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

proc-macro

#![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 2024-05-22.
}

Attribute macro

fn main() {
    println!("{}", at_comptime()); // The program was compiled on 2024-05-22.
}
#[comptime::comptime_fn]
fn at_comptime() -> &'static str {
    format!(concat!(
        "The program was compiled on ",
        comptime::comptime! {
            chrono::Utc::now().format("%Y-%m-%d").to_string()
        },
        "."
    ))
}

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.

Strings generated with comptime_fn will be represented as &'static str as it is known at compile time, to fix this, you need to simply run String::from(comptime_fn()) or comptime_fn().to_string().

Due to how the comptime macro works, writing to stdout will almost definitely cause comptime-rs to fail to build.

The comptime_fn attribute macro still makes the function call to the compile time function, but all calculations inside that function are performed at compile time. e.g.

#[comptime::comptime_fn]
fn costly_calculation() -> i32 {
    2 * 3 * 4 * 5 * 6 * 7 * 8 * 9 // Any calculations
}

will be turned into

#[comptime::comptime_fn]
fn costly_calculation() -> i32 {
    362880
}

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.