[Bug][compiler-v2] Difference in compiler v1 and v2 when actively borrowed variable is destroyed

[vineethk]

🐛 Bug

Consider the following code:

module 0x8675309::M {
    struct R has key, copy, drop { f: u64 }

    fun t4(cond: bool, addr: address): bool acquires R {
        let r = R { f: 0 };
        let f = &borrow_global<R>(addr).f;
        let r1; if (cond) r1 = borrow_global<R>(addr) else r1 = &mut r;
        R { f: _ } = r;
        let res = f == &r1.f;
        res
    }
}

Compiler v1 - provides the following error:

Error: error[E07006]: ambiguous usage of variable
-   ┌─ TEMPFILE:8:22
-   │
- 7 │         let r1; if (cond) r1 = borrow_global<R>(addr) else r1 = &mut r;
-   │                                                                 ------ It is still being borrowed by this reference
- 8 │         R { f: _ } = r;
-   │                      ^
-   │                      │
-   │                      Ambiguous usage of variable 'r'
-   │                      Try an explicit annotation, e.g. 'move r' or 'copy r'
-   │
-   = Ambiguous inference of 'move' or 'copy' for a borrowed variable's last usage: A 'move' would invalidate the borrowing reference, but a 'copy' might not be the expected implicit behavior since this the last direct usage of the variable.

But compiler v2 accepts this code (and inserts a copy of the actively borrowed variable, which is then destroyed). Such a silent copy insertion may not be obvious to the user, so we may want to do what compiler v1 does here.