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Rollup merge of rust-lang#133643 - lcnr:merge-candidates, r=compiler-…
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…errors

-Znext-solver: modify candidate preference rules

This implements the design proposed in the FCP in rust-lang#132325 and matches the old solver behavior. I hope the inline comments are all sufficiently clear, I personally think this is a fairly clear improvement over the existing approach using `fn discard_impls_shadowed_by_env`. This fixes rust-lang/trait-system-refactor-initiative#96.

This also fixes rust-lang#133639 which encounters an ICE in negative coherence when evaluating the where-clause. Given the features required to trigger this ICE 🤷

r? ``@compiler-errors``
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@jhpratt
jhpratt authored Dec 19, 2024
2 parents e018796 + 5fa4b09 commit 4cd157e
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Showing 22 changed files with 332 additions and 181 deletions.
2 changes: 1 addition & 1 deletion compiler/rustc_middle/src/ty/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -971,7 +971,7 @@ pub struct ParamEnv<'tcx> {
}

impl<'tcx> rustc_type_ir::inherent::ParamEnv<TyCtxt<'tcx>> for ParamEnv<'tcx> {
fn caller_bounds(self) -> impl IntoIterator<Item = ty::Clause<'tcx>> {
fn caller_bounds(self) -> impl inherent::SliceLike<Item = ty::Clause<'tcx>> {
self.caller_bounds()
}
}
Expand Down
119 changes: 95 additions & 24 deletions compiler/rustc_next_trait_solver/src/canonicalizer.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@ use std::cmp::Ordering;
use rustc_type_ir::data_structures::HashMap;
use rustc_type_ir::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
use rustc_type_ir::inherent::*;
use rustc_type_ir::solve::{Goal, QueryInput};
use rustc_type_ir::visit::TypeVisitableExt;
use rustc_type_ir::{
self as ty, Canonical, CanonicalTyVarKind, CanonicalVarInfo, CanonicalVarKind, InferCtxtLike,
Expand All @@ -17,8 +18,11 @@ use crate::delegate::SolverDelegate;
/// while canonicalizing the response happens in the context of the
/// query.
#[derive(Debug, Clone, Copy)]
pub enum CanonicalizeMode {
Input,
enum CanonicalizeMode {
/// When canonicalizing the `param_env`, we keep `'static` as merging
/// trait candidates relies on it when deciding whether a where-bound
/// is trivial.
Input { keep_static: bool },
/// FIXME: We currently return region constraints referring to
/// placeholders and inference variables from a binder instantiated
/// inside of the query.
Expand Down Expand Up @@ -59,15 +63,15 @@ pub struct Canonicalizer<'a, D: SolverDelegate<Interner = I>, I: Interner> {
}

impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
pub fn canonicalize<T: TypeFoldable<I>>(
pub fn canonicalize_response<T: TypeFoldable<I>>(
delegate: &'a D,
canonicalize_mode: CanonicalizeMode,
max_input_universe: ty::UniverseIndex,
variables: &'a mut Vec<I::GenericArg>,
value: T,
) -> ty::Canonical<I, T> {
let mut canonicalizer = Canonicalizer {
delegate,
canonicalize_mode,
canonicalize_mode: CanonicalizeMode::Response { max_input_universe },

variables,
variable_lookup_table: Default::default(),
Expand All @@ -80,9 +84,67 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
let value = value.fold_with(&mut canonicalizer);
assert!(!value.has_infer(), "unexpected infer in {value:?}");
assert!(!value.has_placeholders(), "unexpected placeholders in {value:?}");

let (max_universe, variables) = canonicalizer.finalize();
Canonical { max_universe, variables, value }
}

/// When canonicalizing query inputs, we keep `'static` in the `param_env`
/// but erase it everywhere else. We generally don't want to depend on region
/// identity, so while it should not matter whether `'static` is kept in the
/// value or opaque type storage as well, this prevents us from accidentally
/// relying on it in the future.
///
/// We want to keep the option of canonicalizing `'static` to an existential
/// variable in the future by changing the way we detect global where-bounds.
pub fn canonicalize_input<P: TypeFoldable<I>>(
delegate: &'a D,
variables: &'a mut Vec<I::GenericArg>,
input: QueryInput<I, P>,
) -> ty::Canonical<I, QueryInput<I, P>> {
// First canonicalize the `param_env` while keeping `'static`
let mut env_canonicalizer = Canonicalizer {
delegate,
canonicalize_mode: CanonicalizeMode::Input { keep_static: true },

variables,
variable_lookup_table: Default::default(),
primitive_var_infos: Vec::new(),
binder_index: ty::INNERMOST,

cache: Default::default(),
};
let param_env = input.goal.param_env.fold_with(&mut env_canonicalizer);
debug_assert_eq!(env_canonicalizer.binder_index, ty::INNERMOST);
// Then canonicalize the rest of the input without keeping `'static`
// while *mostly* reusing the canonicalizer from above.
let mut rest_canonicalizer = Canonicalizer {
delegate,
canonicalize_mode: CanonicalizeMode::Input { keep_static: false },

variables: env_canonicalizer.variables,
// We're able to reuse the `variable_lookup_table` as whether or not
// it already contains an entry for `'static` does not matter.
variable_lookup_table: env_canonicalizer.variable_lookup_table,
primitive_var_infos: env_canonicalizer.primitive_var_infos,
binder_index: ty::INNERMOST,

// We do not reuse the cache as it may contain entries whose canonicalized
// value contains `'static`. While we could alternatively handle this by
// checking for `'static` when using cached entries, this does not
// feel worth the effort. I do not expect that a `ParamEnv` will ever
// contain large enough types for caching to be necessary.
cache: Default::default(),
};

let predicate = input.goal.predicate.fold_with(&mut rest_canonicalizer);
let goal = Goal { param_env, predicate };
let predefined_opaques_in_body =
input.predefined_opaques_in_body.fold_with(&mut rest_canonicalizer);
let value = QueryInput { goal, predefined_opaques_in_body };

assert!(!value.has_infer(), "unexpected infer in {value:?}");
assert!(!value.has_placeholders(), "unexpected placeholders in {value:?}");
let (max_universe, variables) = rest_canonicalizer.finalize();
Canonical { max_universe, variables, value }
}

Expand Down Expand Up @@ -126,7 +188,7 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
// all information which should not matter for the solver.
//
// For this we compress universes as much as possible.
CanonicalizeMode::Input => {}
CanonicalizeMode::Input { .. } => {}
// When canonicalizing a response we map a universes already entered
// by the caller to the root universe and only return useful universe
// information for placeholders and inference variables created inside
Expand Down Expand Up @@ -290,17 +352,15 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
}
},
ty::Placeholder(placeholder) => match self.canonicalize_mode {
CanonicalizeMode::Input => CanonicalVarKind::PlaceholderTy(PlaceholderLike::new(
placeholder.universe(),
self.variables.len().into(),
)),
CanonicalizeMode::Input { .. } => CanonicalVarKind::PlaceholderTy(
PlaceholderLike::new(placeholder.universe(), self.variables.len().into()),
),
CanonicalizeMode::Response { .. } => CanonicalVarKind::PlaceholderTy(placeholder),
},
ty::Param(_) => match self.canonicalize_mode {
CanonicalizeMode::Input => CanonicalVarKind::PlaceholderTy(PlaceholderLike::new(
ty::UniverseIndex::ROOT,
self.variables.len().into(),
)),
CanonicalizeMode::Input { .. } => CanonicalVarKind::PlaceholderTy(
PlaceholderLike::new(ty::UniverseIndex::ROOT, self.variables.len().into()),
),
CanonicalizeMode::Response { .. } => panic!("param ty in response: {t:?}"),
},
ty::Bool
Expand Down Expand Up @@ -357,29 +417,38 @@ impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I> for Canonicaliz
let kind = match r.kind() {
ty::ReBound(..) => return r,

// We may encounter `ReStatic` in item signatures or the hidden type
// of an opaque. `ReErased` should only be encountered in the hidden
// We don't canonicalize `ReStatic` in the `param_env` as we use it
// when checking whether a `ParamEnv` candidate is global.
ty::ReStatic => match self.canonicalize_mode {
CanonicalizeMode::Input { keep_static: false } => {
CanonicalVarKind::Region(ty::UniverseIndex::ROOT)
}
CanonicalizeMode::Input { keep_static: true }
| CanonicalizeMode::Response { .. } => return r,
},

// `ReErased` should only be encountered in the hidden
// type of an opaque for regions that are ignored for the purposes of
// captures.
//
// FIXME: We should investigate the perf implications of not uniquifying
// `ReErased`. We may be able to short-circuit registering region
// obligations if we encounter a `ReErased` on one side, for example.
ty::ReStatic | ty::ReErased | ty::ReError(_) => match self.canonicalize_mode {
CanonicalizeMode::Input => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
ty::ReErased | ty::ReError(_) => match self.canonicalize_mode {
CanonicalizeMode::Input { .. } => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Response { .. } => return r,
},

ty::ReEarlyParam(_) | ty::ReLateParam(_) => match self.canonicalize_mode {
CanonicalizeMode::Input => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Input { .. } => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Response { .. } => {
panic!("unexpected region in response: {r:?}")
}
},

ty::RePlaceholder(placeholder) => match self.canonicalize_mode {
// We canonicalize placeholder regions as existentials in query inputs.
CanonicalizeMode::Input => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Input { .. } => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Response { max_input_universe } => {
// If we have a placeholder region inside of a query, it must be from
// a new universe.
Expand All @@ -397,7 +466,9 @@ impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I> for Canonicaliz
"region vid should have been resolved fully before canonicalization"
);
match self.canonicalize_mode {
CanonicalizeMode::Input => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
CanonicalizeMode::Input { keep_static: _ } => {
CanonicalVarKind::Region(ty::UniverseIndex::ROOT)
}
CanonicalizeMode::Response { .. } => {
CanonicalVarKind::Region(self.delegate.universe_of_lt(vid).unwrap())
}
Expand Down Expand Up @@ -434,15 +505,15 @@ impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I> for Canonicaliz
ty::InferConst::Fresh(_) => todo!(),
},
ty::ConstKind::Placeholder(placeholder) => match self.canonicalize_mode {
CanonicalizeMode::Input => CanonicalVarKind::PlaceholderConst(
CanonicalizeMode::Input { .. } => CanonicalVarKind::PlaceholderConst(
PlaceholderLike::new(placeholder.universe(), self.variables.len().into()),
),
CanonicalizeMode::Response { .. } => {
CanonicalVarKind::PlaceholderConst(placeholder)
}
},
ty::ConstKind::Param(_) => match self.canonicalize_mode {
CanonicalizeMode::Input => CanonicalVarKind::PlaceholderConst(
CanonicalizeMode::Input { .. } => CanonicalVarKind::PlaceholderConst(
PlaceholderLike::new(ty::UniverseIndex::ROOT, self.variables.len().into()),
),
CanonicalizeMode::Response { .. } => panic!("param ty in response: {c:?}"),
Expand Down
130 changes: 57 additions & 73 deletions compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ use rustc_type_ir::visit::TypeVisitableExt as _;
use rustc_type_ir::{self as ty, Interner, TypingMode, Upcast as _, elaborate};
use tracing::{debug, instrument};

use super::trait_goals::TraitGoalProvenVia;
use crate::delegate::SolverDelegate;
use crate::solve::inspect::ProbeKind;
use crate::solve::{
Expand Down Expand Up @@ -337,15 +338,6 @@ where

self.assemble_param_env_candidates(goal, &mut candidates);

match self.typing_mode() {
TypingMode::Coherence => {}
TypingMode::Analysis { .. }
| TypingMode::PostBorrowckAnalysis { .. }
| TypingMode::PostAnalysis => {
self.discard_impls_shadowed_by_env(goal, &mut candidates);
}
}

candidates
}

Expand Down Expand Up @@ -500,7 +492,7 @@ where
goal: Goal<I, G>,
candidates: &mut Vec<Candidate<I>>,
) {
for (i, assumption) in goal.param_env.caller_bounds().into_iter().enumerate() {
for (i, assumption) in goal.param_env.caller_bounds().iter().enumerate() {
candidates.extend(G::probe_and_consider_implied_clause(
self,
CandidateSource::ParamEnv(i),
Expand Down Expand Up @@ -733,72 +725,64 @@ where
})
}

/// If there's a where-bound for the current goal, do not use any impl candidates
/// to prove the current goal. Most importantly, if there is a where-bound which does
/// not specify any associated types, we do not allow normalizing the associated type
/// by using an impl, even if it would apply.
/// We sadly can't simply take all possible candidates for normalization goals
/// and check whether they result in the same constraints. We want to make sure
/// that trying to normalize an alias doesn't result in constraints which aren't
/// otherwise required.
///
/// Most notably, when proving a trait goal by via a where-bound, we should not
/// normalize via impls which have stricter region constraints than the where-bound:
///
/// ```rust
/// trait Trait<'a> {
/// type Assoc;
/// }
///
/// impl<'a, T: 'a> Trait<'a> for T {
/// type Assoc = u32;
/// }
///
/// fn with_bound<'a, T: Trait<'a>>(_value: T::Assoc) {}
/// ```
///
/// <https://github.com/rust-lang/trait-system-refactor-initiative/issues/76>
// FIXME(@lcnr): The current structure here makes me unhappy and feels ugly. idk how
// to improve this however. However, this should make it fairly straightforward to refine
// the filtering going forward, so it seems alright-ish for now.
#[instrument(level = "debug", skip(self, goal))]
fn discard_impls_shadowed_by_env<G: GoalKind<D>>(
/// The where-bound of `with_bound` doesn't specify the associated type, so we would
/// only be able to normalize `<T as Trait<'a>>::Assoc` by using the impl. This impl
/// adds a `T: 'a` bound however, which would result in a region error. Given that the
/// user explicitly wrote that `T: Trait<'a>` holds, this is undesirable and we instead
/// treat the alias as rigid.
///
/// See trait-system-refactor-initiative#124 for more details.
#[instrument(level = "debug", skip(self), ret)]
pub(super) fn merge_candidates(
&mut self,
goal: Goal<I, G>,
candidates: &mut Vec<Candidate<I>>,
) {
let cx = self.cx();
let trait_goal: Goal<I, ty::TraitPredicate<I>> =
goal.with(cx, goal.predicate.trait_ref(cx));

let mut trait_candidates_from_env = vec![];
self.probe(|_| ProbeKind::ShadowedEnvProbing).enter(|ecx| {
ecx.assemble_param_env_candidates(trait_goal, &mut trait_candidates_from_env);
ecx.assemble_alias_bound_candidates(trait_goal, &mut trait_candidates_from_env);
});
proven_via: Option<TraitGoalProvenVia>,
candidates: Vec<Candidate<I>>,
) -> QueryResult<I> {
let Some(proven_via) = proven_via else {
// We don't care about overflow. If proving the trait goal overflowed, then
// it's enough to report an overflow error for that, we don't also have to
// overflow during normalization.
return Ok(self.make_ambiguous_response_no_constraints(MaybeCause::Ambiguity));
};

if !trait_candidates_from_env.is_empty() {
let trait_env_result = self.merge_candidates(trait_candidates_from_env);
match trait_env_result.unwrap().value.certainty {
// If proving the trait goal succeeds by using the env,
// we freely drop all impl candidates.
//
// FIXME(@lcnr): It feels like this could easily hide
// a forced ambiguity candidate added earlier.
// This feels dangerous.
Certainty::Yes => {
candidates.retain(|c| match c.source {
CandidateSource::Impl(_) | CandidateSource::BuiltinImpl(_) => {
debug!(?c, "discard impl candidate");
false
}
CandidateSource::ParamEnv(_) | CandidateSource::AliasBound => true,
CandidateSource::CoherenceUnknowable => panic!("uh oh"),
});
}
// If it is still ambiguous we instead just force the whole goal
// to be ambig and wait for inference constraints. See
// tests/ui/traits/next-solver/env-shadows-impls/ambig-env-no-shadow.rs
Certainty::Maybe(cause) => {
debug!(?cause, "force ambiguity");
*candidates = self.forced_ambiguity(cause).into_iter().collect();
}
}
}
}
let responses: Vec<_> = match proven_via {
// Even when a trait bound has been proven using a where-bound, we
// still need to consider alias-bounds for normalization, see
// tests/ui/next-solver/alias-bound-shadowed-by-env.rs.
//
// FIXME(const_trait_impl): should this behavior also be used by
// constness checking. Doing so is *at least theoretically* breaking,
// see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound => candidates
.iter()
.filter(|c| {
matches!(c.source, CandidateSource::AliasBound | CandidateSource::ParamEnv(_))
})
.map(|c| c.result)
.collect(),
TraitGoalProvenVia::Misc => candidates.iter().map(|c| c.result).collect(),
};

/// If there are multiple ways to prove a trait or projection goal, we have
/// to somehow try to merge the candidates into one. If that fails, we return
/// ambiguity.
#[instrument(level = "debug", skip(self), ret)]
pub(super) fn merge_candidates(&mut self, candidates: Vec<Candidate<I>>) -> QueryResult<I> {
// First try merging all candidates. This is complete and fully sound.
let responses = candidates.iter().map(|c| c.result).collect::<Vec<_>>();
if let Some(result) = self.try_merge_responses(&responses) {
return Ok(result);
} else {
self.flounder(&responses)
}
self.try_merge_responses(&responses).map_or_else(|| self.flounder(&responses), Ok)
}
}
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