Small @nogc experience report

[Shachar Shemesh]

On 08/09/18 11:07, Peter Alexander wrote:
> I'd love to know if anyone is making good use of @nogc in a larger code 
> base and is happy with it. Weka.io?

No, sorry.

Actually, yes.

Well, sortof.

The main Weka codebase hardly uses any annotations of any kind. Not 
@nogc nor others. This is in the process of being amended, somewhat, but 
is not a high priority. We do use run-time detection of GC use. I.e. - 
we've modified the druntime to invoke a callback if a GC allocation 
takes place, and we then log that fact (with traceback). We then are 
able to search logs for GC allocations and remove them.

So that's the "no" part.

As pointed out, one of the main motivations for running Mecca was to 
clear up the strange solutions that have accumulated over the years. As 
such, the Mecca code does have @nogc in much more wide use.

So, yes.

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.

The good news is that mecca is available, and you can just dub it into 
your project and use it, so you don't have to repeat that whole set of work.

Mecca was advertised mostly around the reactor. While it is a piece of 
work I am very proud of, it is not the only part there, nor is it 
necessary to use mecca's reactor if you want just the library. In fact, 
nothing outside of mecca.reactor depends on the reactor running.

Hope this helps,
Shachar