From 7239d7717188f6dc2b380a40335a0c8571be6502 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Thu, 6 Jul 2017 14:29:55 -0700
Subject: [PATCH] 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.
---
 src/libsyntax/parse/parser.rs                 | 17 +++++++++++++++-
 ...ype-ascription-instead-of-statement-end.rs | 20 +++++++++++++++++++
 ...ascription-instead-of-statement-end.stderr | 16 +++++++++++++++
 3 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/suggestions/type-ascription-instead-of-statement-end.rs
 create mode 100644 src/test/ui/suggestions/type-ascription-instead-of-statement-end.stderr

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 74b2ea1df323a..1b6c3cf94e4f4 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/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
diff --git a/src/test/ui/suggestions/type-ascription-instead-of-statement-end.rs b/src/test/ui/suggestions/type-ascription-instead-of-statement-end.rs
new file mode 100644
index 0000000000000..93de55a39e954
--- /dev/null
+++ b/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;
+}
diff --git a/src/test/ui/suggestions/type-ascription-instead-of-statement-end.stderr b/src/test/ui/suggestions/type-ascription-instead-of-statement-end.stderr
new file mode 100644
index 0000000000000..e4cf78dbb2d9f
--- /dev/null
+++ b/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
+