Apply new lint on clippy source code

Author: GuillaumeGomez

clippy_utils/src/hir_utils.rs

@@ -41,13 +41,15 @@ impl<'a, 'tcx> SpanlessEq<'a, 'tcx> {
     }
 
     /// Consider expressions containing potential side effects as not equal.
+    #[must_use]
     pub fn deny_side_effects(self) -> Self {
         Self {
             allow_side_effects: false,
             ..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 @@ impl<'a> Sugg<'a> {
     /// 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,