Misleading “use of uninitialized value” error when fieldwise struct partially transferred

[aosen]

Hi Mojo team,

I am experimenting with the move semantics and encountered an error message that seems misleading. Here is a minimal reproducible example:

@fieldwise_init
struct MyPair:
    var first: List[String]
    var second: List[Int]

    fn get_sum_of_second(self) -> Int:
        return len(self.second)

def main():
    var pair = MyPair(first=["apple", "banana", "cherry"], second=[1, 2, 3, 4])
    var captureList = pair.first^
    var _ = pair.get_sum_of_second()

Error:

/Users/test/test.mojo:12:35: error: use of uninitialized value 'pair.first'
    var _ = pair.get_sum_of_second()
                                  ^
/Users/test/test.mojo:10:9: note: 'pair' declared here
    var pair = MyPair(first=["apple", "banana", "cherry"], second=[1, 2, 3, 4])

In this case, get_sum_of_second() does not use pair.first at all, but the compiler still reports “use of uninitialized value pair.first.”

It seems that after partially transferring pair.first with the ^ operator, the compiler treats the whole struct as partially moved and forbids using it—even if only unaffected fields are accessed. I understand that partial move support is challenging to implement, but perhaps the diagnostic could be clearer, e.g.:

“cannot call method on partially transferred struct (field ‘first’ moved)”

This would make it clearer that the issue is about partial moves rather than actual uninitialized use.

I love Mojo so far—its combination of safety and expressiveness feels fantastic! Just wanted to share this feedback in case it helps improve the user experience.

Thanks for the amazing work!

[clattner (Chris Lattner)]

This is expected behavior. Mojo doesn’t look into the implementation of “get_sum_of_second()” to understand what fields it accesses, so it must assume it touched everything in “self”. We have ideas on how to improve this in the future (allowing a method to say that it only touches a subset of a value passed into it) but these features aren’t able to be prioritized in the near future, so this is expected behavior for now.

[aosen]

Thanks for the clarification! Looking forward to seeing how Mojo evolves in this area!