[DO NOT MERGE] bootstrap with -Znext-solver=globally

compiler/rustc_middle/src/ty/mod.rs

@@ -23,6 +23,13 @@ pub use adt::*;
 pub use assoc::*;
 pub use generic_args::{GenericArgKind, TermKind, *};
 pub use generics::*;
+// Can't use a glob import here as it would cause
+// ambiguity when importing the actual types implementing
+// the inherent traits from this module.
+// FIXME(clippy#13764): remove this `allow`.
+#[allow(clippy::useless_attribute)]
+#[allow(rustc::non_glob_import_of_type_ir_inherent)]
+use inherent::SliceLike;
 pub use intrinsic::IntrinsicDef;
 use rustc_abi::{Align, FieldIdx, Integer, IntegerType, ReprFlags, ReprOptions, VariantIdx};
 use rustc_ast::expand::StrippedCfgItem;
@@ -972,7 +979,7 @@ 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 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/alias_relate.rs

@@ -15,12 +15,19 @@
 //! (3.) Otherwise, if we end with two rigid (non-projection) or infer types,
 //! relate them structurally.
 
+use rustc_type_ir::data_structures::HashSet;
 use rustc_type_ir::inherent::*;
+use rustc_type_ir::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
 use rustc_type_ir::{self as ty, Interner};
 use tracing::{instrument, trace};
 
 use crate::delegate::SolverDelegate;
-use crate::solve::{Certainty, EvalCtxt, Goal, QueryResult};
+use crate::solve::{Certainty, EvalCtxt, Goal, NoSolution, QueryResult};
+
+enum IgnoreAliases {
+    Yes,
+    No,
+}
 
 impl<D, I> EvalCtxt<'_, D>
 where
@@ -36,6 +43,12 @@ where
         let Goal { param_env, predicate: (lhs, rhs, direction) } = goal;
         debug_assert!(lhs.to_alias_term().is_some() || rhs.to_alias_term().is_some());
 
+        if self.alias_cannot_name_placeholder_in_rigid(param_env, lhs, rhs)
+            || self.alias_cannot_name_placeholder_in_rigid(param_env, rhs, lhs)
+        {
+            return Err(NoSolution);
+        }
+
         // Structurally normalize the lhs.
         let lhs = if let Some(alias) = lhs.to_alias_term() {
             let term = self.next_term_infer_of_kind(lhs);
@@ -96,4 +109,100 @@ where
             }
         }
     }
+
+    /// In case a rigid term refers to a placeholder which is not referenced by the
+    /// alias, the alias cannot be normalized to that rigid term unless it contains
+    /// either inference variables or these placeholders are referenced in a term
+    /// of a `Projection`-clause in the environment.
+    fn alias_cannot_name_placeholder_in_rigid(
+        &mut self,
+        param_env: I::ParamEnv,
+        rigid_term: I::Term,
+        alias: I::Term,
+    ) -> bool {
+        // Check that the rigid term is actually rigid.
+        if rigid_term.to_alias_term().is_some() || alias.to_alias_term().is_none() {
+            return false;
+        }
+
+        // If the alias has any type or const inference variables,
+        // do not try to apply the fast path as these inference variables
+        // may resolve to something containing placeholders.
+        if alias.has_non_region_infer() {
+            return false;
+        }
+
+        let mut referenced_placeholders =
+            self.collect_placeholders_in_term(rigid_term, IgnoreAliases::Yes);
+        for clause in param_env.caller_bounds().iter() {
+            match clause.kind().skip_binder() {
+                ty::ClauseKind::Projection(ty::ProjectionPredicate { term, .. }) => {
+                    if term.has_non_region_infer() {
+                        return false;
+                    }
+
+                    let env_term_placeholders =
+                        self.collect_placeholders_in_term(term, IgnoreAliases::No);
+                    #[allow(rustc::potential_query_instability)]
+                    referenced_placeholders.retain(|p| !env_term_placeholders.contains(p));
+                }
+                ty::ClauseKind::Trait(_)
+                | ty::ClauseKind::HostEffect(_)
+                | ty::ClauseKind::TypeOutlives(_)
+                | ty::ClauseKind::RegionOutlives(_)
+                | ty::ClauseKind::ConstArgHasType(..)
+                | ty::ClauseKind::WellFormed(_)
+                | ty::ClauseKind::ConstEvaluatable(_) => continue,
+            }
+        }
+
+        if referenced_placeholders.is_empty() {
+            return false;
+        }
+
+        let alias_placeholders = self.collect_placeholders_in_term(alias, IgnoreAliases::No);
+        // If the rigid term references a placeholder not mentioned by the alias,
+        // they can never unify.
+        !referenced_placeholders.is_subset(&alias_placeholders)
+    }
+
+    fn collect_placeholders_in_term(
+        &mut self,
+        term: I::Term,
+        ignore_aliases: IgnoreAliases,
+    ) -> HashSet<I::Term> {
+        // Fast path to avoid walking the term.
+        if !term.has_placeholders() {
+            return Default::default();
+        }
+
+        struct PlaceholderCollector<I: Interner> {
+            ignore_aliases: IgnoreAliases,
+            placeholders: HashSet<I::Term>,
+        }
+        impl<I: Interner> TypeVisitor<I> for PlaceholderCollector<I> {
+            type Result = ();
+
+            fn visit_ty(&mut self, t: I::Ty) {
+                match t.kind() {
+                    ty::Placeholder(_) => drop(self.placeholders.insert(t.into())),
+                    ty::Alias(..) if matches!(self.ignore_aliases, IgnoreAliases::Yes) => {}
+                    _ => t.super_visit_with(self),
+                }
+            }
+
+            fn visit_const(&mut self, ct: I::Const) {
+                match ct.kind() {
+                    ty::ConstKind::Placeholder(_) => drop(self.placeholders.insert(ct.into())),
+                    ty::ConstKind::Unevaluated(_) | ty::ConstKind::Expr(_)
+                        if matches!(self.ignore_aliases, IgnoreAliases::Yes) => {}
+                    _ => ct.super_visit_with(self),
+                }
+            }
+        }
+
+        let mut visitor = PlaceholderCollector { ignore_aliases, placeholders: Default::default() };
+        term.visit_with(&mut visitor);
+        visitor.placeholders
+    }
 }