Elaborate on dataflow outputs for region constraints

[amandasystems]

No description provided.

[amandasystems]

I see Zed snuck in some Markdown normalisation too while it was at it. Sorry about that, let me know if that's inexcusable and I will revert those changes.

[lqd]

I will revert those changes

Please do yes, if it's not too hard. It will make review easier.

I'll take a closer look when I have more time, but I noted a couple of things:

• "With -Z dump-mir-graphviz=yes, you will also get Graphviz files for the outlives constraints" is that the case? Did you maybe mean -Zdump-mir=nll?
• I don't think these visualizations are from MIR dataflow, so the new paragraph may need to be moved to somewhere more related to borrowck

[amandasystems]

Please do yes, if it's not too hard. It will make review easier.

Done!

"With -Z dump-mir-graphviz=yes, you will also get Graphviz files for the outlives constraints" is that the case? Did you maybe mean -Zdump-mir=nll?

Huh! Apparently, -Z dump-mir=fn is enough to get those. I assumed it wouldn't drop graphviz files without the Graphviz option, but apparently it does! dump-mir=nll also works and now I'm worried it will dump different graphs.

I don't think these visualizations are from MIR dataflow, so the new paragraph may need to be moved to somewhere more related to borrowck

That's probably true; this should be in the borrowck chapter, maybe? Do you have a suggestion off the top of your head or should I go digging?

[lqd]

Not from the top of my head but I will look for one -- but yeah I agree this should be in the borrowck chapter most likely.

[lqd]

(Sorry for taking so long to get back to this.)

So since these are not dataflow outputs, I agree we should add it to the borrow_check chapter.

To do so, I think we should add a dedicated borrow_check/debugging.md page, and have the "Region constraint graphs and their SCCs" section there. We would mention that these come from -Zdump-mir=nll -- and that will give us the occasion to also describe the NLL dumps there in the future.

We can link to that page from the bottom of src/borrow_check.md instead of src/compiler-debugging.md (but we can leave the unflatten paragraph there, it's a good addition next to the graphviz section).

[lqd]

Suggested change

	The [general instructions on debugging dataflow]() also apply to graphs generated from
	The [general instructions on debugging dataflow](../mir/dataflow.md) also apply to graphs generated from

[lqd]

This line will need to be wrapped as well.

[lqd]

Suggested change

	This is particularly useful for complicated region outlives graphs from
	[MIR Dataflow](mir/dataflow.md).
	This is particularly useful for complicated region outlives graphs from
	[the borrow checker](borrow_check/debugging.md).

[jieyouxu]

@lqd btw, you can also edit @amandasystems's branch directly for trivial things like fixing links. We can also try to get this merged so it doesn't drown from merge conflicts. And it's perfectly fine if it still has inaccuracies/omissions, we can always follow-up, it's just docs after all!