Apply new lint on clippy source code

clippy_utils/src/hir_utils.rs

@@ -41,6 +41,7 @@ pub fn new(cx: &'a LateContext<'tcx>) -> Self {
     }
 
     /// Consider expressions containing potential side effects as not equal.
+    #[must_use]
     pub fn deny_side_effects(self) -> Self {
         Self {
             allow_side_effects: false,
@@ -48,6 +49,7 @@ pub fn deny_side_effects(self) -> Self {
         }
     }
 
+    #[must_use]
     pub fn expr_fallback(self, expr_fallback: impl FnMut(&Expr<'_>, &Expr<'_>) -> bool + 'a) -> Self {
         Self {
             expr_fallback: Some(Box::new(expr_fallback)),

clippy_utils/src/sugg.rs

@@ -294,6 +294,7 @@ pub fn range(self, end: &Self, limit: ast::RangeLimits) -> Sugg<'static> {
     /// Adds parentheses to any expression that might need them. Suitable to the
     /// `self` argument of a method call
     /// (e.g., to build `bar.foo()` or `(1 + 2).foo()`).
+    #[must_use]
     pub fn maybe_par(self) -> Self {
         match self {
             Sugg::NonParen(..) => self,