[WIP] On-demandify region mapping

[cramertj]

Attempt at #41057

This approach wound up introducing lots of churn, and I'm opening this primarily as an opportunity to discuss the change and see if there's a more principled way to break up region maps.

In particular, I'd like to see if there's a way to keep CodeExtents unique so that functions with access to only CodeExtents can lookup the associated function. Without this, I'm not sure of the right way to write things like fmt::Debug for CodeExtent (in src/librustc/middle/region.rs), or the cfg-generating (src/librustc/cfg/construct.rs) or error-reporting (src/librustc/infer/error_reporting/mod.rs), since they all try to access region maps using only a CodeExtent, and doing anything else would require a massive refactor (such as adding a NodeId to note_and_explain_region).

Thoughts?

r? @nikomatsakis

[cramertj]

One possibility that occurred to me was to change CodeExtent from CodeExtent(u32) to CodeExtent(u32, DefId) where DefId is the ID of the top-level function with which it's associated. Thoughts?

[nikomatsakis]

@cramertj those are copied around quite frequently, and that would make the data structures much larger. I'll try to take tomorrow and form a more educated opinion. Thanks for the PR!

[cramertj]

@nikomatsakis Yeah, that's why I didn't do it. :) I opened the PR hoping we could come to a better solution.

[bors]

☔ The latest upstream changes (presumably #41357) made this pull request unmergeable. Please resolve the merge conflicts.

[leonardo-m]

Can't you use something like this to reduce the amount of manual refactoring? http://coccinelle.lip6.fr/

[nikomatsakis]

@leonardo-m plausibly, if it worked on rust, but I think that what @cramertj and I have been talking about is really more what we want the target code to look like, rather than the churn it takes to get there. That said, I've been doing a lot of refactoring on this PR on a side branch, and I think I'm going to close it for now (I've discussed with @cramertj off line), until I reach some point I am happy with.