121 lines
3.7 KiB
Rust
121 lines
3.7 KiB
Rust
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! The AST pointer
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//!
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//! Provides `P<T>`, a frozen owned smart pointer, as a replacement for `@T` in the AST.
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//!
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//! # Motivations and benefits
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//!
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//! * **Identity**: sharing AST nodes is problematic for the various analysis passes
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//! (e.g. one may be able to bypass the borrow checker with a shared `ExprAddrOf`
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//! node taking a mutable borrow). The only reason `@T` in the AST hasn't caused
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//! issues is because of inefficient folding passes which would always deduplicate
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//! any such shared nodes. Even if the AST were to switch to an arena, this would
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//! still hold, i.e. it couldn't use `&'a T`, but rather a wrapper like `P<'a, T>`.
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//!
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//! * **Immutability**: `P<T>` disallows mutating its inner `T`, unlike `Box<T>`
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//! (unless it contains an `Unsafe` interior, but that may be denied later).
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//! This mainly prevents mistakes, but can also enforces a kind of "purity".
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//!
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//! * **Efficiency**: folding can reuse allocation space for `P<T>` and `Vec<T>`,
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//! the latter even when the input and output types differ (as it would be the
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//! case with arenas or a GADT AST using type parameters to toggle features).
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//!
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//! * **Maintainability**: `P<T>` provides a fixed interface - `Deref`,
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//! `and_then` and `map` - which can remain fully functional even if the
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//! implementation changes (using a special thread-local heap, for example).
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//! Moreover, a switch to, e.g. `P<'a, T>` would be easy and mostly automated.
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use std::fmt;
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use std::fmt::Show;
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use std::hash::Hash;
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use std::ptr;
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use serialize::{Encodable, Decodable, Encoder, Decoder};
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/// An owned smart pointer.
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pub struct P<T> {
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ptr: Box<T>
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}
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#[allow(non_snake_case)]
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/// Construct a `P<T>` from a `T` value.
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pub fn P<T: 'static>(value: T) -> P<T> {
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P {
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ptr: box value
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}
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}
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impl<T: 'static> P<T> {
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/// Move out of the pointer.
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/// Intended for chaining transformations not covered by `map`.
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pub fn and_then<U, F>(self, f: F) -> U where
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F: FnOnce(T) -> U,
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{
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f(*self.ptr)
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}
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/// Transform the inner value, consuming `self` and producing a new `P<T>`.
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pub fn map<F>(mut self, f: F) -> P<T> where
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F: FnOnce(T) -> T,
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{
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unsafe {
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let p = &mut *self.ptr;
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// FIXME(#5016) this shouldn't need to zero to be safe.
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ptr::write(p, f(ptr::read_and_zero(p)));
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}
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self
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}
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}
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impl<T> Deref<T> for P<T> {
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fn deref<'a>(&'a self) -> &'a T {
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&*self.ptr
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}
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}
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impl<T: 'static + Clone> Clone for P<T> {
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fn clone(&self) -> P<T> {
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P((**self).clone())
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}
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}
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impl<T: PartialEq> PartialEq for P<T> {
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fn eq(&self, other: &P<T>) -> bool {
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**self == **other
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}
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}
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impl<T: Eq> Eq for P<T> {}
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impl<T: Show> Show for P<T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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(**self).fmt(f)
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}
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}
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impl<S, T: Hash<S>> Hash<S> for P<T> {
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fn hash(&self, state: &mut S) {
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(**self).hash(state);
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}
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}
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impl<E, D: Decoder<E>, T: 'static + Decodable<D, E>> Decodable<D, E> for P<T> {
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fn decode(d: &mut D) -> Result<P<T>, E> {
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Decodable::decode(d).map(P)
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}
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}
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impl<E, S: Encoder<E>, T: Encodable<S, E>> Encodable<S, E> for P<T> {
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fn encode(&self, s: &mut S) -> Result<(), E> {
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(**self).encode(s)
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}
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}
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