58 lines
2.3 KiB
Rust
58 lines
2.3 KiB
Rust
// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
|
|
// file at the top-level directory of this distribution and at
|
|
// http://rust-lang.org/COPYRIGHT.
|
|
//
|
|
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
|
|
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
|
|
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
|
|
// option. This file may not be copied, modified, or distributed
|
|
// except according to those terms.
|
|
|
|
use rustc_data_structures::sync::{RwLock, ReadGuard};
|
|
use std::mem;
|
|
|
|
/// The `Steal` struct is intended to used as the value for a query.
|
|
/// Specifically, we sometimes have queries (*cough* MIR *cough*)
|
|
/// where we create a large, complex value that we want to iteratively
|
|
/// update (e.g., optimize). We could clone the value for each
|
|
/// optimization, but that'd be expensive. And yet we don't just want
|
|
/// to mutate it in place, because that would spoil the idea that
|
|
/// queries are these pure functions that produce an immutable value
|
|
/// (since if you did the query twice, you could observe the
|
|
/// mutations). So instead we have the query produce a `&'tcx
|
|
/// Steal<Mir<'tcx>>` (to be very specific). Now we can read from this
|
|
/// as much as we want (using `borrow()`), but you can also
|
|
/// `steal()`. Once you steal, any further attempt to read will panic.
|
|
/// Therefore we know that -- assuming no ICE -- nobody is observing
|
|
/// the fact that the MIR was updated.
|
|
///
|
|
/// Obviously, whenever you have a query that yields a `Steal` value,
|
|
/// you must treat it with caution, and make sure that you know that
|
|
/// -- once the value is stolen -- it will never be read from again.
|
|
///
|
|
/// FIXME(#41710) -- what is the best way to model linear queries?
|
|
pub struct Steal<T> {
|
|
value: RwLock<Option<T>>
|
|
}
|
|
|
|
impl<T> Steal<T> {
|
|
pub fn new(value: T) -> Self {
|
|
Steal {
|
|
value: RwLock::new(Some(value))
|
|
}
|
|
}
|
|
|
|
pub fn borrow(&self) -> ReadGuard<T> {
|
|
ReadGuard::map(self.value.borrow(), |opt| match *opt {
|
|
None => bug!("attempted to read from stolen value"),
|
|
Some(ref v) => v
|
|
})
|
|
}
|
|
|
|
pub fn steal(&self) -> T {
|
|
let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");
|
|
let value = mem::replace(value_ref, None);
|
|
value.expect("attempt to read from stolen value")
|
|
}
|
|
}
|