Rust would allow you to

let ret = if some_condition {
    <a lot of expensive calculations>
    result_of_operations
} else {
    <a lot of other different expensive calculations>
    result_of_other_operations
};

Now you don't have to declare it inside the blocks.

let ret = someCondition ? expensiveOperation() : otherOperation() 

?

Well, I don't know your use case well enough, but I guess you might have perfect reason for that behavior.

One thing that comes to my mind is the old Try in C#

bool parsedSuccessfully = int.TryParse("123", out int result);

But I guess more popular approach would be to use Error as Values, right?

E.g. something like this

Outcome<Exception, Int> result = int.TotallyNewParse("123");