Auto merge of #43854 - estebank:missing-cond, r=nikomatsakis

Point out missing if conditional

On a case where an else conditional is missing, point this out
instead of the token immediately after the (incorrect) else block:

```
error: missing condition for `if` statemementt push fork -f

  --> $DIR/issue-13483.rs:16:5
   |
13 |    } else if {
   |             ^ expected if condition here
```

instead of

```
error: expected `{`, found `else`
  --> ../../src/test/ui/issue-13483.rs:14:7
   |
14 |     } else {
   |       ^^^^
```

Fix #13483.

src/libsyntax/parse/parser.rs

@@ -2965,6 +2965,18 @@ impl<'a> Parser<'a> {
         }
         let lo = self.prev_span;
         let cond = self.parse_expr_res(RESTRICTION_NO_STRUCT_LITERAL, None)?;
+
+        // Verify that the parsed `if` condition makes sense as a condition. If it is a block, then
+        // verify that the last statement is either an implicit return (no `;`) or an explicit
+        // return. This won't catch blocks with an explicit `return`, but that would be caught by
+        // the dead code lint.
+        if self.eat_keyword(keywords::Else) || !cond.returns() {
+            let sp = lo.next_point();
+            let mut err = self.diagnostic()
+                .struct_span_err(sp, "missing condition for `if` statemement");
+            err.span_label(sp, "expected if condition here");
+            return Err(err)
+        }
         let thn = self.parse_block()?;
         let mut els: Option<P<Expr>> = None;
         let mut hi = thn.span;