Assert size_of type parameter greater than zero

[spearman]

Partial fix for: #15

I couldn't figure out how to make this a compile-time error. It may be possible to do something with CTFE when that is more fleshed out.

[polyfractal]

Heya @spearman, sorry for letting this rot. Do you think this is still relevant? I haven't kept up with Rust lately, and haven't used the library actively for a while so you're a better judge than me :)

[spearman]

There has been some progress in constant evaluation in the last few Rust versions, and there is now a static_assertions crate with some convenience macros: https://crates.io/crates/static_assertions, but I was not able to get this to work in this case for generic 'T':

fn make<T>(capacity: usize) -> (Producer<T>, Consumer<T>) {
const_assert!(std::mem::size_of::<T>() != 0);
...
---
error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
`T` does not have a constant size known at compile-time

One thing that appears to work at least for unit and zero-sized arrays is using an auto trait:

#![feature(optin_builtin_traits)]

pub auto trait Nonzero {}
impl !Nonzero for () {}
impl<T> !Nonzero for [T; 0] {}

...

fn make<T: Nonzero>(capacity: usize) -> (Producer<T>, Consumer<T>) {
...

So that covers only unit and zero-sized arrays, but not any user-created zero-sized types.

Really the main thing with this pull request is the assertion prints out a more informative error message, whereas before the program will just Segfault without explanation.

[spearman]

Closing in favor of #23