let chains in if and while

Summary

• Allow chaining of let expressions in the condition operand of if and while.

Details

Starting with the 2024 Edition, it is now allowed to have chaining of let expressions inside if and while condition operands, where chaining refers to && chains. The let expressions still have to appear at the top level, so if (let Some(hi) = foo || let Some(hi) = bar) is not allowed.

Before 2024, the let had to appear directly after the if or while, forming a if let or while let special variant. Now, if and while allow chains of one or more let expressions, possibly mixed with expressions that are bool typed.

#![allow(unused)]
fn main() {
fn sum_first_two(nums: &[u8]) -> Option<u8> {
    let mut iter = nums.iter();
    if let Some(first) = iter.next()
        && let Some(second) = iter.next()
    {
        first.checked_add(*second)
    } else {
        None
    }
}
}

The feature is edition gated due to requiring if let rescoping, which is a Edition 2024 change.

Migration

The switch to Edition 2024 doesn't neccessitate any migrations due to this feature, as it creates a true extension of the set of allowed Rust programs.