[tor-bugs] #28193 [Core Tor/Tor]: Compile-time assertion

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#28193: Compile-time assertion
-------------------------+------------------------------
 Reporter:  riastradh    |          Owner:  (none)
     Type:  enhancement  |         Status:  new
 Priority:  Medium       |      Component:  Core Tor/Tor
  Version:               |       Severity:  Normal
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
 Reviewer:               |        Sponsor:
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 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__`.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28193>
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