[test] add MutableSpan slicing tests

Author: glessard

test/stdlib/Span/MutableSpanTests.swift

@@ -572,3 +572,130 @@ suite.test("swapAt")
 
   expectEqual(array, (0..<count).reversed())
 }
+
+suite.test("_extracting()")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.2, *) { false } else { true } },
+  reason: "Requires Swift 6.2's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let capacity = 4
+  var b = (0..<capacity).map(Int8.init)
+  b.withUnsafeMutableBufferPointer {
+    var span = MutableSpan(_unsafeElements: $0)
+
+    var sub = span._extracting(0..<2)
+    expectEqual(sub.count, 2)
+    expectEqual(sub[0], 0)
+
+    sub = span._extracting(..<2)
+    expectEqual(sub.count, 2)
+    expectEqual(sub[0], 0)
+
+    sub = span._extracting(...)
+    expectEqual(sub.count, 4)
+    expectEqual(sub[0], 0)
+
+    sub = span._extracting(2...)
+    expectEqual(sub.count, 2)
+    expectEqual(sub[0], 2)
+  }
+}
+
+suite.test("_extracting(unchecked:)")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.2, *) { false } else { true } },
+  reason: "Requires Swift 6.2's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let capacity = 32
+  var b = (0..<capacity).map(UInt8.init)
+  b.withUnsafeMutableBufferPointer {
+    var span = MutableSpan(_unsafeElements: $0.prefix(8))
+    let beyond = span._extracting(unchecked: 16...23)
+    expectEqual(beyond.count, 8)
+    let fromBeyond = beyond[0]
+    expectEqual(fromBeyond, 16)
+  }
+}
+
+suite.test("_extracting prefixes")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.2, *) { false } else { true } },
+  reason: "Requires Swift 6.2's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let capacity = 4
+  var a = Array(0..<UInt8(capacity))
+  a.withUnsafeMutableBufferPointer {
+    var prefix: MutableSpan<UInt8>
+    var span = MutableSpan(_unsafeElements: $0)
+    expectEqual(span.count, capacity)
+
+    prefix = span._extracting(first: 1)
+    expectEqual(prefix[0], 0)
+
+    prefix = span._extracting(first: capacity)
+    expectEqual(prefix[capacity-1], UInt8(capacity-1))
+
+    prefix = span._extracting(droppingLast: capacity)
+    expectEqual(prefix.isEmpty, true)
+
+    prefix = span._extracting(droppingLast: 1)
+    expectEqual(prefix[capacity-2], UInt8(capacity-2))
+  }
+
+  do {
+    let b = UnsafeMutableBufferPointer<Int>(start: nil, count: 0)
+    var span = MutableSpan(_unsafeElements: b)
+    expectEqual(span.count, b.count)
+    expectEqual(span._extracting(first: 1).count, b.count)
+    expectEqual(span._extracting(droppingLast: 1).count, b.count)
+  }
+}
+
+suite.test("_extracting suffixes")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.2, *) { false } else { true } },
+  reason: "Requires Swift 6.2's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let capacity = 4
+  var a = Array(0..<UInt8(capacity))
+  a.withUnsafeMutableBufferPointer {
+    var suffix: MutableSpan<UInt8>
+    var span = MutableSpan(_unsafeElements: $0)
+    expectEqual(span.count, capacity)
+
+    suffix = span._extracting(last: capacity)
+    expectEqual(suffix[0], 0)
+
+    suffix = span._extracting(last: capacity-1)
+    expectEqual(suffix[0], 1)
+
+    suffix = span._extracting(last: 1)
+    expectEqual(suffix[0], UInt8(capacity-1))
+
+    suffix = span._extracting(droppingFirst: capacity)
+    expectTrue(suffix.isEmpty)
+
+    suffix = span._extracting(droppingFirst: 1)
+    expectEqual(suffix[0], 1)
+  }
+
+  do {
+    let b = UnsafeMutableBufferPointer<ObjectIdentifier>(start: nil, count: 0)
+    var span = MutableSpan(_unsafeElements: b)
+    expectEqual(span.count, b.count)
+    expectEqual(span._extracting(last: 1).count, b.count)
+    expectEqual(span._extracting(droppingFirst: 1).count, b.count)
+  }
+}