Point at : when using it instead of ;

When triggering type ascription in such a way that we can infer a
statement end was intended, add a suggestion for the change. Always
point out the reason for the expectation of a type is due to type
ascription.

Author: estebank

src/libsyntax/parse/parser.rs

@@ -2798,7 +2798,22 @@ impl<'a> Parser<'a> {
                 lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Cast)?;
                 continue
             } else if op == AssocOp::Colon {
-                lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Type)?;
+                lhs = match self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Type) {
+                    Ok(lhs) => lhs,
+                    Err(mut err) => {
+                        err.span_label(self.span,
+                                       "expecting a type here because of type ascription");
+                        let cm = self.sess.codemap();
+                        let cur_pos = cm.lookup_char_pos(self.span.lo);
+                        let op_pos = cm.lookup_char_pos(cur_op_span.hi);
+                        if cur_pos.line != op_pos.line {
+                            err.span_suggestion(cur_op_span,
+                                                "did you mean to end the statement here instead?",
+                                                ";".to_string());
+                        }
+                        return Err(err);
+                    }
+                };
                 continue
             } else if op == AssocOp::DotDot || op == AssocOp::DotDotDot {
                 // If we didn’t have to handle `x..`/`x...`, it would be pretty easy to

src/test/ui/suggestions/type-ascription-instead-of-statement-end.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![feature(type_ascription)]
+
+fn main() {
+    println!("test"):
+    0;
+}
+
+fn foo() {
+    println!("test"): 0;
+}

src/test/ui/suggestions/type-ascription-instead-of-statement-end.stderr

@@ -0,0 +1,16 @@
+error: expected type, found `0`
+  --> $DIR/type-ascription-instead-of-statement-end.rs:15:5
+   |
+14 |     println!("test"):
+   |                     - help: did you mean to end the statement here instead? `;`
+15 |     0;
+   |     ^ expecting a type here because of type ascription
+
+error: expected type, found `0`
+  --> $DIR/type-ascription-instead-of-statement-end.rs:19:23
+   |
+19 |     println!("test"): 0;
+   |                       ^ expecting a type here because of type ascription
+
+error: aborting due to 2 previous errors
+