detect wrong number of args when type-checking a closure

Instead of creating inference variables for those argument types, use
the trait error-reporting code to give a nicer error.

src/test/ui/mismatched_types/closure-arg-count-expected-type-issue-47244.rs

@@ -0,0 +1,29 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #47244: in this specific scenario, when the
+// expected type indicated 1 argument but the closure takes two, we
+// would (early on) create type variables for the type of `b`. If the
+// user then attempts to invoke a method on `b`, we would get an error
+// saying that the type of `b` must be known, which was not very
+// helpful.
+
+use std::collections::HashMap;
+fn main() {
+
+    let m = HashMap::new();
+    m.insert( "foo", "bar" );
+
+    m.iter().map( |_, b| {
+        //~^ ERROR closure is expected to take a single 2-tuple
+
+        b.to_string()
+    });
+}

src/test/ui/mismatched_types/closure-arg-count-expected-type-issue-47244.stderr

@@ -0,0 +1,14 @@
+error[E0593]: closure is expected to take a single 2-tuple as argument, but it takes 2 distinct arguments
+  --> $DIR/closure-arg-count-expected-type-issue-47244.rs:24:14
+   |
+24 |     m.iter().map( |_, b| {
+   |              ^^^  ------ takes 2 distinct arguments
+   |              |
+   |              expected closure that takes a single 2-tuple as argument
+help: change the closure to accept a tuple instead of individual arguments
+   |
+24 |     m.iter().map( |(_, b)| {
+   |                   ^^^^^^^^
+
+error: aborting due to previous error
+