Conditional assignment with multiple if/else appears broken

[CLEckhardt]

When attempting a conditional assignment after a statement containing an early return, the compiler seems to think that the assignment type is a return type when using if/else, but this doesn't occur when using the an equivalent match with conditionals.

Example with failing and working code here: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=ad9db532628940de405ab21ff4a5a3a4

The playground example error message doesn't show this, but when working on my code, the error message references the early return line. So it would say something like "note: return type inferred to be () here" and reference line 8 in the example. Unfortunately, I can't reproduce that error message hint, nor can I share my code.

This occurs on stable and nightly.

EDIT:
This actually appears to be simpler and not require an early return.

Simpler example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=8d8d4dada286cad5934bc728c9ea92c4

[asquared31415]

in constructs of the form

if cond {
  expr
}

where there is no else, the if is expected to evaluate to type (). conceptually it's equivalent to

if cond {
   expr
} else {
}

and for the types to match, the type must be ().

The reason the commented out match works is because a match arm, including the if guard is conceptually one part of an if else chain. Rewriting the if-else blocks to the following works for the same reason, since now every possible if and else path returns a value of type Option

if let Some(a_num) = a && a_num == 2{
    Some(3)
} else if let Some(b_num) = b && b_num == 1{
    Some(b_num)
} else {
    None
}

I was under the impression that we had some sort of explanation somewhere about "if blocks cannot return a value without an else" but maybe that isn't working in this case? Or maybe I'm misremembering.

@rustbot label -needs-triage +A-diagnostics +D-terse

[asquared31415]

Oh you edited to add another example while I was typing, that's broken for similar reasons, if let without an else acts like if without an else, it must be type (). Adding the appropriate else makes the added example work as well.

[CLEckhardt]

@asquared31415 Thanks for the explanation. I think I was expecting if/else to be as expressive as a conditional match, but perhaps that's not the case?

[bjorn3]

In

    let c = {
        if let Some(a_num) = a {
            if a_num == 2 {
                Some(3)
            }
        }
        if let Some(b_num) = b {
            if b_num == 1 {
                Some(b_num)
            }
        }
        None
    };

there are no early returns as there are no return statements.

if a_num == 2 {
    Some(3)
}

is trying to return Some(3) from the if, but because there is no else, the if is a statement, not an expression and as such you can't return anything from the if. Statements don't evaluate to any value, only expressions do. By the way for questions about how rust works, please use https://users.rust-lang.org/ or similar, not the issue tracker.

Edit: race condition with asquared31415

[CLEckhardt]

It looks like I can set a mutable variable to None at the beginning and modify that to achieve the same result as the conditional match.
Thanks both for your help!

[asquared31415]

if (let) else can do the same things that matches can, but you have to have that else part. match arms sort of have an else between each arm, which you're missing in your second example.

Perhaps the source of confusion is in:

let x = {
  if cond {
      expr
  }
  expr2
};

when seeing the expr coming from the if, rust does not interpret that as "try to break out of the enclosing block with the value of expr" like you seem to want.

[fmease]

when seeing the expr coming from the if, rust does not interpret that as "try to break out of the enclosing block with the value of expr" like you seem to want.

And if OP would like that, they should use labeled break + block or a return + an immediately invoked closure.