Rust bindings for C++ fundamental types

Crubit maps most C++ fundamental types to the direct Rust equivalent. For example, int32_t becomes i32, int becomes ffi::c_int, double becomes f64, and so on.

The exceptions are the currently-unsupported types nullptr_t, char8_t, wchar_t, and (u)int128_t.

Bidirectional map of C++ types

The following map is bidirectional. If you call a C++ interface from Rust using Crubit, then int32_t in C++ becomes i32 in Rust. Vice versa, if you call a Rust interface from C++ using Crubit, i32 in Rust becomes int32_t in C++.

C++	Rust
`void`	`()` as a return type, `::core::ffi::c_void` otherwise.
`int8_t`	`i8`
`int16_t`	`i16`
`int32_t`	`i32`
`int64_t`	`i64`
`intptr_t`	`isize`
`uint8_t`	`u8`
`uint16_t`	`u16`
`uint32_t`	`u32`
`uint64_t`	`u64`
`uintptr_t`	`usize`
`bool`	`bool`
`double`	`f64`
`float`	`f32`

One-way map of C++ into Rust types

The C++ types below are mapped one-way into the corresponding Rust types. For example size_t maps to usize, but usize maps to uintptr_t.

TODO(b/283258442): ::core::ffi::* should eventually be a bidirectional mapping

C++	Rust
`ptrdiff_t`	`isize`
`size_t`	`usize`
`char16_t`	`u16`
`char32_t`	`u32` [^char32_t]
`char`	`::core::ffi::c_char` [^char]
`signed char`	`::core::ffi::c_schar`
`unsigned char`	`::core::ffi::c_uchar`
`short`	`::core::ffi::c_short`
`unsigned short`	`::core::ffi::c_ushort`
`int`	`::core::ffi::c_int`
`unsigned int`	`::core::ffi::c_uint`
`long`	`::core::ffi::c_long`
`unsigned long`	`::core::ffi::c_ulong`
`long long`	`::core::ffi::c_longlong`
`unsigned long long`	`::core::ffi::c_ulonglong`

Unsupported types

Bindings for the following types are not supported at this point:

nullptr_t and char8_t have not yet been implemented.
b/283268558: wchar_t is currently unsupported, for portability reasons.
b/254094650: int128_t is currently unsupported, because it does not yet have a decided ABI.

[^char32_t]: Unlike Rust char, char16_t and char32_t may contain invalid Unicode characters. [^char]: Note that Rust c_char and C++ char have different signedness in Google, or any other codebase with widespread use of unsigned char in x86.

TODO(jeanpierreda): document this in more detail.