Add '_spi(TokamakCore)' to ideally internal public members. (#386)

Adds `_spi(TokamakCore)` to the modules:
1. TokamakCore
2. TokamakDOM
3. TokamakGTK
4. TokamakStaticHTML

The attribute is applied to:
1. All `View` bodies in TokamakCore — either primitive or regular like `Color`
2. `ViewDeferredToRenderer` bodies
4. `ParentView` `children` members
5. `View` modifiers (such as  `_onMount(perform:)`)
6. Other members of types (like `Color._withScheme`, and `_AnyApp._launch`) 

The attribute semantics (from my brief testing)
1. It can only be applied to `public` declarations
2. It ensures that every SPI declaration is exposed only by other SPI declarations (i.e. `@_spi(Module) public enum A {}; public var a: A` is illegal)
3. Regularly importing a library prohibits clients from accessing SPI declarations that are not protocol witnesses (with an error).
4. Regularly importing a library "discourages" clients from accessing SPI protocol witnesses by hiding them from the autocompletion suggestions (i.e. users can still access `body` of `Text`, but autocompletion hides it).
5. For a declaration marked with `@_spi(Module)`, a client has to write `@_spi(Module) import Library` in order to normally access such SPI declarations.

* Add '_spi(TokamakCore)' to ideally internal public members.

* Remove `_spi` attribute on '_ConditionalContent'.

* Remove spi from 'Path._PathBox', 'Font._Font', and '_TupleScene.body'.

* Remove spi from types.

* Remove trailing whitespace.

* Apply spi to 'View' modifiers.

* Add _spi imports.

* Introduce 'PrimitiveView'.

* Remove 'PrimitiveView' conformances outside of TokamakCore.

* Fix `PrimitiveView` default implementation.

* Remove "BubbleCore" references.

Sources/TokamakCore/App/AppStorage.swift

@@ -173,6 +173,7 @@ struct DefaultAppStorageEnvironmentKey: EnvironmentKey {
 }
 
 public extension EnvironmentValues {
+  @_spi(TokamakCore)
   var _defaultAppStorage: _StorageProvider? {
     get {
       self[DefaultAppStorageEnvironmentKey.self]

Sources/TokamakCore/App/Scenes/WindowGroup.swift

@@ -63,6 +63,7 @@ public struct WindowGroup<Content>: Scene, TitledScene where Content: View {
     self.content = content()
   }
 
+  @_spi(TokamakCore)
   public var body: Never {
     neverScene("WindowGroup")
   }

Sources/TokamakCore/App/Scenes/_SceneModifier.swift

@@ -25,6 +25,7 @@ public struct _SceneModifier_Content<Modifier>: Scene where Modifier: _SceneModi
   public let modifier: Modifier
   public let scene: _AnyScene
 
+  @_spi(TokamakCore)
   public var body: Never {
     neverScene("_SceneModifier_Content")
   }

Sources/TokamakCore/App/_AnyApp.swift

@@ -31,26 +31,32 @@ public struct _AnyApp: App {
     bodyType = A.Body.self
   }
 
+  @_spi(TokamakCore)
   public var body: Never {
     neverScene("_AnyApp")
   }
 
+  @_spi(TokamakCore)
   public init() {
     fatalError("`_AnyApp` cannot be initialized without an underlying `App` type.")
   }
 
+  @_spi(TokamakCore)
   public static func _launch(_ app: Self, _ rootEnvironment: EnvironmentValues) {
     fatalError("`_AnyApp` cannot be launched. Access underlying `app` value.")
   }
 
+  @_spi(TokamakCore)
   public static func _setTitle(_ title: String) {
     fatalError("`title` cannot be set for `AnyApp`. Access underlying `app` value.")
   }
 
+  @_spi(TokamakCore)
   public var _phasePublisher: AnyPublisher<ScenePhase, Never> {
     fatalError("`_AnyApp` cannot monitor scenePhase. Access underlying `app` value.")
   }
 
+  @_spi(TokamakCore)
   public var _colorSchemePublisher: AnyPublisher<ColorScheme, Never> {
     fatalError("`_AnyApp` cannot monitor colorScheme. Access underlying `app` value.")
   }

Sources/TokamakCore/App/_AnyScene.swift

@@ -66,6 +66,7 @@ public struct _AnyScene: Scene {
     }
   }
 
+  @_spi(TokamakCore)
   public var body: Never {
     neverScene("_AnyScene")
   }

Sources/TokamakCore/Modifiers/LifecycleModifier.swift

@@ -14,10 +14,12 @@
 
 // FIXME: these should have standalone implementations
 public extension View {
+  @_spi(TokamakCore)
   func _onMount(perform action: (() -> ())? = nil) -> some View {
     modifier(_AppearanceActionModifier(appear: action))
   }
 
+  @_spi(TokamakCore)
   func _onUnmount(perform action: (() -> ())? = nil) -> some View {
     modifier(_AppearanceActionModifier(disappear: action))
   }

Sources/TokamakCore/Preferences/PreferenceKey.swift

@@ -96,12 +96,6 @@ public protocol _PreferenceWritingModifierProtocol: ViewModifier
   func body(_ content: Self.Content, with preferenceStore: inout _PreferenceStore) -> AnyView
 }
 
-public extension _PreferenceWritingViewProtocol where Self: View {
-  var body: Never {
-    neverBody(String(describing: Self.self))
-  }
-}
-
 public extension _PreferenceWritingModifierProtocol {
   func body(content: Content) -> AnyView {
     content.view

Sources/TokamakCore/Shapes/Shape.swift

@@ -46,7 +46,7 @@ public struct FillStyle: Equatable, ShapeStyle {
   }
 }
 
-public struct _ShapeView<Content, Style>: View where Content: Shape, Style: ShapeStyle {
+public struct _ShapeView<Content, Style>: PrimitiveView where Content: Shape, Style: ShapeStyle {
   @Environment(\.self) public var environment
   @Environment(\.foregroundColor) public var foregroundColor
   public var shape: Content
@@ -58,10 +58,6 @@ public struct _ShapeView<Content, Style>: View where Content: Shape, Style: Shap
     self.style = style
     self.fillStyle = fillStyle
   }
-
-  public var body: Never {
-    neverBody("_ShapeView")
-  }
 }
 
 public extension Shape {

Sources/TokamakCore/State/TargetRef.swift

@@ -33,6 +33,7 @@ public struct _TargetRef<V: View, T>: View, TargetRefType {
 public extension View {
   /** Allows capturing target instance of aclosest descendant host view. The resulting instance
    is written to a given `binding`. */
+  @_spi(TokamakCore)
   func _targetRef<T: Target>(_ binding: Binding<T?>) -> _TargetRef<Self, T> {
     .init(binding: binding, view: self)
   }

Sources/TokamakCore/Styles/ButtonStyle.swift

@@ -15,11 +15,8 @@
 //  Created by Gene Z. Ragan on 07/22/2020.
 
 public struct ButtonStyleConfiguration {
-  public struct Label: View {
+  public struct Label: PrimitiveView {
     let content: AnyView
-    public var body: Never {
-      neverBody("ButtonStyleConfiguration.Label")
-    }
   }
 
   public let label: Label

Sources/TokamakCore/Styles/NavigationLinkStyle.swift

@@ -68,6 +68,7 @@ extension EnvironmentValues {
 }
 
 public extension View {
+  @_spi(TokamakCore)
   func _navigationLinkStyle<S: _NavigationLinkStyle>(_ style: S) -> some View {
     environment(\._navigationLinkStyle, _AnyNavigationLinkStyle(style))
   }

Sources/TokamakCore/Tokens/Color.swift

@@ -241,6 +241,7 @@ public struct Color: Hashable, Equatable {
   }
 
   /// Create a `Color` dependent on the current `ColorScheme`.
+  @_spi(TokamakCore)
   public static func _withScheme(_ resolver: @escaping (ColorScheme) -> Self) -> Self {
     .init(_EnvironmentDependentColorBox {
       resolver($0.colorScheme)
@@ -344,6 +345,7 @@ public extension Color {
 
 extension Color: ShapeStyle {}
 extension Color: View {
+  @_spi(TokamakCore)
   public var body: some View {
     _ShapeView(shape: Rectangle(), style: self)
   }

Sources/TokamakCore/Views/AnyView.swift

@@ -16,7 +16,7 @@
 //
 
 /// A type-erased view.
-public struct AnyView: View {
+public struct AnyView: PrimitiveView {
   /// The type of the underlying `view`.
   let type: Any.Type
 
@@ -67,10 +67,6 @@ public struct AnyView: View {
       }
     }
   }
-
-  public var body: Never {
-    neverBody("AnyView")
-  }
 }
 
 public func mapAnyView<T, V>(_ anyView: AnyView, transform: (V) -> T) -> T? {
@@ -80,6 +76,7 @@ public func mapAnyView<T, V>(_ anyView: AnyView, transform: (V) -> T) -> T? {
 }
 
 extension AnyView: ParentView {
+  @_spi(TokamakCore)
   public var children: [AnyView] {
     (view as? ParentView)?.children ?? []
   }

Sources/TokamakCore/Views/Buttons/Button.swift

@@ -42,12 +42,13 @@ public struct Button<Label>: View where Label: View {
     implementation = _Button(action: action, label: label())
   }
 
+  @_spi(TomkamakCore)
   public var body: some View {
     implementation
   }
 }
 
-public struct _Button<Label>: View where Label: View {
+public struct _Button<Label>: PrimitiveView where Label: View {
   public let label: Label
   public let action: () -> ()
   @State public var isPressed = false
@@ -57,10 +58,6 @@ public struct _Button<Label>: View where Label: View {
     self.label = label
     self.action = action
   }
-
-  public var body: Never {
-    neverBody("_Button")
-  }
 }
 
 public extension Button where Label == Text {
@@ -72,6 +69,7 @@ public extension Button where Label == Text {
 }
 
 extension Button: ParentView {
+  @_spi(TokamakCore)
   public var children: [AnyView] {
     (implementation.label as? GroupView)?.children ?? [AnyView(implementation.label)]
   }

Sources/TokamakCore/Views/Buttons/Link.swift

@@ -17,17 +17,13 @@
 
 import struct Foundation.URL
 
-public struct Link<Label>: View where Label: View {
+public struct Link<Label>: PrimitiveView where Label: View {
   let destination: URL
   let label: Label
 
   public init(destination: URL, @ViewBuilder label: () -> Label) {
     (self.destination, self.label) = (destination, label())
   }
-
-  public var body: Never {
-    neverBody("Link")
-  }
 }
 
 public extension Link where Label == Text {

Sources/TokamakCore/Views/Containers/DisclosureGroup.swift

@@ -15,7 +15,7 @@
 //  Created by Carson Katri on 7/3/20.
 //
 
-public struct DisclosureGroup<Label, Content>: View where Label: View, Content: View {
+public struct DisclosureGroup<Label, Content>: PrimitiveView where Label: View, Content: View {
   @State var isExpanded: Bool = false
   let isExpandedBinding: Binding<Bool>?
 
@@ -39,10 +39,6 @@ public struct DisclosureGroup<Label, Content>: View where Label: View, Content:
     self.label = label()
     self.content = content
   }
-
-  public var body: Never {
-    neverBody("DisclosureGroup")
-  }
 }
 
 public extension DisclosureGroup where Label == Text {

Sources/TokamakCore/Views/Containers/ForEach.swift

@@ -31,7 +31,8 @@ protocol ForEachProtocol: GroupView {
 ///         Text("\($0)")
 ///       }
 ///     }
-public struct ForEach<Data, ID, Content>: View where Data: RandomAccessCollection, ID: Hashable,
+public struct ForEach<Data, ID, Content>: PrimitiveView where Data: RandomAccessCollection,
+  ID: Hashable,
   Content: View
 {
   let data: Data
@@ -47,10 +48,6 @@ public struct ForEach<Data, ID, Content>: View where Data: RandomAccessCollectio
     self.id = id
     self.content = content
   }
-
-  public var body: Never {
-    neverBody("ForEach")
-  }
 }
 
 extension ForEach: ForEachProtocol where Data.Index == Int {
@@ -79,6 +76,7 @@ public extension ForEach where Data == Range<Int>, ID == Int {
 }
 
 extension ForEach: ParentView {
+  @_spi(TokamakCore)
   public var children: [AnyView] {
     data.map { AnyView(IDView(content($0), id: $0[keyPath: id])) }
   }

Sources/TokamakCore/Views/Containers/Group.swift

@@ -19,13 +19,10 @@ public struct Group<Content> {
   }
 }
 
-extension Group: View where Content: View {
-  public var body: Never {
-    neverBody("Group")
-  }
-}
+extension Group: PrimitiveView & View where Content: View {}
 
 extension Group: ParentView where Content: View {
+  @_spi(TokamakCore)
   public var children: [AnyView] { (content as? ParentView)?.children ?? [AnyView(content)] }
 }
 

Sources/TokamakCore/Views/Containers/List.swift

@@ -78,6 +78,7 @@ public struct List<SelectionValue, Content>: View
     }
   }
 
+  @_spi(TokamakCore)
   public var body: some View {
     if let style = style as? ListStyleDeferredToRenderer {
       style.listBody(ScrollView {

Sources/TokamakCore/Views/Containers/OutlineGroup.swift

@@ -86,6 +86,7 @@ public extension OutlineGroup where Parent: View,
 }
 
 extension OutlineGroup: View where Parent: View, Leaf: View, Subgroup: View {
+  @_spi(TokamakCore)
   public var body: some View {
     switch root {
     case let .collection(data):
@@ -124,6 +125,7 @@ extension OutlineGroup: View where Parent: View, Leaf: View, Subgroup: View {
 public struct OutlineSubgroupChildren: View {
   let children: () -> AnyView
 
+  @_spi(TokamakCore)
   public var body: some View {
     children()
   }

Sources/TokamakCore/Views/Containers/Section.swift

@@ -33,6 +33,7 @@ extension Section: View, SectionView where Parent: View, Content: View, Footer:
   }
 
   @ViewBuilder
+  @_spi(TokamakCore)
   public var body: TupleView<(Parent, Content, Footer)> {
     header
     content

Sources/TokamakCore/Views/Containers/TupleView.swift

@@ -18,11 +18,7 @@
 /// A `View` created from a `Tuple` of `View` values.
 ///
 /// Mainly for use with `@ViewBuilder`.
-public struct TupleView<T>: View {
-  public var body: Never {
-    neverBody("TupleView")
-  }
-
+public struct TupleView<T>: PrimitiveView {
   public let value: T
 
   let _children: [AnyView]

Sources/TokamakCore/Views/Image.swift

@@ -17,7 +17,7 @@
 
 import Foundation
 
-public struct Image: View {
+public struct Image: PrimitiveView {
   let label: Text?
   let name: String
   let bundle: Bundle?
@@ -39,10 +39,6 @@ public struct Image: View {
     self.name = name
     self.bundle = bundle
   }
-
-  public var body: Never {
-    neverBody("Image")
-  }
 }
 
 /// This is a helper type that works around absence of "package private" access control in Swift

Sources/TokamakCore/Views/Layout/GeometryReader.swift

@@ -40,13 +40,9 @@ public func makeProxy(from size: CGSize) -> GeometryProxy {
 //   public subscript<T>(anchor: Anchor<T>) -> T {}
 // }
 
-public struct GeometryReader<Content>: View where Content: View {
+public struct GeometryReader<Content>: PrimitiveView where Content: View {
   public let content: (GeometryProxy) -> Content
   public init(@ViewBuilder content: @escaping (GeometryProxy) -> Content) {
     self.content = content
   }
-
-  public var body: Never {
-    neverBody("GeometryReader")
-  }
 }