Adopt let else in more places

compiler/rustc_serialize/src/json.rs

@@ -2306,9 +2306,8 @@ impl crate::Decoder for Decoder {
             }
             json => bad!(ExpectedError("String or Object".to_owned(), json.to_string())),
         };
-        let idx = match names.iter().position(|n| *n == &name[..]) {
-            Some(idx) => idx,
-            None => bad!(UnknownVariantError(name)),
+        let Some(idx) = names.iter().position(|n| *n == &name[..]) else {
+            bad!(UnknownVariantError(name));
         };
         f(self, idx)
     }

compiler/rustc_serialize/src/lib.rs

@@ -15,6 +15,7 @@ Core encoding and decoding interfaces.
 #![feature(min_specialization)]
 #![feature(core_intrinsics)]
 #![feature(maybe_uninit_slice)]
+#![feature(let_else)]
 #![feature(new_uninit)]
 #![cfg_attr(test, feature(test))]
 #![allow(rustc::internal)]