add alias-relate fast path optimization

Author: lcnr

compiler/rustc_next_trait_solver/src/solve/alias_relate.rs

@@ -15,13 +15,21 @@
 //! (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::solve::GoalSource;
-use rustc_type_ir::{self as ty, Interner};
+use rustc_type_ir::{
+    self as ty, Interner, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
+};
 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
@@ -47,6 +55,12 @@ where
                 || rhs.is_error()
         );
 
+        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);
@@ -113,4 +127,132 @@ 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 = Default::default();
+        self.collect_placeholders_in_term(
+            rigid_term,
+            IgnoreAliases::Yes,
+            &mut referenced_placeholders,
+        );
+        if referenced_placeholders.is_empty() {
+            return false;
+        }
+
+        let mut alias_placeholders = Default::default();
+        self.collect_placeholders_in_term(alias, IgnoreAliases::No, &mut alias_placeholders);
+        loop {
+            let mut has_changed = false;
+            for clause in param_env.caller_bounds().iter() {
+                match clause.kind().skip_binder() {
+                    ty::ClauseKind::Projection(ty::ProjectionPredicate {
+                        projection_term,
+                        term,
+                    }) => {
+                        let mut required_placeholders = Default::default();
+                        for term in projection_term.args.iter().filter_map(|arg| arg.as_term()) {
+                            self.collect_placeholders_in_term(
+                                term,
+                                IgnoreAliases::Yes,
+                                &mut required_placeholders,
+                            );
+                        }
+
+                        if !required_placeholders.is_subset(&alias_placeholders) {
+                            continue;
+                        }
+
+                        if term.has_non_region_infer() {
+                            return false;
+                        }
+
+                        has_changed |= self.collect_placeholders_in_term(
+                            term,
+                            IgnoreAliases::No,
+                            &mut alias_placeholders,
+                        );
+                    }
+                    ty::ClauseKind::Trait(_)
+                    | ty::ClauseKind::HostEffect(_)
+                    | ty::ClauseKind::TypeOutlives(_)
+                    | ty::ClauseKind::RegionOutlives(_)
+                    | ty::ClauseKind::ConstArgHasType(..)
+                    | ty::ClauseKind::WellFormed(_)
+                    | ty::ClauseKind::ConstEvaluatable(_) => continue,
+                }
+            }
+
+            if !has_changed {
+                break;
+            }
+        }
+        // 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,
+        placeholders: &mut HashSet<I::Term>,
+    ) -> bool {
+        // Fast path to avoid walking the term.
+        if !term.has_placeholders() {
+            return false;
+        }
+
+        struct PlaceholderCollector<'a, I: Interner> {
+            ignore_aliases: IgnoreAliases,
+            has_changed: bool,
+            placeholders: &'a mut 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(_) => self.has_changed |= 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(_) => {
+                        self.has_changed |= 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, has_changed: false, placeholders };
+        term.visit_with(&mut visitor);
+        visitor.has_changed
+    }
 }

tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.next.stderr

@@ -4,6 +4,20 @@ error[E0308]: mismatched types
 LL |     fn func<const N: u32>() -> [(); { () }] {
    |                                       ^^ expected `usize`, found `()`
 
+error[E0271]: type mismatch resolving `N == { () }`
+  --> $DIR/const-in-impl-fn-return-type.rs:20:5
+   |
+LL |     fn func<const N: u32>() -> [(); { () }] {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
+   |
+note: the requirement `N == { () }` appears on the `impl`'s method `func` but not on the corresponding trait's method
+  --> $DIR/const-in-impl-fn-return-type.rs:12:8
+   |
+LL | trait Trait {
+   |       ----- in this trait
+LL |     fn func<const N: u32>() -> [(); N];
+   |        ^^^^ this trait's method doesn't have the requirement `N == { () }`
+
 error: the constant `N` is not of type `usize`
   --> $DIR/const-in-impl-fn-return-type.rs:12:32
    |
@@ -12,6 +26,7 @@ LL |     fn func<const N: u32>() -> [(); N];
    |
    = note: the length of array `[(); N]` must be type `usize`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0271, E0308.
+For more information about an error, try `rustc --explain E0271`.

tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.rs

@@ -19,6 +19,7 @@ struct S {}
 impl Trait for S {
     fn func<const N: u32>() -> [(); { () }] {
         //~^ ERROR mismatched types
+        //[next]~| ERROR type mismatch resolving `N == { () }`
         N
     }
 }