Filter out anonymous generics in rustdoc.

Author: cjgillot

src/librustdoc/clean/mod.rs

@@ -547,7 +547,12 @@ fn clean_ty_generics(
         .params
         .iter()
         .filter_map(|param| match param.kind {
-            ty::GenericParamDefKind::Lifetime => Some(param.clean(cx)),
+            ty::GenericParamDefKind::Lifetime => {
+                if param.name == kw::UnderscoreLifetime {
+                    return None;
+                }
+                Some(param.clean(cx))
+            }
             ty::GenericParamDefKind::Type { synthetic, .. } => {
                 if param.name == kw::SelfUpper {
                     assert_eq!(param.index, 0);