Fix unused_parens false positive when using binary operations

src/librustc_lint/unused.rs

@@ -380,11 +380,19 @@ 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 = match &inner.kind {
+            ExprKind::Binary(_, lhs, _rhs) => {
+                !rustc_ast::util::classify::expr_requires_semi_to_be_stmt(&*lhs)
             }
+            _ => 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 +1,17 @@
+// check-pass
+// Make sure unused parens lint doesn't emit a false positive.
+// See https://github.com/rust-lang/rust/issues/71290 for details.
+
+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 })
+}
+
+fn main() {}