Fix issue #78496

[wecing]

EarlyOtherwiseBranch finds MIR structures like:

bb0: {
  ...
  _2 = discriminant(X)
  ...
  switchInt(_2) -> [1_isize: bb1, otherwise: bb3]
}
bb1: {
  ...
  _3 = discriminant(Y)
  ...
  switchInt(_3) -> [1_isize: bb2, otherwise: bb3]
}
bb2: {...}
bb3: {...}

And transforms them into something like:

bb0: {
  ...
  _2 = discriminant(X)
  _3 = discriminant(Y)
  _4 = Eq(_2, _3)
  switchInt(_4) -> [true: bb4, otherwise: bb3]
}
bb2: {...} // unchanged
bb3: {...} // unchanged
bb4: {
  switchInt(_2) -> [1_isize: bb2, otherwise: bb3]
}

But that is not always a safe thing to do -- sometimes the early otherwise branch is necessary so the later block could assume the value of discriminant(X).

I am not totally sure what's the best way to detect that, but fixing #78496 should be easy -- we just check if X is a sub-expression of Y. A more precise test might be to check if Y contains a Downcast(1) of X, but I think this might be good enough.

Fix #78496

[rust-highfive]

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[wecing]

r? @oli-obk

[oli-obk]

I think this might be good enough.

yes, I think going with the conservative scheme is for the best. It's not just downcast projections. Deref and index projections have the same issue.

@bors r+

[bors]

📌 Commit 3812f70 has been approved by oli-obk