq, PartialOrd, Ord)]
  48 pub struct P<T: ?Sized> {
  49     ptr: Box<T>
  50 }
  51
  52 #[allow(non_snake_case)]
  53 /// Construct a `P<T>` from a `T` value.
  54 pub fn P<T: 'static>(value: T) -> P<T> {
  55     P {
  56         ptr: Box::new(value)
  57     }
  58 }
  59
  60 impl<T: 'static> P<T> {
  61     /// Move out of the pointer.
  62     /// Intended for chaining transformations not covered by `map`.
  63     pub fn and_then<U, F>(self, f: F) -> U where
  64         F: FnOnce(T) -> U,
  65     {
  66         f(*self.ptr)
  67     }
  68     /// Equivalent to and_then(|x| x)
  69     pub fn unwrap(self) -> T {
  70         *self.ptr
  71     }
  72
  73     /// Transform the inner value, consuming `self` and producing a new `P<T>`.
  74     pub fn map<F>(mut self, f: F) -> P<T> where
  75         F: FnOnce(T) -> T,
  76     {
  77         let p: *mut T = &mut *self.ptr;
  78
  79         // Leak self in case of panic.
  80         // FIXME(eddyb) Use some sort of "free guard" that
  81         // only deallocates, without dropping the pointee,
  82         // in case the call the `f` below ends in a panic.
  83         mem::forget(self);
  84
  85         unsafe {
  86             ptr::write(p, f(ptr::read(p)));
  87
  88             // Recreate self from the raw pointer.
  89             P {
  90                 ptr: Box::from_raw(p)
  91             }
  92         }
  93     }
  94 }
  95
  96 impl<T: ?Sized> Deref for P<T> {
  97     type Target = T;
  98
  99     fn deref(&self) -> &T {
 100         &self.ptr
 101     }
 102 }
 103
 104 impl<T: 'static + Clone> Clone for P<T> {
 105     fn clone(&self) -> P<T> {
 106         P((**self).clone())
 107     }
 108 }
 109
 110 impl<T: ?Sized + Debug> Debug for P<T> {
 111     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 112         Debug::fmt(&self.ptr, f)
 113     }
 114 }
 115
 116 impl<T: Display> Display for P<T> {
 117     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 118         Display::fmt(&**self, f)
 119     }
 120 }
 121
 122 impl<T> fmt::Pointer for P<T> {
 123     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 124         fmt::Pointer::fmt(&self.ptr, f)
 125     }
 126 }
 127
 128 impl<T: 'static + Decodable> Decodable for P<T> {
 129     fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {
 130         Decodable::decode(d).map(P)
 131     }
 132 }
 133
 134 impl<T: Encodable> Encodable for P<T> {
 135     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
 136         (**self).encode(s)
 137     }
 138 }
 139
 140 impl<T> P<[T]> {
 141     pub fn new() -> P<[T]> {
 142         P { ptr: Default::default() }
 143     }
 144
 145     #[inline(never)]
 146     pub fn from_vec(v: Vec<T>) -> P<[T]> {
 147         P { ptr: v.into_boxed_slice() }
 148     }
 149
 150     #[inline(never)]
 151     pub fn into_vec(self) -> Vec<T> {
 152         self.ptr.into_vec()
 153     }
 154 }
 155
 156 impl<T> Default for P<[T]> {
 157     /// Creates an empty `P<[T]>`.
 158     fn default() -> P<[T]> {
 159         P::new()
 160     }
 161 }
 162
 163 impl<T: Clone> Clone for P<[T]> {
 164     fn clone(&self) -> P<[T]> {
 165         P::from_vec(self.to_vec())
 166     }
 167 }
 168
 169 impl<T> From<Vec<T>> for P<[T]> {
 170     fn from(v: Vec<T>) -> Self {
 171         P::from_vec(v)
 172     }
 173 }
 174
 175 impl<T> Into<Vec<T>> for P<[T]> {
 176     fn into(self) -> Vec<T> {
 177         self.into_vec()
 178     }
 179 }
 180
 181 impl<T> FromIterator<T> for P<[T]> {
 182     fn from_iter<I: IntoIterator<Item=T>>(iter: I) -> P<[T]> {
 183         P::from_vec(iter.into_iter().collect())
 184     }
 185 }
 186
 187 impl<T> IntoIterator for P<[T]> {
 188     type Item = T;
 189     type IntoIter = vec::IntoIter<T>;
 190
 191     fn into_iter(self) -> Self::IntoIter {
 192         self.into_vec().into_iter()
 193     }
 194 }
 195
 196 impl<'a, T> IntoIterator for &'a P<[T]> {
 197     type Item = &'a T;
 198     type IntoIter = slice::Iter<'a, T>;
 199     fn into_iter(self) -> Self::IntoIter {
 200         self.ptr.into_iter()
 201     }
 202 }
 203
 204 impl<T: Encodable> Encodable for P<[T]> {
 205     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
 206         Encodable::encode(&**self, s)
 207     }
 208 }
 209
 210 impl<T: Decodable> Decodable for P<[T]> {
 211     fn decode<D: Decoder>(d: &mut D) -> Result<P<[T]>, D::Error> {
 212         Ok(P::from_vec(match Decodable::decode(d) {
 213             Ok(t) => t,
 214             Err(e) => return Err(e)
 215         }))
 216     }
 217 }