macro crimes to evaluate non-const stuff at compile time
Disclaimer: This hijacks the compilation process, so use it with caution and probably not for anything serious.
This only works on windows for now due to a hard coded dll-path, but you should be able to switch the .dll to something else for other platforms.
let v = const_eval!(
let mut res = 0;
for i in 0..200 {
if i % 3 == 0 {
res += 1;
}
}
res.to_string().parse::<i32>().unwrap()
);
assert_eq!(v, 67);The token-tree passed to const_eval! is passed to a proc-macro
that compiles another proc-macro that includes your code.
After your token-tree is evaluated in the proc-macro, the result is converted into a TokenStream.
This stream can then be passed back to the caller of const_eval! - allowing
you to use an already evaluated value!
Because everything is compiled as proc-macros, everything is guaranteed to be evaluated at compile time. This however means that you cannot use dependencies and custom types defined outside of the macro call. I have some ideas around this I would like to explore, but until then you are limited to the code inside the macro.