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::{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         unsafe {
  78             let p = &mut *self.ptr;
  79             // FIXME(#5016) this shouldn't need to drop-fill to be safe.
  80             ptr::write(p, f(ptr::read_and_drop(p)));
  81         }
  82         self
  83     }
  84 }
  85
  86 impl<T> Deref for P<T> {
  87     type Target = T;
  88
  89     fn deref<'a>(&'a self) -> &'a T {
  90         &self.ptr
  91     }
  92 }
  93
  94 impl<T: 'static + Clone> Clone for P<T> {
  95     fn clone(&self) -> P<T> {
  96         P((**self).clone())
  97     }
  98 }
  99
 100 impl<T: Debug> Debug for P<T> {
 101     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 102         Debug::fmt(&**self, f)
 103     }
 104 }
 105 impl<T: Display> Display for P<T> {
 106     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 107         Display::fmt(&**self, f)
 108     }
 109 }
 110
 111 impl<T> fmt::Pointer for P<T> {
 112     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 113         fmt::Pointer::fmt(&self.ptr, f)
 114     }
 115 }
 116
 117 impl<T: 'static + Decodable> Decodable for P<T> {
 118     fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {
 119         Decodable::decode(d).map(P)
 120     }
 121 }
 122
 123 impl<T: Encodable> Encodable for P<T> {
 124     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
 125         (**self).encode(s)
 126     }
 127 }
 128
 129
 130 impl<T:fmt::Debug> fmt::Debug for P<[T]> {
 131     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
 132         self.ptr.fmt(fmt)
 133     }
 134 }
 135
 136 impl<T> P<[T]> {
 137     pub fn new() -> P<[T]> {
 138         P::empty()
 139     }
 140
 141     pub fn empty() -> 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     pub fn as_slice<'a>(&'a self) -> &'a [T] {
 156         &self.ptr
 157     }
 158
 159     pub fn move_iter(self) -> vec::IntoIter<T> {
 160         self.into_vec().into_iter()
 161     }
 162
 163     pub fn map<U, F: FnMut(&T) -> U>(&self, f: F) -> P<[U]> {
 164         self.iter().map(f).collect()
 165     }
 166 }
 167
 168 impl<T> Deref for P<[T]> {
 169     type Target = [T];
 170
 171     fn deref(&self) -> &[T] {
 172         self.as_slice()
 173     }
 174 }
 175
 176 impl<T> Default for P<[T]> {
 177     fn default() -> P<[T]> {
 178         P::empty()
 179     }
 180 }
 181
 182 impl<T: Clone> Clone for P<[T]> {
 183     fn clone(&self) -> P<[T]> {
 184         P::from_vec(self.to_vec())
 185     }
 186 }
 187
 188 impl<T> From<Vec<T>> for P<[T]> {
 189     fn from(v: Vec<T>) -> Self {
 190         P::from_vec(v)
 191     }
 192 }
 193
 194 impl<T> Into<Vec<T>> for P<[T]> {
 195     fn into(self) -> Vec<T> {
 196         self.into_vec()
 197     }
 198 }
 199
 200 impl<T> FromIterator<T> for P<[T]> {
 201     fn from_iter<I: IntoIterator<Item=T>>(iter: I) -> P<[T]> {
 202         P::from_vec(iter.into_iter().collect())
 203     }
 204 }
 205
 206 impl<T> IntoIterator for P<[T]> {
 207     type Item = T;
 208     type IntoIter = vec::IntoIter<T>;
 209
 210     fn into_iter(self) -> Self::IntoIter {
 211         self.into_vec().into_iter()
 212     }
 213 }
 214
 215 impl<'a, T> IntoIterator for &'a P<[T]> {
 216     type Item = &'a T;
 217     type IntoIter = slice::Iter<'a, T>;
 218     fn into_iter(self) -> Self::IntoIter {
 219         self.ptr.into_iter()
 220     }
 221 }
 222
 223 impl<T: Encodable> Encodable for P<[T]> {
 224     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
 225         Encodable::encode(&**self, s)
 226     }
 227 }
 228
 229 impl<T: Decodable> Decodable for P<[T]> {
 230     fn decode<D: Decoder>(d: &mut D) -> Result<P<[T]>, D::Error> {
 231         Ok(P::from_vec(match Decodable::decode(d) {
 232             Ok(t) => t,
 233             Err(e) => return Err(e)
 234         }))
 235     }
 236 }