[SILGen/DI] InitAccessors: Fix handling of nonmutating set when type has other stored properties

[xedin]

The original change supported only the case where all stored properties were controlled
by an init accessor but when other fields were involved SILGen would emitted copies of
loaded "self" for each setter application which is redundant and due to the fact that the
setter was escaping.

These changes make partial application of the setter non-escaping which means that
"self" can be borrowed instead of copied.

Resolves: #67827

[xedin]

@swift-ci please test

[xedin]

@swift-ci please test source compatibility

[xedin]

@swift-ci please build toolchain macOS

[xedin]

@swift-ci please test source compatibility release

[xedin]

@swift-ci please test source compatibility debug

[xedin]

This failure in distributed actors looks unrelated but I am going to wait for main branch suite run to complete.

[mansbernhardt]

Thanks @xedin for fixing this. Would it be possible to get this fix into the 5.9.x branch/release as well?

[xedin]

Unfortunately I think it might be too risky of a change for 5.9.x but I'll see what I can do.

[mansbernhardt]

Hello @xedin, I tried your fix out in the latest Xcode 15.3 beta (15E5178i), and it seem to work fine for the examples I reported, but if I change them slightly to include e.g. let constant it fails to compile again Variable 'self._count' used before being initialized:

struct Test {
    let id: UUID
    private var _count: Int

    var count: Int
    {
        @storageRestrictions(initializes: _count)
        init {
            _count = newValue
        }
        get {
            _count
        }
        nonmutating set {
            // Update store
        }
    }

    init(id: UUID, count: Int) {
        self.id = id
        self.count = count // << Variable 'self._count' used before being initialized
    }
}

[xedin]

I will take a look!

[xedin]

Hm, var id doesn't work either, var and let result in the same SIL so it must be something in DI that I haven't accounted for.

[xedin]

This appears to be related to the UUID, if I change the type of id to String both let and var succeed.

[mansatgigset]

Interesting. Switching UUID to Date also fails.

[xedin]

Right, I think both Date and UUID are big enough to force indirect access to the Test, this is something we didn't account for because for that a copy of Test has to be allocated.

[xedin]

I think I figured it out, the problem is not actually SILGen, it's Sema because it generates a load from "self" of count though it shouldn't be doing that. There is a special handling of that for property wrappers but not for init accessors, I should be able to fix this tomorrow.

[xedin]

Here is the fix - #71248

[mansbernhardt]

@xedin I tested the above with the latest Xcode beta, and it seems to work, but if I modify it slightly it still fails with the same error (the below is a closer match to how my macro is generating its code).

struct Test {
  let id: UUID
  private var _count: Int

  var count: Int
  {
    @storageRestrictions(initializes: _count)
    init {
      _count = newValue
    }
    _read {
      yield _count
    }
    nonmutating _modify {
      var val = _count
      yield &val
    }
  }

  init(id: UUID, count: Int) {
    self.id = id
    self.count = count // << Variable 'self._count' used before being initialized
  } // << Return from initializer without initializing all stored properties
}