add alias-relate fast path optimization

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
+    }
 }