Remove #[rustc_macro_edition_2021].

It was only temporarily used by pin!(), which no longer needs it.

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -28,7 +28,6 @@ pub(crate) mod cfg;
 pub(crate) mod confusables;
 pub(crate) mod deprecation;
 pub(crate) mod repr;
-pub(crate) mod rustc;
 pub(crate) mod stability;
 pub(crate) mod transparency;
 pub(crate) mod util;

compiler/rustc_attr_parsing/src/attributes/rustc.rs

@@ -1,19 +0,0 @@
-use rustc_attr_data_structures::AttributeKind;
-use rustc_span::sym;
-
-use super::{AcceptContext, SingleAttributeParser};
-use crate::parser::ArgParser;
-
-pub(crate) struct RustcMacroEdition2021Parser;
-
-// FIXME(jdonszelmann): make these proper diagnostics
-impl SingleAttributeParser for RustcMacroEdition2021Parser {
-    const PATH: &'static [rustc_span::Symbol] = &[sym::rustc_macro_edition_2021];
-
-    fn on_duplicate(_cx: &crate::context::AcceptContext<'_>, _first_span: rustc_span::Span) {}
-
-    fn convert(_cx: &AcceptContext<'_>, args: &ArgParser<'_>) -> Option<AttributeKind> {
-        assert!(args.no_args());
-        Some(AttributeKind::RustcMacroEdition2021)
-    }
-}