Port #[const_trait] to the new attribute system

compiler/rustc_attr_parsing/src/attributes/traits.rs

@@ -2,9 +2,11 @@ use core::mem;
 
 use rustc_attr_data_structures::AttributeKind;
 use rustc_feature::{AttributeTemplate, template};
-use rustc_span::{Symbol, sym};
+use rustc_span::{Span, Symbol, sym};
 
-use crate::attributes::{AttributeOrder, OnDuplicate, SingleAttributeParser};
+use crate::attributes::{
+    AttributeOrder, NoArgsAttributeParser, OnDuplicate, SingleAttributeParser,
+};
 use crate::context::{AcceptContext, Stage};
 use crate::parser::ArgParser;
 
@@ -52,3 +54,10 @@ impl<S: Stage> SingleAttributeParser<S> for SkipDuringMethodDispatchParser {
         Some(AttributeKind::SkipDuringMethodDispatch { array, boxed_slice, span: cx.attr_span })
     }
 }
+
+pub(crate) struct ConstTraitParser;
+impl<S: Stage> NoArgsAttributeParser<S> for ConstTraitParser {
+    const PATH: &[Symbol] = &[sym::const_trait];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Warn;
+    const CREATE: fn(Span) -> AttributeKind = AttributeKind::ConstTrait;
+}