Add help message for missing right operand in condition

Author: antoyo

src/librustc_parse/parser/expr.rs

@@ -1550,6 +1550,11 @@ impl<'a> Parser<'a> {
             let block = self.parse_block().map_err(|mut err| {
                 if not_block {
                     err.span_label(lo, "this `if` expression has a condition, but no block");
+                    if let ExprKind::Binary(_, _, ref right) = cond.kind {
+                        if let ExprKind::Block(_, _) = right.kind {
+                            err.help("maybe you forgot the right operand of the condition?");
+                        }
+                    }
                 }
                 err
             })?;

src/test/ui/if/if-without-block.stderr

@@ -6,6 +6,8 @@ LL |     if 5 == {
 ...
 LL | }
    | ^ expected `{`
+   |
+   = help: maybe you forgot the right operand of the condition?
 
 error: aborting due to previous error