add recovery to parse_labeled_expr

Author: Centril

src/librustc_parse/parser/expr.rs

@@ -1096,9 +1096,11 @@ impl<'a> Parser<'a> {
         }
 
         let msg = "expected `while`, `for`, `loop` or `{` after a label";
-        let mut err = self.fatal(msg);
-        err.span_label(self.token.span, msg);
-        return Err(err);
+        self.struct_span_err(self.token.span, msg)
+            .span_label(self.token.span, msg)
+            .emit();
+        // Continue as an expression in an effort to recover on `'label: non_block_expr`.
+        self.parse_expr()
     }
 
     /// Returns a string literal if the next token is a string literal.

src/test/ui/parser/recover-labeled-non-block-expr.rs

@@ -0,0 +1,5 @@
+fn main() {
+    'label: 1 + 1; //~ ERROR expected `while`, `for`, `loop` or `{` after a label
+
+    let _recovery_witness: () = 0; //~ ERROR mismatched types
+}

src/test/ui/parser/recover-labeled-non-block-expr.stderr

@@ -0,0 +1,17 @@
+error: expected `while`, `for`, `loop` or `{` after a label
+  --> $DIR/recover-labeled-non-block-expr.rs:2:13
+   |
+LL |     'label: 1 + 1;
+   |             ^ expected `while`, `for`, `loop` or `{` after a label
+
+error[E0308]: mismatched types
+  --> $DIR/recover-labeled-non-block-expr.rs:4:33
+   |
+LL |     let _recovery_witness: () = 0;
+   |                            --   ^ expected `()`, found integer
+   |                            |
+   |                            expected due to this
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.