[stdlib] String.index(before:): Fix index validation issues #41598
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`String.index(before:)` (and methods that rely on it, such as `Substring.index(before:)`, `.distance(from:to:)` etc.) does not currently verify that the given index falls before the `endIndex` before aligning it to a scalar boundary. This allows an out-of-bounds memory access when the provided index points to a position beyond the end of `self`’s storage. Additionally, the `i > startIndex` check needs to be done after scalar alignment, not before, as alignment can round the index down to `startIndex`. rdar://89497074&89495373
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`String.UnicodeScalarView` currently lacks proper index validation in its `index(after:)` and `index(before:)` methods, leading to out-of-bounds memory accesses when index navigation methods in this view are given invalid indices. (Also see apple#41598 and apple#41417) rdar://89498541
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String.index(before:)(and methods that rely on it, such asSubstring.index(before:),.distance(from:to:)etc.) does not currently verify that the given index falls before theendIndexbefore aligning it to a scalar boundary. This allows an out-of-bounds memory access when the provided index points to a position beyond the end ofself’s storage.Additionally, the
i > startIndexcheck needs to be done after scalar alignment, not before, as alignment can round the index down tostartIndex.rdar://89497074&89495373