trait_solver: normalize next-gen region constraints by Dnreikronos · Pull Request #158404 · rust-lang/rust

Dnreikronos · 2026-06-25T15:11:34Z

-zassumptions-on-binders can produce next-gen region constraints that mean the same thing but don't have the same shape. in this case object candidate merging stayed ambiguous and instance resolution later hit the ice. imo normalizing the constraint at the response boundary is the least weird place for this, because candidate selection shouldn't need to know which vtable looks nicer.

canonicalize and evaluate the next-gen region constraint before response canonicalization, then cover the dyn derived

supertrait case from the issue. lgtm locally with the focused test, tests/ui/traits/next-solver, and tests/ui/assumptions_on_binders. idk if there's a better home for the helper call, but ltm this keeps the fix pretty narrow.

rustbot · 2026-06-25T15:11:42Z

r? @camelid

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camelid · 2026-07-01T21:28:20Z

I don't have familiarity with this code, so rerolling. @rustbot reroll

mati865 · 2026-07-02T11:02:19Z

Me neither, @rustbot reroll

hanna-kruppe · 2026-07-02T16:25:09Z

Me three. Let's try something more targeted: r? types

lcnr · 2026-07-03T14:01:48Z

r? BoxyUwU

rustbot · 2026-07-03T14:01:52Z

BoxyUwU is currently at their maximum review capacity.
They may take a while to respond.

BoxyUwU · 2026-07-14T19:21:27Z

so it makes sense to me why having a non-normal form constraint here would cause issues 🤔 what I don't get is why we have a non-normal one here. Though admittedly we don't put much effort into tracking where/when we expect stuff to be normal form lol.

I would add an assertion to this that self.delegate.get_solver_region_constraint() returns something for which evaluating it & computing a canonical form doesn't actually do anything. In theory the only way we should call compute_external_query_constraints should be if we've already called eagerly_handle_placeholders which should evaluate/convert to normal form the constraints and then set that back into the infcx/delegate 🤔

it seems to me that for whatever reason eagerly_handle_placeholders isnt actually giving back normal form constraints for whatever reason, I would expect the "right" fix would be to look into why that's happening and see if we can fix that

View changes since the review

Canonicalize and evaluate next-gen region constraints before query response canonicalization so equivalent constraints merge structurally.

Cover the dyn object supertrait case that used to leave equivalent next-gen region constraints in different shapes.

…ize_next_gen_regions, r=BoxyUwU trait_solver: normalize next-gen region constraints fixes rust-lang#157729 -zassumptions-on-binders can produce next-gen region constraints that mean the same thing but don't have the same shape. in this case object candidate merging stayed ambiguous and instance resolution later hit the ice. imo normalizing the constraint at the response boundary is the least weird place for this, because candidate selection shouldn't need to know which vtable looks nicer. canonicalize and evaluate the next-gen region constraint before response canonicalization, then cover the dyn derived supertrait case from the issue. lgtm locally with the focused test, tests/ui/traits/next-solver, and tests/ui/assumptions_on_binders. idk if there's a better home for the helper call, but ltm this keeps the fix pretty narrow.

Rollup of 16 pull requests Successful merges: - #138618 (Support using const pointers in asm `const` operand) - #157962 (Function item should not be used as const arg) - #158404 (trait_solver: normalize next-gen region constraints) - #158709 (rustdoc: warn on improperly interleaved HTML/MD) - #159720 (document #[global_allocator] constraints) - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled) - #159740 (reuse regular exported_non_generic_symbols logic in Miri) - #159780 (check `extern "custom"` function pointers) - #159786 (rustdoc-js: ignore editor temp files in test folder discovery) - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new) - #155388 (stepping into where-clauses during normalization may be productive) - #155914 (when bailing on ambiguity, don't force other results to ambig) - #159411 ([rustdoc] Correctly handle output options with --show-coverage) - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`) - #159809 (Avoid `#[target_features]`) - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate)

…ize_next_gen_regions, r=BoxyUwU trait_solver: normalize next-gen region constraints fixes rust-lang#157729 -zassumptions-on-binders can produce next-gen region constraints that mean the same thing but don't have the same shape. in this case object candidate merging stayed ambiguous and instance resolution later hit the ice. imo normalizing the constraint at the response boundary is the least weird place for this, because candidate selection shouldn't need to know which vtable looks nicer. canonicalize and evaluate the next-gen region constraint before response canonicalization, then cover the dyn derived supertrait case from the issue. lgtm locally with the focused test, tests/ui/traits/next-solver, and tests/ui/assumptions_on_binders. idk if there's a better home for the helper call, but ltm this keeps the fix pretty narrow.

Rollup of 20 pull requests Successful merges: - #138618 (Support using const pointers in asm `const` operand) - #157962 (Lower paths to functions in const args as ConstKind::Error) - #158404 (trait_solver: normalize next-gen region constraints) - #158709 (rustdoc: warn on improperly interleaved HTML/MD) - #159720 (document #[global_allocator] constraints) - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled) - #159738 (implement `CovariantUnsafeCell`) - #159740 (reuse regular exported_non_generic_symbols logic in Miri) - #159780 (check `extern "custom"` function pointers) - #159786 (rustdoc-js: ignore editor temp files in test folder discovery) - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new) - #155388 (stepping into where-clauses during normalization may be productive) - #155914 (when bailing on ambiguity, don't force other results to ambig) - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`) - #159676 (Update wasm-component-ld to 0.5.27) - #159730 (allow accessing the contents of UnsafeCell without going through get) - #159809 (Avoid `#[target_features]`) - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate) - #159853 (Updated expect messages for `CString` struct and method documentation) - #159877 (Revert "Export `derive` at `core::derive` and `std::derive`")

…n_regions, r=BoxyUwU trait_solver: normalize next-gen region constraints fixes #157729 -zassumptions-on-binders can produce next-gen region constraints that mean the same thing but don't have the same shape. in this case object candidate merging stayed ambiguous and instance resolution later hit the ice. imo normalizing the constraint at the response boundary is the least weird place for this, because candidate selection shouldn't need to know which vtable looks nicer. canonicalize and evaluate the next-gen region constraint before response canonicalization, then cover the dyn derived supertrait case from the issue. lgtm locally with the focused test, tests/ui/traits/next-solver, and tests/ui/assumptions_on_binders. idk if there's a better home for the helper call, but ltm this keeps the fix pretty narrow.

rust-bors · 2026-07-25T12:38:28Z

💔 Test for 7e65599 failed: CI

Rollup of 20 pull requests Successful merges: - #138618 (Support using const pointers in asm `const` operand) - #157962 (Lower paths to functions in const args as ConstKind::Error) - #158404 (trait_solver: normalize next-gen region constraints) - #158709 (rustdoc: warn on improperly interleaved HTML/MD) - #159720 (document #[global_allocator] constraints) - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled) - #159738 (implement `CovariantUnsafeCell`) - #159740 (reuse regular exported_non_generic_symbols logic in Miri) - #159780 (check `extern "custom"` function pointers) - #159786 (rustdoc-js: ignore editor temp files in test folder discovery) - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new) - #155388 (stepping into where-clauses during normalization may be productive) - #155914 (when bailing on ambiguity, don't force other results to ambig) - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`) - #159676 (Update wasm-component-ld to 0.5.27) - #159730 (allow accessing the contents of UnsafeCell without going through get) - #159809 (Avoid `#[target_features]`) - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate) - #159853 (Updated expect messages for `CString` struct and method documentation) - #159877 (Revert "Export `derive` at `core::derive` and `std::derive`")

JonathanBrouwer · 2026-07-25T17:35:04Z

@bors retry
Spurious

…ize_next_gen_regions, r=BoxyUwU trait_solver: normalize next-gen region constraints fixes rust-lang#157729 -zassumptions-on-binders can produce next-gen region constraints that mean the same thing but don't have the same shape. in this case object candidate merging stayed ambiguous and instance resolution later hit the ice. imo normalizing the constraint at the response boundary is the least weird place for this, because candidate selection shouldn't need to know which vtable looks nicer. canonicalize and evaluate the next-gen region constraint before response canonicalization, then cover the dyn derived supertrait case from the issue. lgtm locally with the focused test, tests/ui/traits/next-solver, and tests/ui/assumptions_on_binders. idk if there's a better home for the helper call, but ltm this keeps the fix pretty narrow.

…uwer Rollup of 25 pull requests Successful merges: - #138618 (Support using const pointers in asm `const` operand) - #157962 (Lower paths to functions in const args as ConstKind::Error) - #158404 (trait_solver: normalize next-gen region constraints) - #158709 (rustdoc: warn on improperly interleaved HTML/MD) - #159174 (Fix implicit_provenance_casts warnings on Xous) - #159179 (enable `unreachable_cfg_select_predicates` lint as part of `unused` lint group) - #159518 (iter: extend step_by specialization to cover StepBy<RangeIter<{integer}>>) - #159673 (bootstrap: forward -fdebug-prefix-map when using cc) - #159700 (Split non-local `semicolon_in_expressions_from_macros` into a separate lint) - #159720 (document #[global_allocator] constraints) - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled) - #159738 (implement `CovariantUnsafeCell`) - #159740 (reuse regular exported_non_generic_symbols logic in Miri) - #159780 (check `extern "custom"` function pointers) - #159785 (Share _Unwind_Exception definition between native and wasm) - #159786 (rustdoc-js: ignore editor temp files in test folder discovery) - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new) - #155388 (stepping into where-clauses during normalization may be productive) - #155914 (when bailing on ambiguity, don't force other results to ambig) - #159204 (Add support to caller_location to rustc_public) - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`) - #159676 (Update wasm-component-ld to 0.5.27) - #159695 (proc_macro: Fix cfg_attr inner attrs in file modules) - #159730 (allow accessing the contents of UnsafeCell without going through get) - #159809 (Avoid `#[target_features]`)

…ize_next_gen_regions, r=BoxyUwU trait_solver: normalize next-gen region constraints fixes rust-lang#157729 -zassumptions-on-binders can produce next-gen region constraints that mean the same thing but don't have the same shape. in this case object candidate merging stayed ambiguous and instance resolution later hit the ice. imo normalizing the constraint at the response boundary is the least weird place for this, because candidate selection shouldn't need to know which vtable looks nicer. canonicalize and evaluate the next-gen region constraint before response canonicalization, then cover the dyn derived supertrait case from the issue. lgtm locally with the focused test, tests/ui/traits/next-solver, and tests/ui/assumptions_on_binders. idk if there's a better home for the helper call, but ltm this keeps the fix pretty narrow.

…uwer Rollup of 25 pull requests Successful merges: - #159825 (codegen: handle OperandValue::Uninit in codegen_return_terminator) - #138618 (Support using const pointers in asm `const` operand) - #157962 (Lower paths to functions in const args as ConstKind::Error) - #158404 (trait_solver: normalize next-gen region constraints) - #158709 (rustdoc: warn on improperly interleaved HTML/MD) - #159174 (Fix implicit_provenance_casts warnings on Xous) - #159179 (enable `unreachable_cfg_select_predicates` lint as part of `unused` lint group) - #159518 (iter: extend step_by specialization to cover StepBy<RangeIter<{integer}>>) - #159673 (bootstrap: forward -fdebug-prefix-map when using cc) - #159700 (Split non-local `semicolon_in_expressions_from_macros` into a separate lint) - #159720 (document #[global_allocator] constraints) - #159732 (optimization: don't look for diagnostic/canonical items without rustc_attrs enabled) - #159738 (implement `CovariantUnsafeCell`) - #159740 (reuse regular exported_non_generic_symbols logic in Miri) - #159780 (check `extern "custom"` function pointers) - #159786 (rustdoc-js: ignore editor temp files in test folder discovery) - #159819 (std::sync::poison: disable auto_cfg on PoisonError::new) - #155388 (stepping into where-clauses during normalization may be productive) - #155914 (when bailing on ambiguity, don't force other results to ambig) - #159204 (Add support to caller_location to rustc_public) - #159439 (Fix(lib/fs/win): Fall back on Win32 delete for `Dir::remove_file`) - #159676 (Update wasm-component-ld to 0.5.27) - #159695 (proc_macro: Fix cfg_attr inner attrs in file modules) - #159730 (allow accessing the contents of UnsafeCell without going through get) - #159809 (Avoid `#[target_features]`)

jhpratt · 2026-07-26T04:12:20Z

@bors r- #159926 (comment)

diff of stderr:

20 LL | |     Self::Item: Baz,
21    | |____________________^
22    = note: ...which again requires coherence checking all impls of trait `Foo`, completing the cycle
- note: cycle used when checking that `<impl at $DIR/next-solver-region-resolution.rs:12:1: 14:20>` is well-formed
-   --> $DIR/next-solver-region-resolution.rs:12:1
+ note: cycle used when checking that `<impl at $DIR/next-solver-region-resolution.rs:18:1: 20:21>` is well-formed
+   --> $DIR/next-solver-region-resolution.rs:18:1
25    |
- LL | / impl<'a, T> Foo for &'a T
+ LL | / impl<'a, T> Foo for &T
27 LL | | where
- LL | |     Self::Item: 'a,
-    | |___________________^
+ LL | |     Self::Item: Baz,
+    | |____________________^
30    = note: for more information, see <https://rustc-dev-guide.rust-lang.org/overview.html#queries> and <https://rustc-dev-guide.rust-lang.org/query.html>
31 
32 error: aborting due to 1 previous error

Note: some mismatched output was normalized before being compared
- note: cycle used when checking that `<impl at /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1: 20:21>` is well-formed
-   --> /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1
+ note: cycle used when checking that `<impl at $DIR/next-solver-region-resolution.rs:18:1: 20:21>` is well-formed
+   --> $DIR/next-solver-region-resolution.rs:18:1
+ LL | / impl<'a, T> Foo for &T
+ LL | |     Self::Item: Baz,
+    | |____________________^

Compare output by lines enabled, diff by lines:
Expected contains these lines that are not in actual:
   | |___________________^
  --> $DIR/next-solver-region-resolution.rs:12:1
LL | / impl<'a, T> Foo for &'a T
LL | |     Self::Item: 'a,
note: cycle used when checking that `<impl at $DIR/next-solver-region-resolution.rs:12:1: 14:20>` is well-formed
Actual contains these lines that are not in expected:
   | |____________________^
  --> $DIR/next-solver-region-resolution.rs:18:1
LL | / impl<'a, T> Foo for &T
LL | |     Self::Item: Baz,
note: cycle used when checking that `<impl at $DIR/next-solver-region-resolution.rs:18:1: 20:21>` is well-formed

The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args specialization/min_specialization/next-solver-region-resolution.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-Zthreads=4" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/specialization/min_specialization/next-solver-region-resolution" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-Znext-solver=globally"
stdout: none
--- stderr -------------------------------
error[E0391]: cycle detected when coherence checking all impls of trait `Foo`
##[error]  --> /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:6:1
   |
LL | trait Foo { //~ ERROR cycle detected when coherence checking all impls of trait `Foo`
   | ^^^^^^^^^
   |
   = note: ...which requires building specialization graph of trait `Foo`...
note: ...which requires computing whether impls specialize one another...
  --> /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:12:1
   |
LL | / impl<'a, T> Foo for &'a T
LL | | where
LL | |     Self::Item: 'a,
   | |___________________^
note: ...which requires computing normalized predicates of `<impl at /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1: 20:21>`...
  --> /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1
   |
LL | / impl<'a, T> Foo for &T
LL | | where
LL | |     Self::Item: Baz,
   | |____________________^
   = note: ...which again requires coherence checking all impls of trait `Foo`, completing the cycle
note: cycle used when checking that `<impl at /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1: 20:21>` is well-formed
  --> /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1
   |
LL | / impl<'a, T> Foo for &T
LL | | where
LL | |     Self::Item: Baz,
   | |____________________^
   = note: for more information, see <https://rustc-dev-guide.rust-lang.org/overview.html#queries> and <https://rustc-dev-guide.rust-lang.org/query.html>

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0391`.
------------------------------------------

rust-bors · 2026-07-26T04:12:26Z

This pull request was unapproved.

This PR was contained in a rollup (#159926), which was unapproved.

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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. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Jun 25, 2026

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Dnreikronos force-pushed the trait_solver/canonicalize_next_gen_regions branch 3 times, most recently from 7588b8f to 12ae52a Compare June 27, 2026 01:45

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BoxyUwU mentioned this pull request Jul 13, 2026

Assumptions on Binders rust-lang/rust-project-goals#621

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Dnreikronos added 4 commits July 20, 2026 21:30

Normalize next-gen region constraints

f2476c5

Canonicalize and evaluate next-gen region constraints before query response canonicalization so equivalent constraints merge structurally.

Add regression test for object candidates

cb0e874

Cover the dyn object supertrait case that used to leave equivalent next-gen region constraints in different shapes.

Preserve normalized solver region constraints

30895ee

Normalize eagerly handled region constraints

f754b53

jhpratt mentioned this pull request Jul 25, 2026

Rollup of 16 pull requests #159876

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jhpratt mentioned this pull request Jul 25, 2026

Rollup of 19 pull requests #159879

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Rollup of 20 pull requests #159888

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Rollup of 25 pull requests #159920

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Rollup of 25 pull requests #159926

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