Allow assert() with custom error message

[mdsteele]

Many other languages allow adding a custom error message to assertions, to provide better context if the assertion fails. For example:

assert(x > 0, "x must be positive, but it was {} instead", x);

I've been finding myself missing this in Zig. I don't know whether it's possible to support both assert(cond) and assert(cond, fmt, ...) with the same function name, but if not, then we could have, say, assert(cond) and assertMsg(cond, fmt, ...).

[andrewrk]

I believe this is related to #1304. One consideration here is whether the assert is going to invoke unreachable or @panic. They're the same thing in Debug and ReleaseSafe mode, but in ReleaseFast and ReleaseSmall modes, unreachable is a hint to the optimizer (because it invokes undefined behavior) and @panic is guaranteed to abort.

[PavelVozenilek]

There's precedent for such a desirable feature. Long, long ago someone came up with an idea of advanced assert in C++. Basically, when assert fires, it could also print multiple interesting values.
http://www.drdobbs.com/cpp/enhancing-assertions/184403745

It was very clever, but unfortunately, it severely impacted compilation time, so the effort to get it into Boost was abandoned.

Some notes about Zig implementation:

1. Such assert should evaluate its second and next parameters lazily, to avoid performance/memory impact.

   assert(x > 0, "foo = {}", foo());

   would became (in C like-code):

   #if ASSERTS_ENABLED
   if (x <= 0) {
     assert(false, "foo = {}", foo());
   }
   #endif
   
   

2. Tested values should print out implicitly:

   assert(foo() < bar());

   would be equivalent of:

   #if ASSERTS_ENABLED
   int  foo_result, bar_result;
   foo_result = foo();
   bar_result = bar();
   assert(foo_result < bar_result, "foo = {}, bar = {}", foo_result, bar_result);
   #endif
   
   

3. If some function is called twice, the compiler should rearrange the code to call it only once:

   assert(foo() > 0, "foo() returned ", foo());

   should became:

   #if ASSERTS_ENABLED
   int  foo_result;
   foo_result = foo();
   assert(foo_result > 0, "foo returned", foo_result);
   #endif
   
   
   

4. It should be possible to omit the formatting string altogether:

   assert(x > 0, y, z);

   should automagically transform itself into:

   assert(x > 0, "x = {}, y = {}, z = {}", x, y, z);

[andrewrk]

This is solved in c2db077 which makes the distinction between std.debug.assert and std.testing.expect. The testing module can have any number of handy functions, including ones that do var args printing.