Fix function pointers of different param counts unifying

Author: flodiebold

chalk-ir/src/zip.rs

@@ -524,6 +524,9 @@ impl<I: Interner> Zip<I> for FnSubst<I> {
         b: &Self,
     ) -> Fallible<()> {
         let interner = zipper.interner();
+        if a.0.len(interner) != b.0.len(interner) {
+            return Err(NoSolution);
+        }
         // Parameters
         for (a, b) in a.0.as_slice(interner)[..a.0.len(interner) - 1]
             .iter()

tests/test/unify.rs

@@ -149,6 +149,13 @@ fn forall_equality() {
             expect![["Unique"]]
         }
 
+        goal {
+            // Function pointers with different parameters should not be equal.
+            fn(u32): Eq<fn(u32, u32)>
+        } yields {
+            expect![["No possible solution"]]
+        }
+
         goal {
             // Variadic function pointers should not be equal to non-variadic fn pointers.
             fn(u8, ...): Eq<fn(u8)>