C++ Interop: API importing and semantics

docs/design/interoperability/README.md

@@ -12,6 +12,26 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 -   [Philosophy and goals](#philosophy-and-goals)
 -   [Overview](#overview)
+-   [C++ interoperability model: introduction and principles](#c-interoperability-model-introduction-and-principles)
+    -   [The "successor language" mandate](#the-successor-language-mandate)
+    -   [The C++ interop type](#the-c-interop-type)
+-   [Importing C++ APIs into Carbon](#importing-c-apis-into-carbon)
+    -   [Importing C++ libraries (header-based)](#importing-c-libraries-header-based)
+    -   [TODO: Importing C++ code (inline)](#todo-importing-c-code-inline)
+    -   [Accessing built-in C++ entities (file-less)](#accessing-built-in-c-entities-file-less)
+    -   [The `Cpp` package](#the-cpp-package)
+    -   [TODO: Importing C++ macros](#todo-importing-c-macros)
+-   [Calling C++ code from Carbon](#calling-c-code-from-carbon)
+    -   [Function call syntax and semantics](#function-call-syntax-and-semantics)
+    -   [TODO: Overload resolution](#todo-overload-resolution)
+    -   [TODO: Constructors](#todo-constructors)
+    -   [TODO: Struct literals](#todo-struct-literals)
+-   [TODO: Accessing C++ classes, structs, and members](#todo-accessing-c-classes-structs-and-members)
+-   [TODO: Accessing global variables](#todo-accessing-global-variables)
+-   [TODO: Bi-directional type mapping: primitives and core types](#todo-bi-directional-type-mapping-primitives-and-core-types)
+-   [TODO: Advanced type mapping: pointers, references, and `const`](#todo-advanced-type-mapping-pointers-references-and-const)
+-   [TODO: Bi-directional type mapping: standard library types](#todo-bi-directional-type-mapping-standard-library-types)
+-   [TODO: The operator interoperability model](#todo-the-operator-interoperability-model)
 
 <!-- tocstop -->
 
@@ -29,4 +49,165 @@ more detail.
 
 ## Overview
 
-TODO
+Carbon's bidirectional interoperability with C++ is
+[a cornerstone of its design](/docs/project/goals.md#interoperability-with-and-migration-from-existing-c-code),
+enabling a gradual transition from existing C++ codebases. The goal is not just
+a foreign function interface (FFI), but a seamless, high-fidelity integration
+that supports advanced C++ features, from templates to class hierarchies.
+
+C++ APIs are imported into Carbon using an `import Cpp` directive, which makes
+C++ declarations available within a dedicated `Cpp` package in Carbon. This
+prevents name collisions and makes the origin of symbols explicit. Carbon code
+can then call C++ functions, instantiate C++ classes, and use C++ types, while
+respecting C++'s semantics, including its complex overload resolution rules and
+preserving the nominal distinctions between C++ types like `long` and
+`long long`, or `T*` and `T&`, which is critical for correct overload resolution
+and template instantiation.
+
+Similarly, Carbon APIs can be designed to be callable from C++. The
+interoperability layer is designed to be zero-cost, avoiding unnecessary
+allocations or copies when calling between the two languages.
+
+## C++ interoperability model: introduction and principles
+
+### The "successor language" mandate
+
+The design of Carbon's C++ interoperability is governed by its foundational
+goal: [to be a successor language](/README.md), not merely a language with a
+foreign function interface (FFI). This mandate dictates a design that moves
+beyond the C-style FFI adopted by most modern languages and instead provides
+"seamless, bidirectional interoperability". The objective is to support deep
+integration with existing C++ code, encompassing its most complex features,
+"from inheritance to templates".
+
+This goal has profound implications for the Carbon compiler and language
+semantics. It requires that C++ is not treated as a "foreign" entity. Instead,
+Carbon's semantic model must be _co-designed_ to understand, map, and interact
+with C++'s semantic constructs—including templates, class hierarchies, and
+complex overload resolution—with high fidelity. The interoperability layer must,
+therefore, operate at the semantic analysis level, not just at the linking (ABI)
+level. This document specifies the design of this semantic contract.
+
+### The C++ interop type
+
+A core mechanism in this design is the C++ interop type. This concept defines
+the "trigger" that activates C++-specific semantic rules within the Carbon
+compiler. Any operation involving a type that is designated as a C++ interop
+type could invoke the specialized interoperability logic, such as C++ overload
+resolution or operator overload resolution that involves both Carbon and C++
+operator overloads..
+
+A type is considered a C++ interop type if its definition involves an imported
+C++ type in any of the following ways:
+
+1.  A C++ imported type (for example, `Cpp.Widget`).
+2.  A pointer to a C++ interop type (for example, `Cpp.Widget*`).
+3.  A Carbon generic type parameterized with a C++ interop type (for example,
+    `MyCarbonVector(Cpp.Widget)`).
+4.  A Carbon struct or class containing a C++ interop type as a member (for
+    example, `MyCarbonStruct { x: Cpp.Widget }`).
+
+This "pervasive" model of C++-awareness is a fundamental design choice. The C++
+semantics are not confined to a specific `unsafe` or `extern "C++"` block; they
+affect any Carbon type that composes them. For example, when the Carbon compiler
+instantiates a _Carbon_ generic type like `MyCarbonVector(Cpp.Widget)`, its type
+system must be aware that the `Cpp.Widget` parameter carries C++-specific rules.
+This mandates that Carbon's own generic system, struct layout logic, overload
+resolution and operator lookup must query the type system for the presence of a
+C++ interop type. If present, Carbon must consider C++ rules when operating over
+C++ interop types. This design prioritizes the goal of a "seamless" and
+"intuitive" user experience.
+
+## Importing C++ APIs into Carbon
+
+### Importing C++ libraries (header-based)
+
+The primary mechanism for importing existing, user-defined C++ code is through
+header file inclusion. Carbon must be able to parse and analyze C++ header files
+to make their declarations available within Carbon.
+
+**Syntax:** The syntax for this operation is `import Cpp library "header_name"`.
+This syntax is used for both C-style standard libraries and C++ headers:
+
+-   **C Standard Library:**
+
+    ```carbon
+    import Cpp library "<cstdio>";
+    ```
+
+    This import makes entities like `putchar` available.
+
+-   **C++ User-Defined Header:**
+    ```carbon
+    import Cpp library "circle.h";
+    ```
+    This import makes user-defined declarations and definitions available.
+
+### TODO: Importing C++ code (inline)
+
+### Accessing built-in C++ entities (file-less)
+
+Some C++ entities, particularly built-in primitive types, are not defined in any
+header file. They are "intrinsic" to the C++ language. These entities are
+available in Carbon without an explicit `import` declaration.
+
+### The `Cpp` package
+
+A critical design choice for managing C++ imports is the mandatory use of a
+containing package, `Cpp`. All imported C++ named entities (functions, types,
+namespaces) are contained in the `Cpp` package.
+
+-   **Functions:** `Cpp.putchar(...)`
+-   **Classes/Types:** `Cpp.Circle`, `Cpp.Point`
+-   **Constructors:** `Cpp.Circle.Circle()`
+
+The `Cpp.` prefix makes the _origin_ of every symbol explicit and unambiguous.
+It ensures that C++ entities cannot collide with Carbon code.
+
+### TODO: Importing C++ macros
+
+## Calling C++ code from Carbon
+
+### Function call syntax and semantics
+
+Once imported, C++ functions are invoked using standard Carbon function call
+syntax, prefixed with the `Cpp` name. The Carbon compiler is responsible for
+mapping the Carbon arguments to the types expected by the C++ function's
+signature.
+
+This often requires explicit casting on the Carbon side, using the `as` keyword,
+to satisfy the C++ function's parameter types.
+
+**Example:** The following example imports `cstdio` and calls the C function
+`putchar`. The Carbon `Core.Char` variable `n` must be cast first to `u8` and
+then to `i32` to match the `int` parameter expected by `putchar`.
+
+```carbon
+import Cpp library "<cstdio>";
+
+fn Run() {
+  let hello: array(Core.Char, 6) = ('H', 'e', 'l', 'l', 'o', '!');
+  for (n: Core.Char in hello) {
+    // Carbon 'as' casting is used to match the C++ signature
+    Cpp.putchar((n as u8) as i32);
+  }
+}
+```
+
+### TODO: Overload resolution
+
+### TODO: Constructors
+
+### TODO: Struct literals
+
+## TODO: Accessing C++ classes, structs, and members
+
+## TODO: Accessing global variables
+
+## TODO: Bi-directional type mapping: primitives and core types
+
+## TODO: Advanced type mapping: pointers, references, and `const`
+
+## TODO: Bi-directional type mapping: standard library types
+
+## TODO: The operator interoperability model