[MIR] Make things somewhat prettier

[nagisa]

After the MIR drop PR I was not quite satisfied with the free-standing functions in the mir::build::scope. They are free-standing purely to satisfy the borrowck. This PR satisfies borrowck in a different way by temporarily taking over the ownership of the scopes array (the with_scopes method). Everything else is pretty much unchanged, only moved around.

r? @nikomatsakis

[bors]

☔ The latest upstream changes (presumably #31564) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

So, I personally prefer the free-standing functions. This way, I can read the code and I know that it doesn't use state in a way that would be invalid. Another option of course is to further modify the scopes vector to isolate the state better.

[nikomatsakis]

Another option of course is to further modify the scopes vector to isolate the state better.

To clarify what I meant by this: the reason I initially introduced the cfg variable was precisely the same: so that I could add methods that would be callable even when some parts of the Builder were borrowed. Maybe we need/want a 'scopes' subset, not sure. But in general if subsetting (either via free fn or another type) is an option, I like it because otherwise I get worried we will recursively touch the scopes vector accidentally.

[nikomatsakis]

Going to go ahead and close this PR then. @nagisa let me know if you want to discuss alternatives.