C++ Interop: Import object-like macros

[ivanaivanovska]

Design doc: Carbon: C++ interop for C/C++ object-like macros.

Summary:

An object-like macro that evaluates to a constant expression at compile time is imported as a constant in Carbon if the value can be represented in Carbon.

Supported

• Primitive literals:

  • Integer literals (123, 0xFF, 42L) -> imported as Carbon integer const.
  • Floating-point literals (3.14, 1.0e-9) -> imported as Carbon floating-point const.
  • Boolean literals (true, false) -> imported as Carbon bool const.
  • Character literals ('a', '\n', '\0') -> imported as Carbon char const.
  • String literals ("hello") -> imported as Carbon str const.
  • Null pointer literals (nullptr) -> imported as Carbon Core.CppCompat.NullptrT.

• Constant expressions:

  Any expression that evaluates to a constant of a supported type, including operators like unary, binary, casting etc., is supported. There are no restrictions on the number or type of operands.

  #define ADDITION 1+2+3
  
  #define MyIntLambda (([] { return 7; })())
  
  #define CONCAT_STR(A, B) #A #B
  #define Indirect CONCAT_STR(x, y)

• Macros expanding to an identifier:

  • enum constant: enum Color { Red = 1 }; #define MY_RED ::Red
  • constexpr variables: constexpr int X = 10; #define M ::X

Not supported:

• User-defined literals: 100_km

• Macros expanding to a non-primitive literal:

  • struct or class type: #define ORIGIN MyPoint{0,0}

• Macros expanding to non-constant expressions:

  • int x; #define VAL x

• Macros not expanding to expressions:

  • Type names: #define MY_INT int
  • Statements or keywords: #define RET return, #define FOREVER for(;;)
  • Empty macros: #define EMPTY