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 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:

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.

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 · 2024-09-11T13:56:03Z

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amandasystems · 2024-09-11T13:57:12Z

r? nikomatsakis

amandasystems · 2024-09-11T13:58:10Z

@rustbot author

bors · 2024-09-23T05:19:22Z

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

lqd · 2024-09-27T14:58:15Z

@bors try @rust-timer queue

bors · 2024-09-27T14:59:26Z

⌛ Trying commit 4633c56 with merge b8cc49b...

…mpletely, r=<try> [WIP] Remove placeholders completely 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 rust-lang#123720 started. 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`. 2. 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

amandasystems · 2024-09-27T15:06:30Z

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 · 2024-09-27T16:42:45Z

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

bors · 2025-03-15T12:00:40Z

☔ The latest upstream changes (presumably #138464) made this pull request unmergeable. Please resolve the merge conflicts.

lcnr

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

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

bors · 2025-03-22T10:23:02Z

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

During work on rust-lang#130227, I discovered several situations not covered by any previously existing UI test. This commit introudces tests to cover that.

This rewires SCC annotations to have them be a separate, visitor-type data structure. It was broken out of rust-lang#130227, which needed them to be able to remove unused annotations after computation without recomputing the SCCs themselves. As a drive-by it also removes some redundant code from the hot loop in SCC construction for a performance improvement.

…c-annotations, r=<try> Decouple SCC annotations from SCCs This rewires SCC annotations to have them be a separate, visitor-type data structure. It was broken out of rust-lang#130227, which needed them to be able to remove unused annotations after computation without recomputing the SCCs themselves. As a drive-by it also removes some redundant code from the hot loop in SCC construction for a performance improvement. r? lcnr

During work on rust-lang#130227, I discovered several situations not covered by any previously existing UI test. This commit introudces tests to cover that.

…, r=lcnr Add tests for two untested cases of placeholder relations During work on rust-lang#130227, I discovered several situations not covered by any previously existing UI test. This commit introudces tests to cover that. r? lcnr

During work on rust-lang#130227, I discovered several situations not covered by any previously existing UI test. This commit introudces tests to cover that.

This rewires SCC annotations to have them be a separate, visitor-type data structure. It was broken out of rust-lang#130227, which needed them to be able to remove unused annotations after computation without recomputing the SCCs themselves. As a drive-by it also removes some redundant code from the hot loop in SCC construction for a performance improvement.

…r=lcnr Add tests for two untested cases of placeholder relations During work on rust-lang#130227, I discovered several situations not covered by any previously existing UI test. This commit introudces tests to cover that. r? lcnr

During work on rust-lang#130227, I discovered several situations not covered by any previously existing UI test. This commit introudces tests to cover that.

This commit picks up a few odd ends discovered during the work on rust-lang#130227. It adds some documentation and renames a few methods with too generic names to describe what they actually do. It also adds some debug output that was helpful during bug hunting.

…leywiser Some drive-by housecleaning in `rustc_borrowck` This commit picks up a few odd ends discovered during the work on rust-lang#130227. It adds some documentation and renames a few methods with too generic names to describe what they actually do. It also adds some debug output that was helpful during bug hunting and generally cleans up a few things (for my values of "clean"). r? lcnr

This commit picks up a few odd ends discovered during the work on rust-lang#130227. It adds some documentation and renames a few methods with too generic names to describe what they actually do. It also adds some debug output that was helpful during bug hunting.

Rollup merge of rust-lang#140257 - amandasystems:housecleaning, r=wesleywiser Some drive-by housecleaning in `rustc_borrowck` This commit picks up a few odd ends discovered during the work on rust-lang#130227. It adds some documentation and renames a few methods with too generic names to describe what they actually do. It also adds some debug output that was helpful during bug hunting and generally cleans up a few things (for my values of "clean"). r? lcnr

This commit moves all handling of placeholders, including those in type-tests, into a pre-processing step of the borrowcheck. Along the way it 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 rewrites type-tests that have the same behaviour. Error reporting is handled by introducing a new constraint kind that remembers which original outlives constraint caused an outlives-static. These work a lot like a HTTP redirect response during constraint search. `RegionInferenceContext` is now a lot simpler and only tracks opaque regions without caring about their origin. 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. 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.

This rewires SCC annotations to have them be a separate, visitor-type data structure. It was broken out of rust-lang#130227, which needed them to be able to remove unused annotations after computation without recomputing the SCCs themselves. As a drive-by it also removes some redundant code from the hot loop in SCC construction for a performance improvement.

This commit breaks out the logic of placheolder rewriting into its own preprocessing step. It's one of the more boring parts of rust-lang#130227. The only functional change from this is that the preprocessing step (where extra `r: 'static` constraints are added) is performed upstream of Polonius legacy, finally affecting Polonius. That is mostly a by-product, though.

…c-annotations, r=lcnr Decouple SCC annotations from SCCs This rewires SCC annotations to have them be a separate, visitor-type data structure. It was broken out of rust-lang#130227, which needed them to be able to remove unused annotations after computation without recomputing the SCCs themselves. As a drive-by it also removes some redundant code from the hot loop in SCC construction for a performance improvement. r? lcnr

bors · 2025-05-01T19:30:31Z

☔ The latest upstream changes (presumably #139965) made this pull request unmergeable. Please resolve the merge conflicts.

This commit breaks out the logic of placheolder rewriting into its own preprocessing step. It's one of the more boring parts of rust-lang#130227. The only functional change from this is that the preprocessing step (where extra `r: 'static` constraints are added) is performed upstream of Polonius legacy, finally affecting Polonius. That is mostly a by-product, though.

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

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amandasystems force-pushed the remove-placeholders-completely branch from 07b10ca to 6ab45ce Compare September 12, 2024 13:52

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amandasystems force-pushed the remove-placeholders-completely branch from da9ccef to 4633c56 Compare September 27, 2024 14:56

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amandasystems force-pushed the remove-placeholders-completely branch from 733bb16 to 55e292d Compare March 19, 2025 15:42

lcnr reviewed Mar 19, 2025

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amandasystems mentioned this pull request Apr 17, 2025

Add tests for two untested cases of placeholder relations #139960

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amandasystems mentioned this pull request Apr 17, 2025

Decouple SCC annotations from SCCs #139965

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amandasystems mentioned this pull request Apr 24, 2025

Some drive-by housecleaning in rustc_borrowck #140257

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amandasystems force-pushed the remove-placeholders-completely branch from b5a6590 to 38d5bb5 Compare April 28, 2025 12:02

amandasystems mentioned this pull request Apr 29, 2025

Move placeholder handling to a proper preprocessing step #140466

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Remove placeholders completely #130227

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