Get rid of GSL.

32 files changed, 165 insertions, 182 deletions

diff --git a/.gitmodules b/.gitmodules
index 056b9ef1..c87b5454 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,9 +1,6 @@
 [submodule "lib/double-conversion"]
 	path = lib/double-conversion
 	url = https://github.com/google/double-conversion.git
-[submodule "lib/GSL"]
-	path = lib/GSL
-	url = https://github.com/microsoft/GSL.git
 [submodule "lib/Catch2"]
 	path = lib/Catch2
 	url = https://github.com/catchorg/Catch2.git
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 75da1485..9a5b497d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -32,7 +32,6 @@ set(BUILD_SHARED_LIBS OFF)
 add_subdirectory(lib/Catch2)
 set(BUILD_SHARED_LIBS ${OLD_BUILD_SHARED_LIBS})
 add_subdirectory(lib/double-conversion)
-add_subdirectory(lib/GSL)
 
 enable_testing()
 
diff --git a/demos/InputMethod/main.cpp b/demos/InputMethod/main.cpp
index fc63dd5e..cf42d3c2 100644
--- a/demos/InputMethod/main.cpp
+++ b/demos/InputMethod/main.cpp
@@ -146,7 +146,7 @@ int main() {
     text_layout->SetText(committed_text + composition_text.text);
 
     const auto cursor_pos = composition_text.selection.position +
-                            gsl::narrow_cast<int>(committed_text.size());
+                            static_cast<int>(committed_text.size());
     state->cursor_rect = text_layout->TextSinglePoint(cursor_pos, false);
 
     input_method_context->SetCandidateWindowPosition(
diff --git a/include/cru/common/Base.h b/include/cru/common/Base.h
index 34faf779..8a6a7634 100644
--- a/include/cru/common/Base.h
+++ b/include/cru/common/Base.h
@@ -1,5 +1,10 @@
 #pragma once
-#include "PreConfig.h" // IWYU pragma: keep 
+#include "PreConfig.h"  // IWYU pragma: keep
+
+#include <cassert>
+#include <cstddef>
+#include <functional>
+#include <memory>
 
 #ifdef CRU_PLATFORM_WINDOWS
 #ifdef CRU_BASE_EXPORT_API
@@ -11,8 +16,6 @@
 #define CRU_BASE_API
 #endif
 
-#include <gsl/gsl>
-
 #define CRU_UNUSED(entity) static_cast<void>(entity);
 
 #define CRU__CONCAT(a, b) a##b
@@ -83,7 +86,18 @@ struct CRU_BASE_API Interface {
 
 [[noreturn]] void CRU_BASE_API UnreachableCode();
 
-using Index = gsl::index;
+using Index = std::ptrdiff_t;
+
+inline void Expects(bool condition) { assert(condition); }
+inline void Ensures(bool condition) { assert(condition); }
+template <typename T>
+inline void Expects(const std::shared_ptr<T>& ptr) {
+  assert(ptr != nullptr);
+}
+template <typename T>
+inline void Ensures(const std::shared_ptr<T>& ptr) {
+  assert(ptr != nullptr);
+}
 
 // https://www.boost.org/doc/libs/1_54_0/doc/html/hash/reference.html#boost.hash_combine
 template <class T>
diff --git a/include/cru/common/Event.h b/include/cru/common/Event.h
index 14d68833..18d2c570 100644
--- a/include/cru/common/Event.h
+++ b/include/cru/common/Event.h
@@ -4,6 +4,7 @@
 #include "SelfResolvable.h"
 
 #include <algorithm>
+#include <cassert>
 #include <functional>
 #include <memory>
 #include <utility>
@@ -235,7 +236,7 @@ class EventRevokerGuard {
   EventRevoker Get() {
     // revoker is only null when this is moved
     // you shouldn't use a moved instance
-    Expects(revoker_);
+    assert(revoker_);
     return *revoker_;
   }
 
diff --git a/include/cru/platform/graphics/Base.h b/include/cru/platform/graphics/Base.h
index 3bb9b7b4..3f3ae0b4 100644
--- a/include/cru/platform/graphics/Base.h
+++ b/include/cru/platform/graphics/Base.h
@@ -28,7 +28,7 @@ struct IPainter;
 struct ITextLayout;
 
 struct TextHitTestResult {
-  gsl::index position;
+  Index position;
   bool trailing;
   bool inside_text;
 };
diff --git a/include/cru/platform/graphics/direct2d/WindowRenderTarget.h b/include/cru/platform/graphics/direct2d/WindowRenderTarget.h
index c0251880..f77ce471 100644
--- a/include/cru/platform/graphics/direct2d/WindowRenderTarget.h
+++ b/include/cru/platform/graphics/direct2d/WindowRenderTarget.h
@@ -5,8 +5,7 @@ namespace cru::platform::graphics::direct2d {
 // Represents a window render target.
 class CRU_WIN_GRAPHICS_DIRECT_API D2DWindowRenderTarget : public Object {
  public:
-  D2DWindowRenderTarget(gsl::not_null<DirectGraphicsFactory*> factory,
-                        HWND hwnd);
+  D2DWindowRenderTarget(DirectGraphicsFactory* factory, HWND hwnd);
 
   CRU_DELETE_COPY(D2DWindowRenderTarget)
   CRU_DELETE_MOVE(D2DWindowRenderTarget)
diff --git a/include/cru/platform/gui/win/InputMethod.h b/include/cru/platform/gui/win/InputMethod.h
index 8e28f857..3a9faeaa 100644
--- a/include/cru/platform/gui/win/InputMethod.h
+++ b/include/cru/platform/gui/win/InputMethod.h
@@ -39,7 +39,7 @@ class CRU_WIN_GUI_API WinInputMethodContext
   CRU_DEFINE_CLASS_LOG_TAG(u"WinInputMethodContext")
 
  public:
-  WinInputMethodContext(gsl::not_null<WinNativeWindow*> window);
+  WinInputMethodContext(WinNativeWindow* window);
 
   CRU_DELETE_COPY(WinInputMethodContext)
   CRU_DELETE_MOVE(WinInputMethodContext)
diff --git a/include/cru/ui/components/Menu.h b/include/cru/ui/components/Menu.h
index f4e54c9f..82766aa5 100644
--- a/include/cru/ui/components/Menu.h
+++ b/include/cru/ui/components/Menu.h
@@ -48,13 +48,11 @@ class CRU_UI_API Menu : public Component {
  public:
   controls::Control* GetRootControl() override { return &container_; }
 
-  gsl::index GetItemCount() const {
-    return static_cast<gsl::index>(items_.size());
-  }
+  Index GetItemCount() const { return static_cast<Index>(items_.size()); }
 
   void AddItem(Component* component) { AddItemAt(component, GetItemCount()); }
-  void AddItemAt(Component* component, gsl::index index);
-  Component* RemoveItemAt(gsl::index index);
+  void AddItemAt(Component* component, Index index);
+  Component* RemoveItemAt(Index index);
   void ClearItems();
 
   void AddTextItem(String text, std::function<void()> on_click) {
diff --git a/include/cru/ui/controls/TextBlock.h b/include/cru/ui/controls/TextBlock.h
index 12c9d2f9..ad44ad2d 100644
--- a/include/cru/ui/controls/TextBlock.h
+++ b/include/cru/ui/controls/TextBlock.h
@@ -47,7 +47,7 @@ class CRU_UI_API TextBlock : public NoChildControl,
   }
   void SetTextColor(const Color& color);
 
-  gsl::not_null<render::TextRenderObject*> GetTextRenderObject() override;
+  render::TextRenderObject* GetTextRenderObject() override;
   render::ScrollRenderObject* GetScrollRenderObject() override {
     return nullptr;
   }
diff --git a/include/cru/ui/controls/TextBox.h b/include/cru/ui/controls/TextBox.h
index c3d8fc4d..9388d3fd 100644
--- a/include/cru/ui/controls/TextBox.h
+++ b/include/cru/ui/controls/TextBox.h
@@ -29,7 +29,7 @@ class CRU_UI_API TextBox : public NoChildControl,
 
   render::RenderObject* GetRenderObject() const override;
 
-  gsl::not_null<render::TextRenderObject*> GetTextRenderObject() override;
+  render::TextRenderObject* GetTextRenderObject() override;
   render::ScrollRenderObject* GetScrollRenderObject() override;
 
   bool GetMultiLine() const;
diff --git a/include/cru/ui/controls/TextHostControlService.h b/include/cru/ui/controls/TextHostControlService.h
index ae86e1e9..7efd7760 100644
--- a/include/cru/ui/controls/TextHostControlService.h
+++ b/include/cru/ui/controls/TextHostControlService.h
@@ -23,7 +23,7 @@ namespace cru::ui::controls {
 constexpr int k_default_caret_blink_duration = 500;
 
 struct CRU_UI_API ITextHostControl : virtual Interface {
-  virtual gsl::not_null<render::TextRenderObject*> GetTextRenderObject() = 0;
+  virtual render::TextRenderObject* GetTextRenderObject() = 0;
   // May return nullptr.
   virtual render::ScrollRenderObject* GetScrollRenderObject() = 0;
 };
@@ -48,8 +48,8 @@ class TextControlMovePattern : public Object {
   static std::vector<TextControlMovePattern> kDefaultPatterns;
 
   using MoveFunction =
-      std::function<gsl::index(TextHostControlService* service, StringView text,
-                               gsl::index current_position)>;
+      std::function<Index(TextHostControlService* service, StringView text,
+                          Index current_position)>;
 
   TextControlMovePattern(String name, helper::ShortcutKeyBind key_bind,
                          MoveFunction move_function)
@@ -65,8 +65,8 @@ class TextControlMovePattern : public Object {
  public:
   String GetName() const { return name_; }
   helper::ShortcutKeyBind GetKeyBind() const { return key_bind_; }
-  gsl::index Move(TextHostControlService* service, StringView text,
-                  gsl::index current_position) const {
+  Index Move(TextHostControlService* service, StringView text,
+             Index current_position) const {
     return move_function_(service, text, current_position);
   }
 
@@ -80,7 +80,7 @@ class CRU_UI_API TextHostControlService : public Object {
   CRU_DEFINE_CLASS_LOG_TAG(u"TextControlService")
 
  public:
-  TextHostControlService(gsl::not_null<Control*> control);
+  TextHostControlService(Control* control);
 
   CRU_DELETE_COPY(TextHostControlService)
   CRU_DELETE_MOVE(TextHostControlService)
@@ -105,13 +105,12 @@ class CRU_UI_API TextHostControlService : public Object {
   StringView GetTextView() { return this->text_; }
   void SetText(String text, bool stop_composition = false);
 
-  void InsertText(gsl::index position, StringView text,
+  void InsertText(Index position, StringView text,
                   bool stop_composition = false);
-  void DeleteChar(gsl::index position, bool stop_composition = false);
+  void DeleteChar(Index position, bool stop_composition = false);
 
   // Return the position of deleted character.
-  gsl::index DeleteCharPrevious(gsl::index position,
-                                bool stop_composition = false);
+  Index DeleteCharPrevious(Index position, bool stop_composition = false);
   void DeleteText(TextRange range, bool stop_composition = false);
 
   void CancelComposition();
@@ -124,17 +123,17 @@ class CRU_UI_API TextHostControlService : public Object {
   int GetCaretBlinkDuration() { return caret_blink_duration_; }
   void SetCaretBlinkDuration(int milliseconds);
 
-  gsl::index GetCaretPosition() { return selection_.GetEnd(); }
+  Index GetCaretPosition() { return selection_.GetEnd(); }
   TextRange GetSelection() { return selection_; }
 
   StringView GetSelectedText();
 
-  void SetSelection(gsl::index caret_position);
+  void SetSelection(Index caret_position);
   void SetSelection(TextRange selection, bool scroll_to_caret = true);
 
   void SelectAll();
 
-  void ChangeSelectionEnd(gsl::index new_end);
+  void ChangeSelectionEnd(Index new_end);
   void AbortSelection();
 
   void DeleteSelectedText();
@@ -150,7 +149,7 @@ class CRU_UI_API TextHostControlService : public Object {
 
   IEvent<std::nullptr_t>* TextChangeEvent() { return &text_change_event_; }
 
-  gsl::not_null<render::TextRenderObject*> GetTextRenderObject();
+  render::TextRenderObject* GetTextRenderObject();
   render::ScrollRenderObject* GetScrollRenderObject();
 
  private:
@@ -192,8 +191,8 @@ class CRU_UI_API TextHostControlService : public Object {
   void OpenContextMenu(const Point& position, ContextMenuItem items);
 
  private:
-  gsl::not_null<Control*> control_;
-  gsl::not_null<ITextHostControl*> text_host_control_;
+  Control* control_;
+  ITextHostControl* text_host_control_;
 
   Event<std::nullptr_t> text_change_event_;
 
diff --git a/include/cru/ui/render/ScrollBar.h b/include/cru/ui/render/ScrollBar.h
index c65c0345..20b6e6cf 100644
--- a/include/cru/ui/render/ScrollBar.h
+++ b/include/cru/ui/render/ScrollBar.h
@@ -12,7 +12,6 @@
 #include "cru/ui/controls/Control.h"
 #include "cru/ui/helper/ClickDetector.h"
 
-#include <gsl/pointers>
 #include <memory>
 #include <optional>
 #include <unordered_map>
@@ -45,8 +44,7 @@ String CRU_UI_API GenerateScrollBarThemeColorKey(ScrollBarBrushUsageKind usage,
 
 class CRU_UI_API ScrollBar : public Object {
  public:
-  ScrollBar(gsl::not_null<ScrollRenderObject*> render_object,
-            Direction direction);
+  ScrollBar(ScrollRenderObject* render_object, Direction direction);
 
   CRU_DELETE_COPY(ScrollBar)
   CRU_DELETE_MOVE(ScrollBar)
@@ -69,12 +67,11 @@ class CRU_UI_API ScrollBar : public Object {
   void InstallHandlers(controls::Control* control);
   void UninstallHandlers() { InstallHandlers(nullptr); }
 
-  gsl::not_null<std::shared_ptr<platform::graphics::IBrush>>
-  GetCollapsedThumbBrush();
+  std::shared_ptr<platform::graphics::IBrush> GetCollapsedThumbBrush();
   // Brush could be nullptr to use the theme brush.
   void SetCollapsedThumbBrush(
       std::shared_ptr<platform::graphics::IBrush> brush);
-  gsl::not_null<std::shared_ptr<platform::graphics::IBrush>> GetBrush(
+  std::shared_ptr<platform::graphics::IBrush> GetBrush(
       ScrollBarBrushUsageKind usage, ScrollBarBrushStateKind state);
   // Brush could be nullptr to use the theme brush.
   void SetBrush(ScrollBarBrushUsageKind usage, ScrollBarBrushStateKind state,
@@ -83,14 +80,14 @@ class CRU_UI_API ScrollBar : public Object {
  protected:
   void OnDraw(platform::graphics::IPainter* painter, bool expand);
 
-  virtual void DrawUpArrow(
-      platform::graphics::IPainter* painter, const Rect& area,
-      gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-      gsl::not_null<platform::graphics::IBrush*> background_brush) = 0;
-  virtual void DrawDownArrow(
-      platform::graphics::IPainter* painter, const Rect& area,
-      gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-      gsl::not_null<platform::graphics::IBrush*> background_brush) = 0;
+  virtual void DrawUpArrow(platform::graphics::IPainter* painter,
+                           const Rect& area,
+                           platform::graphics::IBrush* arrow_brush,
+                           platform::graphics::IBrush* background_brush) = 0;
+  virtual void DrawDownArrow(platform::graphics::IPainter* painter,
+                             const Rect& area,
+                             platform::graphics::IBrush* arrow_brush,
+                             platform::graphics::IBrush* background_brush) = 0;
 
   std::optional<ScrollBarAreaKind> ExpandedHitTest(const Point& point);
 
@@ -119,7 +116,7 @@ class CRU_UI_API ScrollBar : public Object {
   ScrollBarBrushStateKind GetState(ScrollBarAreaKind area);
 
  protected:
-  gsl::not_null<ScrollRenderObject*> render_object_;
+  ScrollRenderObject* render_object_;
 
   std::unique_ptr<platform::graphics::IGeometry> arrow_geometry_;
 
@@ -154,8 +151,7 @@ class CRU_UI_API ScrollBar : public Object {
 
 class CRU_UI_API HorizontalScrollBar : public ScrollBar {
  public:
-  explicit HorizontalScrollBar(
-      gsl::not_null<ScrollRenderObject*> render_object);
+  explicit HorizontalScrollBar(ScrollRenderObject* render_object);
 
   CRU_DELETE_COPY(HorizontalScrollBar)
   CRU_DELETE_MOVE(HorizontalScrollBar)
@@ -163,14 +159,12 @@ class CRU_UI_API HorizontalScrollBar : public ScrollBar {
   ~HorizontalScrollBar() override = default;
 
  protected:
-  void DrawUpArrow(
-      platform::graphics::IPainter* painter, const Rect& area,
-      gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-      gsl::not_null<platform::graphics::IBrush*> background_brush) override;
-  void DrawDownArrow(
-      platform::graphics::IPainter* painter, const Rect& area,
-      gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-      gsl::not_null<platform::graphics::IBrush*> background_brush) override;
+  void DrawUpArrow(platform::graphics::IPainter* painter, const Rect& area,
+                   platform::graphics::IBrush* arrow_brush,
+                   platform::graphics::IBrush* background_brush) override;
+  void DrawDownArrow(platform::graphics::IPainter* painter, const Rect& area,
+                     platform::graphics::IBrush* arrow_brush,
+                     platform::graphics::IBrush* background_brush) override;
 
   bool IsShowBar() override;
 
@@ -187,7 +181,7 @@ class CRU_UI_API HorizontalScrollBar : public ScrollBar {
 
 class CRU_UI_API VerticalScrollBar : public ScrollBar {
  public:
-  explicit VerticalScrollBar(gsl::not_null<ScrollRenderObject*> render_object);
+  explicit VerticalScrollBar(ScrollRenderObject* render_object);
 
   CRU_DELETE_COPY(VerticalScrollBar)
   CRU_DELETE_MOVE(VerticalScrollBar)
@@ -195,14 +189,12 @@ class CRU_UI_API VerticalScrollBar : public ScrollBar {
   ~VerticalScrollBar() override = default;
 
  protected:
-  void DrawUpArrow(
-      platform::graphics::IPainter* painter, const Rect& area,
-      gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-      gsl::not_null<platform::graphics::IBrush*> background_brush) override;
-  void DrawDownArrow(
-      platform::graphics::IPainter* painter, const Rect& area,
-      gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-      gsl::not_null<platform::graphics::IBrush*> background_brush) override;
+  void DrawUpArrow(platform::graphics::IPainter* painter, const Rect& area,
+                   platform::graphics::IBrush* arrow_brush,
+                   platform::graphics::IBrush* background_brush) override;
+  void DrawDownArrow(platform::graphics::IPainter* painter, const Rect& area,
+                     platform::graphics::IBrush* arrow_brush,
+                     platform::graphics::IBrush* background_brush) override;
 
   bool IsShowBar() override;
 
@@ -220,7 +212,7 @@ class CRU_UI_API VerticalScrollBar : public ScrollBar {
 // A delegate to draw scrollbar and register related events.
 class CRU_UI_API ScrollBarDelegate : public Object {
  public:
-  explicit ScrollBarDelegate(gsl::not_null<ScrollRenderObject*> render_object);
+  explicit ScrollBarDelegate(ScrollRenderObject* render_object);
 
   CRU_DELETE_COPY(ScrollBarDelegate)
   CRU_DELETE_MOVE(ScrollBarDelegate)
@@ -244,7 +236,7 @@ class CRU_UI_API ScrollBarDelegate : public Object {
   void UninstallHandlers() { InstallHandlers(nullptr); }
 
  private:
-  gsl::not_null<ScrollRenderObject*> render_object_;
+  ScrollRenderObject* render_object_;
 
   HorizontalScrollBar horizontal_bar_;
   VerticalScrollBar vertical_bar_;
diff --git a/include/cru/ui/render/TextRenderObject.h b/include/cru/ui/render/TextRenderObject.h
index f12ac72f..0cb3623d 100644
--- a/include/cru/ui/render/TextRenderObject.h
+++ b/include/cru/ui/render/TextRenderObject.h
@@ -51,7 +51,7 @@ class CRU_UI_API TextRenderObject : public RenderObject {
   Index GetLineIndexFromCharIndex(Index char_index);
   float GetLineHeight(Index line_index);
   std::vector<Rect> TextRangeRect(const TextRange& text_range);
-  Rect TextSinglePoint(gsl::index position, bool trailing);
+  Rect TextSinglePoint(Index position, bool trailing);
   platform::graphics::TextHitTestResult TextHitTest(const Point& point);
 
   std::optional<TextRange> GetSelectionRange() const {
@@ -70,8 +70,8 @@ class CRU_UI_API TextRenderObject : public RenderObject {
 
   // Caret position can be any value. When it is negative, 0 is used. When it
   // exceeds the size of the string, the size of the string is used.
-  gsl::index GetCaretPosition() const { return caret_position_; }
-  void SetCaretPosition(gsl::index position);
+  Index GetCaretPosition() const { return caret_position_; }
+  void SetCaretPosition(Index position);
 
   // Lefttop relative to content lefttop.
   Rect GetCaretRectInContent();
@@ -115,7 +115,7 @@ class CRU_UI_API TextRenderObject : public RenderObject {
   std::shared_ptr<platform::graphics::IBrush> selection_brush_;
 
   bool draw_caret_ = false;
-  gsl::index caret_position_ = 0;
+  Index caret_position_ = 0;
   std::shared_ptr<platform::graphics::IBrush> caret_brush_;
   float caret_width_ = default_caret_width;
 
diff --git a/include/cru/ui/style/StyleRuleSet.h b/include/cru/ui/style/StyleRuleSet.h
index 8f3495bc..080c2eb8 100644
--- a/include/cru/ui/style/StyleRuleSet.h
+++ b/include/cru/ui/style/StyleRuleSet.h
@@ -27,16 +27,16 @@ class CRU_UI_API StyleRuleSet : public Object, public model::IListChangeNotify {
   std::shared_ptr<StyleRuleSet> GetParent() const { return parent_; }
   void SetParent(std::shared_ptr<StyleRuleSet> parent);
 
-  gsl::index GetSize() const { return static_cast<gsl::index>(rules_.size()); }
+  Index GetSize() const { return static_cast<Index>(rules_.size()); }
   const std::vector<StyleRule>& GetRules() const { return rules_; }
 
   void AddStyleRule(StyleRule rule) {
     AddStyleRule(std::move(rule), GetSize());
   }
 
-  void AddStyleRule(StyleRule rule, gsl::index index);
+  void AddStyleRule(StyleRule rule, Index index);
 
-  void RemoveStyleRule(gsl::index index, gsl::index count = 1);
+  void RemoveStyleRule(Index index, Index count = 1);
 
   const StyleRule& GetStyleRule(Index index) const { return rules_[index]; }
   void SetStyleRule(Index index, StyleRule rule);
@@ -45,7 +45,7 @@ class CRU_UI_API StyleRuleSet : public Object, public model::IListChangeNotify {
 
   void Set(const StyleRuleSet& other, bool set_parent = false);
 
-  const StyleRule& operator[](gsl::index index) const { return rules_[index]; }
+  const StyleRule& operator[](Index index) const { return rules_[index]; }
 
   // Triggered whenever a change happened to this (rule add or remove, parent
   // change ...). Subscribe to this and update style change listeners and style.
diff --git a/lib/GSL b/lib/GSL
deleted file mode 160000
-Subproject f1a494cfd2ce55fe88b5134eab985f5852667b8
diff --git a/src/common/CMakeLists.txt b/src/common/CMakeLists.txt
index 45688deb..2414831b 100644
--- a/src/common/CMakeLists.txt
+++ b/src/common/CMakeLists.txt
@@ -73,4 +73,4 @@ else()
 	target_compile_definitions(CruBase PUBLIC CRU_PLATFORM_LINUX)
 endif()
 
-target_link_libraries(CruBase PUBLIC GSL double-conversion)
+target_link_libraries(CruBase PUBLIC double-conversion)
diff --git a/src/common/String.cpp b/src/common/String.cpp
index cac807b0..0d1c38b8 100644
--- a/src/common/String.cpp
+++ b/src/common/String.cpp
@@ -6,7 +6,6 @@
 
 #include <double-conversion/double-conversion.h>
 #include <double-conversion/string-to-double.h>
-#include <gsl/gsl>
 
 #include <algorithm>
 #include <cmath>
diff --git a/src/platform/Color.cpp b/src/platform/Color.cpp
index a35ef535..bbb78628 100644
--- a/src/platform/Color.cpp
+++ b/src/platform/Color.cpp
@@ -1,7 +1,5 @@
 #include "cru/platform/Color.h"
 
-#include <gsl/gsl>
-
 #include <cmath>
 #include <cstdint>
 #include <optional>
@@ -61,9 +59,8 @@ std::optional<Color> Color::Parse(StringView string,
     if (!g) return std::nullopt;
     auto b = get_num_for_two_digit(string.substr(5, 2));
     if (!b) return std::nullopt;
-    return Color(gsl::narrow_cast<std::uint8_t>(*r),
-                 gsl::narrow_cast<std::uint8_t>(*g),
-                 gsl::narrow_cast<std::uint8_t>(*b));
+    return Color(static_cast<std::uint8_t>(*r), static_cast<std::uint8_t>(*g),
+                 static_cast<std::uint8_t>(*b));
   } else if (string_size == 9) {
     if (string[0] != u'#') return std::nullopt;
     auto r = get_num_for_two_digit(string.substr(1, 2));
@@ -74,9 +71,8 @@ std::optional<Color> Color::Parse(StringView string,
     if (!b) return std::nullopt;
     auto a = get_num_for_two_digit(string.substr(7, 2));
     if (!a) return std::nullopt;
-    return Color(
-        gsl::narrow_cast<std::uint8_t>(*r), gsl::narrow_cast<std::uint8_t>(*g),
-        gsl::narrow_cast<std::uint8_t>(*b), gsl::narrow_cast<std::uint8_t>(*a));
+    return Color(static_cast<std::uint8_t>(*r), static_cast<std::uint8_t>(*g),
+                 static_cast<std::uint8_t>(*b), static_cast<std::uint8_t>(*a));
   } else {
     return std::nullopt;
   }
diff --git a/src/platform/graphics/direct2d/WindowRenderTarget.cpp b/src/platform/graphics/direct2d/WindowRenderTarget.cpp
index 0227f084..a8d988f4 100644
--- a/src/platform/graphics/direct2d/WindowRenderTarget.cpp
+++ b/src/platform/graphics/direct2d/WindowRenderTarget.cpp
@@ -4,8 +4,8 @@
 #include "cru/platform/graphics/direct2d/Factory.h"
 
 namespace cru::platform::graphics::direct2d {
-D2DWindowRenderTarget::D2DWindowRenderTarget(
-    gsl::not_null<DirectGraphicsFactory*> factory, HWND hwnd)
+D2DWindowRenderTarget::D2DWindowRenderTarget(DirectGraphicsFactory* factory,
+                                             HWND hwnd)
     : factory_(factory), hwnd_(hwnd) {
   const auto d3d11_device = factory->GetD3D11Device();
   const auto dxgi_factory = factory->GetDxgiFactory();
diff --git a/src/platform/gui/osx/Window.mm b/src/platform/gui/osx/Window.mm
index 8773678d..d6bee564 100644
--- a/src/platform/gui/osx/Window.mm
+++ b/src/platform/gui/osx/Window.mm
@@ -471,7 +471,7 @@ cru::platform::gui::KeyModifier GetKeyModifier(NSEvent* event) {
 - (void)drawRect:(NSRect)dirtyRect {
   auto cg_context = [[NSGraphicsContext currentContext] CGContext];
   auto layer = _p->GetDrawLayer();
-  Ensures(layer);
+  cru::Ensures(layer);
   CGContextDrawLayerAtPoint(cg_context, CGPointMake(0, 0), layer);
 }
 
diff --git a/src/platform/gui/win/InputMethod.cpp b/src/platform/gui/win/InputMethod.cpp
index d9c179ad..4c5b3b8c 100644
--- a/src/platform/gui/win/InputMethod.cpp
+++ b/src/platform/gui/win/InputMethod.cpp
@@ -144,8 +144,7 @@ CompositionText GetCompositionInfo(HIMC imm_context) {
 
 }  // namespace
 
-WinInputMethodContext::WinInputMethodContext(
-    gsl::not_null<WinNativeWindow*> window)
+WinInputMethodContext::WinInputMethodContext(WinNativeWindow* window)
     : native_window_(window) {
   event_guard_ += window->NativeMessageEvent()->AddHandler(
       std::bind(&WinInputMethodContext::OnWindowNativeMessage, this,
diff --git a/src/platform/gui/win/TimerManager.cpp b/src/platform/gui/win/TimerManager.cpp
index 1bc32b51..c6128b85 100644
--- a/src/platform/gui/win/TimerManager.cpp
+++ b/src/platform/gui/win/TimerManager.cpp
@@ -22,7 +22,7 @@ long long TimerManager::SetTimer(TimerType type, int period,
                        std::move(action)};
   if (type == TimerType::Immediate) {
     if (!::PostMessageW(god_window_->GetHandle(), kSetImmediateWindowMessageId,
-                        gsl::narrow<UINT_PTR>(id), 0)) {
+                        static_cast<UINT_PTR>(id), 0)) {
       throw Win32Error(
           ::GetLastError(),
           u"Failed to post window message to god window for set immediate.");
@@ -46,7 +46,7 @@ void TimerManager::CancelTimer(long long id) {
 }
 
 void TimerManager::CreateNativeTimer(TimerInfo* info) {
-  info->native_timer_id = gsl::narrow<UINT_PTR>(info->id);
+  info->native_timer_id = static_cast<UINT_PTR>(info->id);
   ::SetTimer(god_window_->GetHandle(), info->native_timer_id, info->period,
              nullptr);
 }
diff --git a/src/platform/gui/win/UiApplication.cpp b/src/platform/gui/win/UiApplication.cpp
index 90eb20ca..94d6b9c5 100644
--- a/src/platform/gui/win/UiApplication.cpp
+++ b/src/platform/gui/win/UiApplication.cpp
@@ -71,14 +71,14 @@ long long WinUiApplication::SetImmediate(std::function<void()> action) {
 long long WinUiApplication::SetTimeout(std::chrono::milliseconds milliseconds,
                                        std::function<void()> action) {
   return this->timer_manager_->SetTimer(TimerType::Timeout,
-                                        gsl::narrow<int>(milliseconds.count()),
+                                        static_cast<int>(milliseconds.count()),
                                         std::move(action));
 }
 
 long long WinUiApplication::SetInterval(std::chrono::milliseconds milliseconds,
                                         std::function<void()> action) {
   return this->timer_manager_->SetTimer(TimerType::Interval,
-                                        gsl::narrow<int>(milliseconds.count()),
+                                        static_cast<int>(milliseconds.count()),
                                         std::move(action));
 }
 
diff --git a/src/ui/components/Menu.cpp b/src/ui/components/Menu.cpp
index 005147a1..2da38990 100644
--- a/src/ui/components/Menu.cpp
+++ b/src/ui/components/Menu.cpp
@@ -37,14 +37,14 @@ Menu::~Menu() {
   }
 }
 
-void Menu::AddItemAt(Component* item, gsl::index index) {
+void Menu::AddItemAt(Component* item, Index index) {
   Expects(index >= 0 && index <= GetItemCount());
 
   items_.insert(items_.cbegin() + index, item);
   container_.AddChildAt(item->GetRootControl(), index);
 }
 
-Component* Menu::RemoveItemAt(gsl::index index) {
+Component* Menu::RemoveItemAt(Index index) {
   Expects(index >= 0 && index < GetItemCount());
 
   Component* item = items_[index];
@@ -65,7 +65,7 @@ void Menu::ClearItems() {
   container_.ClearChildren();
 }
 
-void Menu::AddTextItemAt(String text, gsl::index index,
+void Menu::AddTextItemAt(String text, Index index,
                          std::function<void()> on_click) {
   MenuItem* item = new MenuItem(std::move(text));
   item->SetOnClick([this, index, on_click = std::move(on_click)] {
diff --git a/src/ui/controls/TextBlock.cpp b/src/ui/controls/TextBlock.cpp
index 79c66865..0dad4f5d 100644
--- a/src/ui/controls/TextBlock.cpp
+++ b/src/ui/controls/TextBlock.cpp
@@ -47,7 +47,7 @@ void TextBlock::SetTextColor(const Color& color) {
       GetUiApplication()->GetGraphicsFactory()->CreateSolidColorBrush(color));
 }
 
-gsl::not_null<render::TextRenderObject*> TextBlock::GetTextRenderObject() {
+render::TextRenderObject* TextBlock::GetTextRenderObject() {
   return text_render_object_.get();
 }
 }  // namespace cru::ui::controls
diff --git a/src/ui/controls/TextBox.cpp b/src/ui/controls/TextBox.cpp
index 66b6bd43..87672b4f 100644
--- a/src/ui/controls/TextBox.cpp
+++ b/src/ui/controls/TextBox.cpp
@@ -53,7 +53,7 @@ bool TextBox::GetMultiLine() const { return service_->IsMultiLine(); }
 
 void TextBox::SetMultiLine(bool value) { service_->SetMultiLine(value); }
 
-gsl::not_null<render::TextRenderObject*> TextBox::GetTextRenderObject() {
+render::TextRenderObject* TextBox::GetTextRenderObject() {
   return text_render_object_.get();
 }
 
diff --git a/src/ui/controls/TextHostControlService.cpp b/src/ui/controls/TextHostControlService.cpp
index d6c40a36..36703986 100644
--- a/src/ui/controls/TextHostControlService.cpp
+++ b/src/ui/controls/TextHostControlService.cpp
@@ -29,7 +29,7 @@ namespace cru::ui::controls {
 TextControlMovePattern TextControlMovePattern::kLeft(
     u"Left", helper::ShortcutKeyBind(platform::gui::KeyCode::Left),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       Utf16PreviousCodePoint(text, current_position, &current_position);
       return current_position;
@@ -37,7 +37,7 @@ TextControlMovePattern TextControlMovePattern::kLeft(
 TextControlMovePattern TextControlMovePattern::kRight(
     u"Right", helper::ShortcutKeyBind(platform::gui::KeyCode::Right),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       Utf16NextCodePoint(text, current_position, &current_position);
       return current_position;
@@ -47,7 +47,7 @@ TextControlMovePattern TextControlMovePattern::kCtrlLeft(
     helper::ShortcutKeyBind(platform::gui::KeyCode::Left,
                             platform::gui::KeyModifiers::ctrl),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       return Utf16PreviousWord(text, current_position);
     });
@@ -56,14 +56,14 @@ TextControlMovePattern TextControlMovePattern::kCtrlRight(
     helper::ShortcutKeyBind(platform::gui::KeyCode::Right,
                             platform::gui::KeyModifiers::ctrl),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       return Utf16NextWord(text, current_position);
     });
 TextControlMovePattern TextControlMovePattern::kUp(
     u"Up", helper::ShortcutKeyBind(platform::gui::KeyCode::Up),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(text)
       auto text_render_object = service->GetTextRenderObject();
       auto rect = text_render_object->TextSinglePoint(current_position, false);
@@ -74,7 +74,7 @@ TextControlMovePattern TextControlMovePattern::kUp(
 TextControlMovePattern TextControlMovePattern::kDown(
     u"Down", helper::ShortcutKeyBind(platform::gui::KeyCode::Down),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(text)
       auto text_render_object = service->GetTextRenderObject();
       auto rect = text_render_object->TextSinglePoint(current_position, false);
@@ -85,7 +85,7 @@ TextControlMovePattern TextControlMovePattern::kDown(
 TextControlMovePattern TextControlMovePattern::kHome(
     u"Home(Line Begin)", helper::ShortcutKeyBind(platform::gui::KeyCode::Home),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       return Utf16BackwardUntil(text, current_position,
                                 [](CodePoint c) { return c == u'\n'; });
@@ -93,7 +93,7 @@ TextControlMovePattern TextControlMovePattern::kHome(
 TextControlMovePattern TextControlMovePattern::kEnd(
     u"End(Line End)", helper::ShortcutKeyBind(platform::gui::KeyCode::End),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       return Utf16ForwardUntil(text, current_position,
                                [](CodePoint c) { return c == u'\n'; });
@@ -103,7 +103,7 @@ TextControlMovePattern TextControlMovePattern::kCtrlHome(
     helper::ShortcutKeyBind(platform::gui::KeyCode::Home,
                             platform::gui::KeyModifiers::ctrl),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       CRU_UNUSED(text)
       CRU_UNUSED(current_position)
@@ -114,7 +114,7 @@ TextControlMovePattern TextControlMovePattern::kCtrlEnd(
     helper::ShortcutKeyBind(platform::gui::KeyCode::End,
                             platform::gui::KeyModifiers::ctrl),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       CRU_UNUSED(text)
       CRU_UNUSED(current_position)
@@ -123,7 +123,7 @@ TextControlMovePattern TextControlMovePattern::kCtrlEnd(
 TextControlMovePattern TextControlMovePattern::kPageUp(
     u"PageUp", helper::ShortcutKeyBind(platform::gui::KeyCode::PageUp),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       CRU_UNUSED(text)
       // TODO: Implement this.
@@ -132,7 +132,7 @@ TextControlMovePattern TextControlMovePattern::kPageUp(
 TextControlMovePattern TextControlMovePattern::kPageDown(
     u"PageDown", helper::ShortcutKeyBind(platform::gui::KeyCode::PageDown),
     [](TextHostControlService* service, StringView text,
-       gsl::index current_position) {
+       Index current_position) {
       CRU_UNUSED(service)
       CRU_UNUSED(text)
       // TODO: Implement this.
@@ -147,9 +147,9 @@ std::vector<TextControlMovePattern> TextControlMovePattern::kDefaultPatterns = {
     TextControlMovePattern::kCtrlHome, TextControlMovePattern::kCtrlEnd,
     TextControlMovePattern::kPageUp,   TextControlMovePattern::kPageDown};
 
-TextHostControlService::TextHostControlService(gsl::not_null<Control*> control)
+TextHostControlService::TextHostControlService(Control* control)
     : control_(control),
-      text_host_control_(dynamic_cast<ITextHostControl*>(control.get())) {
+      text_host_control_(dynamic_cast<ITextHostControl*>(control)) {
   context_menu_ = MakeDeleteLaterPtr<components::PopupMenu>();
 
   SetUpShortcuts();
@@ -224,7 +224,7 @@ void TextHostControlService::SetText(String text, bool stop_composition) {
   text_change_event_.Raise(nullptr);
 }
 
-void TextHostControlService::InsertText(gsl::index position, StringView text,
+void TextHostControlService::InsertText(Index position, StringView text,
                                         bool stop_composition) {
   if (!Utf16IsValidInsertPosition(this->text_, position)) {
     CRU_LOG_ERROR(u"Invalid text insert position.");
@@ -238,21 +238,20 @@ void TextHostControlService::InsertText(gsl::index position, StringView text,
   text_change_event_.Raise(nullptr);
 }
 
-void TextHostControlService::DeleteChar(gsl::index position,
-                                        bool stop_composition) {
+void TextHostControlService::DeleteChar(Index position, bool stop_composition) {
   if (!Utf16IsValidInsertPosition(this->text_, position)) {
     CRU_LOG_ERROR(u"Invalid text delete position.");
     return;
   }
-  if (position == static_cast<gsl::index>(this->text_.size())) return;
+  if (position == static_cast<Index>(this->text_.size())) return;
   Index next;
   Utf16NextCodePoint(this->text_, position, &next);
   this->DeleteText(TextRange::FromTwoSides(position, next), stop_composition);
 }
 
 // Return the position of deleted character.
-gsl::index TextHostControlService::DeleteCharPrevious(gsl::index position,
-                                                      bool stop_composition) {
+Index TextHostControlService::DeleteCharPrevious(Index position,
+                                                 bool stop_composition) {
   if (!Utf16IsValidInsertPosition(this->text_, position)) {
     CRU_LOG_ERROR(u"Invalid text delete position.");
     return 0;
@@ -345,8 +344,7 @@ void TextHostControlService::ScrollToCaret() {
   }
 }
 
-gsl::not_null<render::TextRenderObject*>
-TextHostControlService::GetTextRenderObject() {
+render::TextRenderObject* TextHostControlService::GetTextRenderObject() {
   return this->text_host_control_->GetTextRenderObject();
 }
 
@@ -359,7 +357,7 @@ StringView TextHostControlService::GetSelectedText() {
   return GetTextView().substr(selection.position, selection.count);
 }
 
-void TextHostControlService::SetSelection(gsl::index caret_position) {
+void TextHostControlService::SetSelection(Index caret_position) {
   this->SetSelection(TextRange{caret_position, 0});
 }
 
diff --git a/src/ui/render/ScrollBar.cpp b/src/ui/render/ScrollBar.cpp
index bcef74a2..3e578d37 100644
--- a/src/ui/render/ScrollBar.cpp
+++ b/src/ui/render/ScrollBar.cpp
@@ -84,8 +84,7 @@ std::unique_ptr<platform::graphics::IGeometry> CreateScrollBarArrowGeometry() {
 }
 }  // namespace
 
-ScrollBar::ScrollBar(gsl::not_null<ScrollRenderObject*> render_object,
-                     Direction direction)
+ScrollBar::ScrollBar(ScrollRenderObject* render_object, Direction direction)
     : render_object_(render_object), direction_(direction) {
   arrow_geometry_ = CreateScrollBarArrowGeometry();
 }
@@ -260,12 +259,11 @@ void ScrollBar::InstallHandlers(controls::Control* control) {
   }
 }
 
-gsl::not_null<std::shared_ptr<platform::graphics::IBrush>>
+std::shared_ptr<platform::graphics::IBrush>
 ScrollBar::GetCollapsedThumbBrush() {
-  return collapsed_thumb_brush_
-             ? gsl::not_null(collapsed_thumb_brush_)
-             : gsl::not_null(ThemeManager::GetInstance()->GetResourceBrush(
-                   u"scrollbar.collapse-thumb.color"));
+  return collapsed_thumb_brush_ ? collapsed_thumb_brush_
+                                : ThemeManager::GetInstance()->GetResourceBrush(
+                                      u"scrollbar.collapse-thumb.color");
 }
 
 void ScrollBar::SetCollapsedThumbBrush(
@@ -275,12 +273,12 @@ void ScrollBar::SetCollapsedThumbBrush(
   render_object_->InvalidatePaint();
 }
 
-gsl::not_null<std::shared_ptr<platform::graphics::IBrush>> ScrollBar::GetBrush(
+std::shared_ptr<platform::graphics::IBrush> ScrollBar::GetBrush(
     ScrollBarBrushUsageKind usage, ScrollBarBrushStateKind state) {
   auto b = brushes_[usage][state];
-  return b ? gsl::not_null(b)
-           : gsl::not_null(ThemeManager::GetInstance()->GetResourceBrush(
-                 GenerateScrollBarThemeColorKey(usage, state)));
+  return b ? b
+           : ThemeManager::GetInstance()->GetResourceBrush(
+                 GenerateScrollBarThemeColorKey(usage, state));
 }
 
 // Brush could be nullptr to use the theme brush.
@@ -300,7 +298,7 @@ void ScrollBar::OnDraw(platform::graphics::IPainter* painter,
       auto thumb_state = GetState(ScrollBarAreaKind::Thumb);
       painter->FillRectangle(
           *thumb_rect,
-          GetBrush(ScrollBarBrushUsageKind::Thumb, thumb_state).get().get());
+          GetBrush(ScrollBarBrushUsageKind::Thumb, thumb_state).get());
     }
 
     auto up_slot_rect = GetExpandedAreaRect(ScrollBarAreaKind::UpSlot);
@@ -308,23 +306,23 @@ void ScrollBar::OnDraw(platform::graphics::IPainter* painter,
     if (up_slot_rect)
       painter->FillRectangle(
           *up_slot_rect,
-          GetBrush(ScrollBarBrushUsageKind::Slot, up_slot_state).get().get());
+          GetBrush(ScrollBarBrushUsageKind::Slot, up_slot_state).get());
 
     auto down_slot_rect = GetExpandedAreaRect(ScrollBarAreaKind::DownSlot);
     auto down_slot_state = GetState(ScrollBarAreaKind::DownSlot);
     if (down_slot_rect)
       painter->FillRectangle(
           *down_slot_rect,
-          GetBrush(ScrollBarBrushUsageKind::Slot, down_slot_state).get().get());
+          GetBrush(ScrollBarBrushUsageKind::Slot, down_slot_state).get());
 
     auto up_arrow_rect = GetExpandedAreaRect(ScrollBarAreaKind::UpArrow);
     auto up_arrow_state = GetState(ScrollBarAreaKind::UpArrow);
     if (up_arrow_rect)
       this->DrawUpArrow(
           painter, *up_arrow_rect,
-          GetBrush(ScrollBarBrushUsageKind::Arrow, up_arrow_state).get().get(),
+          GetBrush(ScrollBarBrushUsageKind::Arrow, up_arrow_state).get(),
           GetBrush(ScrollBarBrushUsageKind::ArrowBackground, up_arrow_state)
-              .get()
+
               .get());
 
     auto down_arrow_rect = GetExpandedAreaRect(ScrollBarAreaKind::DownArrow);
@@ -332,17 +330,13 @@ void ScrollBar::OnDraw(platform::graphics::IPainter* painter,
     if (down_arrow_rect)
       this->DrawDownArrow(
           painter, *down_arrow_rect,
-          GetBrush(ScrollBarBrushUsageKind::Arrow, down_arrow_state)
-              .get()
-              .get(),
+          GetBrush(ScrollBarBrushUsageKind::Arrow, down_arrow_state).get(),
           GetBrush(ScrollBarBrushUsageKind::ArrowBackground, down_arrow_state)
-              .get()
               .get());
   } else {
     auto optional_rect = GetCollapsedThumbRect();
     if (optional_rect) {
-      painter->FillRectangle(*optional_rect,
-                             GetCollapsedThumbBrush().get().get());
+      painter->FillRectangle(*optional_rect, GetCollapsedThumbBrush().get());
     }
   }
 }
@@ -416,33 +410,32 @@ ScrollBarBrushStateKind ScrollBar::GetState(ScrollBarAreaKind area) {
   }
 }
 
-HorizontalScrollBar::HorizontalScrollBar(
-    gsl::not_null<ScrollRenderObject*> render_object)
+HorizontalScrollBar::HorizontalScrollBar(ScrollRenderObject* render_object)
     : ScrollBar(render_object, Direction::Horizontal) {}
 
 void HorizontalScrollBar::DrawUpArrow(
     platform::graphics::IPainter* painter, const Rect& area,
-    gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-    gsl::not_null<platform::graphics::IBrush*> background_brush) {
-  painter->FillRectangle(area, background_brush.get());
+    platform::graphics::IBrush* arrow_brush,
+    platform::graphics::IBrush* background_brush) {
+  painter->FillRectangle(area, background_brush);
 
   platform::graphics::util::WithTransform(
       painter, Matrix::Translation(area.GetCenter()),
       [this, arrow_brush](platform::graphics::IPainter* painter) {
-        painter->FillGeometry(arrow_geometry_.get(), arrow_brush.get());
+        painter->FillGeometry(arrow_geometry_.get(), arrow_brush);
       });
 }
 
 void HorizontalScrollBar::DrawDownArrow(
     platform::graphics::IPainter* painter, const Rect& area,
-    gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-    gsl::not_null<platform::graphics::IBrush*> background_brush) {
-  painter->FillRectangle(area, background_brush.get());
+    platform::graphics::IBrush* arrow_brush,
+    platform::graphics::IBrush* background_brush) {
+  painter->FillRectangle(area, background_brush);
 
   platform::graphics::util::WithTransform(
       painter, Matrix::Rotation(180) * Matrix::Translation(area.GetCenter()),
       [this, arrow_brush](platform::graphics::IPainter* painter) {
-        painter->FillGeometry(arrow_geometry_.get(), arrow_brush.get());
+        painter->FillGeometry(arrow_geometry_.get(), arrow_brush);
       });
 }
 
@@ -565,33 +558,32 @@ bool HorizontalScrollBar::CanScrollDown() {
   return render_object_->HorizontalCanScrollDown();
 }
 
-VerticalScrollBar::VerticalScrollBar(
-    gsl::not_null<ScrollRenderObject*> render_object)
+VerticalScrollBar::VerticalScrollBar(ScrollRenderObject* render_object)
     : ScrollBar(render_object, Direction::Vertical) {}
 
 void VerticalScrollBar::DrawUpArrow(
     platform::graphics::IPainter* painter, const Rect& area,
-    gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-    gsl::not_null<platform::graphics::IBrush*> background_brush) {
-  painter->FillRectangle(area, background_brush.get());
+    platform::graphics::IBrush* arrow_brush,
+    platform::graphics::IBrush* background_brush) {
+  painter->FillRectangle(area, background_brush);
 
   platform::graphics::util::WithTransform(
       painter, Matrix::Rotation(90) * Matrix::Translation(area.GetCenter()),
       [this, arrow_brush](platform::graphics::IPainter* painter) {
-        painter->FillGeometry(arrow_geometry_.get(), arrow_brush.get());
+        painter->FillGeometry(arrow_geometry_.get(), arrow_brush);
       });
 }
 
 void VerticalScrollBar::DrawDownArrow(
     platform::graphics::IPainter* painter, const Rect& area,
-    gsl::not_null<platform::graphics::IBrush*> arrow_brush,
-    gsl::not_null<platform::graphics::IBrush*> background_brush) {
-  painter->FillRectangle(area, background_brush.get());
+    platform::graphics::IBrush* arrow_brush,
+    platform::graphics::IBrush* background_brush) {
+  painter->FillRectangle(area, background_brush);
 
   platform::graphics::util::WithTransform(
       painter, Matrix::Rotation(270) * Matrix::Translation(area.GetCenter()),
       [this, arrow_brush](platform::graphics::IPainter* painter) {
-        painter->FillGeometry(arrow_geometry_.get(), arrow_brush.get());
+        painter->FillGeometry(arrow_geometry_.get(), arrow_brush);
       });
 }
 
@@ -712,8 +704,7 @@ bool VerticalScrollBar::CanScrollDown() {
   return render_object_->VerticalCanScrollDown();
 }
 
-ScrollBarDelegate::ScrollBarDelegate(
-    gsl::not_null<ScrollRenderObject*> render_object)
+ScrollBarDelegate::ScrollBarDelegate(ScrollRenderObject* render_object)
     : render_object_(render_object),
       horizontal_bar_(render_object),
       vertical_bar_(render_object) {
diff --git a/src/ui/render/TextRenderObject.cpp b/src/ui/render/TextRenderObject.cpp
index 11548106..135c2d02 100644
--- a/src/ui/render/TextRenderObject.cpp
+++ b/src/ui/render/TextRenderObject.cpp
@@ -77,7 +77,7 @@ std::vector<Rect> TextRenderObject::TextRangeRect(const TextRange& text_range) {
   return text_layout_->TextRangeRect(text_range);
 }
 
-Rect TextRenderObject::TextSinglePoint(gsl::index position, bool trailing) {
+Rect TextRenderObject::TextSinglePoint(Index position, bool trailing) {
   return text_layout_->TextSinglePoint(position, trailing);
 }
 
@@ -107,7 +107,7 @@ void TextRenderObject::SetDrawCaret(bool draw_caret) {
   }
 }
 
-void TextRenderObject::SetCaretPosition(gsl::index position) {
+void TextRenderObject::SetCaretPosition(Index position) {
   if (position != caret_position_) {
     caret_position_ = position;
     if (draw_caret_) {
@@ -136,7 +136,7 @@ void TextRenderObject::SetCaretWidth(const float width) {
 
 Rect TextRenderObject::GetCaretRectInContent() {
   auto caret_pos = this->caret_position_;
-  gsl::index text_size = this->GetText().size();
+  Index text_size = this->GetText().size();
   if (caret_pos < 0) {
     caret_pos = 0;
   } else if (caret_pos > text_size) {
diff --git a/src/ui/style/StyleRuleSet.cpp b/src/ui/style/StyleRuleSet.cpp
index 4deaaefa..7b6454ec 100644
--- a/src/ui/style/StyleRuleSet.cpp
+++ b/src/ui/style/StyleRuleSet.cpp
@@ -37,7 +37,7 @@ void StyleRuleSet::SetParent(std::shared_ptr<StyleRuleSet> parent) {
   RaiseChangeEvent();
 }
 
-void StyleRuleSet::AddStyleRule(StyleRule rule, gsl::index index) {
+void StyleRuleSet::AddStyleRule(StyleRule rule, Index index) {
   Expects(index >= 0 && index <= GetSize());
 
   rules_.insert(rules_.cbegin() + index, std::move(rule));
@@ -45,7 +45,7 @@ void StyleRuleSet::AddStyleRule(StyleRule rule, gsl::index index) {
   RaiseChangeEvent(model::ListChange::ItemAdd(index));
 }
 
-void StyleRuleSet::RemoveStyleRule(gsl::index index, gsl::index count) {
+void StyleRuleSet::RemoveStyleRule(Index index, Index count) {
   Expects(index >= 0);
   Expects(count >= 0 && index + count <= GetSize());
 
diff --git a/test/common/StringUtilTest.cpp b/test/common/StringUtilTest.cpp
index 2dd980f9..613243c5 100644
--- a/test/common/StringUtilTest.cpp
+++ b/test/common/StringUtilTest.cpp
@@ -3,12 +3,13 @@
 
 #include <catch2/catch_test_macros.hpp>
 
+using cru::Index;
 using cru::k_invalid_code_point;
 
 TEST_CASE("StringUtil Utf8NextCodePoint", "[string]") {
   using cru::Utf8NextCodePoint;
   std::string_view text = "aπ你🤣!";
-  gsl::index current = 0;
+  Index current = 0;
   REQUIRE(Utf8NextCodePoint(text.data(), text.size(), current, &current) ==
           0x0061);
   REQUIRE(Utf8NextCodePoint(text.data(), text.size(), current, &current) ==
@@ -21,13 +22,13 @@ TEST_CASE("StringUtil Utf8NextCodePoint", "[string]") {
           0x0021);
   REQUIRE(Utf8NextCodePoint(text.data(), text.size(), current, &current) ==
           k_invalid_code_point);
-  REQUIRE(current == static_cast<gsl::index>(text.size()));
+  REQUIRE(current == static_cast<Index>(text.size()));
 }
 
 TEST_CASE("StringUtil Utf16NextCodePoint", "[string]") {
   using cru::Utf16NextCodePoint;
   std::u16string_view text = u"aπ你🤣!";
-  gsl::index current = 0;
+  Index current = 0;
   REQUIRE(Utf16NextCodePoint(text.data(), text.size(), current, &current) ==
           0x0061);
   REQUIRE(Utf16NextCodePoint(text.data(), text.size(), current, &current) ==
@@ -40,13 +41,13 @@ TEST_CASE("StringUtil Utf16NextCodePoint", "[string]") {
           0x0021);
   REQUIRE(Utf16NextCodePoint(text.data(), text.size(), current, &current) ==
           k_invalid_code_point);
-  REQUIRE(current == static_cast<gsl::index>(text.size()));
+  REQUIRE(current == static_cast<Index>(text.size()));
 }
 
 TEST_CASE("StringUtil Utf16PreviousCodePoint", "[string]") {
   using cru::Utf16PreviousCodePoint;
   std::u16string_view text = u"aπ你🤣!";
-  gsl::index current = text.size();
+  Index current = text.size();
   REQUIRE(Utf16PreviousCodePoint(text.data(), text.size(), current, &current) ==
           0x0021);
   REQUIRE(Utf16PreviousCodePoint(text.data(), text.size(), current, &current) ==