src/librustc_data_structures/ivar.rs

   1 // Copyright 2015 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 use std::fmt;
  12 use std::cell::Cell;
  13
  14 /// A write-once variable. When constructed, it is empty, and
  15 /// can only be set once.
  16 ///
  17 /// Ivars ensure that data that can only be initialised once. A full
  18 /// implementation is used for concurrency and blocks on a read of an
  19 /// unfulfilled value. This implementation is more minimal and panics
  20 /// if you attempt to read the value before it has been set. It is also
  21 /// not `Sync`, but may be extended in the future to be usable as a true
  22 /// concurrency type.
  23 ///
  24 /// The `T: Copy` bound is not strictly needed, but it is required by
  25 /// Cell (so removing it would require using UnsafeCell), and it
  26 /// suffices for the current purposes.
  27 #[derive(PartialEq)]
  28 pub struct Ivar<T: Copy> {
  29     data: Cell<Option<T>>,
  30 }
  31
  32 impl<T: Copy> Ivar<T> {
  33     pub fn new() -> Ivar<T> {
  34         Ivar { data: Cell::new(None) }
  35     }
  36
  37     pub fn get(&self) -> Option<T> {
  38         self.data.get()
  39     }
  40
  41     pub fn fulfill(&self, value: T) {
  42         assert!(self.data.get().is_none(), "Value already set!");
  43         self.data.set(Some(value));
  44     }
  45
  46     pub fn is_fulfilled(&self) -> bool {
  47         self.data.get().is_some()
  48     }
  49
  50     pub fn unwrap(&self) -> T {
  51         self.get().unwrap()
  52     }
  53 }
  54
  55 impl<T: Copy + fmt::Debug> fmt::Debug for Ivar<T> {
  56     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  57         match self.get() {
  58             Some(val) => write!(f, "Ivar({:?})", val),
  59             None => f.write_str("Ivar(<unfulfilled>)"),
  60         }
  61     }
  62 }
  63
  64 impl<T: Copy> Clone for Ivar<T> {
  65     fn clone(&self) -> Ivar<T> {
  66         match self.get() {
  67             Some(val) => Ivar { data: Cell::new(Some(val)) },
  68             None => Ivar::new(),
  69         }
  70     }
  71 }