enable eager param_env norm in new solver

[lcnr]

https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/goodbye.20proper.20param_env.20normalization/near/594260464

Slightly unfortunate that this is what we ended up with. but for now, this fixes issues and landing the new solver with this is certainly not worse than keeping the old impl.

This PR only eagerly normalizes, it does not yet treat aliases in param_env candidates as rigid, which is the worse part of this change.

fixes rust-lang/trait-system-refactor-initiative#265
fixes #136661

r? @BoxyUwU

[rustbot]

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

[lcnr]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 13060c6 (13060c6b2274ff62e47ad00d65d3f92f610a1d67, parent: b7e97a98f24cdb1335850209991455cb9734d333)

[rust-timer]

Finished benchmarking commit (13060c6): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.7%	[0.2%, 3.0%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -0.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.3%, 2.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-3.5%, -0.6%]	5
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.5%, secondary 2.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.3%, 2.8%]	2
Regressions ❌ (secondary)	2.6%	[2.3%, 2.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.3%, 2.8%]	2

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 511.751s -> 510.766s (-0.19%)
Artifact size: 400.67 MiB -> 400.56 MiB (-0.03%)

[lcnr]

r? @jackh726 as Boxy is on leave this week

[jackh726]

r=me with green CI

[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[lcnr]

The only user facing change covered by this FCP is dd884e4: we now entirely ignore region errors when normalizing the ParamEnv, even in the old solver.

This change causes #136661 to compile with the old solver, see the added tests in the last commit:

//@ check-pass
//@ revisions: current next
//@ ignore-compare-mode-next-solver (explicit revisions)
//@[next] compile-flags: -Znext-solver

// In `fn hrtb` normalizing the elaborated where-clause
// `T: for<'a> Supertrait<<&'a () as WithAssoc>::As>` results in
// a region error as `&'a ()` is not `&'static ()`.
//
// This happens even though the where-clauses themselves are well-formed
// as we never have to prove `&'a (): WithAssoc` as `'a` is a bound variable.
//
// Unlike `normalize-param-env-missing-implied-bound-1.rs` this snippet caused
// an ICE with the old trait solver, as we did register region constraints from
// relating types. This ICE was tracked in #136661.

#![allow(unused)]

trait Supertrait<T> {}

trait Other {
    fn method(&self) {}
}

impl WithAssoc for &'static () {
    type As = ();
}

trait WithAssoc {
    type As;
}

trait Trait<P: WithAssoc>: Supertrait<P::As> {
    fn method(&self) {}
}

fn hrtb<T: for<'a> Trait<&'a ()>>() {}

With this PR, we entirely ignore region constraints from ParamEnv normalization. This is necessary to avoid breakage with the new solver, see rust-lang/trait-system-refactor-initiative#166.

The core issue here is that the old solver already accepted code like this if the constraint was an explicit outlives clause instead, see tests/ui/traits/normalize/normalize-param-env-missing-implied-bound-1.rs for a test that already compiled with the old solver.

I am slightly unhappy about this as it feels like fully ignoring region constraints here could be unsound in theory. I haven't been able to come up with an exploit for this issue and even if one exists, it should only be easier to trigger as we already ignore explicit outlives where-clauses on stable. This PR only causes us to also ignore region constraints emitted by type relations. In the medium term this should be fixed by proper assumptions on binders.

@steffahn @theemathas would be interesting to see whether u can exploit this issue. To summarize:

• when proving that where-clauses are well-formed we ignore requirements referencing escaping bound variables, so we don't check &'a (): WithAssoc
• we can then normalize <&'a () as WithAssoc>::As in an elaborated super trait (or associated item) bound, ignoring the constraints required by normalization. The closest I can get to making this problematic is https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b8080c65c19259d2a3ab5d9e2ad1cc84
• can we now use this "incorrectly normalized" elaborated bound in a way that's unsound, e.g. we use them when proving that the ParamEnv is well-formed in the first place
• can we actually call fn hrtb here

Given my reasoning in the above paragraph, I would merge this as is:

I am slightly unhappy about this as it feels like fully ignoring region constraints here could be unsound in theory. I haven't been able to come up with an exploit for this issue and even if one exists, it should only be easier to trigger as we already ignore explicit outlives where-clauses. This PR only causes us to also ignore region constraints emitted by type relations. In the medium term this should be fixed by proper assumptions on binders.

@rfcbot fcp merge types

[rust-rfcbot]

@lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @jackh726
• @lcnr
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rustbot]

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.

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

15 LL | | where
16 LL | |     Self::Item: 'a,
17    | |___________________^
-    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+    = 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>
19 
20 error: aborting due to 1 previous error
21 


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/aarch64-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/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-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" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-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/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Znext-solver=globally"
stdout: none
--- stderr -------------------------------
error[E0391]: cycle detected when computing normalized predicates of `<impl at /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1: 21:21>`
##[error]  --> /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1
   |
LL | / impl<'a, T> Foo for &T
LL | | //~^ ERROR: cycle detected when computing normalized predicates of `<impl at $DIR/next-solver-region-resolution.rs:18:1: 21:21>`
LL | | where
LL | |     Self::Item: Baz,
   | |____________________^
   |
   = note: ...which immediately requires computing normalized predicates of `<impl at /checkout/tests/ui/specialization/min_specialization/next-solver-region-resolution.rs:18:1: 21:21>` again
note: cycle used when 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: 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`.
------------------------------------------