[MetricExpansion]

If I understood all the semantic properties, including the separate compilation requirements, correctly, here’s how I think it would be done in Swift with the proposed nonescapable types features (needed to safely express the AsRef concept here). (Note that this doesn’t quite compile today and the syntax for nonescaping types is still a proposal.)

  @usableFromInline
  func _read(pathView: PathView) throws(IOError) -> [UInt8] {
      var file = try File(pathView)
      var bytes: [UInt8] = []
      try file.readToEnd(into: &bytes)
      return bytes
  }
  
  @inlinable
  public func read<Path>(path: borrowing Path) throws(IOError) -> [UInt8] where Path: PathViewable, Path: ~Copyable {
      try _read(pathView: path.view())
  }
  
  // Definitions...
  
  public enum IOError: Error {}
  
  public protocol PathViewable: ~Copyable {
      func view() -> PathView
  }
  
  public struct PathView: ~Escapable {}
  
  public struct File: ~Copyable {
      public init(_ pathView: borrowing PathView) throws(IOError) {
          fatalError("unimplemented")
      }
  
      public mutating func readToEnd(into buffer: inout [UInt8]) throws(IOError) {
          fatalError("unimplemented")
      }
  }