Fix clippy warning

Allow `many_single_char_names` on `SpanlessHash::hash_expr`. Each
variable has a small scope and the method is readable.

clippy_lints/src/utils/hir_utils.rs

@@ -316,6 +316,7 @@ impl<'a, 'tcx: 'a> SpanlessHash<'a, 'tcx> {
         b.rules.hash(&mut self.s);
     }
 
+    #[allow(many_single_char_names)]
     pub fn hash_expr(&mut self, e: &Expr) {
         if let Some(e) = constant_simple(self.cx, e) {
             return e.hash(&mut self.s);