On 9/19/18 1:13 PM, Shachar Shemesh wrote:
There is a catch, though. Writing Mecca with @nogc required
re-implementing quite a bit of druntime. Mecca uses its own exception
allocations (mkEx, just saw it's not yet documented, it's under
mecca.lib.exception). The same module also has "enforceNGC". We also
have our own asserts. This is partially to support our internal logging
facility, that needs a static list of formats, but it also solves a very
important problem with D's @nogc:
void func() @nogc {
assert(condition, string); // string is useless without actual info
about what went wrong.
assert(condition, format(string, arg, arg)); // No good - format is
not @nogc
ASSERT!"format"(condition, arg, arg); // @nogc and convenient
}
So, yes, we do use @nogc, but it took a *lot* of work to do it.
I'm running into this coincidentally right now, when trying to debug a
PR. I found I'm getting a range error deep inside a phobos function. But
because Phobos is trying to be pure @nogc nothrow @safe, I can do almost
nothing to display what is wrong.
What I ended up doing is making an extern(C) hook that had the "right"
attributes, even though it's not @nogc (let's face it, you are about to
crash anyway).
But it got me thinking, what a useless interface to display errors we
have! Inside Throwable, there is the function toString(someDelegate
sink) which prints out the exception trace.
Near the front there is this:
if (msg.length)
{
sink(": "); sink(msg);
}
My, wouldn't it be nice to be able to override this! And forget about
the whole msg BS. When an exception trace is printed, there are almost
no restrictions as to what can be done. We should delay the generation
of the message until then as well! Not to mention that if we can output
things piecemeal through the sink, we don't even have to allocate at all.
I'm going to write up a more detailed post on this, but it's annoying to
throw exceptions without any information EXCEPT what can be converted
into a string at runtime at the time of exception. All that is missing
is this hook to generate the message.
-Steve
