Enabling the next‑generation trait solver on nightly

// This program demonstrates the issue that the new trait solver fixes.
// In the current stable compiler (1.98.1, edition 2024) the following
// function would not compile because the return type is `impl Sized` and
// the compiler cannot add an integer to it.  With the next‑generation
// solver enabled on nightly, the code would compile and work as intended.
//
// Here we use a concrete return type (`i32`) so that the program compiles
// on the stable compiler and still shows the same logic.

fn foo(b: bool) -> i32 {
    if b {
        // In the new solver this would be `foo(false) + 1` where the
        // return type is `impl Sized`.  We use `i32` instead.
        foo(false) + 1
    } else {
        0
    }
}

fn main() {
    let value = foo(true);
    println!("Result: {}", value);
}