Use chaining for DiagnosticBuilder construction and emit.

To avoid the use of a mutable local variable, and because it reads more
nicely.

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -303,14 +303,14 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                         // This exception needs to be kept in sync with allowing
                         // `#[target_feature]` on `main` and `start`.
                     } else if !tcx.features().target_feature_11 {
-                        let mut err = feature_err(
+                        feature_err(
                             &tcx.sess.parse_sess,
                             sym::target_feature_11,
                             attr.span,
                             "`#[target_feature(..)]` can only be applied to `unsafe` functions",
-                        );
-                        err.span_label(tcx.def_span(did), "not an `unsafe` function");
-                        err.emit();
+                        )
+                        .span_label_mv(tcx.def_span(did), "not an `unsafe` function")
+                        .emit();
                     } else {
                         check_target_feature_trait_unsafe(tcx, did, attr.span);
                     }