[region-isolation] Assigns to var with structs/tuples with multiple fields should be merges for now.
The reason that this is being done is that since currently region based
isolation is not field sensitive, when we assign to the struct or tuple field of
the var, the new region relationship is set for the entire struct, not just a
specific field. This means that we effectively lose any region information from
the other fields. For example in the following at (1), given the current rules, we
lose that s.y was assigned to x:
```swift
struct S {
var x: NonSendableKlass
var y: NonSendableKlass
}
func foo() {
var s = S()
// Regions: [s]
let x = NonSendableKlass()
let y = NonSendableKlass()
// Regions: [(s), (x), (y)]
s.y = x
// Regions: [(s, x), (y)]
s.x = y (1)
// Regions: [(x), (s, y)]
}
```
The best solution to this is to track such var like bindings in a field
sensitive manner where the regions of the aggregate are the union of the
individual fields. This would let us represent the regions of the above as
follows:
```swift
func foo() {
var s = S()
// Regions: [((s.x), (s.y))]
let x = NonSendableKlass()
let y = NonSendableKlass()
// Regions: [((s.x), (s.y)), (x), (y)]
s.y = x
// Regions: [((s.x), (s.y, x)), (y)]
s.x = y (1)
// Regions: [((s.x, y), (s.y, x))]
}
```
We cannot do this today so to plug this hole, we instead treat these operations
as merges. This provides a conservative answer. Thus we would have:"
```swift
func foo() {
var s = S()
// Regions: [s]
let x = NonSendableKlass()
let y = NonSendableKlass()
// Regions: [(s), (x), (y)]
s.y = x
// Regions: [(s, x), (y)]
s.x = y (1)
// Regions: [(s, x, y])
}
```
This is because we are treating the assignment into s.y and s.x as merges into
s, so we do not lose that y was assigned into s before we assigned y into
it. The unfortunate side-effect of this is that if a struct or tuple has
multiple fields, the merge makes it so that if we assign over the same field, we
do not lose the region of the old value:
```swift
func foo() {
var s = S()
// Regions: [s]
let x = NonSendableKlass()
let y = NonSendableKlass()
// Regions: [(s), (x), (y)]
s.y = x
// Regions: [(s, x), (y)]
s.y = y (1)
// Regions: [(s, x, y])
}
```
If we were not to do this, then the s.y at (1) would result in s and x being in
different regions.
rdar://117273952
swift
345b4cc0 - [region-isolation] Assigns to var with structs/tuples with multiple fields should be merges for now.
