Make mutating the same value that is returned, in a defer, an error

[Tetralux]

f :: proc() -> int {
	x := 1
	defer x = 99
	return x
}

main :: proc() {
    result := f()
    // what is 'result' ?
}

Spoiler: result == 1.

I think this is should be a compile error.
I'm struggling to think of even a single case where this isn't a bug: Mutating the same value that is returned, in a defer statement.

If you internalise that defers run after the return statement, then it might be easier to resolve this in your mind - but I think it's a bit error prone, as it is easy to fall into the trap here. And, more importantly, if you do fall into the trap, it's likely a bugger to debug!

You could also extend this to procedure calls in defers, as well.

f :: proc() -> int {
	x := 1
	defer g(&x)
	return x
}

g :: proc(ptr: ^int) {
    ptr^ = 101
}

Perhaps this can also be an error.

If, when checking procedure bodies, the 'mutability' of parameters is flagged, then detection for this within f is trivial:

• g writes through its pointer parameter ptr
• &x is passed to it in the defer; and
• x is returned from f.