Add feature gate macro_attr for declarative macro_rules! attribute macros

Author: joshtriplett

compiler/rustc_feature/src/unstable.rs

@@ -555,6 +555,8 @@ declare_features! (
     (unstable, link_arg_attribute, "1.76.0", Some(99427)),
     /// Allows fused `loop`/`match` for direct intraprocedural jumps.
     (incomplete, loop_match, "CURRENT_RUSTC_VERSION", Some(132306)),
+    /// Allow `macro_rules!` attribute rules
+    (unstable, macro_attr, "CURRENT_RUSTC_VERSION", Some(83527)),
     /// Give access to additional metadata about declarative macro meta-variables.
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
     /// Provides a way to concatenate identifiers using metavariable expressions.

compiler/rustc_span/src/symbol.rs

@@ -1310,6 +1310,7 @@ symbols! {
         lt,
         m68k_target_feature,
         macro_at_most_once_rep,
+        macro_attr,
         macro_attributes_in_derive_output,
         macro_concat,
         macro_escape,