Add AccumulateVec, a potentially stack-allocated vector.

AccumulateVec is generic over the Array trait, which is currently only
implemented for [T; 8].

Author: Mark-Simulacrum

src/librustc_data_structures/accumulate_vec.rs

@@ -0,0 +1,52 @@
+// Copyright 2016 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.
+
+//! A vector type intended to be used for collecting from iterators onto the stack.
+//!
+//! Space for up to N elements is provided on the stack.  If more elements are collected, Vec is
+//! used to store the values on the heap. This type does not support re-allocating onto the heap,
+//! and there is no way to push more elements onto the existing storage.
+//!
+//! The N above is determined by Array's implementor, by way of an associatated constant.
+
+use std::ops::Deref;
+use std::iter::{IntoIterator, FromIterator};
+
+use array_vec::{Array, ArrayVec};
+
+#[derive(Debug)]
+pub enum AccumulateVec<A: Array> {
+    Array(ArrayVec<A>),
+    Heap(Vec<A::Element>)
+}
+
+impl<A: Array> Deref for AccumulateVec<A> {
+    type Target = [A::Element];
+    fn deref(&self) -> &Self::Target {
+        match *self {
+            AccumulateVec::Array(ref v) => &v[..],
+            AccumulateVec::Heap(ref v) => &v[..],
+        }
+    }
+}
+
+impl<A: Array> FromIterator<A::Element> for AccumulateVec<A> {
+    fn from_iter<I>(iter: I) -> AccumulateVec<A> where I: IntoIterator<Item=A::Element> {
+        let iter = iter.into_iter();
+        if iter.size_hint().1.map_or(false, |n| n <= A::LEN) {
+            let mut v = ArrayVec::new();
+            v.extend(iter);
+            AccumulateVec::Array(v)
+        } else {
+            AccumulateVec::Heap(iter.collect())
+        }
+    }
+}
+

src/librustc_data_structures/array_vec.rs

@@ -0,0 +1,106 @@
+// Copyright 2016 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.
+
+//! A stack-allocated vector, allowing storage of N elements on the stack.
+//!
+//! Currently, only the N = 8 case is supported (due to Array only being impl-ed for [T; 8]).
+
+use std::marker::Unsize;
+use std::iter::Extend;
+use std::ptr::drop_in_place;
+use std::ops::{Deref, DerefMut};
+use std::slice;
+use std::fmt;
+
+pub unsafe trait Array {
+    type Element;
+    type PartialStorage: Default + Unsize<[ManuallyDrop<Self::Element>]>;
+    const LEN: usize;
+}
+
+unsafe impl<T> Array for [T; 8] {
+    type Element = T;
+    type PartialStorage = [ManuallyDrop<T>; 8];
+    const LEN: usize = 8;
+}
+
+pub struct ArrayVec<A: Array> {
+    count: usize,
+    values: A::PartialStorage
+}
+
+impl<A: Array> ArrayVec<A> {
+    pub fn new() -> Self {
+        ArrayVec {
+            count: 0,
+            values: Default::default(),
+        }
+    }
+}
+
+impl<A> fmt::Debug for ArrayVec<A>
+    where A: Array,
+          A::Element: fmt::Debug {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self[..].fmt(f)
+    }
+}
+
+impl<A: Array> Deref for ArrayVec<A> {
+    type Target = [A::Element];
+    fn deref(&self) -> &Self::Target {
+        unsafe {
+            slice::from_raw_parts(&self.values as *const _ as *const A::Element, self.count)
+        }
+    }
+}
+
+impl<A: Array> DerefMut for ArrayVec<A> {
+    fn deref_mut(&mut self) -> &mut [A::Element] {
+        unsafe {
+            slice::from_raw_parts_mut(&mut self.values as *mut _ as *mut A::Element, self.count)
+        }
+    }
+}
+
+impl<A: Array> Drop for ArrayVec<A> {
+    fn drop(&mut self) {
+        unsafe {
+            drop_in_place(&mut self[..])
+        }
+    }
+}
+
+impl<A: Array> Extend<A::Element> for ArrayVec<A> {
+    fn extend<I>(&mut self, iter: I) where I: IntoIterator<Item=A::Element> {
+        for el in iter {
+            unsafe {
+                let arr = &mut self.values as &mut [ManuallyDrop<_>];
+                arr[self.count].value = el;
+            }
+            self.count += 1;
+        }
+    }
+}
+
+// FIXME: This should use repr(transparent) from rust-lang/rfcs#1758.
+#[allow(unions_with_drop_fields)]
+pub union ManuallyDrop<T> {
+    value: T,
+    #[allow(dead_code)]
+    empty: (),
+}
+
+impl<T> Default for ManuallyDrop<T> {
+    fn default() -> Self {
+        ManuallyDrop { empty: () }
+    }
+}
+

src/librustc_data_structures/lib.rs

@@ -30,6 +30,9 @@
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
 #![feature(fn_traits)]
+#![feature(untagged_unions)]
+#![feature(associated_consts)]
+#![feature(unsize)]
 
 #![cfg_attr(unix, feature(libc))]
 #![cfg_attr(test, feature(test))]
@@ -41,6 +44,8 @@ extern crate serialize as rustc_serialize; // used by deriving
 #[cfg(unix)]
 extern crate libc;
 
+pub mod array_vec;
+pub mod accumulate_vec;
 pub mod bitslice;
 pub mod blake2b;
 pub mod bitvec;