remove "constant" functions

The goal here is that ALL `Id` values come from a `Table`

components/salsa-macros/src/tracked_fn.rs

@@ -104,8 +104,8 @@ impl Macro {
         }
 
         let needs_interner = match function_type {
-            FunctionType::RequiresInterning => true,
-            FunctionType::Constant | FunctionType::SalsaStruct => false,
+            FunctionType::Constant | FunctionType::RequiresInterning => true,
+            FunctionType::SalsaStruct => false,
         };
 
         let lru = Literal::usize_unsuffixed(self.args.lru.unwrap_or(0));

src/id.rs

@@ -118,20 +118,6 @@ impl FromId for Id {
     }
 }
 
-/// As a special case, we permit `Singleton` to be converted to an `Id`.
-/// This is useful for declaring functions with no arguments.
-impl AsId for () {
-    fn as_id(&self) -> Id {
-        Id::from_u32(0)
-    }
-}
-
-impl FromId for () {
-    fn from_id(id: Id) -> Self {
-        assert_eq!(0, id.as_u32());
-    }
-}
-
 impl<'db, ID: FromId> LookupId<'db> for ID {
     fn lookup_id(id: Id, _db: &'db dyn Database) -> Self {
         Self::from_id(id)