Auto merge of #42578 - estebank:recover-binop, r=nikomatsakis

Learn to parse `a as usize < b`

Parsing `a as usize > b` always works, but `a as usize < b` was a
parsing error because the parser would think the `<` started a generic
type argument for `usize`. The parser now attempts to parse as before,
and if a DiagnosticError is returned, try to parse again as a type with
no generic arguments. If this fails, return the original
`DiagnosticError`.

Fix #22644.

src/test/ui/issue-22644.rs

@@ -0,0 +1,18 @@
+// 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 a : u32 = 0;
+    let b : usize = 0;
+
+    println!("{}", a as usize > b);
+    println!("{}", a as usize < b);
+    println!("{}", a as usize < 4);
+}

src/test/ui/issue-22644.stderr

@@ -0,0 +1,24 @@
+error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
+  --> $DIR/issue-22644.rs:16:33
+   |
+16 |     println!("{}", a as usize < b);
+   |                               - ^ interpreted as generic argument
+   |                               |
+   |                               not interpreted as comparison
+   |
+help: if you want to compare the casted value then write:
+   |     println!("{}", (a as usize) < b);
+
+error: `<` is interpreted as a start of generic arguments for `usize`, not a comparison
+  --> $DIR/issue-22644.rs:17:33
+   |
+17 |     println!("{}", a as usize < 4);
+   |                               - ^ interpreted as generic argument
+   |                               |
+   |                               not interpreted as comparison
+   |
+help: if you want to compare the casted value then write:
+   |     println!("{}", (a as usize) < 4);
+
+error: aborting due to previous error(s)
+