[swift-evolution] [Review] SE-0176: Enforce Exclusive Access to Memory

[Nevin Brackett-Rozinsky]

On Sun, May 7, 2017 at 2:04 PM, Xiaodi Wu via swift-evolution <
swift-evolution at swift.org> wrote:

> Actually, `swapAt` does have a precondition that the elements differ.


Looking at the source code
<https://github.com/apple/swift/blob/master/stdlib/public/core/MutableCollection.swift>,
the “swapAt” method is documented to have no effect when the indices are
equal:

/// Exchange the values at indices `i` and `j`.
///
/// Has no effect when `i` and `j` are equal.
mutating func swapAt(_ i: Index, _ j: Index)

Furthermore, the default implementation simply returns early when given the
same index twice:

extension MutableCollection {
  @_inlineable
  public mutating func swapAt(_ i: Index, _ j: Index) {
    guard i != j else { return }
    let tmp = self[i]
    self[i] = self[j]
    self[j] = tmp
  }
}

Contrast that with “swap” (found here
<https://github.com/apple/swift/blob/master/stdlib/public/core/Sort.swift>)
which is documented with “Precondition: `a` and `b` do not alias each
other.” And the implementation enforces that (at least in debug builds)
with:

_debugPrecondition(p1 != p2, "swapping a location with itself is not
supported")

So it looks like the precondition is only found on “swap”, and “swapAt”
should work properly (by doing nothing) when asked to exchange a single
index with itself.

Nevin
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