Show which test case was found to be meaningless

Author: dtolnay

tests/ui-fulldeps/pprust-parenthesis-insertion.rs

@@ -1,5 +1,6 @@
 //@ run-pass
 //@ ignore-cross-compile
+//@ edition: 2021
 
 // This test covers the AST pretty-printer's automatic insertion of parentheses
 // into unparenthesized syntax trees according to precedence and various grammar
@@ -170,7 +171,9 @@ fn main() -> ExitCode {
         let psess = &ParseSess::new(vec![rustc_parse::DEFAULT_LOCALE_RESOURCE]);
 
         for &source_code in EXPRS {
-            let expr = parse_expr(psess, source_code).unwrap();
+            let Some(expr) = parse_expr(psess, source_code) else {
+                panic!("Failed to parse original test case: {source_code}");
+            };
 
             // Check for FALSE POSITIVE: pretty-printer inserting parentheses where not needed.
             // Pseudocode: