Inline const expressions and patterns

[ecstatic-morse]

Rendered

TL; DR: Allows users to request compile-time evaluation without requiring promotion or a named const.

fn foo() -> &'static i32 {
    &const { 
        let x = 4i32;
        x.pow(5)
    }
}

fn main() {
    match *x {
        0 ..= const { u32::MAX / 2 } => println!("low"),
        const { u32::MAX / 2 + 1 } ..= u32::MAX => println!("high"),
    }
}

This idea has been in the ether for a while now as a way to reduce dependence on promotion for evaluating simple expressions at compile-time. It was discussed most recently on Zulip.

Credit to @scottmcm, who discussed this with me on Zulip and was one of the people who discovered this idea. I believe both @RalfJung and @oli-obk have considered this as well.