comptime

Lightweight compile-time expression evaluation. This crate is inspired by Zig's comptime.

The expression returned by the contents of the comptime macro invocation will be parsed as Rust source code and inserted back into the call site.

tl;dr: comptime! gives you no-context anonynmous proc macros.

Example

#![feature(proc_macro_hygiene)]
fn main() {
    println!(concat!(
        "The program was compiled on ",
        comptime::comptime! {
            chrono::Utc::now().format("%Y-%m-%d").to_string()
        },
        "."
    )); // The program was compiled on 2019-08-30.
}

Limitations

Unlike the real comptime, comptime! does not have access to the scope in which it is invoked. The code in comptime! is run as its own script. Though, technically, you could interpolate static values using quote!.

Also, comptime! requires you to run cargo build at least once before cargo (clippy|check) will work since comptime! does not compile dependencies.

Contributing

Please do! Ideally, rustc would also have (real) comptime which would have access to type information and other static values. In the meantime, this should be a nice way to approximate and experiment with such functionality.