Enable extra warnings required by rust-lang/rust

crates/mbe/src/lib.rs

@@ -6,6 +6,8 @@
 //! The tes for this functionality live in another crate:
 //! `hir_def::macro_expansion_tests::mbe`.
 
+#![warn(rust_2018_idioms, unused_lifetimes, semicolon_in_expressions_from_macros)]
+
 mod parser;
 mod expander;
 mod syntax_bridge;