Small @nogc experience report

[Peter Alexander]

I recently wrote a small program of ~600 lines of code to solve 
an optimisation puzzle. Profiling showed that GC allocations were 
using non-trivial CPU, so I decided to try and apply @nogc to 
remove allocations. This is a small experience report of my 
efforts.

1. My program does some initialisation before the main solver. I 
don't care about allocations in the initialisation. Since not all 
of my code needed to be @nogc, I couldn't add `@nogc:` to the top 
of the file and instead had to refactor my code into 
initialisation parts and main loop parts and wrap the latter in 
@nogc { ... }. This wasn't a major issue, but inconvenient.

2. For my code the errors were quite good. I was immediately able 
to see where GC allocations were occurring and fix them.

3. It was really frustrating that I had to make the compiler 
happy before I was able to run anything again. Due to point #1 I 
had to move code around to restructure things and wanted to make 
sure everything continued working before all GC allocations were 
removed.

4. I used std.algorithm.topNCopy, which is not @nogc. The error 
just says "cannot call non- at nogc function [...]". I know there 
are efforts to make Phobos more @nogc friendly, but seeing this 
error is like hitting a brick wall. I wouldn't expect topNCopy to 
use GC, but as a user, what do I do with the error? Having to dig 
into Phobos source is unpleasant. Should I file a bug? What if it 
is intentionally not @nogc for some subtle reason? Do I rewrite 
topNCopy?

5. Sometimes I wanted to add writeln to my code to debug things, 
but writeln is not @nogc, so I could not. I could have used 
printf in hindsight, but was too frustrated to continue.

6. In general, peppering my code with @nogc annotations was just 
unpleasant.

7. In the end I just gave up and used -vgc flag, which worked 
great. I had to ignore allocations from initialisation, but that 
was easy. It might be nice to have some sort of `ReportGC` RAII 
struct to scope when -vgc reports the GC.