static analysis: how to test code reachability at compile time

[Timothee Cour]

 I'd like to have the following tools to help with static analysis & code
coverage:

1) test whether code is reachable or not.

Of course it only works for executables and may have some caveats (eg:
goto), but its still useful diagnostic tool.

__traits(isReachable): true when the current context can be reached via at
least one code path starting from main():

----
void fun1(){
//__traits(isReachable) would be false here because even though fun1 is
called by fun2, it can never be called starting from main().
}
void fun2(){fun1;}
void fun3(){} // would also be false, because of static if(temp); with temp
being known to be 0 at compile time.
void fun4(){
return;
//__traits(isReachable) would be false here because all code paths return
above this point.
}
void main(){enum temp=0; static if(temp) fun3; fun4;}
----

When a pointer to a function is taken, we may want to assume by convention
that __traits(isReachable) is true.

This would enable for example to make sure a given code is never called.
Note, static assert(0) is not the same.

2)
static code call graph: know caller/calling functions for a given function.


3) static code coverage
this would allow us to tell at compile time the code coverage of a module;
it is larger than runtime coverage, but still useful to find dead code.
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