Fix false positive in unused_parens caused by break

compiler/rustc_lint/src/unused.rs

@@ -794,7 +794,10 @@ trait UnusedDelimLint {
 
                 ExprKind::Break(_label, None) => return false,
                 ExprKind::Break(_label, Some(break_expr)) => {
-                    return matches!(break_expr.kind, ExprKind::Block(..));
+                    // `if (break 'label i) { ... }` removing parens would make `i { ... }`
+                    // be parsed as a struct literal, so keep parentheses if the break value
+                    // ends with a path (which could be mistaken for a struct name).
+                    return matches!(break_expr.kind, ExprKind::Block(..) | ExprKind::Path(..));
                 }
 
                 ExprKind::Range(_lhs, Some(rhs), _limits) => {

tests/ui/lint/unused-parens-labeled-break-issue-143256.rs

@@ -0,0 +1,25 @@
+//@ check-pass
+// testcase for https://github.com/rust-lang/rust/issues/143256
+
+#![deny(unused_parens)]
+#![allow(unreachable_code, unused_variables, dead_code)]
+
+fn foo() {
+    let _x = || 'outer: loop {
+        let inner = 'inner: loop {
+            let i = Default::default();
+            // the parentheses here are necessary
+            if (break 'outer i) {
+                loop {
+                    break 'inner 5i8;
+                }
+            } else if true {
+                break 'inner 6;
+            }
+            break 7;
+        };
+        break inner < 8;
+    };
+}
+
+fn main() {}