doc: writing design doc

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+# C-friendly FFI Design
+
+This document is a guide for authoring Rust libraries that expose a C-friendly FFI
+and distributing them as ordinary CMake/pkg-config packages. It covers how to design
+C-friendly FFI signatures, how to pass primitive types, enums, strings and
+Rust-specific constructs such as `Option` and `Result` across the boundary, and how
+to write the accompanying C/C++ header files.
+
+## Allowed Data Types
+
+Under this section we discuss which data types are allowed to appear in an FFI
+interface, how to pass them across the boundary, and how to convert Rust data types
+into these allowed types.
+
+### Primitive Type Conversion
+
+This subsection covers the conversion of primitive types. The rules described here
+apply to both parameter and return value positions.
+
+#### Types Allowed to Cross the Boundary Directly
+
+The following types are allowed to be passed directly across the FFI boundary. The
+left column lists the Rust type and the right column lists the corresponding C/C++
+type.
+
+| Rust | C/C++ |
+|------|-------|
+| `bool` | `bool` |
+| `f32` | `float` |
+| `f64` | `double` |
+| `i8` | `int8_t` |
+| `i16` | `int16_t` |
+| `i32` | `int32_t` |
+| `i64` | `int64_t` |
+| `u8` | `uint8_t` |
+| `u16` | `uint16_t` |
+| `u32` | `uint32_t` |
+| `u64` | `uint64_t` |
+| `usize` | `uintptr_t` |
+| `isize` | `intptr_t` |
+
+#### Required Header Files in the Generated C/C++ Headers
+
+The generated C/C++ header files must include the appropriate header files for the
+types listed above.
+
+- In C, the `bool` type is provided by `<stddef.h>`. C++ supports `bool` natively
+  and does not need any header file.
+- In C, the integer types are provided by `<stdint.h>`. In C++, they are provided
+  by `<cstdint>`.
+- In C, the `uintptr_t` and `intptr_t` types are provided by `<stdint.h>`. In C++,
+  they are provided by `<cstdint>`.
+- The floating-point types do not require any header file.
+
+#### The u128 and i128 Problem
+
+The `u128` and `i128` types have very poor support in the C/C++ standard libraries.
+Therefore they cannot be passed directly across the FFI boundary.
+
+To transfer data of these types, refer to the later section about passing opaque
+structs.
+
+#### The char Problem
+
+In C/C++, the type closest to the Rust `char` type is `char32_t`. However, the Rust
+`char` type only holds valid Unicode scalar values, which conflicts with `char32_t`
+being any 32-bit unsigned integer.
+
+In addition, the Rust official documentation explicitly states that the `char` type
+does not have FFI safety, so it cannot be used as a value passed across the FFI
+boundary.
+
+The solution is to use the Rust `u32` type for passing. Under this scheme:
+
+- On the Rust side:
+  - The function signature uses `u32` as the parameter type.
+  - For input parameters, use `char::try_from` to perform a fallible conversion that
+    validates the incoming character.
+  - For output parameters, use `u32::from` for the conversion.
+- On the C/C++ header side:
+  - The function signature uses `char32_t` as the parameter type.