Add a nicer error message for missing in for loop, fixes #40782.

src/libsyntax/parse/parser.rs

@@ -3154,7 +3154,13 @@ impl<'a> Parser<'a> {
         // Parse: `for <src_pat> in <src_expr> <src_loop_block>`
 
         let pat = self.parse_pat()?;
-        self.expect_keyword(keywords::In)?;
+        if !self.eat_keyword(keywords::In) {
+            let in_span = self.prev_span.between(self.span);
+            let mut err = self.sess.span_diagnostic
+                .struct_span_err(in_span, "missing `in` in `for` loop");
+            err.span_suggestion_short(in_span, "try adding `in` here", " in ".into());
+            err.emit();
+        }
         let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
         let (iattrs, loop_block) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);

src/test/ui/issue-40782.rs

@@ -0,0 +1,15 @@
+// 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() {
+    for i 0..2 {
+    }
+}
+

src/test/ui/issue-40782.stderr

@@ -0,0 +1,8 @@
+error: missing `in` in `for` loop
+  --> $DIR/issue-40782.rs:12:10
+   |
+12 |     for i 0..2 {
+   |          ^ help: try adding `in` here
+
+error: aborting due to previous error
+