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

Author: jhpratt

compiler/rustc_middle/src/ty/mod.rs

@@ -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()
     }
 }

compiler/rustc_next_trait_solver/src/canonicalizer.rs

@@ -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,
@@ -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.
@@ -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(),
@@ -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 }
     }
 
@@ -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
@@ -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
@@ -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.
@@ -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())
                     }
@@ -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:?}"),

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -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::{
@@ -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
     }
 
@@ -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),
@@ -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)
     }
 }