Assignments in conditionals can be handy handy, but I think it's better when there's a keyword for it. The Rust/Swift `if let` syntax is pretty nice for this.
Since you're using Rust as an example there, worth noting that unlike in C the assignment operator in Rust does not evaluate to the assigned value (it evaluates to the unit value `()` instead). In combination with the fact that `if` expressions in Rust require their conditions to be bools (the language has no automatic coercion to bool), this means that `if foo = 0` is guaranteed to be a type error.
(This difference in the behavior of the assignment operator is a result of Rust's ownership semantics; since assignment transfers ownership, having the assignment operator evaluate to the assigned value would actually result in the original assignment being entirely undone!)
