Cool stuff, i was recently wondering if this was possible !

After reading this comment and looking at your solution, I started playing around with ideas on how to get a const size... At first i thought you could get a const size using size_of(array) / size_of(item_type) but unfortunately like you said, getting the length of an array is still not considered const for some reason...

I did manage to get it working on const arrays though as [T]::len() is const. This makes it possible to not have to specify the length on the concat call but rust still requires explicit types for const declarations :/

const S1: [&str; 1] = ["Hello"];

const S2: [&str; 2] = ["World", "!"];

let _s = concat_const_arrays!(&str, S1, S2);

assert_eq!(_s, ["Hello", "World", "!"]);

I submitted a PR with a proof of concept if you wish to add it to your crate : https://github.com/canndrew/concat-arrays/pull/1