Auto merge of #43096 - estebank:ascription-help, r=nikomatsakis

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.

Fix #42057, #41928.

1 files changed, 16 insertions, 1 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 01b72343b5d..d6a57c2874f 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_short(cur_op_span,
+                                                      "did you mean to use `;` here?",
+                                                      ";".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