[WIP] Remove placeholders completely #130227

amandasystems · 2024-09-11T13:55:54Z

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 all logic used to detect placeholder violations during error reporting. This finishes what #123720 started.

The new method works like this:

during SCC construction, some information about SCC membership and reachability is retained
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.
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.

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 SCC.
currently, if constraints are added the entire set of SCCs are recomputed. This is of course rather wasteful, and we could maybe do better.

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

This version is maximally naive, and currently unsound.

This update extends the SCC metadata tracking a lot and uses the extra information to add p: 'static for any placeholder that reaches another placeholder. It also inlines the few measly bits of `init_free_and_bound_regions()` that still remain as relevant. This increases the constructor for `RegionInferenceContext`s somewhat, but I still think it's readable. The documentation for `init_free_and_bound_regions()` was out of date, and the correct, up to date version is available in the various places where the logic was moved.

help the search for who is to blame.

rustbot · 2024-09-11T13:56:03Z

r? @TaKO8Ki

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amandasystems · 2024-09-11T13:57:12Z

r? nikomatsakis

amandasystems · 2024-09-11T13:58:10Z

@rustbot author

This commit makes the search for blamable outlives constraints treat an added `x: 'static` edge as a redirect to figure out why it reached an invalid placeholder. As a drive-by it also refactors the blame search somewhat, renames a few methods, and allows iterating over outgoing constraints without the implied edges from 'static.

I've switched to encoding our new outlives-static constraints with two extra parameters: the source and drain of a path in the original constraint graph that caused a placeholder issue. This handles all of the soundness issues, leaving 16 failing diagnostics.

amandasystems added 3 commits August 30, 2024 12:25

Preliminary attempt at removing placeholders out of the borrow checker.

698631c

This version is maximally naive, and currently unsound.

Use annotions on the outlives-static constraints to

4214d06

help the search for who is to blame.

rustbot assigned TaKO8Ki Sep 11, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 11, 2024

rustbot assigned nikomatsakis and unassigned TaKO8Ki Sep 11, 2024

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 11, 2024