[stack;vec]

A rust crate to use stack-allocated vectors (to improve performance and/or when there is no std)

Motivation

Rust stack/inline arrays don't implement 2 very useful iterator-related interfaces:

1. IntoIterator<Item = T> for [T; n]

   • Allows using .into_iter() instead of .iter().cloned() (which, by the way, can only be used when T: Clone, and requires cloning, which may be expensive)

   • extern crate stackvec; use ::stackvec::prelude::*;
     
     fn main ()
     {
         // An array of vectors (potentially expensive to clone)
         let vecs_array = [
             vec![1, 2, 3, 4],
             vec![],
             vec![5, 6],
         ];
     
         // Collect / chain all the vectors together
         let flattened: Vec<u8> = vecs_array
                                     .into_iter()  // Needs stackvec (line 1)
                                     .flatten()
                                     .collect()
         ;
         assert_eq!(flattened, vec![1, 2, 3, 4, 5, 6]);
     }
     

2. FromIterator for [T; n]

   • Allows collecting into an array
   • Since it is unsound to have an incomplete array, the collecting fails when the iterator does not have enough elements to fill the array. Hence the new TryFromIterator trait (providing try_collect).

   • extern crate stackvec; use ::stackvec::prelude::*;
     
     fn main ()
     {
         let array: [_; 3] = [1, 2, 3];
     
         let doubled: [_; 3] = array
                                 .iter()
                                 .map(|&x| 2 * x)
                                 .try_collect() // Needs stackvec (line 1)
                                 .expect("Missing elements to collect")
         ;
         assert_eq!(doubled, [2, 4, 6]);
     } 
     

The reason for that is that both interfaces need a structure being able to hold the partially iterated state: i.e., incomplete arrays. Those have (statically-allocated) memory that might not be initialized: so they are, in a way, like Vectors (except for the fact that their (initial) capacity is fixed and cannot be changed)

That's why having those nice iterator interfaces requires writing down a slot-accurate memory ownership management logic very similar to Vec's : hence the StackVec.

Bonus

By exposing the underlying StackVec needed by the aformentioned interfaces, we get full access to a stack-allocated Vec, which can also be useful on its own, since it avoids heap allocation:

• the heap is a mutable global state and in multi-threaded environments locks are involved,

• it may require (slow) system allocation

• heap allocation is not always available

Disclaimer

The performance gain (from using StackVec instead of Vec) is not always guaranteed, since:

1. Vec is the cornerstone of Rust's std library collection and has extremely efficient code written so that LLVM can easily optimize its usage

2. Rust's allocator is also incredibly well optimised so the performance penalties from bins management and system allocations (and the locks in a multi-threaded environment) are quite well amortized on average.

Vec vs StackVec basic benchmark

$ cargo +nightly bench --features nightly

test vec_extend             ... bench:      64,129 ns/iter (+/- 3,069)
test vec_from_iter          ... bench:      65,569 ns/iter (+/- 3,761)
test array_from_iter        ... bench:     358,993 ns/iter (+/- 6,916)
test stackvec_extend        ... bench:     360,105 ns/iter (+/- 17,489)
test stackvec_from_iter     ... bench:     369,585 ns/iter (+/- 40,894)
test stackvec_extend_by_ref ... bench:     374,226 ns/iter (+/- 11,686)
test vec_extend_by_ref      ... bench:     863,362 ns/iter (+/- 32,483)

Usage

• Add this line to your Cargo.toml (under [dependencies]):

  stackvec = "0.2.1"
  

  • Note: By default stackvec improves all the arrays with less than 1000 elements. This leads to longer compilation times. If this is an issue, and you don't really plan on using arbitrary-length arrays but at fixed multiples of 100 or powers of 2, you can depend on a "lighter" stackvec using the following line instead:

    stackvec = { version = "0.2.1", default-features = false }
    

• Add this to your .rs code:

  extern crate stackvec;
  
  use ::stackvec::prelude::*;
  

Examples

See the source files for the examples

You can run each example (example_name.rs) with:

$ cargo run --example example_name

WIP

1. no_std support

2. More Vec-like methods