pragma(breakpoint)

[Monkyyy]

On Friday, 22 August 2025 at 09:11:55 UTC, Ben Jones wrote:
> In my dconf talk I mentioned an idea for helping to debug 
> template metaprogramming issues and took a first pass at 
> implementing my idea here: 
> https://github.com/dlang/dmd/pull/21758
>
> The insight I had is that when I debug TMP code, I spray my 
> code with pragma(msg) and am doing basically printf debugging.  
> What would a "real" debugger for metaprograms look like?
>
> I've implemented the first pieces that I think would be in such 
> a tool:
>
> * a way to specify breakpoints (pragma(breakpoint)
> * a way to print a "backtrace"
>
> In my implementation those two are intertwined and 
> pragma(breakpoint) just prints the "backtrace" at that point in 
> semantic analysis which is computed by tracing the scopes.
>
> Ideally (eventually) pragma(breakpoint) would pause the 
> compiler and provide analogues to normal debugger functions 
> such as examining values (I think this would be the ability to 
> info about various AST nodes), and once you're done inspecting 
> the state of the compiler, you can "continue" to keep compiling.
>
> Something that fell out naturally in the implementation is that 
> you can make the breakpoints conditional by putting them in a 
> static if block.
>
> Does this seem like a useful addition?  What's missing from my 
> outline and/or implementation.  How do people think they might 
> use a tool like this?

Upstream is terrified of the concept of exposing th order of 
compilation believing it's something that could change and maybe 
one day they will update the compiler away from the current 
model. (I find this notion laughable)

Compile values are all "bugs" if they change, but such bugs exist 
all over the place, it's only useful to keep track of the ones 
that don't step on valid syntax *and* don't crash.

If you a) got comfortable with the bugs and b) used them is a 
sensible way, you could implement CT values that mutate and you 
can grab snap shots of when you want.