Specialize the (0, 0) case too.

Author: obi1kenobi

compiler/rustc_type_ir/src/fast_reject.rs

@@ -247,12 +247,12 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_
             }
         };
 
-        // Specialize the common `(1, 1)` case to avoid iterator machinery.
-        if let ([obl], [imp]) = (obligation_args.as_slice(), impl_args.as_slice()) {
-            return may_unify((*obl, *imp));
+        // Specialize the common `(1, 1)` and `(0, 0)` cases to avoid iterator machinery.
+        match (obligation_args.as_slice(), impl_args.as_slice()) {
+            ([obl], [imp]) => may_unify((*obl, *imp)),
+            ([], []) => true,
+            _ => iter::zip(obligation_args.iter(), impl_args.iter()).all(may_unify),
         }
-
-        iter::zip(obligation_args.iter(), impl_args.iter()).all(may_unify)
     }
 
     fn types_may_unify_inner(self, lhs: I::Ty, rhs: I::Ty, depth: usize) -> bool {