Michael Gottesman c3d445831b [region-isolation] Fix an off by one error when mapping AST capture indices to SIL level parameter indices. ...

This problem comes up with the following example:

```swift
class A {
    var description = ""
}

class B {
    let a = A()

    func b() {
        let asdf = ""
        Task { @MainActor in
            a.description = asdf // Sending 'asdf' risks causing data races
        }
    }
}
```

The specific issue is that the closure we generate actually includes an
implicit(any) parameter at the SIL level which occurs after the callee operand
but before the captures. This caused the captured variable index from the AST
and the one we compute from the partial_apply to differ by 1. So we need to
subtract 1 in such a case. That is why we used to print 'asdf' instead of 'a'
above.

DISCUSSION: This shows an interesting difference between SIL applied arg indices
and AST indices. SIL applied arg indices would include the implicit(any)
parameter since it is a parameter in the SIL function type. In contrast, this
doesn't show up in the formal AST parameters or captures. To make it easier to
reason about this, I added a new API to ApplySite called
ApplySite::getASTAppliedArgIndex and added large comments to
getASTAppliedArgIndex and getAppliedArgIndex that explains the issue.

rdar://136593706
https://github.com/swiftlang/swift/issues/76648

2024-10-30 18:32:45 -07:00

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