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Allocationless constant String -> NSString bridging via a new tagged pointer type #1232

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Merged
merged 4 commits into from
Mar 28, 2025
Merged

Allocationless constant String -> NSString bridging via a new tagged pointer type #1232

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98f421d
Revert "Revert "rdar://142693100 (Allocationless constant String -> N…
Catfish-Man Feb 25, 2025
b28673d
Adopt the new entry point for bridging the new tagged pointers
Catfish-Man Mar 7, 2025
4cfd8d9
All remaining callsites passed null-terminated strings, so just elimi…
Catfish-Man Mar 8, 2025
46ebb36
Address review comments
Catfish-Man Mar 14, 2025
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13 changes: 13 additions & 0 deletions Sources/FoundationEssentials/String/String+Bridging.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,16 @@ extension String : _ObjectiveCBridgeable {
return String(unsafeUninitializedCapacity: SMALL_STRING_CAPACITY) {
_NSTaggedPointerStringGetBytes(source, $0.baseAddress!)
}
} else if tag == OBJC_TAG_NSAtom {
var len = UInt16(0)
let contentsPtr = _CFIndirectTaggedPointerStringGetContents(source, &len)
let contents = UnsafeBufferPointer(start: contentsPtr, count: Int(len))
// Will only fail if contents aren't valid UTF8/ASCII
if let result = _SwiftCreateImmortalString_ForFoundation(buffer: contents, isASCII: true) {
return result
}
// Since our contents are invalid, force a real copy of the string and bridge that instead. This should basically never be hit in practice
return source.mutableCopy() as! String
} else if tag.rawValue == 22 /* OBJC_TAG_Foundation_1 */ {
let cStr = source.utf8String!
return String.init(utf8String: cStr)!
Expand All @@ -88,6 +98,9 @@ extension String : _ObjectiveCBridgeable {

if constant {
if ascii {
// We would like to use _SwiftCreateImmortalString_ForFoundation here, but we can't because we need to maintain the invariant
// (constantString as String as NSString) === constantString
// and using _SwiftCreateImmortalString_ForFoundation would make an indirect tagged string instead on the way back
return String(_immortalCocoaString: source, count: len, encoding: Unicode.ASCII.self)
} else {
return String(_immortalCocoaString: source, count: len, encoding: Unicode.UTF16.self)
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