[Language] for-else and while-else

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when binding to a bindable, break in for and while isn't allowed, this proposal allow it using a else

• when break is not reached, evaluates to value in else (because the condition is false? I'm not sure)

• semantic similar to Zig

https://ziglang.org/documentation/master/#for

read the comment:

// For loops can also be used as expressions.
// Similar to while loops, when you break from a for loop, the else branch is not evaluated.

• semantic different of Python (break and the proposed else block outputs an expression in Rust, in Python you can't do it)

Example:

fn main() {
  let result = for counter in 0..5 {
    if counter == 3 {
      break counter
    }
  } else { -7 };

  println!("The result is {result}") // 3
}

fn main() {
  let result = for counter in 0..5 {
    if counter == 90 {
      break counter
    }
  } else { -7 };

  println!("The result is {result}") // -7
}

Alternatives

https://rust-lang.github.io/rfcs/2046-label-break-value.html

Alternatives to else keyword

IDK

Start by reading the proposal in issue #3361, including its Zig reference and the alternative RFC 2046 on labeled break values. Clarify the semantics of for-else and while-else, especially break values and when the else expression runs. Done means the behavior and alternatives are resolved well enough to specify an RFC.