Merge pull request #29094 from Lukasa/cb-substring-fast-access

[stdlib] Add withContiguousStorageIfAvailable to SubString.UTF8View

Author: milseman

stdlib/public/core/Substring.swift

@@ -389,6 +389,13 @@ extension Substring.UTF8View: BidirectionalCollection {
     return _slice.distance(from: start, to: end)
   }
 
+  @_alwaysEmitIntoClient
+  public func withContiguousStorageIfAvailable<R>(
+    _ body: (UnsafeBufferPointer<Element>) throws -> R
+  ) rethrows -> R? {
+    return try _slice.withContiguousStorageIfAvailable(body)
+  }
+
   @inlinable
   public func _failEarlyRangeCheck(_ index: Index, bounds: Range<Index>) {
     _slice._failEarlyRangeCheck(index, bounds: bounds)

test/stdlib/subString.swift

@@ -12,6 +12,19 @@ func checkMatch<S: Collection, T: Collection>(_ x: S, _ y: T, _ i: S.Index)
   expectEqual(x[i], y[i])
 }
 
+func checkMatchContiguousStorage<S: Collection, T: Collection>(_ x: S, _ y: T)
+  where S.Element == T.Element, S.Element: Equatable
+{
+  let xElement = x.withContiguousStorageIfAvailable { $0.first }
+  let yElement = y.withContiguousStorageIfAvailable { $0.first }
+  expectEqual(xElement, yElement)
+}
+
+func checkHasContiguousStorage<S: Collection>(_ x: S) {
+  let hasStorage = x.withContiguousStorageIfAvailable { _ in true } ?? false
+  expectTrue(hasStorage)
+}
+
 SubstringTests.test("Equality") {
   let s = "abcdefg"
   let s1 = s[s.index(s.startIndex, offsetBy: 2) ..<
@@ -228,6 +241,13 @@ SubstringTests.test("UTF8View") {
     expectEqual("", String(t.dropLast(100))!)
     expectEqual("", String(u.dropFirst(100))!)
     expectEqual("", String(u.dropLast(100))!)
+
+    checkHasContiguousStorage(s.utf8)
+    checkHasContiguousStorage(t)
+    checkHasContiguousStorage(u)
+    checkMatchContiguousStorage(Array(s.utf8), s.utf8)
+    checkMatchContiguousStorage(Array(t), t)
+    checkMatchContiguousStorage(Array(u), u)
   }
 }