From 25c107a642c49792b02863391302d5b732c6b556 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Fri, 26 Apr 2024 08:37:58 -0700
Subject: [PATCH 01/11] Make SytemString and SystemChar public

---
 Sources/System/FilePath/FilePath.swift        |   2 +-
 Sources/System/FilePath/FilePathWindows.swift |   2 +-
 Sources/System/SystemString.swift             | 229 +++++++++++-------
 Tests/SystemTests/SystemCharTest.swift        |   8 +-
 Tests/SystemTests/SystemStringTests.swift     |   1 -
 5 files changed, 142 insertions(+), 100 deletions(-)

diff --git a/Sources/System/FilePath/FilePath.swift b/Sources/System/FilePath/FilePath.swift
index 13064b5a..47498650 100644
--- a/Sources/System/FilePath/FilePath.swift
+++ b/Sources/System/FilePath/FilePath.swift
@@ -62,7 +62,7 @@ public struct FilePath: Sendable {
 @available(/*System 0.0.1: macOS 11.0, iOS 14.0, watchOS 7.0, tvOS 14.0*/iOS 8, *)
 extension FilePath {
   /// The length of the file path, excluding the null terminator.
-  public var length: Int { _storage.length }
+  public var length: Int { _storage.count }
 }
 
 @available(/*System 0.0.1: macOS 11.0, iOS 14.0, watchOS 7.0, tvOS 14.0*/iOS 8, *)
diff --git a/Sources/System/FilePath/FilePathWindows.swift b/Sources/System/FilePath/FilePathWindows.swift
index b725dd17..11b94d80 100644
--- a/Sources/System/FilePath/FilePathWindows.swift
+++ b/Sources/System/FilePath/FilePathWindows.swift
@@ -88,7 +88,7 @@ struct _Lexer {
   // Try to consume a drive letter and subsequent `:`.
   mutating func eatDrive() -> SystemChar? {
     let copy = slice
-    if let d = slice._eat(if: { $0.isLetter }), slice._eat(.colon) != nil {
+    if let d = slice._eat(if: { $0.isASCIILetter }), slice._eat(.colon) != nil {
       return d
     }
     // Restore slice
diff --git a/Sources/System/SystemString.swift b/Sources/System/SystemString.swift
index 8ca643b0..a0bf99fc 100644
--- a/Sources/System/SystemString.swift
+++ b/Sources/System/SystemString.swift
@@ -7,162 +7,207 @@
  See https://swift.org/LICENSE.txt for license information
 */
 
-// A platform-native character representation, currently used for file paths
-internal struct SystemChar:
+/// A platform-native character representation.
+///
+/// A SystemChar is a `CChar` on Linux and Darwin, and a `UInt16` on Windows.
+///
+/// Note that no particular encoding is assumed.
+@frozen
+public struct SystemChar:
   RawRepresentable, Sendable, Comparable, Hashable, Codable {
-  internal typealias RawValue = CInterop.PlatformChar
+  public typealias RawValue = CInterop.PlatformChar
 
-  internal var rawValue: RawValue
+  public var rawValue: RawValue
 
-  internal init(rawValue: RawValue) { self.rawValue = rawValue }
+  @inlinable
+  public init(rawValue: RawValue) { self.rawValue = rawValue }
 
-  internal init(_ rawValue: RawValue) { self.init(rawValue: rawValue) }
+  @_alwaysEmitIntoClient
+  public init(_ rawValue: RawValue) {
+    self.init(rawValue: rawValue)
+  }
 
-  static func < (lhs: SystemChar, rhs: SystemChar) -> Bool {
+  @inlinable
+  public static func < (lhs: SystemChar, rhs: SystemChar) -> Bool {
     lhs.rawValue < rhs.rawValue
   }
 }
 
 extension SystemChar {
-  internal init(ascii: Unicode.Scalar) {
-    self.init(rawValue: numericCast(UInt8(ascii: ascii)))
+  /// Create a SystemChar from an ASCII scalar.
+  @_alwaysEmitIntoClient
+  public init(ascii: Unicode.Scalar) {
+    precondition(ascii.isASCII)
+    self.init(rawValue: numericCast(ascii.value))
   }
-  internal init(codeUnit: CInterop.PlatformUnicodeEncoding.CodeUnit) {
-    self.init(rawValue: codeUnit._platformChar)
+
+  /// Cast `x` to a `SystemChar`
+  @_alwaysEmitIntoClient
+  public init(_ x: some FixedWidthInteger) {
+    self.init(numericCast(x))
   }
 
-  internal static var null: SystemChar { SystemChar(0x0) }
-  internal static var slash: SystemChar { SystemChar(ascii: "/") }
-  internal static var backslash: SystemChar { SystemChar(ascii: #"\"#) }
-  internal static var dot: SystemChar { SystemChar(ascii: ".") }
-  internal static var colon: SystemChar { SystemChar(ascii: ":") }
-  internal static var question: SystemChar { SystemChar(ascii: "?") }
+  /// The NULL character `\0`
+  @_alwaysEmitIntoClient
+  public static var null: SystemChar { SystemChar(0x0) }
 
-  internal var codeUnit: CInterop.PlatformUnicodeEncoding.CodeUnit {
-    rawValue._platformCodeUnit
-  }
+  /// The slash character `/`
+  @_alwaysEmitIntoClient
+  public static var slash: SystemChar { SystemChar(ascii: "/") }
+
+  /// The backslash character `\`
+  @_alwaysEmitIntoClient
+  public static var backslash: SystemChar { SystemChar(ascii: #"\"#) }
+
+  /// The dot character `.`
+  @_alwaysEmitIntoClient
+  public static var dot: SystemChar { SystemChar(ascii: ".") }
+
+  /// The colon character `:`
+  @_alwaysEmitIntoClient
+  public static var colon: SystemChar { SystemChar(ascii: ":") }
 
-  internal var asciiScalar: Unicode.Scalar? {
+  /// The question mark character `?`
+  @_alwaysEmitIntoClient
+  public static var question: SystemChar { SystemChar(ascii: "?") }
+
+  /// Returns `self` as a `Unicode.Scalar` if ASCII, else `nil`
+  @_alwaysEmitIntoClient
+  public var asciiScalar: Unicode.Scalar? {
     guard isASCII else { return nil }
     return Unicode.Scalar(UInt8(truncatingIfNeeded: rawValue))
   }
 
-  internal var isASCII: Bool {
+  /// Whether `self` is ASCII
+  @_alwaysEmitIntoClient
+  public var isASCII: Bool {
     (0...0x7F).contains(rawValue)
   }
 
-  internal var isLetter: Bool {
+  /// Whether `self` is an ASCII letter, i.e. in `[a-zA-Z]`
+  @_alwaysEmitIntoClient
+  public var isASCIILetter: Bool {
     guard isASCII else { return false }
     let asciiRaw: UInt8 = numericCast(rawValue)
-    return (UInt8(ascii: "a") ... UInt8(ascii: "z")).contains(asciiRaw) ||
-           (UInt8(ascii: "A") ... UInt8(ascii: "Z")).contains(asciiRaw)
+    switch asciiRaw {
+    case UInt8(ascii: "a")...UInt8(ascii: "z"): return true
+    case UInt8(ascii: "A")...UInt8(ascii: "Z"): return true
+    default: return false
+    }
   }
 }
 
-// A platform-native string representation, currently for file paths
-//
-// Always null-terminated.
-internal struct SystemString: Sendable {
-  internal typealias Storage = [SystemChar]
-  internal var nullTerminatedStorage: Storage
-}
 
-extension SystemString {
-  internal init() {
-    self.nullTerminatedStorage = [.null]
+/// A platform-native string representation. A `SystemString` is a collection
+/// of non-NULL `SystemChar`s followed by a NULL terminator.
+///
+/// TODO: example use or two, showing that innards are not NULL, but there's
+/// always a null at the end, NULL is not part of the count
+@frozen
+public struct SystemString: Sendable {
+  public typealias Storage = [SystemChar]
+
+  @usableFromInline
+  internal var _nullTerminatedStorage: Storage
+
+  /// Access the back storage, including the null terminator. Note that
+  /// `nullTerminatedStorage.count == self.count + 1`, due
+  /// to the null terminator.
+  @_alwaysEmitIntoClient
+  public var nullTerminatedStorage: Storage { _nullTerminatedStorage }
+
+  /// Create a SystemString from pre-existing null-terminated storage
+  @usableFromInline
+  internal init(_nullTerminatedStorage storage: [SystemChar]) {
+    self._nullTerminatedStorage = storage
     _invariantCheck()
   }
+}
 
-  internal var length: Int {
-    let len = nullTerminatedStorage.count - 1
-    assert(len == self.count)
-    return len
-  }
-
-  // Common funnel point. Ensure all non-empty inits go here.
-  internal init(nullTerminated storage: Storage) {
-    self.nullTerminatedStorage = storage
-    _invariantCheck()
+extension SystemString {
+  /// Create an empty `SystemString`
+  public init() {
+    self.init(_nullTerminatedStorage: [.null])
   }
 
-  // Ensures that result is null-terminated
-  internal init<C: Collection>(_ chars: C) where C.Element == SystemChar {
-    var rawChars = Storage(chars)
+  /// Create a `SystemString` from a collection of `SystemChar`s.
+  /// A NULL terminator will be added if `chars` lacks one. `chars` must not
+  /// include any interior NULLs.
+  @_alwaysEmitIntoClient
+  public init<C: Collection>(_ chars: C) where C.Element == SystemChar {
+    var rawChars = Array(chars)
     if rawChars.last != .null {
       rawChars.append(.null)
     }
-    self.init(nullTerminated: rawChars)
+    precondition(
+      rawChars.dropLast(1).allSatisfy { $0 != .null },
+      "Embedded NULL detected")
+    self.init(_nullTerminatedStorage: rawChars)
   }
 }
 
 extension SystemString {
-  fileprivate func _invariantCheck() {
+  @_alwaysEmitIntoClient
+  internal func _invariantCheck() {
     #if DEBUG
-    precondition(nullTerminatedStorage.last! == .null)
-    precondition(nullTerminatedStorage.firstIndex(of: .null) == length)
+    precondition(_nullTerminatedStorage.last! == .null)
+    precondition(_nullTerminatedStorage.firstIndex(of: .null) == endIndex)
     #endif // DEBUG
   }
 }
 
 extension SystemString: RandomAccessCollection, MutableCollection {
-  internal typealias Element = SystemChar
-  internal typealias Index = Storage.Index
-  internal typealias Indices = Range<Index>
+  public typealias Element = SystemChar
+  public typealias Index = Int
+  public typealias Indices = Range<Index>
 
-  internal var startIndex: Index {
-    nullTerminatedStorage.startIndex
+  @inlinable
+  public var startIndex: Index {
+    _nullTerminatedStorage.startIndex
   }
 
-  internal var endIndex: Index {
-    nullTerminatedStorage.index(before: nullTerminatedStorage.endIndex)
+  @inlinable
+  public var endIndex: Index {
+    _nullTerminatedStorage.index(before: _nullTerminatedStorage.endIndex)
   }
 
-  internal subscript(position: Index) -> SystemChar {
+  @inlinable
+  public subscript(position: Index) -> SystemChar {
     _read {
       precondition(position >= startIndex && position <= endIndex)
-      yield nullTerminatedStorage[position]
+      yield _nullTerminatedStorage[position]
     }
     set(newValue) {
       precondition(position >= startIndex && position <= endIndex)
-      nullTerminatedStorage[position] = newValue
+      _nullTerminatedStorage[position] = newValue
       _invariantCheck()
     }
   }
 }
 extension SystemString: RangeReplaceableCollection {
-  internal mutating func replaceSubrange<C: Collection>(
+  @inlinable
+  public mutating func replaceSubrange<C: Collection>(
     _ subrange: Range<Index>, with newElements: C
   ) where C.Element == SystemChar {
     defer { _invariantCheck() }
-    nullTerminatedStorage.replaceSubrange(subrange, with: newElements)
+    _nullTerminatedStorage.replaceSubrange(subrange, with: newElements)
   }
 
-  internal mutating func reserveCapacity(_ n: Int) {
+  @inlinable
+  public mutating func reserveCapacity(_ n: Int) {
     defer { _invariantCheck() }
-    nullTerminatedStorage.reserveCapacity(1 + n)
+    _nullTerminatedStorage.reserveCapacity(1 + n)
   }
 
-  internal func withContiguousStorageIfAvailable<R>(
+  @inlinable
+  public func withContiguousStorageIfAvailable<R>(
     _ body: (UnsafeBufferPointer<SystemChar>) throws -> R
   ) rethrows -> R? {
     // Do not include the null terminator, it is outside the Collection
-    try nullTerminatedStorage.withContiguousStorageIfAvailable {
+    try _nullTerminatedStorage.withContiguousStorageIfAvailable {
       try body(.init(start: $0.baseAddress, count: $0.count-1))
     }
   }
-
-  internal mutating func withContiguousMutableStorageIfAvailable<R>(
-    _ body: (inout UnsafeMutableBufferPointer<SystemChar>) throws -> R
-  ) rethrows -> R? {
-    defer { _invariantCheck() }
-    // Do not include the null terminator, it is outside the Collection
-    return try nullTerminatedStorage.withContiguousMutableStorageIfAvailable {
-      var buffer = UnsafeMutableBufferPointer<SystemChar>(
-        start: $0.baseAddress, count: $0.count-1
-      )
-      return try body(&buffer)
-    }
-  }
 }
 
 extension SystemString: Hashable, Codable {}
@@ -173,7 +218,7 @@ extension SystemString {
   internal func withSystemChars<T>(
     _ f: (UnsafeBufferPointer<SystemChar>) throws -> T
   ) rethrows -> T {
-    try nullTerminatedStorage.withContiguousStorageIfAvailable(f)!
+    try _nullTerminatedStorage.withContiguousStorageIfAvailable(f)!
   }
 
   internal func withCodeUnits<T>(
@@ -231,11 +276,11 @@ extension String {
 }
 
 extension SystemString: ExpressibleByStringLiteral {
-  internal init(stringLiteral: String) {
+  public init(stringLiteral: String) {
     self.init(stringLiteral)
   }
 
-  internal init(_ string: String) {
+  public init(_ string: String) {
     // TODO: can avoid extra strlen
     self = string.withPlatformString {
       SystemString(platformString: $0)
@@ -246,26 +291,24 @@ extension SystemString: ExpressibleByStringLiteral {
 extension SystemString: CustomStringConvertible, CustomDebugStringConvertible {
   internal var string: String { String(decoding: self) }
 
-  internal var description: String { string }
-  internal var debugDescription: String { description.debugDescription }
+  public var description: String { string }
+  public var debugDescription: String { description.debugDescription }
 }
 
 extension SystemString {
-  /// Creates a system string by copying bytes from a null-terminated platform string.
+  /// Creates a `SystemString` by copying bytes from a null-terminated platform string.
   ///
   /// - Parameter platformString: A pointer to a null-terminated platform string.
-  internal init(platformString: UnsafePointer<CInterop.PlatformChar>) {
+  public init(platformString: UnsafePointer<CInterop.PlatformChar>) {
     let count = 1 + system_platform_strlen(platformString)
 
     // TODO: Is this the right way?
     let chars: Array<SystemChar> = platformString.withMemoryRebound(
       to: SystemChar.self, capacity: count
     ) {
-      let bufPtr = UnsafeBufferPointer(start: $0, count: count)
-      return Array(bufPtr)
+      Array(UnsafeBufferPointer(start: $0, count: count))
     }
-
-    self.init(nullTerminated: chars)
+    self.init(_nullTerminatedStorage: chars)
   }
 
   /// Calls the given closure with a pointer to the contents of the sytem string,
@@ -280,7 +323,7 @@ extension SystemString {
   /// The pointer passed as an argument to `body` is valid
   /// only during the execution of this method.
   /// Don't try to store the pointer for later use.
-  internal func withPlatformString<T>(
+  public func withPlatformString<T>(
     _ f: (UnsafePointer<CInterop.PlatformChar>) throws -> T
   ) rethrows -> T {
     try withSystemChars { chars in
diff --git a/Tests/SystemTests/SystemCharTest.swift b/Tests/SystemTests/SystemCharTest.swift
index 0ec5411e..e910e453 100644
--- a/Tests/SystemTests/SystemCharTest.swift
+++ b/Tests/SystemTests/SystemCharTest.swift
@@ -20,20 +20,20 @@ final class SystemCharTest: XCTestCase {
     let valid = SystemString(
       "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
     for char in valid {
-      XCTAssertTrue(char.isLetter)
+      XCTAssertTrue(char.isASCIILetter)
     }
 
     // non printable
     for value in 0..<(UInt8(ascii: " ")) {
-      XCTAssertFalse(SystemChar(codeUnit: CInterop.PlatformUnicodeEncoding.CodeUnit(value)).isLetter)
+      XCTAssertFalse(SystemChar(CInterop.PlatformUnicodeEncoding.CodeUnit(value)).isASCIILetter)
     }
-    XCTAssertFalse(SystemChar(codeUnit: 0x7F).isLetter) // DEL
+    XCTAssertFalse(SystemChar(0x7F).isASCIILetter) // DEL
 
     // misc other
     let invalid = SystemString(
       ##" !"#$%&'()*+,-./0123456789:;<=>?@[\]^_`{|}~"##)
     for char in invalid {
-      XCTAssertFalse(char.isLetter)
+      XCTAssertFalse(char.isASCIILetter)
     }
   }
 }
diff --git a/Tests/SystemTests/SystemStringTests.swift b/Tests/SystemTests/SystemStringTests.swift
index a1891714..9e24aaba 100644
--- a/Tests/SystemTests/SystemStringTests.swift
+++ b/Tests/SystemTests/SystemStringTests.swift
@@ -248,7 +248,6 @@ final class SystemStringTest: XCTestCase {
     str.append(SystemChar(ascii: "d"))
     XCTAssert(str == "abcd")
     XCTAssert(str.count == 4)
-    XCTAssert(str.count == str.length)
 
     str.reserveCapacity(100)
     XCTAssert(str == "abcd")

From cd5fe4effc7109cd55cae84342c711a280c28846 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Fri, 26 Apr 2024 08:55:15 -0700
Subject: [PATCH 02/11] cleanup

---
 .../System/FilePath/FilePathComponents.swift  |  7 +---
 Sources/System/SystemString.swift             | 38 +++++++++++++------
 2 files changed, 27 insertions(+), 18 deletions(-)

diff --git a/Sources/System/FilePath/FilePathComponents.swift b/Sources/System/FilePath/FilePathComponents.swift
index 6b8a9408..d50af35b 100644
--- a/Sources/System/FilePath/FilePathComponents.swift
+++ b/Sources/System/FilePath/FilePathComponents.swift
@@ -150,11 +150,6 @@ extension _StrSlice {
       try f(UnsafeBufferPointer(rebasing: $0[_range]))
     }
   }
-  internal func _withCodeUnits<T>(
-    _ f: (UnsafeBufferPointer<CInterop.PlatformUnicodeEncoding.CodeUnit>) throws -> T
-  ) rethrows -> T {
-    try _slice.withCodeUnits(f)
-  }
 
   internal init?(_platformString s: UnsafePointer<CInterop.PlatformChar>) {
     self.init(SystemString(platformString: s))
@@ -163,7 +158,7 @@ extension _StrSlice {
   internal func _withPlatformString<Result>(
     _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
   ) rethrows -> Result {
-    try _slice.withPlatformString(body)
+    try _slice._withPlatformString(body)
   }
 
   internal var _systemString: SystemString { SystemString(_slice) }
diff --git a/Sources/System/SystemString.swift b/Sources/System/SystemString.swift
index a0bf99fc..17669a9b 100644
--- a/Sources/System/SystemString.swift
+++ b/Sources/System/SystemString.swift
@@ -238,19 +238,14 @@ extension SystemString {
 }
 
 extension Slice where Base == SystemString {
-  internal func withCodeUnits<T>(
-    _ f: (UnsafeBufferPointer<CInterop.PlatformUnicodeEncoding.CodeUnit>) throws -> T
-  ) rethrows -> T {
-    try base.withCodeUnits {
-      try f(UnsafeBufferPointer(rebasing: $0[indices]))
-    }
-  }
-
   internal var string: String {
-    withCodeUnits { String(decoding: $0, as: CInterop.PlatformUnicodeEncoding.self) }
+    base.withCodeUnits {
+      String(decoding: $0[indices],
+             as: CInterop.PlatformUnicodeEncoding.self)
+    }
   }
 
-  internal func withPlatformString<T>(
+  internal func _withPlatformString<T>(
     _ f: (UnsafePointer<CInterop.PlatformChar>) throws -> T
   ) rethrows -> T {
     // FIXME: avoid allocation if we're at the end
@@ -260,13 +255,32 @@ extension Slice where Base == SystemString {
 }
 
 extension String {
-  internal init(decoding str: SystemString) {
+  /// Creates a string by interpreting `str`'s content as UTF-8 on Unix
+  /// and UTF-16 on Windows.
+  ///
+  /// - Parameter str: The system string to be interpreted as
+  /// `CInterop.PlatformUnicodeEncoding`.
+  ///
+  /// If the content of the system string isn't a well-formed Unicode string,
+  /// this initializer replaces invalid bytes with U+FFFD.
+  /// This means that conversion to a string and back to a system string
+  /// might result in a value that's different from the original system string.
+  public init(decoding str: SystemString) {
     // TODO: Can avoid extra strlen
     self = str.withPlatformString {
       String(platformString: $0)
     }
   }
-  internal init?(validating str: SystemString) {
+
+  /// Creates a string from a system string, validating its contents as UTF-8 on
+  /// Unix and UTF-16 on Windows.
+  ///
+  /// - Parameter str: The system string to be interpreted as
+  ///   `CInterop.PlatformUnicodeEncoding`.
+  ///
+  /// If the contents of the system string isn't well-formed Unicode,
+  /// this initializer returns `nil`.
+  public init?(validating str: SystemString) {
     // TODO: Can avoid extra strlen
     guard let str = str.withPlatformString(String.init(validatingPlatformString:))
     else { return nil }

From 780f8c3c4388f062e01ec5825dda5fef01fab593 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Fri, 26 Apr 2024 09:00:12 -0700
Subject: [PATCH 03/11] cleanup

---
 Sources/System/FilePath/FilePathSyntax.swift  |  4 ++--
 Sources/System/FilePath/FilePathWindows.swift |  4 ++--
 Sources/System/SystemString.swift             | 15 +++++----------
 Tests/SystemTests/SystemStringTests.swift     |  2 +-
 4 files changed, 10 insertions(+), 15 deletions(-)

diff --git a/Sources/System/FilePath/FilePathSyntax.swift b/Sources/System/FilePath/FilePathSyntax.swift
index 1c3fc097..f94f4453 100644
--- a/Sources/System/FilePath/FilePathSyntax.swift
+++ b/Sources/System/FilePath/FilePathSyntax.swift
@@ -267,7 +267,7 @@ extension FilePath.Component {
   ///   * `..         => nil`
   public var `extension`: String? {
     guard let range = _extensionRange() else { return nil }
-    return _slice[range].string
+    return String(decoding: .init(_slice[range]))
   }
 
   /// The non-extension portion of this file or directory  component.
@@ -279,7 +279,7 @@ extension FilePath.Component {
   ///   * `.hidden => .hidden`
   ///   * `..      => ..`
   public var stem: String {
-    _slice[_stemRange()].string
+    String(decoding: .init(_slice[_stemRange()]))
   }
 }
 
diff --git a/Sources/System/FilePath/FilePathWindows.swift b/Sources/System/FilePath/FilePathWindows.swift
index 11b94d80..0967d4b0 100644
--- a/Sources/System/FilePath/FilePathWindows.swift
+++ b/Sources/System/FilePath/FilePathWindows.swift
@@ -218,7 +218,7 @@ extension _ParsedWindowsRoot {
 
     // TODO: check for GUID
     // TODO: check for drive
-    return .volume(root[vol].string)
+    return .volume(String(decoding: .init(root[vol])))
   }
 }
 
@@ -227,7 +227,7 @@ extension WindowsRootInfo {
     self.volume = parsed.volumeInfo(root)
 
     if let host = parsed.host {
-      self.host = root[host].string
+      self.host = String(decoding: .init(root[host]))
     } else {
       self.host = nil
     }
diff --git a/Sources/System/SystemString.swift b/Sources/System/SystemString.swift
index 17669a9b..3e6a36f2 100644
--- a/Sources/System/SystemString.swift
+++ b/Sources/System/SystemString.swift
@@ -238,13 +238,6 @@ extension SystemString {
 }
 
 extension Slice where Base == SystemString {
-  internal var string: String {
-    base.withCodeUnits {
-      String(decoding: $0[indices],
-             as: CInterop.PlatformUnicodeEncoding.self)
-    }
-  }
-
   internal func _withPlatformString<T>(
     _ f: (UnsafePointer<CInterop.PlatformChar>) throws -> T
   ) rethrows -> T {
@@ -303,10 +296,12 @@ extension SystemString: ExpressibleByStringLiteral {
 }
 
 extension SystemString: CustomStringConvertible, CustomDebugStringConvertible {
-  internal var string: String { String(decoding: self) }
 
-  public var description: String { string }
-  public var debugDescription: String { description.debugDescription }
+  public var description: String { String(decoding: self) }
+
+  public var debugDescription: String {
+    description.debugDescription
+  }
 }
 
 extension SystemString {
diff --git a/Tests/SystemTests/SystemStringTests.swift b/Tests/SystemTests/SystemStringTests.swift
index 9e24aaba..612b9836 100644
--- a/Tests/SystemTests/SystemStringTests.swift
+++ b/Tests/SystemTests/SystemStringTests.swift
@@ -75,7 +75,7 @@ struct StringTest: TestCase {
 
     // Test String, SystemString, FilePath construction
     let sysStr = SystemString(string)
-    expectEqualSequence(string.unicodeScalars, sysStr.string.unicodeScalars)
+    expectEqualSequence(string.unicodeScalars, String(decoding: sysStr).unicodeScalars)
     expectEqual(string, String(decoding: sysStr))
     expectEqual(string, String(validating: sysStr))
 

From 9513e51350760974bca557fcb6d97fe639fec2ba Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Fri, 26 Apr 2024 09:39:46 -0700
Subject: [PATCH 04/11] wip

---
 Sources/System/FilePath/FilePathComponentView.swift | 10 +++++++---
 Sources/System/FilePath/FilePathComponents.swift    |  8 ++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/Sources/System/FilePath/FilePathComponentView.swift b/Sources/System/FilePath/FilePathComponentView.swift
index 39381b4b..644dbbc3 100644
--- a/Sources/System/FilePath/FilePathComponentView.swift
+++ b/Sources/System/FilePath/FilePathComponentView.swift
@@ -33,6 +33,10 @@ extension FilePath {
     internal var _path: FilePath
     internal var _start: SystemString.Index
 
+    internal var _slice: Slice<SystemString> {
+      _path._storage[_start...]
+    }
+
     internal init(_ path: FilePath) {
       self._path = path
       self._start = path._relativeStart
@@ -50,7 +54,7 @@ extension FilePath {
       // TODO(perf): Small-form root (especially on Unix). Have Root
       // always copy out (not worth ref counting). Make sure that we're
       // not needlessly sliding values around or triggering a COW
-      let rootStr = self.root?._systemString ?? SystemString()
+      let rootStr = SystemString(root?._slice ?? SystemString()[...])
       var comp = ComponentView(self)
       self = FilePath()
       defer {
@@ -156,7 +160,7 @@ extension FilePath {
   public init<C: Collection>(
     root: Root?, _ components: C
   ) where C.Element == Component {
-    var str = root?._systemString ?? SystemString()
+    var str = SystemString(root?._slice ?? SystemString()[...])
     str.appendComponents(components: components)
     self.init(str)
   }
@@ -169,7 +173,7 @@ extension FilePath {
   /// Create a file path from an optional root and a slice of another path's
   /// components.
   public init(root: Root?, _ components: ComponentView.SubSequence) {
-    var str = root?._systemString ?? SystemString()
+    var str = SystemString(root?._slice ?? SystemString()[...])
     let (start, end) =
       (components.startIndex._storage, components.endIndex._storage)
     str.append(contentsOf: components.base._slice[start..<end])
diff --git a/Sources/System/FilePath/FilePathComponents.swift b/Sources/System/FilePath/FilePathComponents.swift
index d50af35b..108efd25 100644
--- a/Sources/System/FilePath/FilePathComponents.swift
+++ b/Sources/System/FilePath/FilePathComponents.swift
@@ -33,6 +33,10 @@ extension FilePath {
     internal var _path: FilePath
     internal var _rootEnd: SystemString.Index
 
+    internal var _slice: Slice<SystemString> {
+      _path._storage[..<_rootEnd]
+    }
+
     internal init(_ path: FilePath, rootEnd: SystemString.Index) {
       self._path = path
       self._rootEnd = rootEnd
@@ -60,6 +64,10 @@ extension FilePath {
     internal var _path: FilePath
     internal var _range: Range<SystemString.Index>
 
+    internal var _slice: Slice<SystemString> {
+      _path._storage[_range]
+    }
+
     // TODO: Make a small-component form to save on ARC overhead when
     // extracted from a path, and especially to save on allocation overhead
     // when constructing one from a String literal.

From 47966f4866a44dbb289d63f5e0e65f03d065f0d3 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Fri, 26 Apr 2024 09:41:01 -0700
Subject: [PATCH 05/11] wip

---
 .../System/FilePath/FilePathComponents.swift  |  2 +-
 Sources/System/FilePath/FilePathString.swift  | 24 +++++++++----------
 Sources/System/SystemString.swift             |  1 -
 3 files changed, 13 insertions(+), 14 deletions(-)

diff --git a/Sources/System/FilePath/FilePathComponents.swift b/Sources/System/FilePath/FilePathComponents.swift
index 108efd25..6c44515d 100644
--- a/Sources/System/FilePath/FilePathComponents.swift
+++ b/Sources/System/FilePath/FilePathComponents.swift
@@ -130,7 +130,7 @@ extension SystemString {
 
     for idx in components.indices {
       let component = components[idx]
-      component._withSystemChars { self.append(contentsOf: $0) }
+      self.append(contentsOf: component._slice)
       self.append(platformSeparator)
     }
   }
diff --git a/Sources/System/FilePath/FilePathString.swift b/Sources/System/FilePath/FilePathString.swift
index 4fe9c1fd..0e0a46f6 100644
--- a/Sources/System/FilePath/FilePathString.swift
+++ b/Sources/System/FilePath/FilePathString.swift
@@ -17,7 +17,7 @@ extension FilePath {
   /// - Parameter platformString: A pointer to a null-terminated platform
   ///   string.
   public init(platformString: UnsafePointer<CInterop.PlatformChar>) {
-    self.init(_platformString: platformString)
+    self.init(SystemString(platformString: platformString))
   }
 
   /// Creates a file path by copying bytes from a null-terminated platform
@@ -104,7 +104,7 @@ extension FilePath {
   public func withPlatformString<Result>(
     _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
   ) rethrows -> Result {
-    return try _withPlatformString(body)
+    return try _storage._withPlatformString(body)
   }
 #endif
 }
@@ -120,7 +120,7 @@ extension FilePath.Component {
   /// - Parameter platformString: A pointer to a null-terminated platform
   ///   string.
   public init?(platformString: UnsafePointer<CInterop.PlatformChar>) {
-    self.init(_platformString: platformString)
+    self.init(SystemString(platformString: platformString))
   }
 
   /// Creates a file path component by copying bytes from a null-terminated
@@ -194,7 +194,7 @@ extension FilePath.Component {
   public func withPlatformString<Result>(
     _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
   ) rethrows -> Result {
-    try _withPlatformString(body)
+    try _slice._withPlatformString(body)
   }
 }
 
@@ -208,7 +208,7 @@ extension FilePath.Root {
   /// - Parameter platformString: A pointer to a null-terminated platform
   ///   string.
   public init?(platformString: UnsafePointer<CInterop.PlatformChar>) {
-    self.init(_platformString: platformString)
+    self.init(SystemString(platformString: platformString))
   }
 
   /// Creates a file path root by copying bytes from a null-terminated platform
@@ -281,7 +281,7 @@ extension FilePath.Root {
   public func withPlatformString<Result>(
     _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
   ) rethrows -> Result {
-    try _withPlatformString(body)
+    try _slice._withPlatformString(body)
   }
 }
 
@@ -459,7 +459,7 @@ extension String {
   /// conversion to a string and back to a path
   /// might result in a value that's different from the original path.
   public init(decoding path: FilePath) {
-    self.init(_decoding: path)
+    self.init(decoding: path._storage)
   }
 
   /// Creates a string from a file path, validating its contents as UTF-8 on
@@ -471,7 +471,7 @@ extension String {
   /// If the contents of the file path isn't a well-formed Unicode string,
   /// this initializer returns `nil`.
   public init?(validating path: FilePath) {
-    self.init(_validating: path)
+    self.init(validating: path._storage)
   }
 }
 
@@ -489,7 +489,7 @@ extension String {
   /// conversion to a string and back to a path component
   /// might result in a value that's different from the original path component.
   public init(decoding component: FilePath.Component) {
-    self.init(_decoding: component)
+    self.init(decoding: SystemString(component._slice))
   }
 
   /// Creates a string from a path component, validating its contents as UTF-8
@@ -501,7 +501,7 @@ extension String {
   /// If the contents of the path component isn't a well-formed Unicode string,
   /// this initializer returns `nil`.
   public init?(validating component: FilePath.Component) {
-    self.init(_validating: component)
+    self.init(validating: SystemString(component._slice))
   }
 }
 
@@ -521,7 +521,7 @@ extension String {
   /// conversion to a string and back to a path root
   /// might result in a value that's different from the original path root.
   public init(decoding root: FilePath.Root) {
-    self.init(_decoding: root)
+    self.init(decoding: SystemString(root._slice))
   }
 
   /// On Unix, creates the string `"/"`
@@ -597,7 +597,7 @@ extension FilePath {
   public func withCString<Result>(
     _ body: (UnsafePointer<CChar>) throws -> Result
   ) rethrows -> Result {
-    return try _withPlatformString(body)
+    return try _storage.withPlatformString(body)
   }
 }
 #endif
diff --git a/Sources/System/SystemString.swift b/Sources/System/SystemString.swift
index 3e6a36f2..52adf8eb 100644
--- a/Sources/System/SystemString.swift
+++ b/Sources/System/SystemString.swift
@@ -244,7 +244,6 @@ extension Slice where Base == SystemString {
     // FIXME: avoid allocation if we're at the end
     return try SystemString(self).withPlatformString(f)
   }
-
 }
 
 extension String {

From 4fe4d476e96edd7aa7a2e8f1c0cf38ce9a9adb42 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Fri, 26 Apr 2024 20:14:38 -0700
Subject: [PATCH 06/11] cleanup

---
 .../System/FilePath/FilePathComponents.swift  | 12 ------------
 Sources/System/FilePath/FilePathString.swift  | 19 +------------------
 Sources/System/Internals/Exports.swift        |  7 +------
 Sources/System/PlatformString.swift           |  7 ++++++-
 4 files changed, 8 insertions(+), 37 deletions(-)

diff --git a/Sources/System/FilePath/FilePathComponents.swift b/Sources/System/FilePath/FilePathComponents.swift
index 6c44515d..b98a4dd2 100644
--- a/Sources/System/FilePath/FilePathComponents.swift
+++ b/Sources/System/FilePath/FilePathComponents.swift
@@ -199,18 +199,6 @@ extension FilePath.Root: _PathSlice {
   }
 }
 
-@available(/*System 0.0.1: macOS 11.0, iOS 14.0, watchOS 7.0, tvOS 14.0*/iOS 8, *)
-extension FilePath: _PlatformStringable {
-  func _withPlatformString<Result>(_ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result) rethrows -> Result {
-    try _storage.withPlatformString(body)
-  }
-
-  init(_platformString: UnsafePointer<CInterop.PlatformChar>) {
-    self.init(SystemString(platformString: _platformString))
-  }
-
-}
-
 @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
 extension FilePath.Component {
   // The index of the `.` denoting an extension
diff --git a/Sources/System/FilePath/FilePathString.swift b/Sources/System/FilePath/FilePathString.swift
index 0e0a46f6..404e438f 100644
--- a/Sources/System/FilePath/FilePathString.swift
+++ b/Sources/System/FilePath/FilePathString.swift
@@ -535,24 +535,7 @@ extension String {
   /// On Windows, if the contents of the path root isn't a well-formed Unicode
   /// string, this initializer returns `nil`.
   public init?(validating root: FilePath.Root) {
-    self.init(_validating: root)
-  }
-}
-
-// MARK: - Internal helpers
-
-extension String {
-  fileprivate init<PS: _PlatformStringable>(_decoding ps: PS) {
-    self = ps._withPlatformString { String(platformString: $0) }
-  }
-
-  fileprivate init?<PS: _PlatformStringable>(_validating ps: PS) {
-    guard let str = ps._withPlatformString(
-      String.init(validatingPlatformString:)
-    ) else {
-      return nil
-    }
-    self = str
+    self.init(validating: SystemString(root._slice))
   }
 }
 
diff --git a/Sources/System/Internals/Exports.swift b/Sources/System/Internals/Exports.swift
index e20454ee..d1283197 100644
--- a/Sources/System/Internals/Exports.swift
+++ b/Sources/System/Internals/Exports.swift
@@ -104,12 +104,7 @@ extension String {
   internal func _withPlatformString<Result>(
     _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
   ) rethrows -> Result {
-    // Need to #if because CChar may be signed
-    #if os(Windows)
-    return try withCString(encodedAs: CInterop.PlatformUnicodeEncoding.self, body)
-    #else
-    return try withCString(body)
-    #endif
+    fatalError()
   }
 
   internal init?(_platformString platformString: UnsafePointer<CInterop.PlatformChar>) {
diff --git a/Sources/System/PlatformString.swift b/Sources/System/PlatformString.swift
index 4e2e7ddf..e2a9e05b 100644
--- a/Sources/System/PlatformString.swift
+++ b/Sources/System/PlatformString.swift
@@ -159,7 +159,12 @@ extension String {
   public func withPlatformString<Result>(
     _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
   ) rethrows -> Result {
-    try _withPlatformString(body)
+    // Need to #if because CChar may be signed
+    #if os(Windows)
+    return try withCString(encodedAs: CInterop.PlatformUnicodeEncoding.self, body)
+    #else
+    return try withCString(body)
+    #endif
   }
 
 }

From 8bafa5d619d25b7184be65ca5dda89dce4ecb70c Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Fri, 26 Apr 2024 20:20:32 -0700
Subject: [PATCH 07/11] cleanup

---
 .../FilePath/FilePathComponentView.swift      | 13 ----
 .../System/FilePath/FilePathComponents.swift  | 59 +++----------------
 Sources/System/PlatformString.swift           |  9 ---
 3 files changed, 9 insertions(+), 72 deletions(-)

diff --git a/Sources/System/FilePath/FilePathComponentView.swift b/Sources/System/FilePath/FilePathComponentView.swift
index 644dbbc3..b8ed69fa 100644
--- a/Sources/System/FilePath/FilePathComponentView.swift
+++ b/Sources/System/FilePath/FilePathComponentView.swift
@@ -181,19 +181,6 @@ extension FilePath {
   }
 }
 
-// MARK: - Internals
-
-@available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
-extension FilePath.ComponentView: _PathSlice {
-  internal var _range: Range<SystemString.Index> {
-    _start ..< _path._storage.endIndex
-  }
-
-  internal init(_ str: SystemString) {
-    fatalError("TODO: consider dropping proto req")
-  }
-}
-
 // MARK: - Invariants
 
 @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
diff --git a/Sources/System/FilePath/FilePathComponents.swift b/Sources/System/FilePath/FilePathComponents.swift
index b98a4dd2..46b98358 100644
--- a/Sources/System/FilePath/FilePathComponents.swift
+++ b/Sources/System/FilePath/FilePathComponents.swift
@@ -136,42 +136,12 @@ extension SystemString {
   }
 }
 
-// Unifying protocol for common functionality between roots, components,
-// and views onto SystemString and FilePath.
-internal protocol _StrSlice: _PlatformStringable, Hashable, Codable {
-  var _storage: SystemString { get }
-  var _range: Range<SystemString.Index> { get }
-
-  init?(_ str: SystemString)
-
-  func _invariantCheck()
+// Protocol for types which hash and compare as their underlying
+// SystemString slices
+internal protocol _SystemStringBacked: Hashable, Codable {
+  var _slice: Slice<SystemString> { get }
 }
-extension _StrSlice {
-  internal var _slice: Slice<SystemString> {
-    Slice(base: _storage, bounds: _range)
-  }
-
-  internal func _withSystemChars<T>(
-    _ f: (UnsafeBufferPointer<SystemChar>) throws -> T
-  ) rethrows -> T {
-    try _storage.withSystemChars {
-      try f(UnsafeBufferPointer(rebasing: $0[_range]))
-    }
-  }
-
-  internal init?(_platformString s: UnsafePointer<CInterop.PlatformChar>) {
-    self.init(SystemString(platformString: s))
-  }
-
-  internal func _withPlatformString<Result>(
-    _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
-  ) rethrows -> Result {
-    try _slice._withPlatformString(body)
-  }
-
-  internal var _systemString: SystemString { SystemString(_slice) }
-}
-extension _StrSlice {
+extension _SystemStringBacked {
   public static func == (lhs: Self, rhs: Self) -> Bool {
     lhs._slice.elementsEqual(rhs._slice)
   }
@@ -182,22 +152,11 @@ extension _StrSlice {
     }
   }
 }
-internal protocol _PathSlice: _StrSlice {
-  var _path: FilePath { get }
-}
-extension _PathSlice {
-  internal var _storage: SystemString { _path._storage }
-}
-
-@available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
-extension FilePath.Component: _PathSlice {
-}
-@available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
-extension FilePath.Root: _PathSlice {
-  internal var _range: Range<SystemString.Index> {
-    (..<_rootEnd).relative(to: _path._storage)
-  }
+extension FilePath: _SystemStringBacked {
+  var _slice: Slice<SystemString> { _storage[...] }
 }
+extension FilePath.Component: _SystemStringBacked {}
+extension FilePath.Root: _SystemStringBacked {}
 
 @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
 extension FilePath.Component {
diff --git a/Sources/System/PlatformString.swift b/Sources/System/PlatformString.swift
index e2a9e05b..4561cd4b 100644
--- a/Sources/System/PlatformString.swift
+++ b/Sources/System/PlatformString.swift
@@ -190,12 +190,3 @@ extension CInterop.PlatformUnicodeEncoding.CodeUnit {
     #endif
   }
 }
-
-internal protocol _PlatformStringable {
-  func _withPlatformString<Result>(
-    _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
-  ) rethrows -> Result
-
-  init?(_platformString: UnsafePointer<CInterop.PlatformChar>)
-}
-extension String: _PlatformStringable {}

From e80ea343432e471c407bde2684d87060385fa54b Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Sun, 28 Apr 2024 18:27:08 -0700
Subject: [PATCH 08/11] wip: cleanup

---
 Sources/System/FilePath/FilePath.swift        |  4 +-
 .../FilePath/FilePathComponentView.swift      | 14 +++----
 .../System/FilePath/FilePathComponents.swift  | 12 +++---
 Sources/System/FilePath/FilePathParsing.swift | 41 ++++++++-----------
 Sources/System/FilePath/FilePathSyntax.swift  |  2 +-
 Sources/System/FilePath/FilePathWindows.swift |  8 ++--
 Sources/System/SystemString.swift             | 27 +-----------
 .../FilePathTests/FilePathExtras.swift        |  2 +-
 Tests/SystemTests/SystemStringTests.swift     |  2 +-
 9 files changed, 43 insertions(+), 69 deletions(-)

diff --git a/Sources/System/FilePath/FilePath.swift b/Sources/System/FilePath/FilePath.swift
index 47498650..7c2764db 100644
--- a/Sources/System/FilePath/FilePath.swift
+++ b/Sources/System/FilePath/FilePath.swift
@@ -52,7 +52,9 @@ public struct FilePath: Sendable {
   // In addition to the empty init, this init will properly normalize
   // separators. All other initializers should be implemented by
   // ultimately deferring to a normalizing init.
-  internal init(_ str: SystemString) {
+
+  /// TODO: docs
+  public init(_ str: SystemString) {
     self._storage = str
     self._normalizeSeparators()
     _invariantCheck()
diff --git a/Sources/System/FilePath/FilePathComponentView.swift b/Sources/System/FilePath/FilePathComponentView.swift
index b8ed69fa..446e7c0f 100644
--- a/Sources/System/FilePath/FilePathComponentView.swift
+++ b/Sources/System/FilePath/FilePathComponentView.swift
@@ -74,15 +74,15 @@ extension FilePath.ComponentView: BidirectionalCollection {
 
   @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
   public struct Index: Sendable, Comparable, Hashable {
-    internal typealias Storage = SystemString.Index
+    internal typealias _Storage = SystemString.Index
 
-    internal var _storage: Storage
+    internal var _storage: _Storage
 
     public static func < (lhs: Self, rhs: Self) -> Bool {
       lhs._storage < rhs._storage
     }
 
-    fileprivate init(_ idx: Storage) {
+    fileprivate init(_ idx: _Storage) {
       self._storage = idx
     }
   }
@@ -144,11 +144,11 @@ extension FilePath.ComponentView: RangeReplaceableCollection {
     var str = SystemString()
     let atEnd = subrange.lowerBound == endIndex
     if atEnd {
-      str.append(platformSeparator)
+      str.append(_platformSeparator)
     }
-    str.appendComponents(components: newElements)
+    str._appendComponents(components: newElements)
     if !atEnd {
-      str.append(platformSeparator)
+      str.append(_platformSeparator)
     }
     _path._storage.replaceSubrange(range, with: str)
   }
@@ -161,7 +161,7 @@ extension FilePath {
     root: Root?, _ components: C
   ) where C.Element == Component {
     var str = SystemString(root?._slice ?? SystemString()[...])
-    str.appendComponents(components: components)
+    str._appendComponents(components: components)
     self.init(str)
   }
 
diff --git a/Sources/System/FilePath/FilePathComponents.swift b/Sources/System/FilePath/FilePathComponents.swift
index 46b98358..22f043e2 100644
--- a/Sources/System/FilePath/FilePathComponents.swift
+++ b/Sources/System/FilePath/FilePathComponents.swift
@@ -118,7 +118,7 @@ extension FilePath.Root {
 extension SystemString {
   // TODO: take insertLeadingSlash: Bool
   // TODO: turn into an insert operation with slide
-  internal mutating func appendComponents<C: Collection>(
+  internal mutating func _appendComponents<C: Collection>(
     components: C
   ) where C.Element == FilePath.Component {
     // TODO(perf): Consider pre-pass to count capacity, slide
@@ -131,7 +131,7 @@ extension SystemString {
     for idx in components.indices {
       let component = components[idx]
       self.append(contentsOf: component._slice)
-      self.append(platformSeparator)
+      self.append(_platformSeparator)
     }
   }
 }
@@ -189,7 +189,8 @@ internal func _makeExtension(_ ext: String) -> SystemString {
 
 @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
 extension FilePath.Component {
-  internal init?(_ str: SystemString) {
+  /// TODO: docs
+  public init?(_ str: SystemString) {
     // FIXME: explicit null root? Or something else?
     let path = FilePath(str)
     guard path.root == nil, path.components.count == 1 else {
@@ -202,7 +203,8 @@ extension FilePath.Component {
 
 @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
 extension FilePath.Root {
-  internal init?(_ str: SystemString) {
+  /// TODO: docs
+  public init?(_ str: SystemString) {
     // FIXME: explicit null root? Or something else?
     let path = FilePath(str)
     guard path.root != nil, path.components.isEmpty else {
@@ -222,7 +224,7 @@ extension FilePath.Component {
     #if DEBUG
     precondition(!_slice.isEmpty)
     precondition(_slice.last != .null)
-    precondition(_slice.allSatisfy { !isSeparator($0) } )
+    precondition(_slice.allSatisfy { !_isSeparator($0) } )
     precondition(_path._relativeStart <= _slice.startIndex)
     #endif // DEBUG
   }
diff --git a/Sources/System/FilePath/FilePathParsing.swift b/Sources/System/FilePath/FilePathParsing.swift
index 6d014774..5a9ab8dc 100644
--- a/Sources/System/FilePath/FilePathParsing.swift
+++ b/Sources/System/FilePath/FilePathParsing.swift
@@ -11,24 +11,19 @@
 // manages (and hides) the null terminator
 
 // The separator we use internally
-private var genericSeparator: SystemChar { .slash }
+private var _genericSeparator: SystemChar { .slash }
 
 // The platform preferred separator
 //
 // TODO: Make private
-internal var platformSeparator: SystemChar {
-  _windowsPaths ? .backslash : genericSeparator
+internal var _platformSeparator: SystemChar {
+  _windowsPaths ? .backslash : _genericSeparator
 }
 
 // Whether the character is the canonical separator
 // TODO: Make private
-internal func isSeparator(_ c: SystemChar) -> Bool {
-  c == platformSeparator
-}
-
-// Whether the character is a pre-normalized separator
-internal func isPrenormalSeparator(_ c: SystemChar) -> Bool {
-  c == genericSeparator || c == platformSeparator
+internal func _isSeparator(_ c: SystemChar) -> Bool {
+  c == _platformSeparator
 }
 
 // Separator normalization, checking, and root parsing is internally hosted
@@ -42,7 +37,7 @@ extension SystemString {
     guard _relativePathStart != endIndex else { return false }
     assert(!isEmpty)
 
-    return isSeparator(self.last!)
+    return _isSeparator(self.last!)
   }
 
   // Enforce invariants by removing a trailing separator.
@@ -82,7 +77,7 @@ extension SystemString {
       // parsing and (potentially) fixing up semi-formed roots. This
       // normalization reduces the complexity of the task by allowing us to
       // use a read-only lexer.
-      self._replaceAll(genericSeparator, with: platformSeparator)
+      self._replaceAll(_genericSeparator, with: _platformSeparator)
 
       // Windows roots can have meaningful repeated backslashes or may
       // need backslashes inserted for partially-formed roots. Delegate that to
@@ -90,19 +85,19 @@ extension SystemString {
       readIdx = _prenormalizeWindowsRoots()
       writeIdx = readIdx
     } else {
-      assert(genericSeparator == platformSeparator)
+      assert(_genericSeparator == _platformSeparator)
     }
 
     while readIdx < endIndex {
       assert(writeIdx <= readIdx)
 
       // Swap and advance our indices.
-      let wasSeparator = isSeparator(self[readIdx])
+      let wasSeparator = _isSeparator(self[readIdx])
       self.swapAt(writeIdx, readIdx)
       self.formIndex(after: &writeIdx)
       self.formIndex(after: &readIdx)
 
-      while wasSeparator, readIdx < endIndex, isSeparator(self[readIdx]) {
+      while wasSeparator, readIdx < endIndex, _isSeparator(self[readIdx]) {
         self.formIndex(after: &readIdx)
       }
     }
@@ -223,8 +218,8 @@ extension FilePath {
       }
     }
 
-    assert(!isSeparator(_storage[i]))
-    guard let nextSep = _storage[i...].firstIndex(where: isSeparator) else {
+    assert(!_isSeparator(_storage[i]))
+    guard let nextSep = _storage[i...].firstIndex(where: _isSeparator) else {
       return (_storage.endIndex, _storage.endIndex)
     }
     return (nextSep, _storage.index(after: nextSep))
@@ -244,11 +239,11 @@ extension FilePath {
 
     var slice = _storage[..<i]
     if i != _storage.endIndex {
-      assert(isSeparator(slice.last!))
+      assert(_isSeparator(slice.last!))
       slice.removeLast()
     }
     let end = slice.endIndex
-    while slice.endIndex != relStart, let c = slice.last, !isSeparator(c) {
+    while slice.endIndex != relStart, let c = slice.last, !_isSeparator(c) {
       slice.removeLast()
     }
 
@@ -286,7 +281,7 @@ extension SystemString {
     if _windowsPaths { return _parseWindowsRoot() }
 
     // A leading `/` is a root
-    guard isSeparator(self.first!) else { return (startIndex, startIndex) }
+    guard _isSeparator(self.first!) else { return (startIndex, startIndex) }
 
     let next = self.index(after: startIndex)
     return (next, next)
@@ -302,7 +297,7 @@ extension FilePath.Root {
   // relative roots
   //
   // TODO: public
-  internal var isAbsolute: Bool {
+  internal var _isAbsolute: Bool {
     assert(FilePath(SystemString(self._slice)).root == self, "not a root")
 
     guard _windowsPaths else { return true }
@@ -341,7 +336,7 @@ internal var _windowsPaths: Bool {
 extension FilePath {
   // Whether we should add a separator when doing an append
   internal var _needsSeparatorForAppend: Bool {
-    guard let last = _storage.last, !isSeparator(last) else { return false }
+    guard let last = _storage.last, !_isSeparator(last) else { return false }
 
     // On Windows, we can have a path of the form `C:` which is a root and
     // does not need a separator after it
@@ -358,7 +353,7 @@ extension FilePath {
     assert(FilePath(SystemString(content)).root == nil)
     if content.isEmpty { return }
     if _needsSeparatorForAppend {
-      _storage.append(platformSeparator)
+      _storage.append(_platformSeparator)
     }
     _storage.append(contentsOf: content)
   }
diff --git a/Sources/System/FilePath/FilePathSyntax.swift b/Sources/System/FilePath/FilePathSyntax.swift
index f94f4453..d36b352c 100644
--- a/Sources/System/FilePath/FilePathSyntax.swift
+++ b/Sources/System/FilePath/FilePathSyntax.swift
@@ -37,7 +37,7 @@ extension FilePath {
   ///   * `\\?\UNC\server\share\bar.exe`
   ///   * `\\server\share\bar.exe`
   public var isAbsolute: Bool {
-    self.root?.isAbsolute ?? false
+    self.root?._isAbsolute ?? false
   }
 
   /// Returns true if this path is not absolute (see `isAbsolute`).
diff --git a/Sources/System/FilePath/FilePathWindows.swift b/Sources/System/FilePath/FilePathWindows.swift
index 0967d4b0..84bb5b7f 100644
--- a/Sources/System/FilePath/FilePathWindows.swift
+++ b/Sources/System/FilePath/FilePathWindows.swift
@@ -141,7 +141,7 @@ struct _Lexer {
   }
 }
 
-internal struct WindowsRootInfo {
+internal struct _WindowsRootInfo {
   // The "volume" of a root. For UNC paths, this is also known as the "share".
   internal enum Volume: Equatable {
     /// No volume specified
@@ -209,7 +209,7 @@ internal struct WindowsRootInfo {
 }
 
 extension _ParsedWindowsRoot {
-  fileprivate func volumeInfo(_ root: SystemString) -> WindowsRootInfo.Volume {
+  fileprivate func volumeInfo(_ root: SystemString) -> _WindowsRootInfo.Volume {
     if let d = self.drive {
       return .drive(Character(d.asciiScalar!))
     }
@@ -222,7 +222,7 @@ extension _ParsedWindowsRoot {
   }
 }
 
-extension WindowsRootInfo {
+extension _WindowsRootInfo {
   internal init(_ root: SystemString, _ parsed: _ParsedWindowsRoot) {
     self.volume = parsed.volumeInfo(root)
 
@@ -243,7 +243,7 @@ extension WindowsRootInfo {
   }
 }
 
-extension WindowsRootInfo {
+extension _WindowsRootInfo {
   /// NOT `\foo\bar` nor `C:foo\bar`
   internal var isFullyQualified: Bool {
     return form != .traditional(fullyQualified: false)
diff --git a/Sources/System/SystemString.swift b/Sources/System/SystemString.swift
index 52adf8eb..6014a68e 100644
--- a/Sources/System/SystemString.swift
+++ b/Sources/System/SystemString.swift
@@ -212,31 +212,6 @@ extension SystemString: RangeReplaceableCollection {
 
 extension SystemString: Hashable, Codable {}
 
-extension SystemString {
-
-  // withSystemChars includes the null terminator
-  internal func withSystemChars<T>(
-    _ f: (UnsafeBufferPointer<SystemChar>) throws -> T
-  ) rethrows -> T {
-    try _nullTerminatedStorage.withContiguousStorageIfAvailable(f)!
-  }
-
-  internal func withCodeUnits<T>(
-    _ f: (UnsafeBufferPointer<CInterop.PlatformUnicodeEncoding.CodeUnit>) throws -> T
-  ) rethrows -> T {
-    try withSystemChars { chars in
-      let length = chars.count * MemoryLayout<SystemChar>.stride
-      let count = length / MemoryLayout<CInterop.PlatformUnicodeEncoding.CodeUnit>.stride
-      return try chars.baseAddress!.withMemoryRebound(
-        to: CInterop.PlatformUnicodeEncoding.CodeUnit.self,
-        capacity: count
-      ) { pointer in
-        try f(UnsafeBufferPointer(start: pointer, count: count))
-      }
-    }
-  }
-}
-
 extension Slice where Base == SystemString {
   internal func _withPlatformString<T>(
     _ f: (UnsafePointer<CInterop.PlatformChar>) throws -> T
@@ -334,7 +309,7 @@ extension SystemString {
   public func withPlatformString<T>(
     _ f: (UnsafePointer<CInterop.PlatformChar>) throws -> T
   ) rethrows -> T {
-    try withSystemChars { chars in
+    try _nullTerminatedStorage.withUnsafeBufferPointer { chars in
       let length = chars.count * MemoryLayout<SystemChar>.stride
       return try chars.baseAddress!.withMemoryRebound(
         to: CInterop.PlatformChar.self,
diff --git a/Tests/SystemTests/FilePathTests/FilePathExtras.swift b/Tests/SystemTests/FilePathTests/FilePathExtras.swift
index 82f11373..647d9d70 100644
--- a/Tests/SystemTests/FilePathTests/FilePathExtras.swift
+++ b/Tests/SystemTests/FilePathTests/FilePathExtras.swift
@@ -43,7 +43,7 @@ extension FilePath {
     for _ in 0..<baseTail.count {
       prefix.append(.dot)
       prefix.append(.dot)
-      prefix.append(platformSeparator)
+      prefix.append(_platformSeparator)
     }
 
     return FilePath(prefix + tail._storageSlice)
diff --git a/Tests/SystemTests/SystemStringTests.swift b/Tests/SystemTests/SystemStringTests.swift
index 612b9836..f575f311 100644
--- a/Tests/SystemTests/SystemStringTests.swift
+++ b/Tests/SystemTests/SystemStringTests.swift
@@ -101,7 +101,7 @@ struct StringTest: TestCase {
     // Test null insertion
     let rawChars = raw.lazy.map { SystemChar($0) }
     expectEqual(sysRaw, SystemString(rawChars.dropLast()))
-    sysRaw.withSystemChars {  // TODO: assuming we want null in withSysChars
+    sysRaw.nullTerminatedStorage.withUnsafeBufferPointer {
       expectEqualSequence(rawChars, $0, "rawChars")
     }
 

From 3096de9973d414748d2f8b17ba6bc7f709c52356 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Mon, 29 Apr 2024 13:29:20 -0700
Subject: [PATCH 09/11] WIP: proposal

---
 Documentation/system-string.md | 53 ++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)
 create mode 100644 Documentation/system-string.md

diff --git a/Documentation/system-string.md b/Documentation/system-string.md
new file mode 100644
index 00000000..b1cdaa99
--- /dev/null
+++ b/Documentation/system-string.md
@@ -0,0 +1,53 @@
+# SystemString for OS-native string representations
+
+* Authors: [Michael Ilseman](https://github.com/milseman)
+* Implementation: PR (**TODO**)
+
+## Introduction
+
+We introduce `SystemString`, which supports OS-native string operations. `SystemString` is a bag-of-bytes type without a prescribed encoding. It is a collection `SystemChar`s, which is `UInt8` on Unix platforms and `UInt16` on Windows platforms.
+
+
+## Motivation
+
+`SystemString` is the backing storage representation for `FilePath`. `FilePath` normalizes its contents (e.g. `a//b -> a/b`), and so it is insufficient as a OS-preferred bag-of-bytes string representation.
+
+**TODO**: It would be nice to ship with a few syscalls that make use of it.
+
+**TODO**: A little more motivation on `SystemChar`. Also, let's make sure we have clarity on layout equivalence and demonstrate how to get from a null-`SystemChar`-termianted `UBP<SystemChar>` to null-terminated `UBP<UIntX>`.
+
+## Proposed solution
+
+**TODO**: Brief highlights
+
+
+## Detailed design
+
+
+## Source compatibility
+
+This proposal is additive and source-compatible with existing code.
+
+## ABI compatibility
+
+This proposal is additive and ABI-compatible with existing code.
+
+
+## Alternatives considered
+
+**TODO**: Consider not having `SystemChar`
+
+**TODO**: Consider separate `SystemByteString` and `SystemBytePairString` types.
+
+**TODO**: Why we don't want to have a single-byte ASCII representation on Windows and have syscall wrapper adjust/dispatch appropriately.
+
+
+## Future directions
+
+**TODO**: Map out some future syscalls that this would (partially) unblock
+
+## Acknowledgments
+
+**TODO**
+
+

From 570e143ecc2fd22172abc75368be3a896f43b179 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Mon, 29 Apr 2024 13:53:05 -0700
Subject: [PATCH 10/11] Flesh out proposal and docs

---
 Documentation/system-string.md                | 231 ++++++++++++++++++
 Sources/System/FilePath/FilePath.swift        |   2 +-
 .../System/FilePath/FilePathComponents.swift  |   8 +-
 Sources/System/Internals/Exports.swift        |   6 -
 4 files changed, 238 insertions(+), 9 deletions(-)

diff --git a/Documentation/system-string.md b/Documentation/system-string.md
index b1cdaa99..a5638482 100644
--- a/Documentation/system-string.md
+++ b/Documentation/system-string.md
@@ -23,6 +23,237 @@ We introduce `SystemString`, which supports OS-native string operations. `System
 
 ## Detailed design
 
+`SystemChar` is a raw wrapper around `CChar` on Linux and Darwin, and a `UInt16`on Windows. It is layout compatible with those types and exposes convenience interfaces for getting common values.
+
+```swift
+/// A platform-native character representation.
+///
+/// A SystemChar is a `CChar` on Linux and Darwin, and a `UInt16` on Windows.
+///
+/// Note that no particular encoding is assumed.
+@frozen
+public struct SystemChar:
+  RawRepresentable, Sendable, Comparable, Hashable, Codable {
+  public typealias RawValue = CInterop.PlatformChar
+
+  public var rawValue: RawValue
+
+  @inlinable
+  public init(rawValue: RawValue)
+
+  @_alwaysEmitIntoClient
+  public init(_ rawValue: RawValue)
+
+  @inlinable
+  public static func < (lhs: SystemChar, rhs: SystemChar) -> Bool
+}
+
+extension SystemChar {
+  /// Create a SystemChar from an ASCII scalar.
+  @_alwaysEmitIntoClient
+  public init(ascii: Unicode.Scalar)
+
+  /// Cast `x` to a `SystemChar`
+  @_alwaysEmitIntoClient
+  public init(_ x: some FixedWidthInteger)
+
+  /// The NULL character `\0`
+  @_alwaysEmitIntoClient
+  public static var null: SystemChar { get }
+
+  /// The slash character `/`
+  @_alwaysEmitIntoClient
+  public static var slash: SystemChar { get }
+
+  /// The backslash character `\`
+  @_alwaysEmitIntoClient
+  public static var backslash: SystemChar { get }
+
+  /// The dot character `.`
+  @_alwaysEmitIntoClient
+  public static var dot: SystemChar { get }
+
+  /// The colon character `:`
+  @_alwaysEmitIntoClient
+  public static var colon: SystemChar { get }
+
+  /// The question mark character `?`
+  @_alwaysEmitIntoClient
+  public static var question: SystemChar { get }
+
+  /// Returns `self` as a `Unicode.Scalar` if ASCII, else `nil`
+  @_alwaysEmitIntoClient
+  public var asciiScalar: Unicode.Scalar? { get }
+
+  /// Whether `self` is ASCII
+  @_alwaysEmitIntoClient
+  public var isASCII: Bool { get }
+
+  /// Whether `self` is an ASCII letter, i.e. in `[a-zA-Z]`
+  @_alwaysEmitIntoClient
+  public var isASCIILetter: Bool { get }
+}
+```
+
+`SystemString` is a `RangeReplaceableCollection` of `SystemChar`s and ensures that it is always `NULL`-terminated. The `NULL` is not considered part of the count and the string must not contain `NULL`s inside of it.
+
+```swift
+
+/// A platform-native string representation. A `SystemString` is a collection
+/// of non-NULL `SystemChar`s followed by a NULL terminator.
+///
+/// TODO: example use or two, showing that innards are not NULL, but there's
+/// always a null at the end, NULL is not part of the count
+@frozen
+public struct SystemString: Sendable {
+  public typealias Storage = [SystemChar]
+
+  @usableFromInline
+  internal var _nullTerminatedStorage: Storage
+
+  /// Access the back storage, including the null terminator. Note that
+  /// `nullTerminatedStorage.count == self.count + 1`, due
+  /// to the null terminator.
+  @_alwaysEmitIntoClient
+  public var nullTerminatedStorage: Storage { get }
+}
+
+extension SystemString {
+  /// Create an empty `SystemString`
+  public init()
+
+  /// Create a `SystemString` from a collection of `SystemChar`s.
+  /// A NULL terminator will be added if `chars` lacks one. `chars` must not
+  /// include any interior NULLs.
+  @_alwaysEmitIntoClient
+  public init<C: Collection>(_ chars: C) where C.Element == SystemChar 
+}
+
+extension SystemString: RandomAccessCollection, MutableCollection {
+  public typealias Element = SystemChar
+  public typealias Index = Int
+  public typealias Indices = Range<Index>
+
+  @inlinable
+  public var startIndex: Index { get }
+
+  @inlinable
+  public var endIndex: Index { get }
+
+  @inlinable
+  public subscript(position: Index) -> SystemChar {
+    _read, set
+  }
+}
+extension SystemString: RangeReplaceableCollection {
+  @inlinable
+  public mutating func replaceSubrange<C: Collection>(
+    _ subrange: Range<Index>, with newElements: C
+  ) where C.Element == SystemChar
+
+  @inlinable
+  public mutating func reserveCapacity(_ n: Int)
+
+  @inlinable
+  public func withContiguousStorageIfAvailable<R>(
+    _ body: (UnsafeBufferPointer<SystemChar>) throws -> R
+  ) rethrows -> R?
+}
+
+extension SystemString: Hashable, Codable {}
+
+extension SystemString: ExpressibleByStringLiteral {
+  public init(stringLiteral: String)
+
+  public init(_ string: String)
+}
+
+extension SystemString: CustomStringConvertible, CustomDebugStringConvertible {
+
+  public var description: String { get }
+
+  public var debugDescription: String { get }
+}
+
+extension SystemString {
+  /// Creates a `SystemString` by copying bytes from a null-terminated platform string.
+  ///
+  /// - Parameter platformString: A pointer to a null-terminated platform string.
+  public init(platformString: UnsafePointer<CInterop.PlatformChar>)
+
+  /// Calls the given closure with a pointer to the contents of the sytem string,
+  /// represented as a null-terminated platform string.
+  ///
+  /// - Parameter body: A closure with a pointer parameter
+  ///   that points to a null-terminated platform string.
+  ///   If `body` has a return value,
+  ///   that value is also used as the return value for this method.
+  /// - Returns: The return value, if any, of the `body` closure parameter.
+  ///
+  /// The pointer passed as an argument to `body` is valid
+  /// only during the execution of this method.
+  /// Don't try to store the pointer for later use.
+  public func withPlatformString<T>(
+    _ f: (UnsafePointer<CInterop.PlatformChar>) throws -> T
+  ) rethrows -> T
+}
+
+```
+
+You can create `String`s from `SystemString`, either decoding them (i.e. performing Unicode error correction on the contents) or validating them (i.e. returning `nil` if invalidly-encoded Unicode content).
+
+```swift
+
+extension String {
+  /// Creates a string by interpreting `str`'s content as UTF-8 on Unix
+  /// and UTF-16 on Windows.
+  ///
+  /// - Parameter str: The system string to be interpreted as
+  /// `CInterop.PlatformUnicodeEncoding`.
+  ///
+  /// If the content of the system string isn't a well-formed Unicode string,
+  /// this initializer replaces invalid bytes with U+FFFD.
+  /// This means that conversion to a string and back to a system string
+  /// might result in a value that's different from the original system string.
+  public init(decoding str: SystemString)
+
+  /// Creates a string from a system string, validating its contents as UTF-8 on
+  /// Unix and UTF-16 on Windows.
+  ///
+  /// - Parameter str: The system string to be interpreted as
+  ///   `CInterop.PlatformUnicodeEncoding`.
+  ///
+  /// If the contents of the system string isn't well-formed Unicode,
+  /// this initializer returns `nil`.
+  public init?(validating str: SystemString)
+}
+
+
+```
+
+You can create a `FilePath`, `FilePath.Root`, and `FilePath.Component` from a `SystemString`.
+
+```swift
+extension FilePath {
+  /// Create a `FilePath` with the contents of `str`, normalizing separators.
+  public init(_ str: SystemString)
+}
+
+extension FilePath.Component {
+  /// Create a `FilePath.Component` with the contents of `str`.
+  ///
+  /// Returns `nil` if `str` is empty or contains the directory separator.
+  public init?(_ str: SystemString)
+}
+
+extension FilePath.Root {
+  /// Create a `FilePath.Root` with the contents of `str`.
+  ///
+  /// Returns `nil` if `str` is empty or is not a root
+  public init?(_ str: SystemString)
+}
+
+```
 
 ## Source compatibility
 
diff --git a/Sources/System/FilePath/FilePath.swift b/Sources/System/FilePath/FilePath.swift
index 7c2764db..f3a9cfa7 100644
--- a/Sources/System/FilePath/FilePath.swift
+++ b/Sources/System/FilePath/FilePath.swift
@@ -53,7 +53,7 @@ public struct FilePath: Sendable {
   // separators. All other initializers should be implemented by
   // ultimately deferring to a normalizing init.
 
-  /// TODO: docs
+  /// Create a `FilePath` with the contents of `str`, normalizing separators.
   public init(_ str: SystemString) {
     self._storage = str
     self._normalizeSeparators()
diff --git a/Sources/System/FilePath/FilePathComponents.swift b/Sources/System/FilePath/FilePathComponents.swift
index 22f043e2..4ffb822a 100644
--- a/Sources/System/FilePath/FilePathComponents.swift
+++ b/Sources/System/FilePath/FilePathComponents.swift
@@ -189,7 +189,9 @@ internal func _makeExtension(_ ext: String) -> SystemString {
 
 @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
 extension FilePath.Component {
-  /// TODO: docs
+  /// Create a `FilePath.Component` with the contents of `str`.
+  ///
+  /// Returns `nil` if `str` is empty or contains the directory separator.
   public init?(_ str: SystemString) {
     // FIXME: explicit null root? Or something else?
     let path = FilePath(str)
@@ -203,7 +205,9 @@ extension FilePath.Component {
 
 @available(/*System 0.0.2: macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0*/iOS 8, *)
 extension FilePath.Root {
-  /// TODO: docs
+  /// Create a `FilePath.Root` with the contents of `str`.
+  ///
+  /// Returns `nil` if `str` is empty or is not a root
   public init?(_ str: SystemString) {
     // FIXME: explicit null root? Or something else?
     let path = FilePath(str)
diff --git a/Sources/System/Internals/Exports.swift b/Sources/System/Internals/Exports.swift
index d1283197..cd91ea21 100644
--- a/Sources/System/Internals/Exports.swift
+++ b/Sources/System/Internals/Exports.swift
@@ -101,12 +101,6 @@ internal func system_memset(
 
 // Interop between String and platfrom string
 extension String {
-  internal func _withPlatformString<Result>(
-    _ body: (UnsafePointer<CInterop.PlatformChar>) throws -> Result
-  ) rethrows -> Result {
-    fatalError()
-  }
-
   internal init?(_platformString platformString: UnsafePointer<CInterop.PlatformChar>) {
     // Need to #if because CChar may be signed
     #if os(Windows)

From 1faa91bdae9122540da3ce1e0e711287df545ac3 Mon Sep 17 00:00:00 2001
From: Michael Ilseman <michael.ilseman@gmail.com>
Date: Mon, 29 Apr 2024 14:06:31 -0700
Subject: [PATCH 11/11] Explain platform string vs system string

---
 Documentation/system-string.md | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/Documentation/system-string.md b/Documentation/system-string.md
index a5638482..6478d750 100644
--- a/Documentation/system-string.md
+++ b/Documentation/system-string.md
@@ -266,11 +266,15 @@ This proposal is additive and ABI-compatible with existing code.
 
 ## Alternatives considered
 
-**TODO**: Consider not having `SystemChar`
+### The role of `SystemString` and `PlatformString`
 
-**TODO**: Consider separate `SystemByteString` and `SystemBytePairString` types.
+`PlatformString` is a typealias for the corresponding `UnsafePointer` you'd use with a raw syscall. `SystemString` is an owning type which provides memory safety, COW-semantics, and allows mutations and range-replaceable operations (such as appending). You can always get a `PlatformString` directly from a `SystemString`, and you can construct (by copying the memory) a `SystemString` from a `PlatformString`.
 
-**TODO**: Why we don't want to have a single-byte ASCII representation on Windows and have syscall wrapper adjust/dispatch appropriately.
+### **TODO**
+
+- Consider not having `SystemChar`
+- Consider separate `SystemByteString` and `SystemBytePairString` types.
+- Why we don't want to have a single-byte ASCII representation on Windows and have syscall wrapper adjust/dispatch appropriately.
 
 
 ## Future directions