3 //! Provides `P<T>`, a frozen owned smart pointer, as a replacement for `@T` in
6 //! # Motivations and benefits
8 //! * **Identity**: sharing AST nodes is problematic for the various analysis
9 //! passes (e.g., one may be able to bypass the borrow checker with a shared
10 //! `ExprKind::AddrOf` node taking a mutable borrow). The only reason `@T` in the
11 //! AST hasn't caused issues is because of inefficient folding passes which
12 //! would always deduplicate any such shared nodes. Even if the AST were to
13 //! switch to an arena, this would still hold, i.e., it couldn't use `&'a T`,
14 //! but rather a wrapper like `P<'a, T>`.
16 //! * **Immutability**: `P<T>` disallows mutating its inner `T`, unlike `Box<T>`
17 //! (unless it contains an `Unsafe` interior, but that may be denied later).
18 //! This mainly prevents mistakes, but can also enforces a kind of "purity".
20 //! * **Efficiency**: folding can reuse allocation space for `P<T>` and `Vec<T>`,
21 //! the latter even when the input and output types differ (as it would be the
22 //! case with arenas or a GADT AST using type parameters to toggle features).
24 //! * **Maintainability**: `P<T>` provides a fixed interface - `Deref`,
25 //! `and_then` and `map` - which can remain fully functional even if the
26 //! implementation changes (using a special thread-local heap, for example).
27 //! Moreover, a switch to, e.g., `P<'a, T>` would be easy and mostly automated.
29 use std::fmt::{self, Display, Debug};
30 use std::iter::FromIterator;
31 use std::ops::{Deref, DerefMut};
32 use std::{mem, ptr, slice, vec};
34 use serialize::{Encodable, Decodable, Encoder, Decoder};
36 use rustc_data_structures::stable_hasher::{StableHasher, StableHasherResult,
38 /// An owned smart pointer.
39 #[derive(Hash, PartialEq, Eq)]
40 pub struct P<T: ?Sized> {
44 #[allow(non_snake_case)]
45 /// Construct a `P<T>` from a `T` value.
46 pub fn P<T: 'static>(value: T) -> P<T> {
52 impl<T: 'static> P<T> {
53 /// Move out of the pointer.
54 /// Intended for chaining transformations not covered by `map`.
55 pub fn and_then<U, F>(self, f: F) -> U where
60 /// Equivalent to and_then(|x| x)
61 pub fn into_inner(self) -> T {
65 /// Produce a new `P<T>` from `self` without reallocating.
66 pub fn map<F>(mut self, f: F) -> P<T> where
69 let p: *mut T = &mut *self.ptr;
71 // Leak self in case of panic.
72 // FIXME(eddyb) Use some sort of "free guard" that
73 // only deallocates, without dropping the pointee,
74 // in case the call the `f` below ends in a panic.
78 ptr::write(p, f(ptr::read(p)));
80 // Recreate self from the raw pointer.
81 P { ptr: Box::from_raw(p) }
85 /// Optionally produce a new `P<T>` from `self` without reallocating.
86 pub fn filter_map<F>(mut self, f: F) -> Option<P<T>> where
87 F: FnOnce(T) -> Option<T>,
89 let p: *mut T = &mut *self.ptr;
91 // Leak self in case of panic.
92 // FIXME(eddyb) Use some sort of "free guard" that
93 // only deallocates, without dropping the pointee,
94 // in case the call the `f` below ends in a panic.
98 if let Some(v) = f(ptr::read(p)) {
101 // Recreate self from the raw pointer.
102 Some(P { ptr: Box::from_raw(p) })
104 drop(Box::from_raw(p));
111 impl<T: ?Sized> Deref for P<T> {
114 fn deref(&self) -> &T {
119 impl<T: ?Sized> DerefMut for P<T> {
120 fn deref_mut(&mut self) -> &mut T {
125 impl<T: 'static + Clone> Clone for P<T> {
126 fn clone(&self) -> P<T> {
131 impl<T: ?Sized + Debug> Debug for P<T> {
132 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
133 Debug::fmt(&self.ptr, f)
137 impl<T: Display> Display for P<T> {
138 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
139 Display::fmt(&**self, f)
143 impl<T> fmt::Pointer for P<T> {
144 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
145 fmt::Pointer::fmt(&self.ptr, f)
149 impl<T: 'static + Decodable> Decodable for P<T> {
150 fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {
151 Decodable::decode(d).map(P)
155 impl<T: Encodable> Encodable for P<T> {
156 fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
162 pub fn new() -> P<[T]> {
163 P { ptr: Default::default() }
167 pub fn from_vec(v: Vec<T>) -> P<[T]> {
168 P { ptr: v.into_boxed_slice() }
172 pub fn into_vec(self) -> Vec<T> {
177 impl<T> Default for P<[T]> {
178 /// Creates an empty `P<[T]>`.
179 fn default() -> P<[T]> {
184 impl<T: Clone> Clone for P<[T]> {
185 fn clone(&self) -> P<[T]> {
186 P::from_vec(self.to_vec())
190 impl<T> From<Vec<T>> for P<[T]> {
191 fn from(v: Vec<T>) -> Self {
196 impl<T> Into<Vec<T>> for P<[T]> {
197 fn into(self) -> Vec<T> {
202 impl<T> FromIterator<T> for P<[T]> {
203 fn from_iter<I: IntoIterator<Item=T>>(iter: I) -> P<[T]> {
204 P::from_vec(iter.into_iter().collect())
208 impl<T> IntoIterator for P<[T]> {
210 type IntoIter = vec::IntoIter<T>;
212 fn into_iter(self) -> Self::IntoIter {
213 self.into_vec().into_iter()
217 impl<'a, T> IntoIterator for &'a P<[T]> {
219 type IntoIter = slice::Iter<'a, T>;
220 fn into_iter(self) -> Self::IntoIter {
225 impl<T: Encodable> Encodable for P<[T]> {
226 fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
227 Encodable::encode(&**self, s)
231 impl<T: Decodable> Decodable for P<[T]> {
232 fn decode<D: Decoder>(d: &mut D) -> Result<P<[T]>, D::Error> {
233 Ok(P::from_vec(Decodable::decode(d)?))
237 impl<CTX, T> HashStable<CTX> for P<T>
238 where T: ?Sized + HashStable<CTX>
240 fn hash_stable<W: StableHasherResult>(&self,
242 hasher: &mut StableHasher<W>) {
243 (**self).hash_stable(hcx, hasher);