stabilize -Znext-solver=coherence

[lcnr]

r? @compiler-errors

---

This PR stabilizes the use of the next generation trait solver in coherence checking by enabling -Znext-solver=coherence by default. More specifically its use in the implicit negative overlap check. The tracking issue for this is #114862.

Background

The next generation trait solver

The new solver lives in rustc_trait_selection::solve and is intended to replace the existing evaluate, fulfill, and project implementation. It also has a wider impact on the rest of the type system, for example by changing our approach to handling associated types.

For a more detailed explanation of the new trait solver, see the rustc-dev-guide. This does not stabilize the current behavior of the new trait solver, only the behavior impacting the implicit negative overlap check. There are many areas in the new solver which are not yet finalized. We are confident that their final design will not conflict with the user-facing behavior observable via coherence. More on that further down.

Please check out the chapter summarizing the most significant changes between the existing and new implementations.

Coherence and the implicit negative overlap check

Coherence checking detects any overlapping impls. Overlapping trait impls always error while overlapping inherent impls result in an error if they have methods with the same name. Coherence also results in an error if any other impls could exist, even if they are currently unknown. This affects impls which may get added to upstream crates in a backwards compatible way and impls from downstream crates.

Coherence failing to detect overlap is generally considered to be unsound, even if it is difficult to actually get runtime UB this way. It is quite easy to get ICEs due to bugs in coherence.

It currently consists of two checks:

The orphan check validates that impls do not overlap with other impls we do not know about: either because they may be defined in a sibling crate, or because an upstream crate is allowed to add it without being considered a breaking change.

The overlap check validates that impls do not overlap with other impls we know about. This is done as follows:

• Instantiate the generic parameters of both impls with inference variables
• Equate the TraitRefs of both impls. If it fails there is no overlap.
• implicit negative: Check whether any of the instantiated where-bounds of one of the impls definitely do not hold when using the constraints from the previous step. If a where-bound does not hold, there is no overlap.
• explicit negative (still unstable, ignored going forward): Check whether the any negated where-bounds can be proven, e.g. a &mut u32: Clone bound definitely does not hold as an explicit impl<T> !Clone for &mut T exists.

The overlap check has to prove that unifying the impls does not succeed. This means that incorrectly getting a type error during coherence is unsound as it would allow impls to overlap: coherence has to be complete.

Completeness means that we never incorrectly error. This means that during coherence we must only add inference constraints if they are definitely necessary. During ordinary type checking this does not hold, so the trait solver has to behave differently, depending on whether we're in coherence or not.

The implicit negative check only considers goals to "definitely not hold" if they could not be implemented downstream, by a sibling, or upstream in a backwards compatible way. If the goal is is "unknowable" as it may get added in another crate, we add an ambiguous candidate: source.

Motivation

Replacing the existing solver in coherence fixes soundness bugs by removing sources of incompleteness in the type system. The new solver separately strengthens coherence, resulting in more impls being disjoint and passing the coherence check. The concrete changes will be elaborated further down. We believe the stabilization to reduce the likelihood of future bugs in coherence as the new implementation is easier to understand and reason about.

It allows us to remove the support for coherence and implicit-negative reasoning in the old solver, allowing us to remove some code and simplifying the old trait solver. We will only remove the old solver support once this stabilization has reached stable to make sure we're able to quickly revert in case any unexpected issues are detected before then.

Stabilizing the use of the next-generation trait solver expresses our confidence that its current behavior is intended and our work towards enabling its use everywhere will not require any breaking changes to the areas used by coherence checking. We are also confident that we will be able to replace the existing solver everywhere, as maintaining two separate systems adds a significant maintainance burden.

User-facing impact and reasoning

Breakage due to improved handling of associated types

The new solver fixes multiple issues related to associated types. As these issues caused coherence to consider more types distinct, fixing them results in more overlap errors. This is therefore a breaking change.

Structurally relating aliases containing bound vars

Fixes #102048. In the existing solver relating ambiguous projections containing bound variables is structural. This is incomplete and allows overlapping impls. These was mostly not exploitable as the same issue also caused impls to not apply when trying to use them. The new solver defers alias-relating to a nested goal, fixing this issue:

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence
trait Trait {}

trait Project {
    type Assoc<'a>;
}

impl Project for u32 {
    type Assoc<'a> = &'a u32;
}

// Eagerly normalizing `<?infer as Project>::Assoc<'a>` is ambiguous,
// so the old solver ended up structurally relating
//
//     (?infer, for<'a> fn(<?infer as Project>::Assoc<'a>))
//
// with 
//
//     ((u32, fn(&'a u32)))
//
// Equating `&'a u32` with `<u32 as Project>::Assoc<'a>` failed, even
// though these types are equal modulo normalization.
impl<T: Project> Trait for (T, for<'a> fn(<T as Project>::Assoc<'a>)) {}

impl<'a> Trait for (u32, fn(&'a u32)) {}
//[next]~^ ERROR conflicting implementations of trait `Trait` for type `(u32, for<'a> fn(&'a u32))`

A crater run did not discover any breakage due to this change.

Unknowable candidates for higher ranked trait goals

This avoids an unsoundness by attempting to normalize in trait_ref_is_knowable, fixing #114061. This is a side-effect of supporting lazy normalization, as that forces us to attempt to normalize when checking whether a TraitRef is knowable: source.

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence
trait IsUnit {}
impl IsUnit for () {}


pub trait WithAssoc<'a> {
    type Assoc;
}

// We considered `for<'a> <T as WithAssoc<'a>>::Assoc: IsUnit`
// to be knowable, even though the projection is ambiguous.
pub trait Trait {}
impl<T> Trait for T
where
    T: 'static,
    for<'a> T: WithAssoc<'a>,
    for<'a> <T as WithAssoc<'a>>::Assoc: IsUnit,
{
}
impl<T> Trait for Box<T> {}
//[next]~^ ERROR conflicting implementations of trait `Trait`

The two impls of Trait overlap given the following downstream crate:

use dep::*;
struct Local;
impl WithAssoc<'_> for Box<Local> {
    type Assoc = ();
}

There a similar coherence unsoundness caused by our handling of aliases which is fixed separately in #117164.

This change breaks the derive-visitor crate. I have opened an issue in that repo: nikis05/derive-visitor#16.

Evaluating goals to a fixpoint and applying inference constraints

In the old implementation of the implicit-negative check, each obligation is checked separately without applying its inference constraints. The new solver instead uses a FulfillmentCtxt for this, which evaluates all obligations in a loop until there's no further inference progress.

This is necessary for backwards compatibility as we do not eagerly normalize with the new solver, resulting in constraints from normalization to only get applied by evaluating a separate obligation. This also allows more code to compile:

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence
trait Mirror {
    type Assoc;
}
impl<T> Mirror for T {
    type Assoc = T;
}

trait Foo {}
trait Bar {}

// The self type starts out as `?0` but is constrained to `()`
// due to the where-clause below. Because `(): Bar` is known to
// not hold, we can prove the impls disjoint.
impl<T> Foo for T where (): Mirror<Assoc = T> {}
//[current]~^ ERROR conflicting implementations of trait `Foo` for type `()`
impl<T> Foo for T where T: Bar {}

fn main() {}

The old solver does not run nested goals to a fixpoint in evaluation. The new solver does do so, strengthening inference and improving the overlap check:

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence
trait Foo {}
impl<T> Foo for (u8, T, T) {}
trait NotU8 {}
trait Bar {}
impl<T, U: NotU8> Bar for (T, T, U) {}

trait NeedsFixpoint {}
impl<T: Foo + Bar> NeedsFixpoint for T {}
impl NeedsFixpoint for (u8, u8, u8) {}

trait Overlap {}
impl<T: NeedsFixpoint> Overlap for T {}
impl<T, U: NotU8, V> Overlap for (T, U, V) {}
//[current]~^ ERROR conflicting implementations of trait `Foo`

Breakage due to removal of incomplete candidate preference

Fixes #107887. In the old solver we incompletely prefer the builtin trait object impl over user defined impls. This can break inference guidance, inferring ?x in dyn Trait<u32>: Trait<?x> to u32, even if an explicit impl of Trait<u64> also exists.

This caused coherence to incorrectly allow overlapping impls, resulting in ICEs and a theoretical unsoundness. See #107887 (comment). This compiles on stable but results in an overlap error with -Znext-solver=coherence:

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence
struct W<T: ?Sized>(*const T);

trait Trait<T: ?Sized> {
    type Assoc;
}

// This would trigger the check for overlap between automatic and custom impl.
// They actually don't overlap so an impl like this should remain possible
// forever.
//
// impl Trait<u64> for dyn Trait<u32> {}
trait Indirect {}
impl Indirect for dyn Trait<u32, Assoc = ()> {}
impl<T: Indirect + ?Sized> Trait<u64> for T {
    type Assoc = ();
}

// Incomplete impl where `dyn Trait<u32>: Trait<_>` does not hold, but
// `dyn Trait<u32>: Trait<u64>` does.
trait EvaluateHack<U: ?Sized> {}
impl<T: ?Sized, U: ?Sized> EvaluateHack<W<U>> for T
where
    T: Trait<U, Assoc = ()>, // incompletely constrains `_` to `u32`
    U: IsU64,
    T: Trait<U, Assoc = ()>, // incompletely constrains `_` to `u32`
{
}

trait IsU64 {}
impl IsU64 for u64 {}

trait Overlap<U: ?Sized> {
    type Assoc: Default;
}
impl<T: ?Sized + EvaluateHack<W<U>>, U: ?Sized> Overlap<U> for T {
    type Assoc = Box<u32>;
}
impl<U: ?Sized> Overlap<U> for dyn Trait<u32, Assoc = ()> {
//[next]~^ ERROR conflicting implementations of trait `Overlap<_>`
    type Assoc = usize;
}

Considering region outlives bounds in the leak_check

For details on the leak_check, see the FCP proposal in #119820.¹

In both coherence and during candidate selection, the leak_check relies on the region constraints added in evaluate. It therefore currently does not register outlives obligations: source. This was likely done as a performance optimization without considering its impact on the leak_check. This is the case as in the old solver, evaluatation and fulfillment are split, with evaluation being responsible for candidate selection and fulfillment actually registering all the constraints.

This split does not exist with the new solver. The leak_check can therefore eagerly detect errors caused by region outlives obligations. This improves both coherence itself and candidate selection:

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence
trait LeakErr<'a, 'b> {}
// Using this impl adds an `'b: 'a` bound which results
// in a higher-ranked region error. This bound has been
// previously ignored but is now considered.
impl<'a, 'b: 'a> LeakErr<'a, 'b> for () {}

trait NoOverlapDir<'a> {}
impl<'a, T: for<'b> LeakErr<'a, 'b>> NoOverlapDir<'a> for T {}
impl<'a> NoOverlapDir<'a> for () {}
//[current]~^ ERROR conflicting implementations of trait `NoOverlapDir<'_>`

// --------------------------------------

// necessary to avoid coherence unknowable candidates
struct W<T>(T); 

trait GuidesSelection<'a, U> {}
impl<'a, T: for<'b> LeakErr<'a, 'b>> GuidesSelection<'a, W<u32>> for T {}
impl<'a, T> GuidesSelection<'a, W<u8>> for T {}

trait NotImplementedByU8 {}
trait NoOverlapInd<'a, U> {}
impl<'a, T: GuidesSelection<'a, W<U>>, U> NoOverlapInd<'a, U> for T {}
impl<'a, U: NotImplementedByU8> NoOverlapInd<'a, U> for () {}
//[current]~^ conflicting implementations of trait `NoOverlapInd<'_, _>`

Removal of fn match_fresh_trait_refs

The old solver tries to eagerly detect unbounded recursion, forcing the affected goals to be ambiguous. This check is only an approximation and has not been added to the new solver.

The check is not necessary in the new solver and it would be problematic for caching. As it depends on all goals currently on the stack, using a global cache entry would have to always make sure that doing so does not circumvent this check.

This changes some goals to error - or succeed - instead of failing with ambiguity. This allows more code to compile:

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence

// Need to use this local wrapper for the impls to be fully
// knowable as unknowable candidate result in ambiguity.
struct Local<T>(T);

trait Trait<U> {}
// This impl does not hold, but is ambiguous in the old
// solver due to its overflow approximation.
impl<U> Trait<U> for Local<u32> where Local<u16>: Trait<U> {}
// This impl holds.
impl Trait<Local<()>> for Local<u8> {}

// In the old solver, `Local<?t>: Trait<Local<?u>>` is ambiguous,
// resulting in `Local<?u>: NoImpl`, also being ambiguous.
//
// In the new solver the first impl does not apply, constraining
// `?u` to `Local<()>`, causing `Local<()>: NoImpl` to error.
trait Indirect<T> {}
impl<T, U> Indirect<U> for T
where
    T: Trait<U>,
    U: NoImpl
{}

// Not implemented for `Local<()>`
trait NoImpl {}
impl NoImpl for Local<u8> {}
impl NoImpl for Local<u16> {}

// `Local<?t>: Indirect<Local<?u>>` cannot hold, so
// these impls do not overlap.
trait NoOverlap<U> {}
impl<T: Indirect<U>, U> NoOverlap<U> for T {}
impl<T, U> NoOverlap<Local<U>> for Local<T> {}
//~^ ERROR conflicting implementations of trait `NoOverlap<Local<_>>`

Non-fatal overflow

The old solver immediately emits a fatal error when hitting the recursion limit. The new solver instead returns overflow. This both allows more code to compile and is results in performance and potential future compatability issues.

Non-fatal overflow is generally desirable. With fatal overflow, changing the order in which we evaluate nested goals easily causes breakage if we have goal which errors and one which overflows. It is also required to prevent breakage due to the removal of fn match_fresh_trait_refs, e.g. in typenum.

Enabling more code to compile

In the below example, the old solver first tried to prove an overflowing goal, resulting in a fatal error. The new solver instead returns ambiguity due to overflow for that goal, causing the implicit negative overlap check to succeed as Box<u32>: NotImplemented does not hold.

// revisions: current next
//[next] compile-flags: -Znext-solver=coherence
//[current] ERROR overflow evaluating the requirement

trait Indirect<T> {}
impl<T: Overflow<()>> Indirect<T> for () {}

trait Overflow<U> {}
impl<T, U> Overflow<U> for Box<T>
where
    U: Indirect<Box<Box<T>>>,
{}

trait NotImplemented {}

trait Trait<U> {}
impl<T, U> Trait<U> for T
where
    // T: NotImplemented, // causes old solver to succeed
    U: Indirect<T>,
    T: NotImplemented,
{}

impl Trait<()> for Box<u32> {}

Avoiding hangs with non-fatal overflow

Simply returning ambiguity when reaching the recursion limit can very easily result in hangs, e.g.

trait Recur {}
impl<T, U> Recur for ((T, U), (U, T))
where
    (T, U): Recur,
    (U, T): Recur,
{}

trait NotImplemented {}
impl<T: NotImplemented> Recur for T {}

This can happen quite frequently as it's easy to have exponential blowup due to multiple nested goals at each step. As the trait solver is depth-first, this immediately caused a fatal overflow error in the old solver. In the new solver we have to handle the whole proof tree instead, which can very easily hang.

To avoid this we restrict the recursion depth after hitting the recursion limit for the first time. We also ignore all inference constraints from goals resulting in overflow. This is mostly backwards compatible as any overflow in the old solver resulted in a fatal error.

sidenote about normalization

We return ambiguous nested goals of NormalizesTo goals to the caller and ignore their impact when computing the Certainty of the current goal. See the normalization chapter for more details.This means we apply constraints resulting from other nested goals and from equating the impl header when normalizing, even if a nested goal results in overflow. This is necessary to avoid breaking the following example:

trait Trait {
    type Assoc;
}

struct W<T: ?Sized>(*mut T);
impl<T: ?Sized> Trait for W<W<T>>
where
    W<T>: Trait,
{
    type Assoc = ();
}

// `W<?t>: Trait<Assoc = u32>` does not hold as
// `Assoc` gets normalized to `()`. However, proving
// the where-bounds of the impl results in overflow.
//
// For this to continue to compile we must not discard
// constraints from normalizing associated types.
trait NoOverlap {}
impl<T: Trait<Assoc = u32>> NoOverlap for T {}
impl<T: ?Sized> NoOverlap for W<T> {}

Future compatability concerns

Non-fatal overflow results in some unfortunate future compatability concerns. Changing the approach to avoid more hangs by more strongly penalizing overflow can cause breakage as we either drop constraints or ignore candidates necessary to successfully compile. Weakening the overflow penalities instead allows more code to compile and strengthens inference while potentially causing more code to hang.

While the current approach is not perfect, we believe it to be good enough. We believe it to apply the necessary inference constraints to avoid breakage and expect there to not be any desirable patterns broken by our current penalities. Similarly we believe the current constraints to avoid most accidental hangs. Ignoring constraints of overflowing goals is especially useful, as it may allow major future optimizations to our overflow handling. See this summary and the linked documents in case you want to know more.

changes to performance

In general, trait solving during coherence checking is not significant for performance. Enabling the next-generation trait solver in coherence does not impact our compile time benchmarks. We are still unable to compile the benchmark suite when fully enabling the new trait solver.

There are rare cases where the new solver has significantly worse performance due to non-fatal overflow, its reliance on fixpoint algorithms and the removal of the fn match_fresh_trait_refs approximation. We encountered such issues in typenum and believe it should be pretty much as bad as it can get.

Due to an improved structure and far better caching, we believe that there is a lot of room for improvement and that the new solver will outperform the existing implementation in nearly all cases, sometimes significantly. We have not yet spent any time micro-optimizing the implementation and have many unimplemented major improvements, such as fast-paths for trivial goals.

TODO: get some rough results here and put them in a table

Unstable features

Unsupported unstable features

The new solver currently does not support all unstable features, most notably #![feature(generic_const_exprs)], #![feature(associated_const_equality)] and #![feature(adt_const_params)] are not yet fully supported in the new solver. We are confident that supporting them is possible, but did not consider this to be a priority. This stabilization introduces new ICE when using these features in impl headers.

fixes to #![feature(specialization)]

• fixes specialization: default items completely drop candidates instead of ambiguity #105782
• fixes ICE: When translating generic parameters from .. to .., the expected specialization failed to hold #118987

fixes to #![feature(type_alias_impl_trait)]

• fixes ICE: unexpected ambiguity: Canonical.. #119272
• occurs check with aliases results in error, not ambiguity #105787 (comment)
• fixes ICE: Layout::compute: unexpected type '_' #124207

This does not stabilize the whole solver

While this stabilizes the use of the new solver in coherence checking, there are many parts of the solver which will remain fully unstable. We may still adapt these areas while working towards stabilizing the new solver everywhere. We are confident that we are able to do so without negatively impacting coherence.

goals with a non-empty ParamEnv

Coherence always uses an empty environment. We therefore do not depend on the behavior of AliasBound and ParamEnv candidates. We only stabilizes the behavior of user-defined and builtin implementations of traits. There are still many open questions there.

opaque types in the defining scope

The handling of opaque types - impl Trait - in both the new and old solver is still not fully figured out. Luckily this can be ignored for now. While opaque types are reachable during coherence checking by using impl_trait_in_associated_types, the behavior during coherence is separate and self-contained. The old and new solver fully agree here.

normalization is hard

This stabilizes that we equate associated types involving bound variables using deferred-alias-equality. We also stop eagerly normalizing in coherence, which should not have any user-facing impact.

We do not stabilize the normalization behavior outside of coherence, e.g. we currently deeply normalize all types during writeback with the new solver. This may change going forward

how to replace select from the old solver

We sometimes depend on getting a single impl for a given trait bound, e.g. when resolving a concrete method for codegen/CTFE. We do not depend on this during coherence, so the exact approach here can still be freely changed going forward.

Acknowledgements

This work would not have been possible without @compiler-errors. He implemented large chunks of the solver himself but also and did a lot of testing and experimentation, eagerly discovering multiple issues which had a significant impact on our approach. @BoxyUwU has also done some amazing work on the solver. Thank you for the endless hours of discussion resulting in the current approach. Especially the way aliases are handled has gone through multiple revisions to get to its current state.

There were also many contributions from - and discussions with - other members of the community and the rest of @rust-lang/types. This solver builds upon previous improvements to the compiler, as well as lessons learned from chalk and a-mir-formality. Getting to this point would not have been possible without that and I am incredibly thankful to everyone involved. See the list of relevant PRs.

Footnotes

1. which should get moved to the dev-guide once that PR lands :3 ↩

[lcnr]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 936fb93 with merge 5893492...

[bors]

💔 Test failed - checks-actions

[lcnr]

rebased on top of #121853 😅

@bors try @rust-timer queue

[bors]

⌛ Trying commit f899a56 with merge bfe97fc...

[bors]

☀️ Try build successful - checks-actions
Build commit: bfe97fc (bfe97fcae016881ad333f0ff8cea2cadb9cf136c)

[rust-timer]

Finished benchmarking commit (bfe97fc): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.2%, 0.3%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.2%	[-5.1%, -0.3%]	9
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	3
All ❌✅ (primary)	-2.2%	[-5.1%, 0.3%]	13

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.9%, 1.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.8%	[-2.1%, -1.6%]	2
Improvements ✅ (secondary)	-1.7%	[-1.9%, -1.5%]	2
All ❌✅ (primary)	-0.6%	[-2.1%, 1.9%]	3

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.3%	[-4.3%, -2.6%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.3%	[-4.3%, -2.6%]	7

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 651.828s -> 651.877s (0.01%)
Artifact size: 311.10 MiB -> 311.62 MiB (0.17%)

[matthiaskrgr]

I'm somewhat worried because we still have a bunch of ICEs related to next-solver=coherence

https://github.com/rust-lang/rust/issues?q=is%3Aissue+is%3Aopen+label%3AI-ICE+%22%3Dcoherence%22++%22solver%22+

[compiler-errors]

@matthiaskrgr: Almost all of those are related to consts, and I'm not particularly concerned with users hitting these in production. None of them, as far as I am aware, are of the "pass -> ICE", so @lcnr and I will fix them in due time.

[lcnr]

added tests from #124532.

@bors r=compiler-errors rollup=never

[bors]

📌 Commit ef7071d has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit ef7071d with merge 26ba153...

[bors]

💥 Test timed out

[lqd]

@bors retry

[matthiaskrgr]

@bors r-
https://github.com/rust-lang-ci/rust/actions/runs/8918744232/job/24493839585#step:26:3741