Handle casts to integer/float variables

These can happen if prior errors disable defaulting.

Fixes #43825.

src/test/ui/cast-errors-issue-43825.rs

@@ -0,0 +1,17 @@
+// Copyright 2017 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.
+
+fn main() {
+    let error = error; //~ ERROR cannot find value `error`
+
+    // These used to cause errors.
+    0 as f32;
+    0.0 as u32;
+}

src/test/ui/cast-errors-issue-43825.stderr

@@ -0,0 +1,8 @@
+error[E0425]: cannot find value `error` in this scope
+  --> $DIR/cast-errors-issue-43825.rs:12:17
+   |
+12 |     let error = error; //~ ERROR cannot find value `error`
+   |                 ^^^^^ not found in this scope
+
+error: aborting due to previous error
+