Remove register_attr feature

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

compiler/rustc_feature/src/active.rs

@@ -479,8 +479,6 @@ declare_features! (
     (incomplete, raw_dylib, "1.40.0", Some(58713), None),
     /// Allows `&raw const $place_expr` and `&raw mut $place_expr` expressions.
     (active, raw_ref_op, "1.41.0", Some(64490), None),
-    /// Allows using the `#[register_attr]` attribute.
-    (active, register_attr, "1.41.0", Some(66080), None),
     /// Allows using the `#[register_tool]` attribute.
     (active, register_tool, "1.41.0", Some(66079), None),
     /// Allows the `#[repr(i128)]` attribute for enums.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -458,10 +458,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     gated!(ffi_pure, Normal, template!(Word), WarnFollowing, experimental!(ffi_pure)),
     gated!(ffi_const, Normal, template!(Word), WarnFollowing, experimental!(ffi_const)),
-    gated!(
-        register_attr, CrateLevel, template!(List: "attr1, attr2, ..."), DuplicatesOk,
-        experimental!(register_attr),
-    ),
     gated!(
         register_tool, CrateLevel, template!(List: "tool1, tool2, ..."), DuplicatesOk,
         experimental!(register_tool),

compiler/rustc_feature/src/removed.rs

@@ -163,6 +163,9 @@ declare_features! (
     (removed, quad_precision_float, "1.0.0", None, None, None),
     (removed, quote, "1.33.0", Some(29601), None, None),
     (removed, reflect, "1.0.0", Some(27749), None, None),
+    /// Allows using the `#[register_attr]` attribute.
+    (removed, register_attr, "CURRENT_RUSTC_VERSION", Some(66080), None,
+     Some("removed in favor of `#![register_tool]`")),
     /// Allows using the macros:
     /// + `__diagnostic_used`
     /// + `__register_diagnostic`