Fix unused_parens false positive when using binary operations

src/librustc_lint/unused.rs

@@ -380,11 +380,27 @@ trait UnusedDelimLint {
     );
 
     fn is_expr_delims_necessary(inner: &ast::Expr, followed_by_block: bool) -> bool {
-        followed_by_block
-            && match inner.kind {
-                ExprKind::Ret(_) | ExprKind::Break(..) => true,
-                _ => parser::contains_exterior_struct_lit(&inner),
+        // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`
+        let lhs_needs_parens = {
+            let mut innermost = inner;
+            loop {
+                if let ExprKind::Binary(_, lhs, _rhs) = &innermost.kind {
+                    innermost = lhs;
+                    if !rustc_ast::util::classify::expr_requires_semi_to_be_stmt(innermost) {
+                        break true;
+                    }
+                } else {
+                    break false;
+                }
             }
+        };
+
+        lhs_needs_parens
+            || (followed_by_block
+                && match inner.kind {
+                    ExprKind::Ret(_) | ExprKind::Break(..) => true,
+                    _ => parser::contains_exterior_struct_lit(&inner),
+                })
     }
 
     fn emit_unused_delims_expr(

src/test/ui/lint/issue-71290-unused-paren-binop.rs

@@ -0,0 +1,23 @@
+// check-pass
+// Make sure unused parens lint doesn't emit a false positive.
+// See https://github.com/rust-lang/rust/issues/71290 for details.
+#![deny(unused_parens)]
+
+fn x() -> u8 {
+    ({ 0 }) + 1
+}
+
+fn y() -> u8 {
+    ({ 0 } + 1)
+}
+
+pub fn foo(a: bool, b: bool) -> u8 {
+    (if a { 1 } else { 0 } + if b { 1 } else { 0 })
+}
+
+pub fn bar() -> u8 {
+    // Make sure nested expressions are handled correctly as well
+    ({ 0 } + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)
+}
+
+fn main() {}