[DRAFT][stdlib] String.Index: Add custom printing

stdlib/public/core/StringIndex.swift

@@ -512,3 +512,81 @@ extension String.Index: Hashable {
     hasher.combine(orderingValue)
   }
 }
+
+@available(SwiftStdlib 5.7, *)
+extension String.Index: CustomStringConvertible {
+  internal var _encodingDescription: String {
+    switch (_rawBits & Self.__utf8Bit != 0, _rawBits & Self.__utf16Bit != 0) {
+    case (false, false): return "unknown"
+    case (true, false): return "utf8"
+    case (false, true): return "utf16"
+    case (true, true): return "any"
+    }
+  }
+
+  @available(SwiftStdlib 5.7, *)
+  @inline(never)
+  public var description: String {
+    // For the regular `description`, we just print the offset value along with
+    // its encoding and the transcoded offset, in a compact form: `23.utf8+1`.
+    //
+    // This is intended to expose crucial positioning information in a form that
+    // remains relatively readable even when part of a larger printout, e.g., as
+    // with range expressions like `12[utf8]..<56[utf8]+1`.
+    var d = "\(_encodedOffset)[\(_encodingDescription)]"
+    if transcodedOffset != 0 {
+      d += "+\(transcodedOffset)"
+    }
+    return d
+  }
+}
+
+@available(SwiftStdlib 5.7, *)
+extension String.Index: CustomDebugStringConvertible {
+  @available(SwiftStdlib 5.7, *)
+  @inline(never)
+  public var debugDescription: String {
+    var d = "String.Index("
+    d += "offset: \(_encodedOffset)"
+    d += ", encoding: \(_encodingDescription)"
+    if transcodedOffset != 0 {
+      d += ", transcodedOffset: \(transcodedOffset)"
+    }
+    if _isCharacterAligned {
+      d += ", aligned: character"
+    } else if _isScalarAligned {
+      d += ", aligned: scalar"
+    }
+    if let stride = characterStride {
+      d += ", stride: \(stride)"
+    }
+    d += ")"
+    return d
+  }
+}
+
+#if SWIFT_ENABLE_REFLECTION
+@available(SwiftStdlib 5.7, *)
+extension String.Index: CustomReflectable {
+  @available(SwiftStdlib 5.7, *)
+  @inline(never)
+  public var customMirror: Mirror {
+    var children: [(label: String?, value: Any)] = []
+    children.reserveCapacity(5)
+    children.append(("_offset", _encodedOffset))
+    children.append(("_encoding", _encodingDescription))
+    if transcodedOffset > 0 {
+      children.append(("_transcodedOffset", transcodedOffset))
+    }
+    if _isCharacterAligned {
+      children.append(("_aligned", "character"))
+    } else if _isScalarAligned {
+      children.append(("_aligned", "scalar"))
+    }
+    if let stride = characterStride {
+      children.append(("_stride", stride))
+    }
+    return Mirror(self, children: children, displayStyle: .struct)
+  }
+}
+#endif

test/api-digester/stability-stdlib-abi-without-asserts.test

@@ -56,7 +56,6 @@ Func Collection.subranges(where:) has been removed
 Func MutableCollection.moveSubranges(_:to:) has been removed
 Func MutableCollection.removeSubranges(_:) has been removed
 Func RangeReplaceableCollection.removeSubranges(_:) has been removed
-Struct AnyHashable has added a conformance to an existing protocol _HasCustomAnyHashableRepresentation
 Struct DiscontiguousSlice has been removed
 Struct RangeSet has been removed
 Subscript Collection.subscript(_:) has been removed
@@ -67,6 +66,12 @@ Protocol Error has added inherited protocol Sendable
 Protocol Error has generic signature change from to <Self : Swift.Sendable>
 Enum Never has added a conformance to an existing protocol Identifiable
 
+// The ABI checker doesn't understand protocol conformances with availability
+// yet.
+Struct AnyHashable has added a conformance to an existing protocol _HasCustomAnyHashableRepresentation
+Struct String.Index has added a conformance to an existing protocol CustomStringConvertible
+Struct String.Index has added a conformance to an existing protocol CustomDebugStringConvertible
+
 // These haven't actually been removed; they are simply marked unavailable.
 // This seems to be a false positive in the ABI checker. This is not an ABI
 // break because the symbols are still present, and is not a source break

test/stdlib/StringIndex.swift

@@ -28,6 +28,258 @@ let simpleStrings: [String] = [
   "",
 ]
 
+suite.test("CustomStringConvertible/native") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let string = "a👉🏼b"
+
+  func check<S: Collection>(
+    _ s: S,
+    _ expected: [String],
+    file: String = #file, line: UInt = #line
+  ) where S.Index == String.Index {
+    let indices = Array(s.indices) + [s.endIndex]
+    expectEqual(
+      indices.count,
+      expected.count,
+      "count",
+      file: file, line: line)
+    for i in 0 ..< indices.count {
+      expectEqual(
+        "\(indices[i])",
+        expected[i],
+        "i: \(i)",
+        file: file, line: line)
+    }
+  }
+
+  check(string, [
+      "0[any]",
+      "1[utf8]",
+      "9[utf8]",
+      "10[utf8]",
+    ])
+  check(string.unicodeScalars, [
+      "0[any]",
+      "1[utf8]",
+      "5[utf8]",
+      "9[utf8]",
+      "10[utf8]",
+    ])
+  check(string.utf8, [
+      "0[any]",
+      "1[utf8]",
+      "2[utf8]",
+      "3[utf8]",
+      "4[utf8]",
+      "5[utf8]",
+      "6[utf8]",
+      "7[utf8]",
+      "8[utf8]",
+      "9[utf8]",
+      "10[utf8]",
+    ])
+  check(string.utf16, [
+      "0[any]",
+      "1[utf8]",
+      "1[utf8]+1",
+      "5[utf8]",
+      "5[utf8]+1",
+      "9[utf8]",
+      "10[utf8]",
+    ])
+}
+
+#if _runtime(_ObjC)
+suite.test("CustomStringConvertible/bridged") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let string = "a👉🏼b" as NSString as String
+
+  func check<S: Collection>(
+    _ s: S,
+    _ expected: [String],
+    file: String = #file, line: UInt = #line
+  ) where S.Index == String.Index {
+    let indices = Array(s.indices) + [s.endIndex]
+    expectEqual(
+      indices.count,
+      expected.count,
+      "count",
+      file: file, line: line)
+    for i in 0 ..< indices.count {
+      expectEqual(
+        "\(indices[i])",
+        expected[i],
+        "i: \(i)",
+        file: file, line: line)
+    }
+  }
+
+  check(string, [
+      "0[any]",
+      "1[utf16]",
+      "5[utf16]",
+      "6[utf16]",
+    ])
+  check(string.unicodeScalars, [
+      "0[any]",
+      "1[utf16]",
+      "3[utf16]",
+      "5[utf16]",
+      "6[utf16]",
+    ])
+  check(string.utf8, [
+      "0[any]",
+      "1[utf16]",
+      "1[utf16]+1",
+      "1[utf16]+2",
+      "1[utf16]+3",
+      "3[utf16]",
+      "3[utf16]+1",
+      "3[utf16]+2",
+      "3[utf16]+3",
+      "5[utf16]",
+      "6[utf16]",
+    ])
+  check(string.utf16, [
+      "0[any]",
+      "1[utf16]",
+      "2[utf16]",
+      "3[utf16]",
+      "4[utf16]",
+      "5[utf16]",
+      "6[utf16]",
+    ])
+}
+#endif
+
+suite.test("CustomDebugStringConvertible/native") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let string = "a👉🏼b"
+
+  func check<S: Collection>(
+    _ s: S,
+    _ expected: [String],
+    file: String = #file, line: UInt = #line
+  ) where S.Index == String.Index {
+    let indices = Array(s.indices) + [s.endIndex]
+    expectEqual(
+      indices.count,
+      expected.count,
+      "count",
+      file: file, line: line)
+    for i in 0 ..< indices.count {
+      expectEqual(
+        String(reflecting: indices[i]),
+        expected[i],
+        "i: \(i)",
+        file: file, line: line)
+    }
+  }
+
+  check(string, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf8, aligned: character, stride: 8)",
+      "String.Index(offset: 9, encoding: utf8, aligned: character, stride: 1)",
+      "String.Index(offset: 10, encoding: utf8, aligned: character)",
+    ])
+  check(string.unicodeScalars, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf8, aligned: scalar)",
+      "String.Index(offset: 5, encoding: utf8, aligned: scalar)",
+      "String.Index(offset: 9, encoding: utf8, aligned: scalar)",
+      "String.Index(offset: 10, encoding: utf8, aligned: character)",
+    ])
+  check(string.utf8, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf8)",
+      "String.Index(offset: 2, encoding: utf8)",
+      "String.Index(offset: 3, encoding: utf8)",
+      "String.Index(offset: 4, encoding: utf8)",
+      "String.Index(offset: 5, encoding: utf8)",
+      "String.Index(offset: 6, encoding: utf8)",
+      "String.Index(offset: 7, encoding: utf8)",
+      "String.Index(offset: 8, encoding: utf8)",
+      "String.Index(offset: 9, encoding: utf8)",
+      "String.Index(offset: 10, encoding: utf8, aligned: character)",
+    ])
+  check(string.utf16, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf8, aligned: scalar)",
+      "String.Index(offset: 1, encoding: utf8, transcodedOffset: 1)",
+      "String.Index(offset: 5, encoding: utf8, aligned: scalar)",
+      "String.Index(offset: 5, encoding: utf8, transcodedOffset: 1)",
+      "String.Index(offset: 9, encoding: utf8, aligned: scalar)",
+      "String.Index(offset: 10, encoding: utf8, aligned: character)",
+    ])
+}
+
+#if _runtime(_ObjC)
+suite.test("CustomDebugStringConvertible/bridged") {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let string = "a👉🏼b" as NSString as String
+
+  func check<S: Collection>(
+    _ s: S,
+    _ expected: [String],
+    file: String = #file, line: UInt = #line
+  ) where S.Index == String.Index {
+    let indices = Array(s.indices) + [s.endIndex]
+    expectEqual(
+      indices.count,
+      expected.count,
+      "count",
+      file: file, line: line)
+    for i in 0 ..< indices.count {
+      expectEqual(
+        String(reflecting: indices[i]),
+        expected[i],
+        "i: \(i)",
+        file: file, line: line)
+    }
+  }
+
+  check(string, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf16, aligned: character, stride: 4)",
+      "String.Index(offset: 5, encoding: utf16, aligned: character, stride: 1)",
+      "String.Index(offset: 6, encoding: utf16, aligned: character)",
+    ])
+  check(string.unicodeScalars, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf16, aligned: scalar)",
+      "String.Index(offset: 3, encoding: utf16, aligned: scalar)",
+      "String.Index(offset: 5, encoding: utf16, aligned: scalar)",
+      "String.Index(offset: 6, encoding: utf16, aligned: character)",
+    ])
+  check(string.utf8, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf16, aligned: scalar)",
+      "String.Index(offset: 1, encoding: utf16, transcodedOffset: 1)",
+      "String.Index(offset: 1, encoding: utf16, transcodedOffset: 2)",
+      "String.Index(offset: 1, encoding: utf16, transcodedOffset: 3)",
+      "String.Index(offset: 3, encoding: utf16, aligned: scalar)",
+      "String.Index(offset: 3, encoding: utf16, transcodedOffset: 1)",
+      "String.Index(offset: 3, encoding: utf16, transcodedOffset: 2)",
+      "String.Index(offset: 3, encoding: utf16, transcodedOffset: 3)",
+      "String.Index(offset: 5, encoding: utf16, aligned: scalar)",
+      "String.Index(offset: 6, encoding: utf16, aligned: character)",
+    ])
+  check(string.utf16, [
+      "String.Index(offset: 0, encoding: any, aligned: character)",
+      "String.Index(offset: 1, encoding: utf16)",
+      "String.Index(offset: 2, encoding: utf16)",
+      "String.Index(offset: 3, encoding: utf16)",
+      "String.Index(offset: 4, encoding: utf16)",
+      "String.Index(offset: 5, encoding: utf16)",
+      "String.Index(offset: 6, encoding: utf16, aligned: character)",
+    ])
+}
+#endif
+
 suite.test("basic sanity checks")
 .forEach(in: simpleStrings) { s in
   let utf8 = Array(s.utf8)