[SR-11957] withContiguous[Mutable]StorageIfAvailable not implemented on Slice

[stephentyrone]

Previous ID	SR-11957
Radar	rdar://58090587
Original Reporter	@stephentyrone
Type	New Feature
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	New Feature
Assignee	@lorentey
Priority	Medium

md5: 1e0512ad7793abdadd029d3398408f86

Issue Description:

Slice does not implement withContiguous[Mutable]StorageIfAvailable, which means it falls to the default implementation which, unconditionally nil s out, even when Base does provide a custom implementation.

This is most noticeable with Unsafe[Mutable]BufferPointer:

var a = UnsafeMutableBufferPointer<Int>.allocate(capacity: 4)
a[1...].withContiguousStorageIfAvailable { print($0) } // does not print

Providing a good implementation on Slice is somewhat tricky because we are lacking constraints on the index type of Base, but it would be nice to improve this.

[stephentyrone]

Note for anyone hitting this: you can always work around it by slicing the UnsafeMutableBufferPointer passed to the closure instead, though that's not as ergonomic in all cases.

[stephentyrone]

@swift-ci create

[lorentey]

@swift-ci create

[lorentey]

I think it's okay to just rely on Collection.distance(from:to🙂 to get the offsets.

[lorentey]

#28883