[SR-11999] Substring.UTF8View does not provide an implementation for withContiguousStorageIfAvailable

[Lukasa]

Previous ID	SR-11999
Radar	rdar://problem/58663804
Original Reporter	@Lukasa
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	@milseman
Priority	Medium

md5: 0e12af38d6e41ffcc6bfa3efeeec376e

Issue Description:

While String is very good at providing fast access to its backing storage, Substring is less friendly. Try running the program below:

let x = "Hello, world!"

let basePointer = x.utf8.withContiguousStorageIfAvailable {
    UInt(bitPattern: $0.baseAddress!)
}

let slicePointer = x[...].utf8.withContiguousStorageIfAvailable {
    UInt(bitPattern: $0.baseAddress!)
}

if let ptr = basePointer {
    print("✅ String has fast pointer to storage: \(ptr)")
} else {
    print("⚠️  String has no fast pointer to storage")
}

if let ptr = slicePointer {
    print("✅ Substring has fast pointer to storage: \(ptr)")
} else {
    print("⚠️  Substring has no fast pointer to storage")
}

I would expect the output to be:

✅ String has fast pointer to storage: 140732689800216
✅ Substring has fast pointer to storage: 140732689800216

But instead I get

✅ String has fast pointer to storage: 140732689800216
⚠️  Substring has no fast pointer to storage

[Lukasa]

Patch available in #29094

[swift-ci]

Comment by Kyle Macomber (JIRA)

@swift-ci create

[milseman]

Great work @Lukasa!