[SR-15615] [SE-0263] Incorrect buffer count for small capacity?

[benrimmington]

Previous ID	SR-15615
Radar	None
Original Reporter	@benrimmington
Type	Bug

Environment

• macOS 12.0.1 (21A559)

• Xcode version 13.2 (13C90)

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	None
Priority	Medium

md5: 1a2f74d992daac9b9dd8f4d6500c6f1b

Issue Description:

String.init(unsafeUninitializedCapacity:initializingUTF8With:) has different behavior for a small vs large capacity.

// Example 1. Small strings (capacity <= _SmallString.capacity)
for capacity in 0...15 {
  String.init(unsafeUninitializedCapacity: capacity) { buffer in
    // Always prints "false 16"
    print(capacity == buffer.count, buffer.count)
    return 0
  }
}

// Example 2. Large strings (capacity > _SmallString.capacity)
for capacity in 16...255 {
  String.init(unsafeUninitializedCapacity: capacity) { buffer in
    // Always prints "true ..."
    print(capacity == buffer.count, buffer.count)
    return 0
  }
}

The first example is surprising because:

• The actual buffer.count of 16 exceeds the _SmallString.capacity of 15.

• The API documentation suggests the buffer will only have "room for `capacity` UTF-8 code units".

I noticed this issue because I was using buffer.endIndex when initializing in reverse.

[benrimmington]

Cc: @milseman, @Catfish-Man