Remove placeholders completely

[amandasystems]

This PR does shotgun surgery on borrowck to remove all special handling of placeholders, completely replacing them with a preprocessing step that rewrites placeholder leaks into constraints, removing constraint propagation of placeholders and most of the logic used to detect placeholder violations during error reporting. This finishes what #123720 started. Due to the already sprawling scope of this PR, not all the breaks are clean. In particular, some of the error reporting code can almost certainly be further simplified.

The new method works like this:

1. during SCC construction, some information about SCC membership and reachability is retained
2. just after SCC construction, a constraint r - (from: to_invalid) - > 'static is added when r is the representative of an SCC and
   1. that SCC either has had its universe shrunk because it reaches a region with a smaller one (in which case to_invalid is the smallest-universed region it reaches), or if it reaches a region with a too large universe that isn't part of the SCC (in which case to_invalid is the region with a too large universe). In either case, from is also r.
3. some region reaches in r's SCC reaches another placeholder, reached, in which case the added constraint is r -> (reaches: reached) 'static. Through clever choice of defaults (chosing minimum elements), reached will be r if at all possible.

When tracing errors for diagnostics one of these special constraints along a path are treated much like a HTTP redirect: if we are explaining from: to and reach an edge with reaches: invalid we stop the search and start following reaches: invalid instead. When doing this the implicit edges x: 'static for every region x are ignored, since the search would otherwise be able to cheat by going through 'static and re-find the same edge again.

Type-tests are also rewritten to account for placeholder issues. In particular, if a bound implies : 'static, this is flagged using a new variant, and if a test is guaranteed to always fail (e.g. if an equals bound reaches different placeholders), it is replaced with a bound that is always unsatisfied.

A bunch of optimisations are possible:

• Conservatively add constraints, e.g. one per SCC. May worsen error tracing!
• as a final pass, allow fusing the annotations for the SCC after adding the extra constraints to remove unnecessary information and save memory. This could be done cheaply since we already iterate over the entire set of SCCs.
• currently, if constraints are added the entire set of SCCs are recomputed. This is of course rather wasteful, and we could do better. Especially since SCCs are added in dependency order. This would require a fully separate SCC module since the dynamic SCC combo we'd need now shares almost no properties with regular SCC computation. Given that this is meant to be a temporary work-around, that seems like too much work.

There are a bunch of rather nice bonuses:

• We now don't need to expose region indices in MirTypeckRegionConstraints to the entire crate. The only entry point is placeholder_region() so correctness of the indices is now guaranteed
• A lot of things that were previously iterations over lists is now a single lookup
• The constraint graph search functions are simple and at least one of them can now take a proper region as target rather than a predicate function. The only case that needs the predicate argument to find_constraint_path_to() is find_sub_region_live_at(), which may or may not be possible to work around.

r​? nikomatsakis

[rustbot]

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[amandasystems]

r? nikomatsakis

[amandasystems]

@rustbot author

[bors]

☔ The latest upstream changes (presumably #130724) made this pull request unmergeable. Please resolve the merge conflicts.

[lqd]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 4633c56 with merge b8cc49b...

[amandasystems]

Failing test cases left:

"for any two lifetimes" becomes "for any one", with the other replaced by the user-supplied region variable

• tests/ui/associated-types/associated-types-eq-hr.rs
• tests/ui/async-await/return-type-notation/issue-110963-early.rs
• tests/ui/coroutine/auto-trait-regions.rs
• tests/ui/higher-ranked/trait-bounds/hrtb-conflate-regions.rs

missing "note: due to current limitations in the borrow checker, this implies a 'static lifetime":

• tests/ui/generic-associated-types/bugs/hrtb-implied-2.rs

[bors]

☀️ Try build successful - checks-actions
Build commit: b8cc49b (b8cc49b4b6432618a26e2a74fec6b342ee94484e)

[lcnr]

have to take a bit more time to properly review this, some thoughts as i skimmed over it

[bors]

☔ The latest upstream changes (presumably #138719) made this pull request unmergeable. Please resolve the merge conflicts.