Reduces the compile size of Pipelines and ContentContext (#167671)

This reduces the size of ContentContext by introducing generic
superclass versions of `RenderPipelineHandle` and `Variants` which
allows the compiler to put most of the .text in the superclasses.

testing: no new functionality, refactor

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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engine/src/flutter/impeller/entity/contents/content_context.cc

@@ -28,6 +28,56 @@ namespace impeller {
 
 namespace {
 
+/// A generic version of `Variants` which mostly exists to reduce code size.
+class GenericVariants {
+ public:
+  void Set(const ContentContextOptions& options,
+           std::unique_ptr<GenericRenderPipelineHandle> pipeline) {
+    uint64_t p_key = options.ToKey();
+    for (const auto& [key, pipeline] : pipelines_) {
+      if (key == p_key) {
+        return;
+      }
+    }
+    pipelines_.push_back(std::make_pair(p_key, std::move(pipeline)));
+  }
+
+  void SetDefault(const ContentContextOptions& options,
+                  std::unique_ptr<GenericRenderPipelineHandle> pipeline) {
+    default_options_ = options;
+    if (pipeline) {
+      Set(options, std::move(pipeline));
+    }
+  }
+
+  GenericRenderPipelineHandle* Get(const ContentContextOptions& options) const {
+    uint64_t p_key = options.ToKey();
+    for (const auto& [key, pipeline] : pipelines_) {
+      if (key == p_key) {
+        return pipeline.get();
+      }
+    }
+    return nullptr;
+  }
+
+  void SetDefaultDescriptor(std::optional<PipelineDescriptor> desc) {
+    desc_ = std::move(desc);
+  }
+
+  size_t GetPipelineCount() const { return pipelines_.size(); }
+
+  bool IsDefault(const ContentContextOptions& opts) {
+    return default_options_.has_value() &&
+           opts.ToKey() == default_options_.value().ToKey();
+  }
+
+ protected:
+  std::optional<PipelineDescriptor> desc_;
+  std::optional<ContentContextOptions> default_options_;
+  std::vector<std::pair<uint64_t, std::unique_ptr<GenericRenderPipelineHandle>>>
+      pipelines_;
+};
+
 /// Holds multiple Pipelines associated with the same PipelineHandle types.
 ///
 /// For example, it may have multiple
@@ -42,7 +92,7 @@ namespace {
 ///  - impeller::RenderPipelineHandle<> - The type of objects this typically
 ///    contains.
 template <class PipelineHandleT>
-class Variants {
+class Variants : public GenericVariants {
   static_assert(
       ShaderStageCompatibilityChecker<
           typename PipelineHandleT::VertexShader,
@@ -55,32 +105,20 @@ class Variants {
 
   void Set(const ContentContextOptions& options,
            std::unique_ptr<PipelineHandleT> pipeline) {
-    uint64_t p_key = options.ToKey();
-    for (const auto& [key, pipeline] : pipelines_) {
-      if (key == p_key) {
-        return;
-      }
-    }
-    pipelines_.push_back(std::make_pair(p_key, std::move(pipeline)));
+    GenericVariants::Set(options, std::move(pipeline));
   }
 
   void SetDefault(const ContentContextOptions& options,
                   std::unique_ptr<PipelineHandleT> pipeline) {
-    default_options_ = options;
-    if (pipeline) {
-      Set(options, std::move(pipeline));
-    }
-  }
-
-  void SetDefaultDescriptor(std::optional<PipelineDescriptor> desc) {
-    desc_ = std::move(desc);
+    GenericVariants::SetDefault(options, std::move(pipeline));
   }
 
   void CreateDefault(const Context& context,
                      const ContentContextOptions& options,
                      const std::vector<Scalar>& constants = {}) {
-    auto desc = PipelineHandleT::Builder::MakeDefaultPipelineDescriptor(
-        context, constants);
+    std::optional<PipelineDescriptor> desc =
+        PipelineHandleT::Builder::MakeDefaultPipelineDescriptor(context,
+                                                                constants);
     if (!desc.has_value()) {
       VALIDATION_LOG << "Failed to create default pipeline.";
       return;
@@ -96,18 +134,7 @@ class Variants {
   }
 
   PipelineHandleT* Get(const ContentContextOptions& options) const {
-    uint64_t p_key = options.ToKey();
-    for (const auto& [key, pipeline] : pipelines_) {
-      if (key == p_key) {
-        return pipeline.get();
-      }
-    }
-    return nullptr;
-  }
-
-  bool IsDefault(const ContentContextOptions& opts) {
-    return default_options_.has_value() &&
-           opts.ToKey() == default_options_.value().ToKey();
+    return static_cast<PipelineHandleT*>(GenericVariants::Get(options));
   }
 
   PipelineHandleT* GetDefault(const Context& context) {
@@ -123,13 +150,7 @@ class Variants {
     return Get(default_options_.value());
   }
 
-  size_t GetPipelineCount() const { return pipelines_.size(); }
-
  private:
-  std::optional<PipelineDescriptor> desc_;
-  std::optional<ContentContextOptions> default_options_;
-  std::vector<std::pair<uint64_t, std::unique_ptr<PipelineHandleT>>> pipelines_;
-
   Variants(const Variants&) = delete;
 
   Variants& operator=(const Variants&) = delete;

engine/src/flutter/impeller/renderer/pipeline.h

@@ -107,34 +107,24 @@ PipelineFuture<ComputePipelineDescriptor> CreatePipelineFuture(
     const Context& context,
     std::optional<ComputePipelineDescriptor> desc);
 
-/// Holds a reference to a Pipeline used for rendering while also maintaining
-/// the vertex shader and fragment shader types at compile-time.
+/// Holds a reference to a Pipeline used for rendering.
 ///
-/// See also:
-///   - impeller::ContentContext::Variants - the typical container for
-///     RenderPipelineHandles.
-template <class VertexShader_, class FragmentShader_>
-class RenderPipelineHandle {
+/// @see RenderPipelineHandle the templated subclass that stores compile-time
+/// shader information.
+class GenericRenderPipelineHandle {
  public:
-  using VertexShader = VertexShader_;
-  using FragmentShader = FragmentShader_;
-  using Builder = PipelineBuilder<VertexShader, FragmentShader>;
+  explicit GenericRenderPipelineHandle(const Context& context,
+                                       std::optional<PipelineDescriptor> desc,
+                                       bool async = true)
+      : GenericRenderPipelineHandle(
+            CreatePipelineFuture(context, std::move(desc), /*async=*/async)) {}
 
-  explicit RenderPipelineHandle(const Context& context, bool async = true)
-      : RenderPipelineHandle(CreatePipelineFuture(
-            context,
-            Builder::MakeDefaultPipelineDescriptor(context),
-            async)) {}
-
-  explicit RenderPipelineHandle(const Context& context,
-                                std::optional<PipelineDescriptor> desc,
-                                bool async = true)
-      : RenderPipelineHandle(
-            CreatePipelineFuture(context, desc, /*async=*/async)) {}
-
-  explicit RenderPipelineHandle(PipelineFuture<PipelineDescriptor> future)
+  explicit GenericRenderPipelineHandle(
+      PipelineFuture<PipelineDescriptor> future)
       : pipeline_future_(std::move(future)) {}
 
+  virtual ~GenericRenderPipelineHandle() = default;
+
   std::shared_ptr<Pipeline<PipelineDescriptor>> WaitAndGet() {
     if (did_wait_) {
       return pipeline_;
@@ -155,6 +145,40 @@ class RenderPipelineHandle {
   std::shared_ptr<Pipeline<PipelineDescriptor>> pipeline_;
   bool did_wait_ = false;
 
+  GenericRenderPipelineHandle(const GenericRenderPipelineHandle&) = delete;
+
+  GenericRenderPipelineHandle& operator=(const GenericRenderPipelineHandle&) =
+      delete;
+};
+
+/// Holds a reference to a Pipeline used for rendering while also maintaining
+/// the vertex shader and fragment shader types at compile-time.
+///
+/// See also:
+///   - impeller::ContentContext::Variants - the typical container for
+///     RenderPipelineHandles.
+template <class VertexShader_, class FragmentShader_>
+class RenderPipelineHandle : public GenericRenderPipelineHandle {
+ public:
+  using VertexShader = VertexShader_;
+  using FragmentShader = FragmentShader_;
+  using Builder = PipelineBuilder<VertexShader, FragmentShader>;
+
+  explicit RenderPipelineHandle(const Context& context, bool async = true)
+      : GenericRenderPipelineHandle(CreatePipelineFuture(
+            context,
+            Builder::MakeDefaultPipelineDescriptor(context),
+            async)) {}
+
+  explicit RenderPipelineHandle(const Context& context,
+                                std::optional<PipelineDescriptor> desc,
+                                bool async = true)
+      : GenericRenderPipelineHandle(context, desc, async) {}
+
+  explicit RenderPipelineHandle(PipelineFuture<PipelineDescriptor> future)
+      : GenericRenderPipelineHandle(std::move(future)) {}
+
+ private:
   RenderPipelineHandle(const RenderPipelineHandle&) = delete;
 
   RenderPipelineHandle& operator=(const RenderPipelineHandle&) = delete;