Deprecate ArgParser::list

Author: scrabsha

compiler/rustc_attr_parsing/src/parser.rs

@@ -176,6 +176,7 @@ impl ArgParser {
     ///
     /// - `#[allow(clippy::complexity)]`: `(clippy::complexity)` is a list
     /// - `#[rustfmt::skip::macros(target_macro_name)]`: `(target_macro_name)` is a list
+    #[deprecated = "use `AcceptContext::single_element_list` instead"]
     pub fn list(&self) -> Option<&MetaItemListParser> {
         match self {
             Self::List(l) => Some(l),