feat: address concurrency warnings arising from Theme

`Theme` was marked as `Sendable` in gonzalezreal#351. There's nothing wrong
with this per se, but sendability is not inferred for public
types, a structure is only truely `Sendable` if all of its
members are `Sendable`, and most of the members of `Theme` are
not `Sendable`.

This patch address the sendability warnings for Theme's members
that the compiler emits (regardless of `StrictConcurrency`).

Author: cweider

Sources/MarkdownUI/Theme/TextStyle/Styles/FontProperties.swift

@@ -3,7 +3,7 @@ import SwiftUI
 /// The characteristics of a font.
 public struct FontProperties: Hashable {
   /// The font family.
-  public enum Family: Hashable {
+  public enum Family: Hashable, Sendable {
     /// The system font family.
     case system(Font.Design = .default)
 
@@ -12,7 +12,7 @@ public struct FontProperties: Hashable {
   }
 
   /// The font family variant.
-  public enum FamilyVariant: Hashable {
+  public enum FamilyVariant: Hashable, Sendable {
     /// No variant. Use the current font family.
     case normal
 
@@ -21,7 +21,7 @@ public struct FontProperties: Hashable {
   }
 
   /// The font caps variant.
-  public enum CapsVariant: Hashable {
+  public enum CapsVariant: Hashable, Sendable {
     /// Don't use a font caps variant.
     case normal
 
@@ -36,7 +36,7 @@ public struct FontProperties: Hashable {
   }
 
   /// The font digit variant.
-  public enum DigitVariant: Hashable {
+  public enum DigitVariant: Hashable, Sendable {
     /// Don't use a font digit variant.
     case normal
 
@@ -45,7 +45,7 @@ public struct FontProperties: Hashable {
   }
 
   /// The font style.
-  public enum Style {
+  public enum Style: Sendable {
     /// Don't use a font style.
     case normal
 
@@ -98,11 +98,11 @@ public struct FontProperties: Hashable {
   /// The font width.
   @available(iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0, *)
   public var width: Font.Width {
-    get { (self.widthStorage as? Font.Width) ?? .standard }
-    set { self.widthStorage = newValue }
+    get { (self.widthStorage?.base as? Font.Width) ?? .standard }
+    set { self.widthStorage = AnySendableHashableBox(newValue) }
   }
 
-  private var widthStorage: AnyHashable?
+  private var widthStorage: AnySendableHashableBox?
 
   /// The font size.
   public var size: CGFloat = Self.defaultSize
@@ -187,3 +187,14 @@ extension FontProperties: TextStyle {
     attributes.fontProperties = self
   }
 }
+
+// A cheap workaround for `AnyHashable` of a `Sendable` not being `Sendable`.
+// This is necessary for `widthStorage`.
+// TODO: iOS16,macOS13 once minimum deployment target is raised, remove this indirection
+fileprivate struct AnySendableHashableBox: Hashable, @unchecked Sendable {
+  let base: AnyHashable
+
+  init<V: Hashable & Sendable>(_ base: V) {
+    self.base = base
+  }
+}

Sources/MarkdownUI/Theme/TextStyle/Styles/FontSize.swift

@@ -2,7 +2,7 @@ import Foundation
 
 /// A text style that sets the font size.
 public struct FontSize: TextStyle {
-  private enum Size {
+  private enum Size: Sendable {
     case points(CGFloat)
     case relative(RelativeSize)
   }

Sources/MarkdownUI/Theme/TextStyle/TextStyle.swift

@@ -57,6 +57,6 @@ import SwiftUI
 /// ```
 ///
 /// ![](CustomBlockquote)
-public protocol TextStyle {
+public protocol TextStyle : Sendable {
   func _collectAttributes(in attributes: inout AttributeContainer)
 }

Sources/MarkdownUI/Utility/RelativeSize.swift

@@ -21,8 +21,8 @@ import SwiftUI
 ///     FontSize(.em(2))
 ///   }
 /// ```
-public struct RelativeSize: Hashable {
-  enum Unit: Hashable {
+public struct RelativeSize: Hashable, Sendable {
+  enum Unit: Hashable, Sendable {
     case em
     case rem
   }