syntax_ext: deny(unused_lifetimes).

src/libsyntax_ext/cfg.rs

@@ -12,7 +12,7 @@ use syntax::tokenstream;
 use syntax::parse::token;
 use syntax_pos::Span;
 
-pub fn expand_cfg<'cx>(cx: &mut ExtCtxt<'_>,
+pub fn expand_cfg(cx: &mut ExtCtxt<'_>,
                        sp: Span,
                        tts: &[tokenstream::TokenTree])
                        -> Box<dyn base::MacResult + 'static> {

src/libsyntax_ext/lib.rs

@@ -4,6 +4,7 @@
 
 #![deny(rust_2018_idioms)]
 #![deny(internal)]
+#![deny(unused_lifetimes)]
 
 #![feature(in_band_lifetimes)]
 #![feature(proc_macro_diagnostic)]