Support lowering specifics for an imported generic function. (carbon-language#5475)

When lowering a specific function whose generic was defined in a
different file, switch to that other file's `FileContext` and lower the
generic there. Also pass the `FileContext` corresponding to the specific
into the `FunctionContext`, and use that `FileContext` for resolving
requests for constants and types from the specific.

Author: zygoloid

toolchain/lower/context.cpp

@@ -41,7 +41,21 @@ auto Context::GetFileContext(const SemIR::File* file,
   return *insert_result.value();
 }
 
+auto Context::LowerPendingDefinitions() -> void {
+  // Lower function definitions for generics.
+  // This cannot be a range-based loop, as new definitions can be added
+  // while building other definitions.
+  // NOLINTNEXTLINE(modernize-loop-convert)
+  for (size_t i = 0; i != specific_function_definitions_.size(); ++i) {
+    auto [file_context, function_id, specific_id] =
+        specific_function_definitions_[i];
+    file_context->BuildFunctionDefinition(function_id, specific_id);
+  }
+}
+
 auto Context::Finalize() && -> std::unique_ptr<llvm::Module> {
+  LowerPendingDefinitions();
+
   file_contexts_.ForEach(
       [](auto, auto& file_context) { file_context->Finalize(); });
   return std::move(llvm_module_);

toolchain/lower/context.h

@@ -34,6 +34,13 @@ class Context {
     int32_t column_number;
   };
 
+  // A specific function whose definition needs to be lowered.
+  struct PendingSpecificFunctionDefinition {
+    FileContext* context;
+    SemIR::FunctionId function_id;
+    SemIR::SpecificId specific_id;
+  };
+
   explicit Context(llvm::LLVMContext& llvm_context,
                    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> fs,
                    std::optional<llvm::ArrayRef<Parse::GetTreeAndSubtreesFn>>
@@ -48,6 +55,12 @@ class Context {
                       const SemIR::InstNamer* inst_namer = nullptr)
       -> FileContext&;
 
+  // Registers a specific function definition to be lowered later.
+  auto AddPendingSpecificFunctionDefinition(
+      PendingSpecificFunctionDefinition pending) -> void {
+    specific_function_definitions_.push_back(pending);
+  }
+
   // Finishes lowering and takes ownership of the LLVM module. The context
   // cannot be used further after calling this.
   auto Finalize() && -> std::unique_ptr<llvm::Module>;
@@ -98,6 +111,10 @@ class Context {
                           llvm::Module& llvm_module,
                           llvm::DIBuilder& di_builder) -> llvm::DICompileUnit*;
 
+  // Lower any definitions that have been registered for later lowering.
+  // Currently, this lowers specifics for generic functions.
+  auto LowerPendingDefinitions() -> void;
+
   // State for building the LLVM IR.
   llvm::LLVMContext* llvm_context_;
   std::unique_ptr<llvm::Module> llvm_module_;
@@ -126,6 +143,11 @@ class Context {
 
   // Global format string for `printf.int.format` used by the PrintInt builtin.
   llvm::Value* printf_int_format_string_ = nullptr;
+
+  // Tracks which specific functions need to have their definitions lowered.
+  // This list may grow while lowering generic definitions from this list.
+  llvm::SmallVector<PendingSpecificFunctionDefinition>
+      specific_function_definitions_;
 };
 
 }  // namespace Carbon::Lower