Port #[const_trait] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -234,6 +234,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_const_stable_indirect]`.
     ConstStabilityIndirect,
 
+    /// Represents `#[const_trait]`.
+    ConstTrait(Span),
+
     /// Represents [`#[deprecated]`](https://doc.rust-lang.org/stable/reference/attributes/diagnostics.html#the-deprecated-attribute).
     Deprecation { deprecation: Deprecation, span: Span },
 

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -23,6 +23,7 @@ impl AttributeKind {
             ConstContinue(..) => No,
             ConstStability { .. } => Yes,
             ConstStabilityIndirect => No,
+            ConstTrait(..) => No,
             Deprecation { .. } => Yes,
             DocComment { .. } => Yes,
             Dummy => No,