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[stdlib] Switch to using unchecked buffer subscript in low-level Unicode helpers

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We expect indices to be already validated by the time these are
called, and UBP’s debug-mode checks are compiled into opaque parts
of the stdlib.

(The big exception is that Swift <5.7 used to not do proper index
validation and allowed out-of-bounds accesses. We emulate this
behavior for code that was compiled with such a release, and it turns
out that these UnsafeBufferPointer checks are interfering with the
intended (undefined) behavior.)

rdar://93707276
This commit is contained in:
Karoy Lorentey
2022-07-05 18:33:51 -07:00
parent dbc8307b64
commit 64a57e3ade
1 changed files with 3 additions and 3 deletions
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6
stdlib/public/core/UnicodeHelpers.swift
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@@ -64,7 +64,7 @@ internal func _decodeUTF8(
internal func _decodeScalar(
_ utf16: UnsafeBufferPointer<UInt16>, startingAt i: Int
) -> (Unicode.Scalar, scalarLength: Int) {
let high = utf16[i]
let high = utf16[_unchecked: i]
if i + 1 >= utf16.count {
_internalInvariant(!UTF16.isLeadSurrogate(high))
_internalInvariant(!UTF16.isTrailSurrogate(high))
@@ -76,7 +76,7 @@ internal func _decodeScalar(
return (Unicode.Scalar(_unchecked: UInt32(high)), 1)
}
let low = utf16[i+1]
let low = utf16[_unchecked: i+1]
_internalInvariant(UTF16.isLeadSurrogate(high))
_internalInvariant(UTF16.isTrailSurrogate(low))
return (UTF16._decodeSurrogates(high, low), 2)
@@ -207,7 +207,7 @@ extension _StringGuts {
@inlinable
internal func fastUTF8ScalarLength(startingAt i: Int) -> Int {
_internalInvariant(isFastUTF8)
let len = _utf8ScalarLength(self.withFastUTF8 { $0[i] })
let len = _utf8ScalarLength(self.withFastUTF8 { $0[_unchecked: i] })
_internalInvariant((1...4) ~= len)
return len
}