Suppress compiler warnings about Errors with non-Sendable members

Sources/_RegexParser/Regex/Parse/CompilerInterface.swift

@@ -17,9 +17,19 @@ public let currentRegexLiteralFormatVersion = 1
 
 @_spi(CompilerInterface)
 public struct CompilerLexError: Error {
+  var underlyingLocation: UnsafeSourceLocation
+
   public var message: String
-  public var location: UnsafeRawPointer
+  public var location: UnsafeRawPointer { return underlyingLocation.ptr }
   public var completelyErroneous: Bool
+
+  init(
+    message: String, location: UnsafeRawPointer, completelyErroneous: Bool
+  ) {
+    self.message = message
+    self.underlyingLocation = UnsafeSourceLocation(location)
+    self.completelyErroneous = completelyErroneous
+  }
 }
 
 /// Interface for the Swift compiler.

Sources/_RegexParser/Regex/Parse/DelimiterLexing.swift

@@ -70,6 +70,20 @@ extension Delimiter {
   }
 }
 
+/// A wrapper for an `UnsafeRawPointer` representing a source location, marked
+/// `@unchecked Sendable` to help errors carrying such information satisfy their
+/// `Sendable` requirement.
+///
+/// TODO: consider further refactoring to avoid needing to lie to the compiler
+/// about Sendability.
+struct UnsafeSourceLocation: @unchecked Sendable {
+  let ptr: UnsafeRawPointer
+
+  init(_ ptr: UnsafeRawPointer) {
+    self.ptr = ptr
+  }
+}
+
 public struct DelimiterLexError: Error, CustomStringConvertible {
   public enum Kind: Hashable {
     case unterminated
@@ -81,12 +95,14 @@ public struct DelimiterLexError: Error, CustomStringConvertible {
 
   public var kind: Kind
 
+  var resumeLocation: UnsafeSourceLocation
+
   /// The pointer at which to resume lexing.
-  public var resumePtr: UnsafeRawPointer
+  public var resumePtr: UnsafeRawPointer { resumeLocation.ptr }
 
   init(_ kind: Kind, resumeAt resumePtr: UnsafeRawPointer) {
     self.kind = kind
-    self.resumePtr = resumePtr
+    self.resumeLocation = UnsafeSourceLocation(resumePtr)
   }
 
   public var description: String {

Sources/_StringProcessing/Engine/MEBuiltins.swift

@@ -316,9 +316,10 @@ extension String {
   ) -> String.Index? {
     // TODO: Branch here on scalar semantics
     // Don't want to pay character cost if unnecessary
-    guard var (char, next) =
+    guard let (char, nextIndex) =
             characterAndEnd(at: currentPosition, limitedBy: end)
     else { return nil }
+    var next = nextIndex
     let scalar = unicodeScalars[currentPosition]
 
     let asciiCheck = !isStrictASCII