-Zassumptions-on-binders: rewrite alias outlives constraints more goodly by BoxyUwU · Pull Request #163938 · rust-lang/rust

BoxyUwU · 2026-10-07T12:23:06Z

best reviewed commit by commit. there are two improvements here.

First, we no longer rewrite alias outlives constraints if they're not in the current universe. Previously we did so and it resulted in rewriting alias outlives constraints to false because of there being no assumptions available (because we only know assumptions for the current universe). I think this was just a whoopsie from my original PR because the other places we compute candidates do check the universe first lol.

Second I merged the two codepaths which compute candidates via replacing current-universe-placeholders with bound vars. One of them did this for the whole constraint, the other did this just for the alias and elaborated the outlived region in Alias: 'a into an OR of all the regions larger than 'a. e.g. OR(Alias: 'b, Alias: 'c) given assumptions that 'a: 'b and 'a: 'c hold.

Merging them together makes the code a lot nicer but also is a slight correctness improvement as we can now get more specific constraints back when rewriting alias outlives, which means we need less general assumptions to prove them :3 See added test in the commit.

Though, after updating the expect in that test it still passed on main before this PR because of rust-lang/project-assumptions-on-binders#26

rustbot · 2026-10-07T12:23:13Z

khyperia is not on the review rotation at the moment.
They may take a while to respond.

BoxyUwU · 2026-10-07T13:03:00Z

is there nothing good in this world of ours

BoxyUwU · 2026-10-07T13:06:06Z

blocked on #163937 I guess lol

khyperia

new version is so much easier to read, nice! lgtm, r=me when test bless garbage blah blah

View changes since this review

rustbot · 2026-10-09T12:17:19Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

BoxyUwU · 2026-10-09T13:45:00Z

@bors r=khyperia

rust-bors · 2026-10-09T13:45:04Z

📌 Commit 7d43809 has been approved by khyperia

It is now in the queue for this repository.

@khyperia

…writing, r=khyperia -Zassumptions-on-binders: rewrite alias outlives constraints more goodly r? @khyperia best reviewed commit by commit. there are two improvements here. First, we no longer rewrite alias outlives constraints if they're not in the current universe. Previously we did so and it resulted in rewriting alias outlives constraints to `false` because of there being no assumptions available (because we only know assumptions for the current universe). I think this was just a whoopsie from my original PR because the other places we compute candidates *do* check the universe first lol. Second I merged the two codepaths which compute candidates via replacing current-universe-placeholders with bound vars. One of them did this for the whole constraint, the other did this just for the alias and elaborated the outlived region in `Alias: 'a` into an OR of all the regions larger than `'a`. e.g. `OR(Alias: 'b, Alias: 'c)` given assumptions that `'a: 'b` and `'a: 'c` hold. Merging them together makes the code a lot nicer but also is a slight correctness improvement as we can now get more specific constraints back when rewriting alias outlives, which means we need less general assumptions to prove them :3 See added test in the commit. Though, after updating the `expect` in that test it still passed on main before this PR because of rust-lang/project-assumptions-on-binders#26

@khyperia

…writing, r=khyperia -Zassumptions-on-binders: rewrite alias outlives constraints more goodly r? @khyperia best reviewed commit by commit. there are two improvements here. First, we no longer rewrite alias outlives constraints if they're not in the current universe. Previously we did so and it resulted in rewriting alias outlives constraints to `false` because of there being no assumptions available (because we only know assumptions for the current universe). I think this was just a whoopsie from my original PR because the other places we compute candidates *do* check the universe first lol. Second I merged the two codepaths which compute candidates via replacing current-universe-placeholders with bound vars. One of them did this for the whole constraint, the other did this just for the alias and elaborated the outlived region in `Alias: 'a` into an OR of all the regions larger than `'a`. e.g. `OR(Alias: 'b, Alias: 'c)` given assumptions that `'a: 'b` and `'a: 'c` hold. Merging them together makes the code a lot nicer but also is a slight correctness improvement as we can now get more specific constraints back when rewriting alias outlives, which means we need less general assumptions to prove them :3 See added test in the commit. Though, after updating the `expect` in that test it still passed on main before this PR because of rust-lang/project-assumptions-on-binders#26

…uwer Rollup of 24 pull requests Successful merges: - #161998 ( Support type-relative assoc item paths in generic param defaults & const param types) - #162106 (Helpful suggestions for incorrect address-of mutability (2)) - #162652 (Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`)) - #163337 (MIR move elimination [3/6]: PreciseLiveness) - #163938 (-Zassumptions-on-binders: rewrite alias outlives constraints more goodly) - #163939 (Better debug impls for some assumptions on binders types) - #163954 (fix(bootstrap/darwin): fix rpath for distributed LLD) - #163956 (Pass the unremapped path to the `rustc` invocation for doctests) - #164042 (Allow testing cg-gcc on any target) - #162443 (Do not retain `Normalization` goal errors in nested goals for `BestObligationVisitor:: non_trivial_candidates `) - #162908 (Fix - const parameters rejected when identical) - #163193 (cfi: mangle `f128` as `e` rather than `g` on platforms without `_Float128`) - #163634 (move overflow lint computation into decorator) - #163666 (Updates the expect message library/core/src/time.rs) - #163727 (rigid aliases to non-rigid for fully normalized check) - #163745 (replace `fully_monomorphized` with `cx.typing_env()`) - #163912 (Fix debug assert failure in `note_obligation_cause_code_inner`) - #163950 (don't treat inherited opaques as defining) - #163972 (const-eval: ICE when we hit a non-const fn) - #164000 (When mentioning that closure doesn't implement trait, point at closure) - #164007 ([rustdoc] Prefer local paths over remote ones when foreign item is locally reexported) - #164008 (properly ignore the current goal's usages) - #164017 (cg_llvm: Avoid some explicit casts to `*const c_char`) - #164025 (Less `CanonicalVarValues`)

@khyperia

Rollup merge of #163938 - BoxyUwU:abby_more_correct_alias_rewriting, r=khyperia -Zassumptions-on-binders: rewrite alias outlives constraints more goodly r? @khyperia best reviewed commit by commit. there are two improvements here. First, we no longer rewrite alias outlives constraints if they're not in the current universe. Previously we did so and it resulted in rewriting alias outlives constraints to `false` because of there being no assumptions available (because we only know assumptions for the current universe). I think this was just a whoopsie from my original PR because the other places we compute candidates *do* check the universe first lol. Second I merged the two codepaths which compute candidates via replacing current-universe-placeholders with bound vars. One of them did this for the whole constraint, the other did this just for the alias and elaborated the outlived region in `Alias: 'a` into an OR of all the regions larger than `'a`. e.g. `OR(Alias: 'b, Alias: 'c)` given assumptions that `'a: 'b` and `'a: 'c` hold. Merging them together makes the code a lot nicer but also is a slight correctness improvement as we can now get more specific constraints back when rewriting alias outlives, which means we need less general assumptions to prove them :3 See added test in the commit. Though, after updating the `expect` in that test it still passed on main before this PR because of rust-lang/project-assumptions-on-binders#26

…uwer Rollup of 24 pull requests Successful merges: - rust-lang/rust#161998 ( Support type-relative assoc item paths in generic param defaults & const param types) - rust-lang/rust#162106 (Helpful suggestions for incorrect address-of mutability (2)) - rust-lang/rust#162652 (Syntactically reject leading parenthesized precise capturing lists in bare trait object types (`(use<…>)+`)) - rust-lang/rust#163337 (MIR move elimination [3/6]: PreciseLiveness) - rust-lang/rust#163938 (-Zassumptions-on-binders: rewrite alias outlives constraints more goodly) - rust-lang/rust#163939 (Better debug impls for some assumptions on binders types) - rust-lang/rust#163954 (fix(bootstrap/darwin): fix rpath for distributed LLD) - rust-lang/rust#163956 (Pass the unremapped path to the `rustc` invocation for doctests) - rust-lang/rust#164042 (Allow testing cg-gcc on any target) - rust-lang/rust#162443 (Do not retain `Normalization` goal errors in nested goals for `BestObligationVisitor:: non_trivial_candidates `) - rust-lang/rust#162908 (Fix - const parameters rejected when identical) - rust-lang/rust#163193 (cfi: mangle `f128` as `e` rather than `g` on platforms without `_Float128`) - rust-lang/rust#163634 (move overflow lint computation into decorator) - rust-lang/rust#163666 (Updates the expect message library/core/src/time.rs) - rust-lang/rust#163727 (rigid aliases to non-rigid for fully normalized check) - rust-lang/rust#163745 (replace `fully_monomorphized` with `cx.typing_env()`) - rust-lang/rust#163912 (Fix debug assert failure in `note_obligation_cause_code_inner`) - rust-lang/rust#163950 (don't treat inherited opaques as defining) - rust-lang/rust#163972 (const-eval: ICE when we hit a non-const fn) - rust-lang/rust#164000 (When mentioning that closure doesn't implement trait, point at closure) - rust-lang/rust#164007 ([rustdoc] Prefer local paths over remote ones when foreign item is locally reexported) - rust-lang/rust#164008 (properly ignore the current goal's usages) - rust-lang/rust#164017 (cg_llvm: Avoid some explicit casts to `*const c_char`) - rust-lang/rust#164025 (Less `CanonicalVarValues`)

rustbot assigned khyperia Oct 7, 2026

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 Oct 7, 2026

This comment has been minimized.

Sign in to view

khyperia reviewed Oct 8, 2026

View reviewed changes

khyperia 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 Oct 8, 2026

This comment has been minimized.

Sign in to view

BoxyUwU added 2 commits October 9, 2026 13:11

Don't rewrite alias outlives in smaller universes

9bc17fd

Produce less general higher ranked alias constraints

7d43809

BoxyUwU force-pushed the abby_more_correct_alias_rewriting branch from 94bd998 to 7d43809 Compare October 9, 2026 12:17

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 9, 2026

This was referenced Oct 9, 2026

We don't handle false in root contexts lol rust-lang/project-assumptions-on-binders#26

Open

-Zassumptions-on-binders: replace region vars with bound vars when handling alias outlives' #164045

Merged

JonathanBrouwer mentioned this pull request Oct 9, 2026

Rollup of 10 pull requests #164048

Closed

This was referenced Oct 9, 2026

Rollup of 10 pull requests #164050

Closed

Rollup of 24 pull requests #164052

Merged

rust-bors Bot merged commit 46d4e1d into rust-lang:main Oct 9, 2026
14 checks passed

rustbot added this to the 1.101.0 milestone Oct 9, 2026

Navigation Menu

Repository navigation

-Zassumptions-on-binders: rewrite alias outlives constraints more goodly - #163938

-Zassumptions-on-binders: rewrite alias outlives constraints more goodly#163938
rust-bors[bot] merged 2 commits into
rust-lang:mainfrom
BoxyUwU:abby_more_correct_alias_rewriting

BoxyUwU commented Oct 7, 2026

Uh oh!

rustbot commented Oct 7, 2026

Uh oh!

This comment has been minimized.

BoxyUwU commented Oct 7, 2026

Uh oh!

BoxyUwU commented Oct 7, 2026

Uh oh!

khyperia left a comment •

edited by rustbot

Loading

Uh oh!

This comment has been minimized.

rustbot commented Oct 9, 2026

Uh oh!

BoxyUwU commented Oct 9, 2026

Uh oh!

rust-bors Bot commented Oct 9, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Conversation

BoxyUwU commented Oct 7, 2026

Uh oh!

rustbot commented Oct 7, 2026

Uh oh!

This comment has been minimized.

BoxyUwU commented Oct 7, 2026

Uh oh!

BoxyUwU commented Oct 7, 2026

Uh oh!

khyperia left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

rustbot commented Oct 9, 2026

Uh oh!

BoxyUwU commented Oct 9, 2026

Uh oh!

rust-bors Bot commented Oct 9, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

khyperia left a comment •

edited by rustbot

Loading