From 4b7a92838c98929a9f4c7a64583b5ea49ebb3a35 Mon Sep 17 00:00:00 2001
From: mibac138 <5672750+mibac138@users.noreply.github.com>
Date: Wed, 6 May 2020 01:50:55 +0200
Subject: [PATCH] Fix unused_parens nested binary op false positive

---
 src/librustc_lint/unused.rs                      | 16 ++++++++++++----
 .../ui/lint/issue-71290-unused-paren-binop.rs    |  5 +++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index 4f7a1f9c31091..c24079a6e4be2 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -381,12 +381,20 @@ trait UnusedDelimLint {
 
     fn is_expr_delims_necessary(inner: &ast::Expr, followed_by_block: bool) -> bool {
         // 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)
+        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;
+                }
             }
-            _ => false,
         };
+
         lhs_needs_parens
             || (followed_by_block
                 && match inner.kind {
diff --git a/src/test/ui/lint/issue-71290-unused-paren-binop.rs b/src/test/ui/lint/issue-71290-unused-paren-binop.rs
index f11d2d377378c..24d77e36d94f5 100644
--- a/src/test/ui/lint/issue-71290-unused-paren-binop.rs
+++ b/src/test/ui/lint/issue-71290-unused-paren-binop.rs
@@ -15,4 +15,9 @@ 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() {}