doc attributes on macro invocations can contain arbitrary expressions

[mejrs]

I tried this code:

fn main() {
    #[doc = {
        let a = 1;
        let b = 1;
        let sum = a + b;
        assert_eq!(sum, 2);
    }]
    println!();
}

I expected to see this happen: compiler error

Instead, this happened: program compiles, no diagnostic issued

This works on rust 1.94 and newer.

[mejrs]

Since #161003 this now emits an unused doc warning, but nothing about the expression