Add hard error and migration lint for unsafe attrs

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1145,10 +1145,6 @@ pub fn is_valid_for_get_attr(name: Symbol) -> bool {
     })
 }
 
-pub fn is_unsafe_attr(name: Symbol) -> bool {
-    BUILTIN_ATTRIBUTE_MAP.get(&name).is_some_and(|attr| attr.safety == AttributeSafety::Unsafe)
-}
-
 pub static BUILTIN_ATTRIBUTE_MAP: LazyLock<FxHashMap<Symbol, &BuiltinAttribute>> =
     LazyLock::new(|| {
         let mut map = FxHashMap::default();

compiler/rustc_feature/src/lib.rs

@@ -125,7 +125,7 @@ pub use accepted::ACCEPTED_FEATURES;
 pub use builtin_attrs::AttributeDuplicates;
 pub use builtin_attrs::{
     deprecated_attributes, encode_cross_crate, find_gated_cfg, is_builtin_attr_name,
-    is_unsafe_attr, is_valid_for_get_attr, AttributeGate, AttributeTemplate, AttributeType,
+    is_valid_for_get_attr, AttributeGate, AttributeSafety, AttributeTemplate, AttributeType,
     BuiltinAttribute, GatedCfg, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
 };
 pub use removed::REMOVED_FEATURES;