Flounder in function trait when inference var or type alias is encoun…

…tered

chalk-solve/src/clauses.rs

@@ -295,7 +295,7 @@ fn program_clauses_that_could_match<I: Interner>(
             }
 
             if let Some(well_known) = trait_datum.well_known {
-                builtin_traits::add_builtin_program_clauses(db, builder, well_known, trait_ref);
+                builtin_traits::add_builtin_program_clauses(db, builder, well_known, trait_ref)?;
             }
         }
         DomainGoal::Holds(WhereClause::AliasEq(alias_eq)) => match &alias_eq.alias {
@@ -370,7 +370,7 @@ fn program_clauses_that_could_match<I: Interner>(
                 if let Some(well_known) = trait_datum.well_known {
                     builtin_traits::add_builtin_assoc_program_clauses(
                         db, builder, well_known, self_ty,
-                    );
+                    )?;
                 }
 
                 push_program_clauses_for_associated_type_values_in_impls_of(

chalk-solve/src/clauses/builder.rs

@@ -114,7 +114,11 @@ impl<'me, I: Interner> ClauseBuilder<'me, I> {
     /// The new binders are always pushed onto the end of the internal
     /// list of binders; this means that any extant values where were
     /// created referencing the *old* list of binders are still valid.
-    pub fn push_binders<V>(&mut self, binders: &Binders<V>, op: impl FnOnce(&mut Self, V::Result))
+    pub fn push_binders<R, V>(
+        &mut self,
+        binders: &Binders<V>,
+        op: impl FnOnce(&mut Self, V::Result) -> R,
+    ) -> R
     where
         V: Fold<I> + HasInterner<Interner = I>,
         V::Result: std::fmt::Debug,
@@ -134,10 +138,11 @@ impl<'me, I: Interner> ClauseBuilder<'me, I> {
 
         let value = binders.substitute(self.interner(), &self.parameters[old_len..]);
         debug!("push_binders: value={:?}", value);
-        op(self, value);
+        let res = op(self, value);
 
         self.binders.truncate(old_len);
         self.parameters.truncate(old_len);
+        res
     }
 
     /// Push a single binder, for a type, at the end of the binder

chalk-solve/src/clauses/builtin_traits.rs

@@ -1,6 +1,6 @@
 use super::{builder::ClauseBuilder, generalize};
 use crate::{Interner, RustIrDatabase, TraitRef, WellKnownTrait};
-use chalk_ir::{Substitution, Ty};
+use chalk_ir::{Floundered, Substitution, Ty};
 
 mod clone;
 mod copy;
@@ -14,7 +14,7 @@ pub fn add_builtin_program_clauses<I: Interner>(
     builder: &mut ClauseBuilder<'_, I>,
     well_known: WellKnownTrait,
     trait_ref: &TraitRef<I>,
-) {
+) -> Result<(), Floundered> {
     // If `trait_ref` contains bound vars, we want to universally quantify them.
     // `Generalize` collects them for us.
     let generalized = generalize::Generalize::apply(db.interner(), trait_ref);
@@ -26,20 +26,21 @@ pub fn add_builtin_program_clauses<I: Interner>(
             if force_impl {
                 builder.push_fact(trait_ref.clone());
             }
-            return;
+            return Ok(());
         }
 
         match well_known {
             WellKnownTrait::Sized => sized::add_sized_program_clauses(db, builder, &trait_ref, ty),
             WellKnownTrait::Copy => copy::add_copy_program_clauses(db, builder, &trait_ref, ty),
             WellKnownTrait::Clone => clone::add_clone_program_clauses(db, builder, &trait_ref, ty),
             WellKnownTrait::FnOnce | WellKnownTrait::FnMut | WellKnownTrait::Fn => {
-                fn_family::add_fn_trait_program_clauses(db, builder, trait_ref.trait_id, self_ty)
+                fn_family::add_fn_trait_program_clauses(db, builder, trait_ref.trait_id, self_ty)?
             }
             // Drop impls are provided explicitly
             WellKnownTrait::Drop => (),
         }
-    });
+        Ok(())
+    })
 }
 
 /// Like `add_builtin_program_clauses`, but for `DomainGoal::Normalize` involving
@@ -49,14 +50,15 @@ pub fn add_builtin_assoc_program_clauses<I: Interner>(
     builder: &mut ClauseBuilder<'_, I>,
     well_known: WellKnownTrait,
     self_ty: Ty<I>,
-) {
+) -> Result<(), Floundered> {
     match well_known {
         WellKnownTrait::FnOnce => {
             let trait_id = db.well_known_trait_id(well_known).unwrap();
-            fn_family::add_fn_trait_program_clauses(db, builder, trait_id, self_ty);
+            fn_family::add_fn_trait_program_clauses(db, builder, trait_id, self_ty)?;
         }
         _ => {}
     }
+    Ok(())
 }
 
 /// Given a trait ref `T0: Trait` and a list of types `U0..Un`, pushes a clause of the form

chalk-solve/src/clauses/builtin_traits/fn_family.rs

@@ -3,8 +3,8 @@ use crate::infer::instantiate::IntoBindersAndValue;
 use crate::rust_ir::WellKnownTrait;
 use crate::{Interner, RustIrDatabase, TraitRef};
 use chalk_ir::{
-    AliasTy, ApplicationTy, Binders, Normalize, ProjectionTy, Substitution, TraitId, Ty, TyData,
-    TypeName, VariableKinds,
+    AliasTy, ApplicationTy, Binders, Floundered, Normalize, ProjectionTy, Substitution, TraitId,
+    Ty, TyData, TypeName, VariableKinds,
 };
 
 /// Handles clauses for FnOnce/FnMut/Fn.
@@ -21,7 +21,7 @@ pub fn add_fn_trait_program_clauses<I: Interner>(
     builder: &mut ClauseBuilder<'_, I>,
     trait_id: TraitId<I>,
     self_ty: Ty<I>,
-) {
+) -> Result<(), Floundered> {
     let interner = db.interner();
     match self_ty.data(interner) {
         TyData::Function(fn_val) => {
@@ -81,7 +81,10 @@ pub fn add_fn_trait_program_clauses<I: Interner>(
                     builder.push_fact(normalize);
                 }
             });
+            Ok(())
         }
-        _ => {}
+        // Function traits are non-enumerable
+        TyData::InferenceVar(..) | TyData::Alias(..) => Err(Floundered),
+        _ => Ok(()),
     }
 }

tests/test/functions.rs

@@ -14,6 +14,12 @@ fn function_implement_fn_traits() {
 
             #[lang(fn)]
             trait Fn<Args> where Self: FnMut<Args> { }
+
+            struct Ty { }
+
+            trait Clone { }
+            opaque type MyOpaque: Clone = Ty;
+
         }
 
         // Simple test: make sure a fully monomorphic type implements FnOnce
@@ -156,6 +162,22 @@ fn function_implement_fn_traits() {
         } yields {
             "No possible solution"
         }
+
+        // Tests that we flounder for inference variables
+        goal {
+            exists<T> {
+                T: FnOnce<()>
+            }
+        } yields_first[SolverChoice::slg(3, None)] {
+            "Floundered"
+        }
+
+        // Tests that we flounder for alias type (opaque)
+        goal {
+            MyOpaque: FnOnce<()>
+        } yields_first[SolverChoice::slg(3, None)] {
+            "Floundered"
+        }
     }
 }