Skip method calls with arity mismatch

Author: martingms

compiler/rustc_hir_typeck/src/demand.rs

@@ -298,6 +298,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // call's arguments and we can provide a more explicit span.
                 let sig = self.tcx.fn_sig(def_id).subst_identity();
                 let def_self_ty = sig.input(0).skip_binder();
+                let param_tys = sig.inputs().skip_binder().iter().skip(1);
+                // If there's an arity mismatch, pointing out the call as the source of an inference
+                // can be misleading, so we skip it.
+                if param_tys.len() != args.len() {
+                    continue;
+                }
                 let rcvr_ty = self.node_ty(rcvr.hir_id);
                 // Get the evaluated type *after* calling the method call, so that the influence
                 // of the arguments can be reflected in the receiver type. The receiver
@@ -323,7 +329,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let mut param_found = FxHashMap::default();
                 if self.can_eq(self.param_env, ty, found).is_ok() {
                     // We only point at the first place where the found type was inferred.
-                    for (param_ty, arg) in sig.inputs().skip_binder().iter().skip(1).zip(args) {
+                    for (param_ty, arg) in param_tys.zip(args) {
                         if def_self_ty.contains(*param_ty) && let ty::Param(_) = param_ty.kind() {
                             // We found an argument that references a type parameter in `Self`,
                             // so we assume that this is the argument that caused the found

tests/ui/type/type-check/point-at-inference-4.rs

@@ -13,8 +13,6 @@ fn main() {
     //~^ ERROR this method takes 2 arguments but 1 argument was supplied
     //~| NOTE an argument is missing
     //~| HELP provide the argument
-    //~| NOTE this is of type `i32`, which causes `s` to be inferred as `S<i32, _>`
-    //~| HELP change the type of the numeric literal from `i32` to `u32`
     let t: S<u32, _> = s;
     //~^ ERROR mismatched types
     //~| NOTE expected `S<u32, _>`, found `S<i32, _>`

tests/ui/type/type-check/point-at-inference-4.stderr

@@ -15,22 +15,15 @@ LL |     s.infer(0i32, /* b */);
    |            ~~~~~~~~~~~~~~~
 
 error[E0308]: mismatched types
-  --> $DIR/point-at-inference-4.rs:18:24
+  --> $DIR/point-at-inference-4.rs:16:24
    |
-LL |     s.infer(0i32);
-   |             ---- this is of type `i32`, which causes `s` to be inferred as `S<i32, _>`
-...
 LL |     let t: S<u32, _> = s;
    |            ---------   ^ expected `S<u32, _>`, found `S<i32, _>`
    |            |
    |            expected due to this
    |
    = note: expected struct `S<u32, _>`
               found struct `S<i32, _>`
-help: change the type of the numeric literal from `i32` to `u32`
-   |
-LL |     s.infer(0u32);
-   |              ~~~
 
 error: aborting due to 2 previous errors