Unwarranted unsafe optional unwrap in MutableSpan implementation

[glessard]

Description

In https://forums.swift.org/t/80854/4, @benrimmington asked an implementation question:

I'm not sure what happens to _start() when the span is empty.

extension MutableSpan where Element: ~Copyable {
  @lifetime(&self)
  mutating public func extracting(_: UnboundedRange) -> Self {
    let newSpan = unsafe Self(_unchecked: _start(), count: _count)
    return unsafe _overrideLifetime(newSpan, mutating: &self)
  }
}

The answer is probably nothing good. This is an error in the implementation.

Expected behavior

There shouldn't be an unsafe unwrap in this code path.

Environment

swift-DEVELOPMENT-SNAPSHOT-2025-06-22-a

Additional information

Also tracked in rdar://155198823

[glessard]

Some additional auditing uncovered the following:

• MutableSpan.update(repeating:) needs a guard statement.

• RawSpan.init(_mutableRawSpan:) needs to use the _pointer.

• MutableRawSpan.update(fromContentsOf:) needs a _precondition.

• MutableRawSpan.extracting(_:) needs to use the _pointer.

• MutableSpan.withUnsafeMutableBytes(_:) doesn't need to use _start()

• MutableSpan._start() and MutableRawSpan._start() should have been marked @unsafe

[glessard]

This was fixed on main in #82828, and on release/6.2 in #82844