Improve const generics parsing

- Handle const generics type args
- Fix issue with const generic as first parameter in trait impl

xtask/src/ast_src.rs

@@ -206,6 +206,7 @@ pub(crate) const KINDS_SRC: KindsSrc = KindsSrc {
         "LIFETIME_ARG",
         "TYPE_ARG",
         "ASSOC_TYPE_ARG",
+        "CONST_ARG",
         "PARAM_LIST",
         "PARAM",
         "SELF_PARAM",
@@ -511,10 +512,12 @@ pub(crate) const AST_SRC: AstSrc = AstSrc {
             type_args: [TypeArg],
             lifetime_args: [LifetimeArg],
             assoc_type_args: [AssocTypeArg],
+            const_arg: [ConstArg],
         }
         struct TypeArg { TypeRef }
         struct AssocTypeArg { NameRef, TypeRef }
         struct LifetimeArg {}
+        struct ConstArg { Literal, BlockExpr }
 
         struct MacroItems: ModuleItemOwner, FnDefOwner { }