alt Hacker News> Borrow Checking
It's very confusing name for this feature. It suggest that some sort of borrowing takes place and that it's just an optional check, which isn't the case. It should be named something like "enforced static usage analysis" instead.
In my programming language I have similar mechanism. But it isn't just checking, since it affects code generation by tracking which variables are still in use and which can be destroyed.
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You can very likely borrow check in languages that don't have it in the type system. Exactly the way you suggest, as an optional add-in. It's still WIP but in my side project I haven't found cases that can't be handled yet.
In Rust variables are not destroyed after the last borrow ends but instead when it goes out of scope. I guess that is why it us called borrow checking.
In rust it's a lot closer to optional: you can in principle compile rust without doing any borrow checking at all (and I believe in practice mrustc does not bother to implement it, because it's primarily used for bootstrapping and so assumes it is already being passed code that compiles with regular rustc).