Assert in apply_substs that the number of parameters doesn't change

... and fix a small bug revealed by that.

crates/ra_hir/src/ty.rs

@@ -148,6 +148,10 @@ impl Substs {
         self.0.iter()
     }
 
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
     pub fn walk_mut(&mut self, f: &mut impl FnMut(&mut Ty)) {
         // Without an Arc::make_mut_slice, we can't avoid the clone here:
         let mut v: Vec<_> = self.0.iter().cloned().collect();
@@ -286,7 +290,8 @@ impl Ty {
     /// `Option<u32>` afterwards.)
     pub fn apply_substs(self, substs: Substs) -> Ty {
         match self {
-            Ty::Apply(ApplicationTy { ctor, .. }) => {
+            Ty::Apply(ApplicationTy { ctor, parameters: previous_substs }) => {
+                assert_eq!(previous_substs.len(), substs.len());
                 Ty::Apply(ApplicationTy { ctor, parameters: substs })
             }
             _ => self,

crates/ra_hir/src/ty/infer.rs

@@ -817,7 +817,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                             Some(func.generic_params(self.db)),
                         )
                     }
-                    None => (Ty::Unknown, receiver_ty, None),
+                    None => (receiver_ty, Ty::Unknown, None),
                 };
                 let substs = self.substs_for_method_call(def_generics, generic_args);
                 let method_ty = method_ty.apply_substs(substs);