Support FnPtr trait

chalk-integration/src/lowering.rs

@@ -1141,6 +1141,7 @@ impl Lower for WellKnownTrait {
             WellKnownTrait::DispatchFromDyn => rust_ir::WellKnownTrait::DispatchFromDyn,
             WellKnownTrait::Tuple => rust_ir::WellKnownTrait::Tuple,
             WellKnownTrait::Pointee => rust_ir::WellKnownTrait::Pointee,
+            WellKnownTrait::FnPtr => rust_ir::WellKnownTrait::FnPtr,
         }
     }
 }

chalk-parse/src/ast.rs

@@ -163,6 +163,7 @@ pub enum WellKnownTrait {
     DispatchFromDyn,
     Tuple,
     Pointee,
+    FnPtr,
 }
 
 #[derive(Clone, PartialEq, Eq, Debug)]

chalk-parse/src/parser.lalrpop

@@ -71,6 +71,7 @@ WellKnownTrait: WellKnownTrait = {
      "#" "[" "lang" "(" "dispatch_from_dyn" ")" "]" => WellKnownTrait::DispatchFromDyn,
      "#" "[" "lang" "(" "tuple_trait" ")" "]" => WellKnownTrait::Tuple,
      "#" "[" "lang" "(" "pointee_trait" ")" "]" => WellKnownTrait::Pointee,
+     "#" "[" "lang" "(" "fn_ptr_trait" ")" "]" => WellKnownTrait::FnPtr,
 };
 
 AdtReprAttr: AdtReprAttr = {

chalk-solve/src/clauses/builtin_traits.rs

@@ -2,7 +2,7 @@ use super::{builder::ClauseBuilder, generalize};
 use crate::{
     rust_ir::AdtKind, CanonicalVarKinds, Interner, RustIrDatabase, TraitRef, WellKnownTrait,
 };
-use chalk_ir::{Floundered, Substitution, Ty};
+use chalk_ir::{Floundered, Substitution, Ty, TyKind};
 
 mod clone;
 mod copy;
@@ -60,6 +60,11 @@ pub fn add_builtin_program_clauses<I: Interner>(
             WellKnownTrait::Pointee => {
                 pointee::add_pointee_program_clauses(db, builder, self_ty)?;
             }
+            WellKnownTrait::FnPtr => {
+                if let TyKind::Function(_) = self_ty.kind(db.interner()) {
+                    builder.push_fact(trait_ref);
+                }
+            }
             // There are no builtin impls provided for the following traits:
             WellKnownTrait::Unpin
             | WellKnownTrait::Drop

chalk-solve/src/display/items.rs

@@ -207,6 +207,7 @@ impl<I: Interner> RenderAsRust<I> for TraitDatum<I> {
                 WellKnownTrait::DispatchFromDyn => "dispatch_from_dyn",
                 WellKnownTrait::Tuple => "tuple_trait",
                 WellKnownTrait::Pointee => "pointee",
+                WellKnownTrait::FnPtr => "fn_ptr_trait",
             };
             writeln!(f, "#[lang({})]", name)?;
         }

chalk-solve/src/rust_ir.rs

@@ -278,6 +278,7 @@ pub enum WellKnownTrait {
     DispatchFromDyn,
     Tuple,
     Pointee,
+    FnPtr,
 }
 
 chalk_ir::const_visit!(WellKnownTrait);

chalk-solve/src/wf.rs

@@ -437,7 +437,8 @@ where
             | WellKnownTrait::DiscriminantKind
             | WellKnownTrait::Generator
             | WellKnownTrait::Pointee
-            | WellKnownTrait::Tuple => false,
+            | WellKnownTrait::Tuple
+            | WellKnownTrait::FnPtr => false,
         };
 
         if is_legal {

tests/test/functions.rs

@@ -66,7 +66,6 @@ fn function_implement_fn_traits() {
             impl Clone for Ty { }
 
             opaque type MyOpaque: Clone = Ty;
-
         }
 
         // Simple test: make sure a fully monomorphic type implements FnOnce
@@ -246,3 +245,74 @@ fn function_implement_fn_traits() {
         }
     }
 }
+
+#[test]
+fn functions_implement_fn_ptr_trait() {
+    test! {
+        program {
+            #[lang(fn_ptr_trait)]
+            trait FnPtr {}
+
+            closure closure_ty(self,) {}
+            fn fn_def();
+        }
+
+        goal {
+            fn(()) -> (): FnPtr
+        } yields {
+            expect![[r#"Unique"#]]
+        }
+
+        goal {
+            unsafe fn(): FnPtr
+        } yields {
+            expect![[r#"Unique"#]]
+        }
+
+        goal {
+            extern "C" fn(u32, ...): FnPtr
+        } yields {
+            expect![[r#"Unique"#]]
+        }
+
+        goal {
+            for<'a> fn(&'a ()): FnPtr
+        } yields {
+            expect![[r#"Unique"#]]
+        }
+
+        goal {
+            forall<T, U> {
+                fn(T) -> U: FnPtr
+            }
+        } yields {
+            expect![[r#"Unique"#]]
+        }
+
+        goal {
+            i32: FnPtr
+        } yields {
+            expect![[r#"No possible solution"#]]
+        }
+
+        goal {
+            closure_ty: FnPtr
+        } yields {
+            expect![[r#"No possible solution"#]]
+        }
+
+        goal {
+            fn_def: FnPtr
+        } yields {
+            expect![[r#"No possible solution"#]]
+        }
+
+        goal {
+            exists<T> {
+                T: FnPtr
+            }
+        } yields {
+            expect![[r#"Ambiguous; no inference guidance"#]]
+        }
+    }
+}