Compile-time assertion

The enclosed patch implements a CTASSERT(condition) macro that, at the top level of a file, causes a compiler error if the constant expression condition evaluates to false. This is conciser than

#if !condition
#error Condition was false.
#endif

and applicable in situations that #if cannot handle, because CTASSERT allows any constant expressions, including, e.g., sizeof, while #if is limited to C preprocessor conditional expansion. (Conversely, CTASSERT can't be used with defined(...), so it does not subsume #if.)

nickm suggested that it should be in src/lib/cc, so I put it there. If you think it should be in a different file, go for it.

The patch uses a couple of different mechanisms to implement it, depending on what the compiler supports:

• If C11 is available, it expands to _Static_assert(condition, #condition). Obviously if you have a C11 compiler this is the best way to do it, because it is most likely to give the best error message.
• If any of __COUNTER__, or __INCLUDE_LEVEL__ and ___LINE__, or just __LINE__, is available, their macro values are expanded and appended to a name tor_ctassert_ which is typedef'd to an array type with negative length if the condition is false, and positive length if the condition is true. This has zero run-time overhead; the use of __COUNTER__, &c., is to attain a unique name, which is guaranteed with __COUNTER__, and highly likely with __INCLUDE_LEVEL__ and __LINE__.